OBJECTIVE To evaluate the effects of 4 gas compositions at various volumes (simulated tidal volumes [VTs]) on accuracy of measurements obtained with 2 types of flow sensors and accuracy of gas volume delivery by a piston-driven ventilator.
SAMPLE 4 gas mixtures (medical air [21% O2:79% N2], > 95% O2, O2-enriched air [30% O2:70% N2], and heliox [30% O2:70% He]).
PROCEDURES For each gas mixture, reference VTs of 1 to 8 L were delivered into an anesthetic breathing circuit via calibration syringe; measurements recorded by a Pitot tube-based flow sensor (PTFS) connected to a multiparameter host anesthesia monitor and by a thermal mass flow and volume meter (TMFVM) were compared with the reference values. Following leak and compliance testing, the ventilator was preset to deliver each gas at VTs of 1 to 8 L into the calibration syringe. Effects of gas volume and composition on accuracy of VT measurement and delivery were assessed by ANOVA. Agreements between delivered and flow sensor-measured VT and preset versus ventilator-delivered VT were determined by Bland-Altman analysis.
RESULTS Flow sensor measurements were accurate and not influenced by gas composition. Mean measurement error ranges for the PTFS and TMFVM were −4.99% to 4.21% and −4.50% to 0.17%, respectively. There were no significant differences between ventilator-delivered and reference VTs regardless of gas volume or composition. Bland-Altman analysis yielded biases of −0.046 L, −0.007 L, −0.002 L, and 0.031 L for medical air, > 95% O2, O2-enriched air, and heliox, respectively.
CONCLUSIONS AND CLINICAL RELEVANCE The PTFS and the TMFVM measured VTs and the piston-driven ventilator delivered VTs with error rates of < 5% for all gas compositions and volumes tested.
OBJECTIVE To evaluate efficacy of an alveolar recruitment maneuver (ARM) with positive end-expiratory pressures (PEEPs) in anesthetized horses ventilated with oxygen or heliox (70% helium and 30% oxygen).
ANIMALS 6 healthy adult horses.
PROCEDURES In a randomized crossover study, horses were anesthetized and positioned in dorsal recumbency. Volume-controlled ventilation was performed with heliox or oxygen (fraction of inspired oxygen [Fio2] > 90%). Sixty minutes after mechanical ventilation commenced, an ARM with PEEP (0 to 30 cm H2O in steps of 5 cm H2O every 5 minutes, followed by incremental steps back to 0 cm H2O) was performed. Peak inspiratory pressure, dynamic lung compliance (Cdyn), and Pao2 were measured during each PEEP. Indices of pulmonary oxygen exchange and alveolar dead space were calculated. Variables were compared with baseline values (PEEP, 0 cm H2O) and between ventilation gases by use of repeated-measures ANOVAs.
RESULTS For both ventilation gases, ARM significantly increased pulmonary oxygen exchange indices and Cdyn. Mean ± SD Cdyn (506 ± 35 mL/cm H2O) and Pao2-to-Fio2 ratio (439 ± 36) were significantly higher and alveolar-arterial difference in Pao2 (38 ± 11 mm Hg) was significantly lower for heliox, compared with values for oxygen (357 ± 50 mL/cm H2O, 380 ± 92, and 266 ± 88 mm Hg, respectively).
CONCLUSIONS AND CLINICAL RELEVANCE An ARM in isoflurane-anesthetized horses ventilated with heliox significantly improved pulmonary oxygen exchange and respiratory mechanics by decreasing resistive properties of the respiratory system and reducing turbulent gas flow in small airways.
To assess effects of nitrogen and helium on efficacy of an alveolar recruitment maneuver (ARM) for improving pulmonary mechanics and oxygen exchange in anesthetized horses.
6 healthy adult horses.
Horses were anesthetized twice in a randomized crossover study. Isoflurane-anesthetized horses in dorsal recumbency were ventilated with 30% oxygen and 70% nitrogen (treatment N) or heliox (30% oxygen and 70% helium; treatment H) as carrier gas. After 60 minutes, an ARM was performed. Optimal positive end-expiratory pressure was identified and maintained for 120 minutes. Throughout the experiment, arterial blood pressures, heart rate, peak inspiratory pressure, dynamic compliance (Cdyn), and Pao2 were measured. Variables were compared with baseline values and between treatments by use of an ANOVA.
The ARM resulted in significant increases in Pao2 and Cdyn and decreases in the alveolar-arterial gradient in the partial pressure of oxygen in all horses. After the ARM and during the subsequent 120-minute phase, mean values were significantly lower for treatment N than treatment H for Pao2 and Cdyn. Optimal positive end-expiratory pressure was consistently 15 cm H2O for treatment N, but it was 10 cm H2O (4 horses) and 15 cm H2O (2 horses) for treatment H.
CONCLUSIONS AND CLINICAL RELEVANCE
An ARM in anesthetized horses might be more efficacious in improving Pao2 and Cdyn when animals breathe helium instead of nitrogen as the inert gas.
To determine the accuracy of tidal volume (VT) delivery among 5 different models of large-animal ventilators when tested at various settings for VT delivery, peak inspiratory flow (PIF) rate, and fresh gas flow (FGF) rate.
4 different models of pneumatically powered ventilators and 1 electrically powered piston-driven ventilator.
After a leak flow check, each ventilator was tested 10 times for each experimental setting combination of 5 levels of preset VT, 3 PIF rates, and 4 FGF rates. A thermal mass flow and volume meter was used as the gold-standard method to measure delivered VT. In addition, circuit systems of rubber versus polyvinyl chloride breathing hoses were evaluated with the piston-driven ventilator. Differences between preset and delivered VT (volume error [δVT]) were calculated as a percentage of preset VT, and ANOVA was used to compare results across devices. Pearson correlation coefficient analyses and the coefficient of determination (r) were used to assess potential associations between the δVT and the preset VT, PIF rate, and FGF rate.
For each combination of experimental settings, ventilators had δVT values that ranged from 1.2% to 22.2%. Mean ± SD δVT was 4.8 ± 2.5% for the piston-driven ventilator, compared with 6.6 ± 3.2%, 10.6 ± 2.9%, 13.8 ± 2.97%, and 15.2 ± 2.6% for the 4 pneumatic ventilators. The δVT increased with higher PIF rates (r = 0.69), decreased with higher FGF rates (r = 0.62), and decreased with higher preset VT (r = 0.58).
CONCLUSIONS AND CLINICAL RELEVANCE
Results indicated that the tested ventilators all had δVT but that the extent of each of δVT varied among ventilators. Close monitoring of delivered VT with external flow and volume meters is warranted, particularly when pneumatic ventilators are used or when very precise VT delivery is required.
To compare the accuracy and precision of cardiac output (CO) measurements derived from 4 thermodilution protocols that used different injectate temperatures and volumes in healthy adult horses.
8 healthy adult horses.
Horses were anesthetized and instrumented with Swan-Ganz catheters. The CO was derived from each of 4 thermodilution protocols (IV injection of physiologic saline [0.9% NaCl] solution chilled to < 5 °C at volumes of 1 mL/15 kg of body weight [protocol A; control], 1 mL/25 kg [protocol B], and 1 mL/35 kg [protocol C] or maintained at 17 °C at a volume of 1 mL/15 kg [protocol D]) 3 times during each of 5 measurement cycles, with a 30-minute interval between cycles. During each measurement cycle, protocol A was performed first, and protocols B, C, and D were performed in a randomized order. Mean CO and within-subject variance in CO were compared among the 4 protocols.
Mean CO did not differ significantly among the 4 protocols. The within-subject variance for CO measurements derived from protocols C and D, but not protocol B, was significantly greater than that for protocol A (control).
CONCLUSIONS AND CLINICAL RELEVANCE
Results suggested that, in healthy adult horses, decreasing the thermodilution injectate volume to 1 mL/25 kg from the recommended volume of 1 mL/15 kg did not adversely affect the accuracy or precision of CO measurements. However, use of smaller injectate volumes or use of injectate at approximately room temperature is not recommended owing to a clinically unacceptable increase in CO measurement variability.
Objective—To determine characteristics of the inflammatory reaction in the jejunum of horses in response to various mechanical manipulations.
Animals—12 adult warmblood horses without gastrointestinal tract disorders.
Procedures—The proximal aspect of the jejunum in each horse was divided into 5 segments, and the following manipulations were performed: manual emptying, placement of Doyen forceps, enterotomy alone, enterotomy with mucosal abrasion, and serosal abrasion. Jejunum samples were collected before (control), immediately after, and 30 minutes after the end of manipulations and histologically evaluated to determine distribution of neutrophils and eosinophils.
Results—Macroscopically, all manipulations resulted in jejunal hemorrhage and edema. Compared with control samples, neutrophil numbers were significantly higher after manipulations in the serosa (after all manipulation types), circular muscle layer (after manual emptying), submucosa (after placement of Doyen forceps), and mucosa (after all manipulations except enterotomy alone). Eosinophil numbers were significantly higher in the submucosa after mechanical abrasion of the serosa and manual emptying versus control samples.
Conclusions and Clinical Relevance—Results indicated mechanical manipulation of the jejunum resulted in local inflammatory reactions characterized predominantly by infiltration of neutrophils. This could contribute to the development of postoperative ileus or adhesions in horses without macroscopically detectable injury of the jejunum during surgery.
Objective—To evaluate the use of a micro-lightguide tissue spectrophotometer for measurement of tissue oxygenation and blood flow in the small and large intestines of horses under anesthesia.
Animals—13 adult horses without gastrointestinal disease.
Procedures—Horses were anesthetized and placed in dorsal recumbency. Ventral midline laparotomy was performed. Intestinal segments were exteriorized to obtain measurements. Spectrophotometric measurements of tissue oxygenation and regional blood flow of the jejunum and pelvic flexure were obtained under various conditions that were considered to have a potential effect on measurement accuracy. In addition, arterial oxygen saturation at the measuring sites was determined by use of pulse oximetry.
Results—12,791 single measurements of oxygen saturation, relative amount of hemoglobin, and blood flow were obtained. Errors occurred in 381 of 12,791 (2.98%) measurements. Most measurement errors occurred when surgical lights were directed at the measuring site; covering the probe with the surgeon's hand did not eliminate this error source. No measurement errors were observed when the probe was positioned on the intestinal wall with room light, at the mesenteric side, or between the mesenteric and antimesenteric side. Values for blood flow had higher variability, and this was most likely caused by motion artifacts of the intestines.
Conclusions and Clinical Relevance—The micro-lightguide spectrophotometry system was easy to use on the small and large intestines of horses and provided rapid evaluation of the microcirculation. Results indicated that measurements should be performed with room light only and intestinal motion should be minimized.
Objective—To determine whether administration of lidocaine during ischemia and reperfusion in horses results in concentrations in smooth muscle sufficient to protect against the negative consequences of ischemia-reperfusion injury on smooth muscle motility.
Procedures—Artificial ischemia and reperfusion injury of jejunal segments was induced in vivo in conjunction with lidocaine treatment during ischemia (IRL) or without lidocaine treatment (IR). Isometric force performance was measured in vitro in IRL and IR smooth muscle preparations with and without additional in vitro application of lidocaine. Lidocaine concentrations in smooth muscle were determined by means of high-performance liquid chromatography. To assess the influence of lidocaine on membrane permeability, activity of creatine kinase and lactate dehydrogenase released by in vitro incubated tissues was determined biochemically.
Results—In vivo administration of lidocaine allowed maintenance of contractile performance after an ischemia and reperfusion injury. Basic contractility and frequency of contractions were significantly increased in IRL smooth muscle tissues in vitro. Additionally, in vitro application of lidocaine achieved further improvement of contractility of IR and IRL preparations. Only in vitro application of lidocaine was able to ameliorate membrane permeability in smooth muscle of IR and IRL preparations. Lidocaine accumulation could be measured in in vivo treated samples and serum.
Conclusions and Clinical Relevance—In vivo lidocaine administration during ischemia and reperfusion had beneficial effects on smooth muscle motility. Initiating lidocaine treatment during surgery to treat colic in horses may improve lidocaine's prokinetic features by protecting smooth muscle from effects of ischemia and reperfusion injury.
OBJECTIVE To determine global and peripheral perfusion and oxygenation during anesthesia with equipotent doses of desflurane and propofol combined with a constant rate infusion of dexmedetomidine in horses.
ANIMALS 6 warmblood horses.
PROCEDURES Horses were premedicated with dexmedetomidine (3.5 μg•kg−1, IV). Anesthesia was induced with propofol or ketamine and maintained with desflurane or propofol (complete crossover design) combined with a constant rate infusion of dexmedetomidine (7 μg•kg−1 •h−1). Microperfusion and oxygenation of the rectal, oral, and esophageal mucosa were measured before and after sedation and during anesthesia at the minimal alveolar concentration and minimal infusion rate. Heart rate, mean arterial blood pressure, respiratory rate, cardiac output, and blood gas pressures were recorded during anesthesia.
RESULTS Mean ± SD minimal alveolar concentration and minimal infusion rate were 2.6 ± 0.9% and 0.04 ± 0.01 mg•kg−1 •min−1, respectively. Peripheral microperfusion and oxygenation decreased significantly after dexmedetomidine administration for both treatments. Oxygenation returned to baseline values, whereas tissue microperfusion remained low during anesthesia. There were no differences in peripheral tissue microperfusion and oxygenation between treatments. Cardiac index was significantly higher and systemic vascular resistance was significantly lower for desflurane treatment than for propofol treatment. For the propofol treatment, Pao2 was significantly higher and there was less dead space and venous admixture than for the desflurane treatment.
CONCLUSIONS AND CLINICAL RELEVANCE Dexmedetomidine decreased blood flow and oxygen saturation in peripheral tissues. Peripheral tissues were well oxygenated during anesthesia with desflurane and propofol combined with dexmedetomidine, whereas blood flow was reduced.