• View in gallery
    Figure 1—

    Mean ± SEM heart rate (HR), CI, MAP, DO2, and SVR in 8 horses that were anesthetized with halothane (open circles) and isoflurane (closed circles) and underwent stifle joint arthroscopy in a crossover study. *Significant (P<0.05) difference between treatment groups at this sampling interval. tValue significantly (P < 0.05) different from value at sampling interval 1 (baseline).

  • View in gallery
    Figure 2—

    Box-and-whisker plot of gastrointestinal motility score determined via abdominal auscultation before induction of anesthesia (B) and at intervals after the end of anesthesia in 8 horses anesthetized with halothane and isoflurane in a crossover study. At each time point, 50% of the data is contained within the box, the horizontal bold line represents the median value, and the whiskers represent the range of score. *Significant (P < 0.05) difference between halothane-treated horses and isoflurane-treated horses at this postanesthetic assessment.

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    Figure 3—

    Mean ± SEM fecal chromium concentration determined at intervals after induction of anesthesia in 8 horses that received halothane (open circles) and isoflurane (closed circles) and underwent stifle joint arthroscopy in a crossover study. Fecal samples were collected at 2-hour intervals until 16 hours and then at 6-hour intervals until 106 hours (data from 5 horses available at 100 and 106 hours).

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Comparison of hemodynamic, clinicopathologic, and gastrointestinal motility effects and recovery characteristics of anesthesia with isoflurane and halothane in horses undergoing arthroscopic surgery

Sumit DurongphongtornDepartment of Clinical Studies, Ontario Veterinary College, University of Guelph, Guelph, ON N1G 2W1, Canada.
Department of Clinical Studies, Ontario Veterinary College, University of Guelph, Guelph, ON N1G 2W1, Canada.
Department of Clinical Studies, Ontario Veterinary College, University of Guelph, Guelph, ON N1G 2W1, Canada.
Department of Clinical Studies, Ontario Veterinary College, University of Guelph, Guelph, ON N1G 2W1, Canada.

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Wayne N. McDonellDepartment of Clinical Studies, Ontario Veterinary College, University of Guelph, Guelph, ON N1G 2W1, Canada.

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Carolyn L. KerrDepartment of Clinical Studies, Ontario Veterinary College, University of Guelph, Guelph, ON N1G 2W1, Canada.

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Francisco J. Teixeira NetoDepartment of Clinical Studies, Ontario Veterinary College, University of Guelph, Guelph, ON N1G 2W1, Canada.

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Kuldip K. MirakhurDepartment of Clinical Studies, Ontario Veterinary College, University of Guelph, Guelph, ON N1G 2W1, Canada.

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Abstract

Objective—To compare hemodynamic, clinicopathologic, and gastrointestinal motility effects and recovery characteristics of halothane and isoflurane in horses undergoing arthroscopic surgery.

Animals—8 healthy adult horses.

Procedure—Anesthesia was maintained with isoflurane or halothane (crossover study). At 6 intervals during anesthesia and surgery, cardiopulmonary variables and related derived values were recorded. Recovery from anesthesia was assessed; gastrointestinal tract motility was subjectively monitored for 72 hours after anesthesia. Horses were administered chromium, and fecal chromium concentration was used to assess intestinal transit time. Venous blood samples were collected for clinicopathologic analyses before and 2, 24, and 48 hours after anesthesia.

Results—Compared with halothane-anesthetized horses, cardiac index, oxygen delivery, and heart rate were higher and systemic vascular resistance was lower in isoflurane-anesthetized horses. Mean arterial blood pressure and the dobutamine dose required to maintain blood pressure were similar for both treatments. Duration and quality of recovery from anesthesia did not differ between treatments, although the recovery periods were somewhat shorter with isoflurane. After isoflurane anesthesia, gastrointestinal motility normalized earlier and intestinal transit time of chromium was shorter than that detected after halothane anesthesia. Compared with isoflurane, halothane was associated with increases in serum aspartate transaminase and glutamate dehydrogenase activities, but there were no other important differences in clinicopathologic variables between treatments.

Conclusions and Clinical Relevance—Compared with halothane, isoflurane appears to be associated with better hemodynamic stability during anesthesia, less hepatic and muscle damage, and more rapid return of normal intestinal motility after anesthesia in horses undergoing arthroscopic procedures.

Abstract

Objective—To compare hemodynamic, clinicopathologic, and gastrointestinal motility effects and recovery characteristics of halothane and isoflurane in horses undergoing arthroscopic surgery.

Animals—8 healthy adult horses.

Procedure—Anesthesia was maintained with isoflurane or halothane (crossover study). At 6 intervals during anesthesia and surgery, cardiopulmonary variables and related derived values were recorded. Recovery from anesthesia was assessed; gastrointestinal tract motility was subjectively monitored for 72 hours after anesthesia. Horses were administered chromium, and fecal chromium concentration was used to assess intestinal transit time. Venous blood samples were collected for clinicopathologic analyses before and 2, 24, and 48 hours after anesthesia.

Results—Compared with halothane-anesthetized horses, cardiac index, oxygen delivery, and heart rate were higher and systemic vascular resistance was lower in isoflurane-anesthetized horses. Mean arterial blood pressure and the dobutamine dose required to maintain blood pressure were similar for both treatments. Duration and quality of recovery from anesthesia did not differ between treatments, although the recovery periods were somewhat shorter with isoflurane. After isoflurane anesthesia, gastrointestinal motility normalized earlier and intestinal transit time of chromium was shorter than that detected after halothane anesthesia. Compared with isoflurane, halothane was associated with increases in serum aspartate transaminase and glutamate dehydrogenase activities, but there were no other important differences in clinicopathologic variables between treatments.

Conclusions and Clinical Relevance—Compared with halothane, isoflurane appears to be associated with better hemodynamic stability during anesthesia, less hepatic and muscle damage, and more rapid return of normal intestinal motility after anesthesia in horses undergoing arthroscopic procedures.

Compared with anesthesia in humans and small animals, anesthesia in horses is associated with much higher risk and has a morbidity rate as high as 1%.1–3 Many of the perianesthetic problems that are observed in this species, such as poor recovery or myopathy following anesthesia, are related to the development of hypotension and presumably inadequate tissue perfusion during anesthesia.4–7 Anesthesia and surgery in horses have also been associated with the development of postoperative ileus, clinical signs of colic, and intestinal impaction.8,9 In 1 retrospective study8 of horses with impaction of the large colon, previous anesthesia and arthroscopic surgery were contributing factors in 19 of 147 cases. In another study10 designed to determine prevalence and risk factors for development of ileus of the large intestine after surgery in horses, 10 of 85 (12%) horses developed signs of colic after orthopedic surgery. These delayed postoperative complications may have been associated with the choice of anesthetic agents, but this factor was not investigated in any detail. Although inhalation anesthesia is the most practical and effective means of management of prolonged anesthetic episodes in many species, horses are particularly sensitive to the cardiopulmonary-depressant effects of inhalant anesthetics.11–14

Halothane and isoflurane are commonly used for maintenance of anesthesia in horses. Both agents produce dose-related depression of cardiovascular function, which is less severe during spontaneous ventilation and associated hypercapnia.12 The 2 anesthetic agents produce similar circulatory effects during controlled ventilation with eucapnia12; however, the period of recovery from anesthesia with isoflurane is significantly shorter than recovery from anesthesia with halothane.15 Doppler ultrasonographic studies5,14 have revealed that muscular blood flow to the hind limbs is better maintained with isoflurane than halothane. In a comparative clinical study16 of the 2 agents involving 38 juvenile and adult horses undergoing surgery because of colic, there was no detectable difference in the cardiovascular responses to these agents, although recovery from isoflurane was more rapid; other studies17,18 in 18 pregnant mares and 58 foals yielded similar results. Unfortunately, none of these studies included CO measurements, which is an important limitation because CO is a major determinant of systemic (and local organ) DO2.19 Decreases in muscle DO2 may be a major factor in the development of postanes-thetic myopathy, and in a recent report,20 the importance of CO in maintaining adequate intramuscular blood flow was confirmed. In an important stress study,21 horses underwent a deliberate hypoxic challenge (PaO2, 50 mm Hg) for 3 hours during either isoflurane or halothane anesthesia. In isoflurane-anesthetized horses, there was less postanesthetic muscle dysfunction, less evidence of postoperative hepatorenal dysfunction, and an improved mental attitude after anesthesia, compared with findings in halothane-anesthetized horses.21

Cardiac output is generally higher in horses anesthetized with isoflurane than in horses anesthetized with halothane. A highly clinically relevant investigation22 was carried out as a randomized crossover study involving 8 research horses; anesthesia was induced by use of xylazine, guaifenesin, and ketamine, and the horses were placed in right lateral recumbency and allowed to breathe spontaneously. Anesthesia was maintained via inhalation of either isoflurane or halothane. Cardiac output was higher in isoflurane-anesthetized horses than in halothane-anesthetized horses at 45 and 60 minutes after the onset of anesthesia but not at other times up to 90 minutes. It is quite possible, however, that this improvement in CO may have been attributable to more pronounced hypercapnia at the time points in question.23,24

In contrast to studies involving experimental animals wherein a minimum of other drugs are typically used, most clinical abdominal and arthroscopic surgeries in horses are performed after the patients have been sedated with α2-adrenoceptor agonists and anesthesia is induced via an IV regimen; horses are placed in dorsal recumbency, and IPPV is applied to maintain eucapnia. Surgical stimuli may improve arterial blood pressure as a result of increased SVR.25 However, this increase in blood pressure may be associated with a decrease in CO, and the improvement in blood pressure may not result in adequate blood flow to vital organs and muscles.23–26 Positive inotropic agents, such as dobutamine, are now commonly used to treat anesthetic-induced hypotension in clinical settings. In horses, dobutamine administered at low infusion rates (3.0 to 5.0 μg/kg/min) increases myocardial contractility, CO, and arterial blood pressure, without significantly changing SVR.27 It is not yet known whether isoflurane would improve hemodynamic function, compared with halothane, when equipotent doses are used as part of a typical clinical anesthesia regimen in horses undergoing surgery (with inotrope administration to maintain blood pressure). In a recent epidemiologic multicenter study,3 there was no evidence that the use of isoflurane decreased anesthesia-associated mortality rate, compared with use of halothane; in fact, isoflurane appeared to be associated with a higher mortality rate.

The purpose of the study reported here was to compare intraoperative hemodynamic effects, recovery characteristics, and postanesthetic clinicopathologic and gastrointestinal motility effects of halothane and isoflurane in horses undergoing arthroscopic surgery. Each horse was anesthetized once with each of the inhalant anesthetics in a crossover study. For each anesthetic episode, horses were sedated and anesthesia was induced in a routine manner; a standardized analgesia protocol was used postoperatively.

Materials and Methods

The study was approved by the University of Guelph Animal Care Committee. Eight horses that were free of clinically apparent cardiopulmonary disease (determined on the basis of findings of a clinical examination and arterial blood gas analysis) were initially used in a randomized crossover study. Each horse was anesthetized once with halothane and once with isoflurane, with an interval of at least 3 months between each anesthetic episode. During periods of anesthesia, the horses underwent arthroscopic procedures; the first surgery involved arthroscopy of both stifle joints, and the second surgery involved arthroscopy of 1 stifle joint. Two of these initial 8 horses were euthanatized during the study; therefore, an additional 2 horses were included in the investigational group to provide a minimal sample size of 8 horses. These additional horses received the inhalant anesthetics in the same order of exposure as the horses they replaced. Four female and 4 castrated male horses that weighed 380 to 616 kg (mean ± SEM weight, 467.4 ± 18 kg) completed the study.

Preparation and instrumentation—Food was withheld from the horses for 18 hours prior to each anesthetic episode. Before instrumentation, each horse was assigned a subjective gastrointestinal motility score via auscultation, according to a previously described and validated method (Appendix 1).28,29 Briefly, each of the 4 abdominal quadrants (ie, upper, lower, left, and right quadrants) was auscultated at 2 sites for at least 2 minutes. Gastrointestinal motility scores were assigned by a person (FJTN) who was unaware of the anesthetic treatments and had experience with the scoring system and abdominal auscultation; a subjective score of 0 to 4 was assigned for each quadrant. The gastrointestinal tract motility score represented the sum of all quadrants, ranging from 0 (absence of motility) to 16 (normal intestinal sounds in all quadrants). After application of local anesthesia (0.5 mL of 2% lidocaine, SC) to the selected site, a 14-gauge over-the-needle catheter was introduced into the left jugular vein for IV administration of drugs and fluids. A 120-cm-long polyethylene catheter with an internal diameter of 1.5 mm was introduced through a preplaced 10-gauge jugular catheter into the right atrium for the injection of lithium chloride and measurement of CVP. Catheter placement was confirmed by observation of the pressure waveform. Thirty to 45 minutes before induction of anesthesia, chromium oxide (a solid-phase marker) was administered as slurry (15 g of chromium oxide in 5 L of water) through a nasogastric tube to provide an estimate of intestinal transit time.30

Experimental protocol—For each anesthetic episode, horses were premedicated with romifidinea (0.1 mg/kg, IV). After 5 minutes, anesthesia was induced with ketamine (2 mg/kg, IV) and diazepam (0.04 mg/kg, IV); horses were intubated, and the endotracheal tube was connected to a large-animal anesthetic circuit.b Anesthesia was main tained by use of halothanec or isofluraned in oxygen. The horses were positioned in dorsal recumbency on a surgical table covered with a 30-cm pad. Horses were monitored continuously by use of an ECGe (base-apex lead) to determine heart rate and rhythm. A 20-gauge catheter was placed in the facial artery for measurement of blood pressure and for withdrawal of arterial blood samples for blood gas analysis and lithium detection. Approximately 15 minutes after induction of anesthesia, IPPV (respiratory rate, 5 breaths/min; tidal volume, 15 mL/kg) was commenced and adjusted to achieve an end-tidal carbon dioxide concentration of 40 to 45 mm Hg. By use of a sidestream infrared gas analyzer,e airway gases were continuously measured in gas samples taken from the distal end of the endotracheal tube. Vaporizer dial settings were adjusted to maintain anesthesia at 1.3 times the minimal alveolar concentration for each agent (1.2% end-tidal concentration for halothane and 1.6% end-tidal concentration for isoflurane). When there was a need to deepen the level of anesthesia (determined on the basis of a sharp increase in blood pressure, detection of nystagmus, or movement of the horse), small incremental doses of ketamine (0.1 to 0.2 mg/kg) were administered IV. The frequency and amount of ketamine supplementation were recorded. After the first sample collection and just before the start of surgery, butorphanol (0.04 mg/kg, IV) was administered for analgesia. The arterial pressure and CVP transducers were calibrated before each experiment by use of a mercury column, and the airway gas monitor was simultaneously calibrated by use of a commercial reference gas. Lactated Ringer's solution was administered (5 mL/kg/h, IV) throughout each anesthetic episode. Mean arterial blood pressure was maintained at a mean value of ≥ 70 mm Hg via dobutamine infusions (given to effect)starting after the interval 1data collection;; the infusion rate and duration of dobutamine administration were recorded. To standardize the dobutamine infusion, the initial infusion rate was 1.0 μg/kg/min. If the MAP value was not increased to acceptable levels (ie, 70 to 80 mm Hg) within 5 minutes after the beginning of the infusion, then the infusion rate was increased in 0.5 to 1 μg/kg/min increments every 2 minutes until the desired effect was attained. If MAP values increased above the maximum desired value (80 mm Hg), the infusion rate was decreased or discontinued until stabilization of MAP within the desired range. The mean infusion rate of dobutamine was recorded.

Cardiac output was estimated by use of a LiDCO technique31–33 and a commercial computer.f Recommendations for inputting plasma sodium and blood hemoglobin concentrations were followed with each CO determination. The sensor (a flow-through cell housing a lithium-selective electrode) was connected to the arterial catheter. When a CO determination was made, 15 mL (2.25 mmol) of lithium chloride was injected through the right atrial catheter while arterial blood passed through the sensor at a flow rate (4 mL/min) that was controlled by a small battery-operated pump. A single CO determination was made at each sampling interval during a brief period of apnea achieved by turning off the ventilator.

In a previous studyg in our laboratory, it was determined that the mean difference between 35 duplicate LiDCO estimations of CO was 8.7%. The CO determinations were made after determination of blood pressure values and collection of arterial blood samples for blood gas analysis.

Cardiovascular variables (heart rate, CO, CVP, MAP, SAP, and DAP) were assessed, and arterial blood gas samples were obtained at 6 intervals. Interval 1 was 30 minutes after induction of anesthesia; interval 2 was before the start of surgery; interval 3 was immediately after the commencement of surgery (following skin incision and joint distension); intervals 4 and 5 were during maximal surgical stimulation (rongeuring, drilling, hammering, or cartilage biopsy), and interval 6 was after the completion of surgery but before the anesthetic dose was decreased for recovery from anesthesia. Times of data collections did not differ between isoflurane- and halothane-anesthetized horses. Blood gas analyses (including assessment of pH, PaO2, PaCO2 [by measurement], and base excess [derived value]) were carried out within 5 minutes of blood collection by use of an automated system that was calibrated daily with reference samples. Values were corrected to body temperature. Arterial oxygen concentration was obtained by use of a hemoximeter.h Derived hemodynamic and respiratory parameters were calculated according to standard formulas as follows: stroke volume (mL/beat) = CO/heart rate; CI (mL/kg/min) = CO/body weight; SVR (dyn·s/cm5) = (MAP - CVP)/CO × 79.9; and DO2 (mL/kg/min) = CI × CaO2/100.

Jugular venous blood samples were collected for CBC and serum biochemical analyses before induction of anesthesia and at 2, 24, and 48 hours after discontinuation of anesthesia. Hematocrit was assessed, and serum biochemical analyses included determinations of total protein, urea, crea-tinine, total bilirubin, and free bilirubin concentrations and alkaline phosphatase, γ-glutamyltransferase, aspartate transaminase, creatine kinase, and glutamate dehydrogenase activities.

After an arthroscopic procedure was completed, the stifle joint was infused with 10 mL of 0.5% bupivacaine; phenylbutazone (2.2 mg/kg, IV) was administered to each horse to assist in providing analgesia. After the return of spontaneous respiration, the horses were transferred to a 3.6 × 3.6-m dimly lit padded recovery room. The endotracheal tube was left in place, and 100% oxygen (15 L/min) was insufflated into the endotracheal tube until the horse stood or disconnected the oxygen line through movement. Romifidine (0.02 mg/kg, IV) was administered soon after the transfer to the recovery room to provide sedation during the recovery period. Recovery characteristics were recorded by a person (FJTN) who was unaware of the inhalant anesthetic used and included time of first movement, time of first attempt to stand, time of standing, number of attempts to stand, and subjective recovery score. The recovery score (1 to 5) was assigned as previously described; a score of 1 represented an excellent recovery, and 5 represented a very poor recovery (Appendix 2).34Total surgical time (the time from the first skin incision until placement of the last skin suture) and total anesthetic time (the time from anesthetic induction to disconnection from the anesthetic machine) were also recorded.

After surgery and recovery from anesthesia, each horse was monitored at 2-hour intervals for 24 hours and at 6-hour intervals from 24 to 72 hours. Monitoring consisted of assessment of the horse's general demeanor, heart rate, respiratory rate, signs of abdominal discomfort (if any), and appetite as per routine postanesthetic care. As described, gastrointestinal tract motility was evaluated via auscultation at 2, 4, 6, 8, 10, 12, 24, 48, and 72 hours after the end of the anesthetic episode. Fecal samples for chromium determination were collected from the rectum starting 2 hours after anesthesia and at 2-hour intervals until 16 hours; then, samples were collected at 6-hour intervals until at least 94 hours after anesthesia. After fecal chromium analysis of the first 3 horses, the collection time for the remaining 5 horses was extended to 106 hours. The chromium concentration in dry fecal matter was determined by use of an atomic absorption spectrometer.i Time-concentration curves were prepared for each horse and the group of horses. An estimate of the actual peak concentration time was obtained through the use of curve-fitting softwarej to establish the best equation that approximated the curves. Then, the curve was drawn by use of a commercial software programk to determine the time of peak fecal chromium concentration for that horse. The group mean time for peak fecal chromium concentration was based on the peaks of the individual curves.

Statistical analysis—Mean ± SEM values of cardiopul-monary, biochemical, hematologic, and recovery data were calculated. A repeated-measures ANOVA was used to compare mean values of cardiopulmonary and serum biochemical data from isoflurane- and halothane-anesthetized horses and determine whether temporal responses between agents existed. Statistical analyses were performed with a statistical software packagek on raw cardiopulmonary and serum biochemical data; however, some biochemical data were logarithmically transformed for the analysis. For the analyses of the cardiopulmonary data, baseline data were those obtained 30 minutes after induction of anesthesia; for serum biochemical data analyses, baseline data were those obtained before anesthesia. When a significant time effect was detected, data were compared with baseline values by use of a Dunnett adjustment. When a significant treatment and time interaction was detected, a Tukey adjustment was used to compare the treatments at each time interval. The subjective gastrointestinal motility scores for isoflurane- and halothane-anesthetized horses were analyzed and compared with an independent test (paired t test or Wilcoxon signed rank if data were not normally distributed) at each time interval. The time to peak fecal chromium concentration was analyzed by use of a paired t test. Time events (time at each sampling interval, total surgical and anesthetic time, time until first attempt to stand, and time until standing) and the mean infusion rate of dobutamine required were analyzed by use of a paired t test. Recovery characteristics (number of attempts to stand and recovery score) and ketamine supplementation were analyzed by use of a Wilcoxon signed rank test. Results are stated as mean values ± SEM; significance for all statistical tests was set at a value of P < 0.05.

Results

Induction of anesthesia was smooth in all horses. During the transfer to inhalation anesthesia, 6 horses in both the halothane and isoflurane treatment groups required additional doses of ketamine (mean dose, 115 mg; range, 100 to 150 mg) because signs of a light plane of anesthesia (ie, nystagmus and eye blinking) were detected. During surgery, 2 horses of each group required additional doses of ketamine (100 to 450 mg) to control minor movement. The times of data collections did not differ between isoflurane- and halothane-anesthetized horses. Relative to the time of induction of anesthesia (0 minutes) in isoflurane- and halothane-anesthetized horses, interval 1 measurements of cardiopulmonary variables were completed at 33.1 ± 4.5 minutes and 33.1 ± 5.0 minutes, respectively; interval 2 measurements were completed at 55 ± 12.3 minutes and 54.1 ±5.7 minutes, respectively; interval 3 measurements were completed at 68.6 ± 16.2 minutes and 71.1 ± 11.3 minutes, respectively; interval 4 measurements were completed at 94 ± 20.8 minutes and 86.7 ± 13.2 minutes, respectively; interval 5 measurements were completed at 112.4 ± 26.7 minutes and 103 ± 12.1 minutes, respectively; and interval 6 measurements (end of surgery) were completed at 145.2 ± 44.5 minutes and 140 ± 22.1 minutes, respectively.

Horses with postoperative complications—Data from 2 horses were not included in the statistical analyses because the horses were euthanatized after the first anesthetic episode. Both horses had received halothane and had undergone bilateral stifle joint arthroscopy; throughout the period of anesthesia, the hemodynamic function of each horse had been stable. At 6 hours after the end of the anesthetic episode, 1 horse was unable to stand despite assistance and treatment with an analgesic. The surgical procedures in that horse had been difficult and prolonged, and muscle or nerve injury was suspected. Euthanasia was carried out for humane reasons by use of an overdose of pentobarbital administered IV. At necropsy, there was evidence of unilateral femoral nerve hemorrhage. The other horse developed signs of abdominal discomfort 48 hours after the end of anesthesia and did not respond to administration of flunixin meglumine (1.1 mg/kg, IV), fluids, and walking. At 8 hours after anesthesia, the gastrointestinal motility score was 14 (maximum possible score, 16). Euthanasia was carried out by use of an overdose of pentobarbital administered IV 52 hours after the end of anesthesia because of intractable colic, and necropsy findings indicated cecal impaction and cecal rupture.

Horses for which the investigation was completed—Because 2 study horses were euthanatized after their first anesthetic episode (in which they received halothane), their replacement with 2 additional horses disrupted the randomization of isoflurane and halothane administrations with regard to the surgical procedures (unilateral or bilateral arthroscopy). As a result of the desire to maintain randomization of the order of anesthetic administration, 5 of the 8 horses anesthetized with isoflurane underwent arthroscopy of both stifle joints, whereas 3 of the 8 horses anesthetized with halothane underwent arthroscopy of both stifle joints.

During the period of surgical stimulation, heart rate, CI, and DO2 were higher and SVR was lower in isoflurane-anesthetized horses, compared with findings in halothane-anesthetized horses (Figure 1). Inotrope administration was needed in all horses, generally from immediately after the first sampling interval to the start of the period of maximal surgical stimulation. The dose of dobutamine required to maintain the target MAP value was not significantly different between treatments (mean dobutamine dose was 0.9 ± 1 μg/kg/min and 0.6 ± 1 μg/kg/min for isoflurane- and halothane-anesthetized horses, respectively). Values of MAP, SAP, and DAP in horses treated with isoflurane or halothane were not different during any sampling interval, but arterial blood pressures in both treatment groups were higher than baseline values (measured at 30 minutes after induction of anesthesia) at all subsequent sampling intervals (Table 1). Overall, there was no difference in CVP or stroke volume between treatment groups; however, in isoflurane- and halothane-anesthetized horses, CVP was higher at interval 2 (before surgery) and stroke volume was higher at intervals 3 and 4 (during early surgery and the first maximal surgical stimulation), compared with baseline values recorded at interval 1. During anesthetic episodes, mean ± SEM PaCO2 values ranged from 42.8 ± 1 to 47.1 ± 1.1; in isoflurane- and halothane-anesthetized horses, values during the first and second maximal surgical stimuli and at the end of surgery were slightly higher (albeit significantly) than baseline values recorded at interval 1. Mean PaO2 values in horses anesthetized with isoflurane or halothane did not differ, and within either treatment group, there was no difference between the baseline value and any sampling interval, with the exception of mean PaO2 value in halothane-anesthetized horses at interval 5. With both treatments, CaO2was significantly higher at intervals 3 and 4, compared with baseline values obtained at interval 1. Base excess and arterial blood pH were within reference ranges and did not differ between treatments at any sampling interval; however, minor changes within treatment groups were detected over time. In isoflurane- and halothane-anesthetized horses, base excess values increased from baseline values throughout the anesthetic episode, and at interval 6, this difference was significant in both groups.

Table 1—

Cardiovascular variables (mean ± SEM) determined in 8 horses that were anesthetized with isoflurane (I) and halothane (H) and underwent stifle joint arthroscopy in a crossover study. Variables were assessed 30 minutes after induction of anesthesia (interval 1 [baseline]), before the start of surgery (interval 2), immediately after the start of surgery (interval 3), twice during maximal surgical stimulation (intervals 4 and 5), and at the end of surgery (interval 6).

VariableTreatmentBefore surgeryAfter beginning of surgery
Interval 1Interval 2Interval 3Interval 4Interval 5Interval 6
SAP (mm Hg)I95.7 ± 4.9125.7 ± 5.9*130.4 ± 8.1*125.6 ± 5.8*112.6 ± 3.5*108.4 ± 3.2*
H96.2 ± 4.5111.6 ± 3.4*119.9 ± 3.1*121 ± 7*114 ± 3.6*108.5 ± 2.9*
DAP (mm Hg)I47.9 ± 4.256.6 ± 1.7*54.7 ± 1.8*62.1 ± 1.8*64.5 ± 3.1*67.2 ± 3*
H46.4 ± 2.556.1 ± 2.6*63.1 ± 3.6*63.7 ± 1.6*66.6 ± 2.6*67 ± 2.3*
CVP (mm Hg)I4.2 ± 1.86.2 ± 1.6*5.9 ± 1.55.2 ± 0.94.4 ± 0.94.4 ± 1
H3.6 ± 1.18.7 ± 1.2*6.4 ± 0.94.7 ± 14.5 ± 1.24.6 ± 1 ± .3
CO (L/min)I22.5 ± 1.623.4 ± 1.529.5 ± 2.333.7 ± 2.9*31.4 ± 4.4*27.6 ± 3.2
H18.6 ± 1.618.9 ± 2.121.1 ± 1.419.4 ± 1.916.2 ± 1.216.6 ± 1.3
Stroke volume (mL/beat)I674 ± 59779 ± 49908 ± 62*875 ± 76*734 ± 93640 ± 75
H599 ± 66698 ± 80757 ± 71*661 ± 83*580 ± 68531 ± 57
CaO2 (dL/L)I18.9 ± 0.418.3 ± 0.520.4 ± 0.6*20.6 ± 0.5*19.2 ± 0.618 ± 0.7
H17.7 ± 0.618.8 ± 0.720.7 ± 0.5*21.4 ± 0.8*18.5 ± 0.618.3 ± 0.7
Arterial pHI7.42 ± 0.017.42 ± 0.017.39 ± 0.01*7.40 ± 0.01*7.41 ± 0.017.41 ± 0.01
H7.41 ± 0.017.42 ± 0.017.41 ± 0.01*7.38 ± 0.02*7.40 ± 0.017.41 ± 0.01
PaCO2 (mm Hg)I42.8 ± 143.1 ± 1.245.4 ± 1.445.5 ± 1.2*44.6 ± 0.9*44.9 ± 0.8*
H43.3 ± 0.943.4 ± 0.645.8 ± 1.346.7 ± 0.9*47.1 ± 1.1*46.9 ± 1.1*
PaO2 (mm Hg)I340 ± 57313 ± 62326 ± 58359 ± 60344 ± 52342 ± 50
H349 ± 38349 ± 43319 ± 56337 ± 42284 ± 43*315 ± 49
Base excess (mmol/L)I2.9 ± 0.63.2 ± 0.62.1 ± 0.62.8 ± 0.53.4 ± 0.54 ± 0.4*
H2.6 ± 0.63 ± 0.72.9 ± 0.72.1 ± 0.93.4 ± 0.74.2 ± 0.7*

Value significantly (P<0.05) different from baseline (interval 1) value within treatment group.

Value in horses administered isoflurane significantly (P<0.05) different from value in horses administered halothane.

Figure 1—
Figure 1—

Mean ± SEM heart rate (HR), CI, MAP, DO2, and SVR in 8 horses that were anesthetized with halothane (open circles) and isoflurane (closed circles) and underwent stifle joint arthroscopy in a crossover study. *Significant (P<0.05) difference between treatment groups at this sampling interval. tValue significantly (P < 0.05) different from value at sampling interval 1 (baseline).

Citation: American Journal of Veterinary Research 67, 1; 10.2460/ajvr.67.1.32

Total surgical time, total anesthetic time, and characteristics assessed during recovery from anesthesia with either of the 2 agents did not differ significantly. In isoflurane- and halothane-anesthetized horses, total surgical time was 83.3 ± 11.7 minutes and 57.3 ± 7.9 minutes, respectively; total anesthetic time was 168.8 ± 13.7 minutes and 155 ± 6.7 minutes, respectively; time to first movement was 34.3 ± 5.9 minutes and 37.5 ± 5.4 minutes, respectively; and time to first attempt to stand was 44.6 ± 5.6 minutes and 61.4 ± 8.5 minutes, respectively. In halothane-anesthetized horses, time to standing was longer (albeit not significantly) than results for isoflurane-anesthetized horses (63.5 ± 8.7 minutes vs 55.9 ±8.1 minutes). Compared with recovery from anesthesia with halothane, recovery from anesthesia with isoflurane appeared to be associated with more attempts to stand (1.9 ± 0.2 vs 1.3 ± 0.2), but this difference was not significant. The subjective recovery score for horses anesthetized with isoflurane was 1.8 ± 0.2, which was not significantly different from that for horses anesthetized with halothane (1.3 ± 0.2).

At 4 and 10 hours after anesthesia, the mean gastrointestinal motility score for halothane-anesthetized horses was significantly lower than that for isoflurane-anesthetized horses (Figure 2). Individual horses anesthetized with halothane had a slow return of intestinal sounds detectable via auscultation. Results of fecal chromium analysis indicated that horses anesthetized with isoflurane had an early onset of detectable chromium in the feces and a more rapid decrease from peak fecal chromium concentration, compared with horses anesthetized with halothane (Figure 3). Mean time to peak fecal chromium concentration was significantly shorter in isoflurane-anesthetized horses (41.5 ± 6.6 hours), compared with findings in halothane-anesthetized horses (55.3 ± 10.2 hours).

Figure 2—
Figure 2—

Box-and-whisker plot of gastrointestinal motility score determined via abdominal auscultation before induction of anesthesia (B) and at intervals after the end of anesthesia in 8 horses anesthetized with halothane and isoflurane in a crossover study. At each time point, 50% of the data is contained within the box, the horizontal bold line represents the median value, and the whiskers represent the range of score. *Significant (P < 0.05) difference between halothane-treated horses and isoflurane-treated horses at this postanesthetic assessment.

Citation: American Journal of Veterinary Research 67, 1; 10.2460/ajvr.67.1.32

Figure 3—
Figure 3—

Mean ± SEM fecal chromium concentration determined at intervals after induction of anesthesia in 8 horses that received halothane (open circles) and isoflurane (closed circles) and underwent stifle joint arthroscopy in a crossover study. Fecal samples were collected at 2-hour intervals until 16 hours and then at 6-hour intervals until 106 hours (data from 5 horses available at 100 and 106 hours).

Citation: American Journal of Veterinary Research 67, 1; 10.2460/ajvr.67.1.32

All measured hematologic and serum biochemical values were within reference ranges before the onset of anesthesia in both treatment groups. Mean values for serum urea concentration; serum alkaline phosphatase activity; and counts of RBCs, platelets, neutrophils, band neutrophils, and basophils remained within reference limits and did not differ significantly between treatment groups or over time. At 24 and 48 hours after anesthesia, mean serum aspartate transaminase and glutamate dehydrogenase activities were higher in halothane-anesthetized horses than isoflurane-anesthetized horses (Table 2). In both treatment groups, serum creatinine, total bilirubin, and free bilirubin concentrations and serum creatine kinase activity were significantly increased from baseline (preanesthesia) values beginning 2 hours after the end of anesthesia and returned toward baseline values 48 hours after anesthesia. In both treatment groups, serum total protein concentrations and γ-glutamyltransferase activity were significantly decreased from baseline values at 2 hours after the end of anesthesia but all variables increased back to baseline values by 24 or 48 hours after anesthesia. Counts of WBCs, lymphocytes, mono-cytes, and eosinophils remained lower than baseline values from 2 to 48 hours after anesthesia.

Table 2—

Clinicopathologic variables (mean ± SEM) determined in 8 horses that were anesthetized with isoflurane (I) and halothane (H) and underwent stifle joint arthroscopy in a crossover study. Variables were assessed before induction of anesthesia (baseline) and at intervals after the end of anesthesia.

VariableTreatmentBaselineTime after anesthesia
2 h24 h48 h
Hct (L/L)I0.36 ± 0.020.27 ± 0.02*0.33 ± 0.02*0.33 ± 0.0*
H0.41 ± 0.010.27 ± 0.02*0.36 ± 0.01*0.35 ± 0.01*
Total protein (g/L)I66.9 ± 1.756.4 ± 2.1*62.1 ± 4.564.5 ± 1.3
H72 ± 1.557.8 ± 1.7*70.9 ± 1.568.9 ± 1.6
Creatine kinase (U/L)I219 ± 22544 ± 188*334 ± 56*254 ± 27
H223 ± 35673 ± 221695 ± 161406 ± 71
Aspartate transaminase (U/L)I318 ± 18282 ± 21348 ± 34377 ± 34
H305 ± 20259 ± 20569 ± 47*535 ± 45*
γ-Glutamyltransferase (U/L)I10.3 ± 1.48 ± 1.3*10.3 ± 1.712.5 ± 0.8
H12 ± 0.610.4 ± 1*21.9 ± 8.520.4 ± 7.4
Glutamate dehydrogenase (U/L)I2.4 ± 0.22.9 ± 0.3*3.3 ± 0.4*2.9 ± 0.4*
H1.9 ± 0.23.5 ± 1.1*25.8 ± 17.5*16.5 ± 8.1*
Creatinine (U/L)I94 ± 4108 ± 5*110 ± 13*108 ± 13*
H93 ± 3108 ± 6*108 ± 6*109 ± 5*
Total bilirubin (m mol/L)I35.6 ± 7.451.3 ± 7.8*56.6 ± 13.3*43.1 ± 5.8*
H32.3 ± 7.645.9 ± 5*69.8 ± 9.1*46.5 ± 7.1*
Free bilirubin (m mol/L)I33.3 ± 7.549.1 ± 7.8*54.1 ± 13.1*40.9 ± 5.7*
H29.9 ± 7.740.1 ± 5.8*67 ± 9*44.4 ± 7*

Value significantly (P < 0.05) different from baseline (interval 1) value within treatment group.

Value in horses administered isoflurane significantly (P < 0.05) different from value in horses administered halothane.

Discussion

During the period of maximum surgical stimulation (intervals 4 and 5), CO and DO2 were 68% to 87% and 59% to 87% higher, respectively, in horses anesthetized with isoflurane than in horses anesthetized with halothane. In addition, compared with horses anesthetized with halothane, there was a more rapid return of gastrointestinal motility during the postanes-thetic period and less biochemical evidence of subclin-ical muscle and hepatic damage in horses anesthetized with isoflurane. However, these apparent isoflurane-related improvements in cardiovascular stability and postoperative physiologic parameters were not associated with a significant difference in the horses' quality of recovery from anesthesia when analgesic and sedative agents were administered in a routine manner postoperatively.

In the present study, measurement intervals during the anesthetic period were standardized primarily with regard to the presence or absence and level of surgical stimulation; this was based on results of another of our studies,g which indicated that the degree of surgical stimulation associated with arthroscopy can significantly alter hemodynamic function in horses. If the replicate measurements are standardized by time alone, it is inevitable that there will be differences in the degree of ongoing surgical stimulation. The first hemodynamic measurement was standardized at 30 minutes after induction of anesthesia because we wanted to compare the inhalant anesthetic effect in reasonable time proximity to the induction period, once a stable inhalant level was established and before administration of inotropic support was started. As the horses were recovering from anesthesia, inotropic support was provided after 30 minutes to minimize the risk of postanesthetic myopathy.7,35 Administration of an inotrope obviously affects comparisons of subsequent hemodynamic measurements with data obtained at the first sampling interval for both anesthetic agents, but inotrope administration in both treatment groups was based on identical criteria.

Although there were no significant differences in total surgery and anesthetic times between treatment groups, those times were slightly longer in isoflurane-anesthetized horses than in halothane-anesthetized horses. In part, this was because 2 study horses were euthanatized after their first anesthetic episode (in which they received halothane) and their replacement with 2 additional horses disrupted the randomization of isoflurane and halothane administrations with regard to the surgical procedures (unilateral or bilateral arthroscopy). As a result of the desire to maintain randomization of the order of anesthetic administration, 5 of the 8 horses anesthetized with isoflurane underwent arthroscopy of both stifle joints, whereas 3 of the 8 horses anesthetized with halothane underwent arthroscopy of both stifle joints. If anything, the greater surgical stimulation and time for completion of the procedure bilaterally should have biased the results against isoflurane, especially among postoperative assessments.

In our study, comparisons of the cardiovascular effects of halothane and isoflurane in horses revealed findings that were similar to those reported12,14 previously; CI, DO2, and heart rate were higher and SVR was lower in horses during anesthesia with isoflurane, compared with values during anesthesia with halothane (albeit not significantly different at all sampling intervals). Early in the anesthetic period (ie, at sampling interval 1), the differences in CI and DO2 were not significant, perhaps reflecting the influence of the drugs used to induce anesthesia. Nevertheless, at sampling interval 1, mean CI and DO2 values in isoflurane-anesthetized horses were 18% and 26% higher, respectively, compared with findings in halothane-anesthetized horses. With the onset of surgery (sampling interval 3), the differences in mean CI and DO2 values between treatment groups increased to 34% and 30%, respectively, but this degree of difference was not significant because of the variability among horses. Significant and quite profound differences in CI and DO2were detected during and after the periods of maximal surgical stimulation (sampling intervals 4 to 6); the values in horses anesthetized with isoflurane were 58% to 87% greater than those in horses anesthetized with halothane.

There was no difference in SAP, MAP, or DAP measurements between the 2 treatment groups after the first sampling interval, which was expected because blood pressures were stabilized after sampling interval 1 via administration of an inotrope. It is very common for horses that are positioned in dorsal recumbency and maintained at a surgical plane of anesthesia by use of IPPV to become hypotensive, irrespective of the regimen used for induction of anesthesia or the inhalant anesthetic administered.36,37 As there is a well-recognized link between longer periods of hypotension and development of postanesthetic myopathy,4,7 it is our normal clinical practice to administer dobutamine (to effect) to maintain MAP within the range of 70 to 90 mm Hg. For the present study, we followed that clinical protocol but planned to maintain MAP within the range of 70 to 80 mm Hg and measure the amount of inotrope administered. Inotrope administration was needed in all horses, generally from immediately after the first sampling interval to the onset of the period of maximal surgical stimulation; typically, less dobutamine was required when surgery started. Comparing treatments, there was no significant difference in the amount of inotrope administered to anesthetized horses, although isoflurane-anesthetized horses appeared to require a higher dose of dobutamine despite having higher CI values. This was probably because of the lower SVR values in this group. Although administration of dobutamine significantly increased arterial blood pressures from baseline values at sampling interval 2 (about 20 minutes after the first administration of the inotrope), it is interesting to note that stroke volume, CI, SVR, and DO2 values were not significantly increased at this sampling period in horses receiving either anesthetic and heart rate either did not change (in isoflurane-anesthetized horses) or decreased (in halothane-anesthetized horses), compared with baseline values.

During anesthesia of horses in clinical practice, heart rate and arterial blood pressure are commonly monitored and the usual assumption is that the increase in MAP detected when dobutamine is administered equates to an overall improvement in hemodynamic status.13,27 Other investigators38,39 have detected increases in blood pressure in association with surgical interventions, and it is recognized that such a blood pressure increase may not equate to an increase in CI, but rather to an increase in SVR. In the present study, surgical stimulation per se seemed to produce a major effect on hemodynamic function. During anesthesia with halothane, increases in arterial blood pressure and SVR but not CI or DO2 were detected; in sampling intervals 5 and 6, the latter 2 variables were slightly (but not significantly) lower, compared with values in sampling interval 1, whereas heart rate did not change. Previous studies26,38,39,g in which surgical stimulation was determined to be associated with increases in MAP and SVR and a decrease in CI were performed in halothane-anesthetized horses that were not receiving inotropic drugs. To our knowledge, there are no published reports of studies to evaluate the impact of surgical stimulation on hemodynamic function in isoflurane-anesthetized horses. In contrast with reported findings in halothane-anesthetized horses, maximal surgical stimulation in isoflurane-anesthetized horses in our study was associated with an improvement in CI and heart rate, compared with baseline (presurgery) values, whereas SVR did not increase. In general, DO2 was also higher in horses anesthetized with isoflurane, compared with halothane-anesthetized horses, albeit only significantly so during maximal surgical stimulation (intervals 4 and 5); at these sampling intervals, the DO2 values in isoflurane-anesthetized horses were significantly greater than the baseline value. In the present study, changes in DO2 paralleled changes in CI in both treatment groups; this parallelism was not surprising because DO2 is dependent on CO to a large extent. In a study of foals17 undergoing surgical stimulation, heart rate was maintained close to the conscious rate during anesthesia with isoflurane, whereas the rate decreased during anesthesia with halothane. In some comparative drug studies12,22 in mature horses, heart rate remained stable during anesthesia, but Raisis et al14 reported that heart rate was slightly higher during anesthesia with isoflurane than with halothane. During surgical stimulation in the present study, PaCO2 values increased minimally; however, Khanna et al24 reported that such minimal increases of PaCO2 do not alter plasma catecholamine concentrations. Interestingly, the horses in the present study had no temporal improvement in CI during anesthesia with halothane, despite the administration of dobutamine and fluids.

In our study, we did not detect a significant difference between agents in terms of duration of recovery from anesthesia, unlike findings of other studies15,16,22,40 in which isoflurane was associated with shorter recovery times, compared with halothane. However, compared with horses treated with halothane, horses anesthetized with isoflurane in the present study appeared to make a greater number of attempts to stand and had shorter time to the first attempt to stand and time to standing, although these differences were not significant. During recovery from inhalant anesthesia, early attempts to stand made by a horse may increase its risk of injuries as a result of motor incoordination or excessive ataxia. Some clinicians believe that halothane may be preferred over isoflurane because the former is associated with slower recoveries from anesthesia, and thus, by the time the horse first attempts to stand, it is likely less ataxic. Low doses of sedative agents (ie, xylazine and romifidine) have been administered to horses during their recovery from isoflurane anesthesia to slightly prolong the recovery period and delay the time to first attempt to stand because it is believed that these characteristics may improve the overall recovery quality.41 In the present study, administration of romifidine (0.02 mg/kg, IV) to horses for sedation in the recovery period (although clinically relevant) presumably affected data comparisons between the 2 treatment groups. It must also be considered that the sample population (8 horses) may not have been large enough to show a difference in the parameters used to evaluate recovery from anesthesia.

Postoperative ileus has been identified as one of the causes of the high incidence of perianesthetic complications in horses.8–10 Even though prolonged gastrointestinal stasis may be expected as a major complication in the postoperative period of horses undergoing gastrointestinal tract surgery (eg, in the treatment of colic),9,10 a high incidence of intestinal motility—related problems, such as large colon impactions, has been identified in horses undergoing procedures that are not related to the gastrointestinal system, such as arthroscopic surgery.8–10 Multiple factors can be involved in the high incidence of intestinal motility—related problems in horses, including postoperative pain and the motility-depressant effects of anesthetics.8–10,42 Although anesthetics may suppress intestinal motility via multiple mechanisms, pain-related suppression of intestinal motility appears to be attributable to a change in autonomic balance toward a greater prevalence of sympathetic outflow, ultimately resulting in inhibition of intestinal motility. In a previous study,42 anesthesia inhibited gastrointestinal tract motility in horses, as indicated by suppression of the migrating myoelectric complex. One of the horses initially included in the present study developed cecal impaction after its first anesthetic episode (involving halothane) and surgery. It is possible that surgical and anesthetic-related factors contributed to the development of this complication.

Although auscultation of intestinal sounds is a subjective method with which to assess gastrointestinal tract motility, it is a noninvasive method of evaluation and has been used effectively to assess the effects of drugs and anesthesia on intestinal motility.28–30,43 Results of the present study indicated that the period required for gastrointestinal motility (as determined via abdominal auscultation) to return to normal levels was shorter in horses anesthetized with isoflurane, compared with findings in horses anesthetized with halothane. This difference in the rate of return of gastrointestinal motility was also statistically present if evaluated on the basis of the number of horses that had 50% or 75% recovery of their baseline auscultation score at the sampling intervals. Furthermore, intestinal transit time (assessed by use of chromium oxide as a solid-phase marker) was shorter in horses receiving isoflurane. Thus, it appears that horses anesthetized with isoflurane have a more rapid return to normal gastrointestinal tract function after anesthesia and surgery than horses anesthetized with halothane. Interestingly, in a previous study43 of research horses that received similar preanesthetic chromium oxide administration and underwent anesthesia of 3 hours' duration but did not undergo surgery, fecal clearance of the chromium marker was detected in all horses by 96 hours after anesthesia, whereas a more prolonged clearance was detected in the horses of our study. Surgery, the anesthetic regimen, and postoperative conditions may have influenced this result.

In both treatment groups in the present study, Hct was lower than baseline (preanesthesia) values during the 48-hour period following discontinuation of administration of the inhalant anesthetic, whereas serum total protein concentration was lower than baseline only 2 hours after the end of anesthesia. Despite administration of crystalloid solution at a rate of only 5 mL/kg/h, the administration of fluid during the experiments may have caused some hemodilution, which may explain this effect. Increases in serum creatine kinase and aspartate transaminase activities (compared with baseline values) in horses after anesthesia suggest muscle cell injury.6,44 The increases in serum creatine kinase and aspartate transaminase activities in our study were similar to those detected in anesthetized horses after a hypoxic challenge in which the increases were higher in halothane-treated horses than isoflurane-treated horses.21 As expected, the increase (above baseline value) in serum creatine kinase activity was detected earlier than the increase in serum aspartate transaminase activity, but the duration of the latter increase was longer.4 Skeletal muscle blood flow in equids is reported5,14,45 to be higher during anesthesia with isoflurane than during anesthesia with halothane. Maintenance of circulatory function during anesthesia is a crucial factor that affects skeletal muscle perfusion.1,4,46,47 Compared with findings in horses anesthetized with halothane in the present study, anesthesia with isoflurane was associated with less evidence of muscle injury after arthroscopic surgery, as suggested by the smaller increases in serum aspartate transaminase activity at 24 and 48 hours after the end of anesthesia. After anesthesia, serum glutamate dehydroge-nase activity in the halothane-treated horses was notably greater than that detected in isoflurane-treated horses; serum γ-glutamyltransferase activity was also slightly greater, but this difference was not significant. These results are similar to findings of previous studies21,48 in horses and suggest that more hepatocellular damage is associated with halothane anesthesia than isoflurane anesthesia in this species.

In the present comparative study of the effects of anesthesia with isoflurane and with halothane in horses, isoflurane appeared to be associated with better hemodynamic function during arthroscopic surgery, a more rapid return to normal gastrointestinal tract function in the postanesthetic period, and less evidence of muscle injury and hepatic dysfunction after anesthesia. Further large-scale studies involving a homogenous sample population are recommended to evaluate whether these advantages of isoflurane anesthesia would result in better long-term outcomes (ie, less incidence of complications such as cecal or large colon impactions and lower morbidity and mortality rates) in horses, compared with halothane anesthesia.

CO

Cardiac output

DO2

Delivery of oxygen

IPPV

Intermittent positive-pressure ventilation

SVR

Systemic vascular resistance

CVP

Central venous pressure

MAP

Mean arterial blood pressure

LiDCO

Lithium chloride dilution CO

SAP

Systolic arterial blood pressure

DAP

Diastolic arterial blood pressure

CaO2

Arterial oxygen concentration

CI

Cardiac index

a

Sedivet, Boerhinger Ingelheim Canada, Burlington, ON, Canada.

b

Large animal control center, North America Dragger, Telford, Pa.

c

Halothane BP, Bimeda-MCT Pharmaceuticals, Cambridge, ON, Canada.

d

Isoflo, Abbott Laboratories Ltd, Montreal, QC, Canada.

e

Criticare 1100, Criticare System Inc, Waukesha, Wis.

f

LiDCO Ltd, London, UK.

g

Teixeira FN, McDonell WN, Pearce SG, et al. Evaluation of anesthetic maintained with halothane and epidural xylazine for hind limb surgery in horses (abstr), in Proceedings. 25th Annu Am Coll Vet Anesth Meet 2000;33.

h

Co-oximeter OSM 3, Radiometer, Copenhagen, Denmark,

i

Spectra AA-10, Varian Inc, Mississauga, ON, Canada,

j

Table curve 2D, SYSSTAT Software Inc, Richmond, Calif,

k

SAS, SAS Institute Inc, Cary, NC.

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Appendix 1

Appendix 1

Criteria for assignment of a gastrointestinal motility score to each abdominal quadrant (determined via auscultation at 2 sites/quadrant) in horses.

ScoreCriteria
4Sustained, loud gurgling sound audible at both sites in each quadrant (detected at least 2 to 4 times/min).
3Sustained, loud gurgling sound audible at both sites in each quadrant, audible only once per minute at both sides or more than once per minute but only at 1 site in a particular quadrant.
2Low-pitched crepitationlike sounds audible at both sites in a quadrant (detected 1 time/min).
1Low-pitched crepitationlike sounds audible only once per minute at both sites of quadrant.
0No intestinal sounds detected throughout the quadrant.

Appendix 2

Appendix 2

Criteria for subjective scoring of recovery from anesthesia in horses.

ScoreCriteria
1Stands on first real attempt with a good coordinated effort. A real attempt is when the horse makes a genuine attempt to stand; lifting its head, rolling into sternal recumbency, assumption of a dog-sitting position, or changing position while sitting without a lunge are not considered a real attempt.
2Requires 2 or 3 efforts to stand, or stands with strong first effort but is unstable after standing.
3Several attempts to stand with some danger of injury. When horse stands, it does so with a strong effort and remains standing, or alternatively, the horse stands on the first to third attempt but then falls; or a recovery period of 2 hours before the horse stands.
4Stormy recovery with minor injuries (eg, lacerations, dehiscence of surgical wound, or bleeding from tongue or nose). Several unsuccessful weak attempts to rise.
5Stormy recovery with major injuries (eg, fractures).

Contributor Notes

Address correspondence to Dr. McDonell.

Dr. Durongphongtorn's present address is Department of Veterinary Surgery, Faculty of Veterinary Science, Chulalongkorn University, Patumwan, Bangkok 10330, Thailand.

Dr. Teixeira Neto's present address is Department of Veterinary Surgery and Anesthesiology, Faculdade de Medicina Veterinaria e Zootecnia, Unesp, Botucatu, SP 14870-000, Brazil.

Dr. Mirakhur's present address is VIDO, University of Saskatchewan, 120 Veterinary Rd, Saskatoon, SK S7N 5E3, Canada.

This manuscript represents a portion of a thesis submitted by the senior author to the Department of Clinical Studies, Ontario Veterinary College, University of Guelph for the Doctor in Veterinary Science degree.

Supported in part by a grant from Equine Guelph; the Ontario Ministry of Agriculture, Food, and Rural Affairs; and the Ontario Horse Racing Industry Association.

Presented in part as an abstract at the 8thWorld Congress of Veterinary Anesthesia, Knoxville, Tenn, September 2003.