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    Mean ± SEM plasma cTnI concentration in 9 black-tailed prairie dogs anesthetized by an IM injection of dexmedetomidine (0.25 mg/kg), ketamine (40 mg/kg), and midazolam (1.5 mg/kg [black circles and solid line]) or via inhalation of 2% isoflurane and oxygen (white circles and dashed line) in a randomized, crossover design study. Three days after the first anesthetic episode, each prairie dog underwent the alternative anesthetic protocol. Blood samples (for assessment of plasma cTnI concentration) were collected within 5 minutes after initiation and just prior to cessation of each anesthetic protocol (duration of anesthesia, 45 minutes). There was no significant (P = 0.71) difference in mean cTnI concentration when prairie dogs underwent the DKM or isoflurane anesthetic protocol as the first treatment. Regarding the second treatment, the mean cTnI concentration when prairie dogs were anesthetized via inhalation of isoflurane was significantly (P = 0.01) increased, compared with the value when prairie dogs were anesthetized by injection of the DKM combination. During the second anesthetic episode, 1 prairie dog was removed from the study shortly after the induction of anesthesia with the DKM combination because of difficulties during blood sample collection that disrupted the planned course of the experimental procedure; data for this prairie dog were not included in the analysis.

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Comparison of the effects of a dexmedetomidine-ketamine-midazolam anesthetic protocol versus isoflurane inhalation anesthesia on echocardiography variables and plasma cardiac troponin I concentration in black-tailed prairie dogs (Cynomys ludovicianus)

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  • 1 1Department of Clinical Sciences, College of Veterinary Medicine, Kansas State University, Manhattan, KS 66502.
  • | 2 2Department of Clinical Studies, Ontario Veterinary College, University of Guelph, Guelph, ON NIG 2WI Canada.

Abstract

OBJECTIVE

To compare the effects of a dexmedetomidine-ketamine-midazolam (DKM) anesthetic protocol versus isoflurane inhalation anesthesia on echocardiographic variables and plasma cardiac troponin 1 (cTnI) concentration in black-tailed prairie dogs (BTPDs; Cynomys ludovicianus).

ANIMALS

Nine 6-month-old sexually intact male captive BTPDs.

PROCEDURES

Each BTPD was randomly assigned to be anesthetized by IM administration of dexmedetomidine (0.25 mg/kg), ketamine (40 mg/kg), and midazolam (1.5 mg/kg) or via inhalation of isoflurane and oxygen. Three days later, each BTPD underwent the alternative anesthetic protocol. Echocardiographic data and a blood sample were collected within 5 minutes after initiation and just prior to cessation of each 45-minute-long anesthetic episode.

RESULTS

Time or anesthetic protocol had no significant effect on echocardiographic variables. For either protocol, plasma cTnI concentration did not differ with time. When administered as the first treatment, neither anesthetic protocol significantly affected plasma cTnI concentration. However, with regard to findings for the second treatments, plasma cTnI concentrations in isoflurane-treated BTPDs (n = 4; data for 1 animal were not analyzed because of procedural problems) were higher than values in DKM-treated BTPDs (4), which was suspected to be a carryover effect from prior DKM treatment.

CONCLUSIONS AND CLINICAL RELEVANCE

The DKM and isoflurane anesthetic protocols did not have any significant effect on echocardiographic measurements in the BTPDs. Increases in plasma cTnI concentration during the second anesthetic episode were evident when BTPDs underwent the DKM anesthetic protocol as the first of the 2 treatments, suggestive of potential myocardial injury associated with that anesthetic protocol. Clinicians should consider these findings, especially when evaluating BTPDs with known or suspected cardiac disease.

Abstract

OBJECTIVE

To compare the effects of a dexmedetomidine-ketamine-midazolam (DKM) anesthetic protocol versus isoflurane inhalation anesthesia on echocardiographic variables and plasma cardiac troponin 1 (cTnI) concentration in black-tailed prairie dogs (BTPDs; Cynomys ludovicianus).

ANIMALS

Nine 6-month-old sexually intact male captive BTPDs.

PROCEDURES

Each BTPD was randomly assigned to be anesthetized by IM administration of dexmedetomidine (0.25 mg/kg), ketamine (40 mg/kg), and midazolam (1.5 mg/kg) or via inhalation of isoflurane and oxygen. Three days later, each BTPD underwent the alternative anesthetic protocol. Echocardiographic data and a blood sample were collected within 5 minutes after initiation and just prior to cessation of each 45-minute-long anesthetic episode.

RESULTS

Time or anesthetic protocol had no significant effect on echocardiographic variables. For either protocol, plasma cTnI concentration did not differ with time. When administered as the first treatment, neither anesthetic protocol significantly affected plasma cTnI concentration. However, with regard to findings for the second treatments, plasma cTnI concentrations in isoflurane-treated BTPDs (n = 4; data for 1 animal were not analyzed because of procedural problems) were higher than values in DKM-treated BTPDs (4), which was suspected to be a carryover effect from prior DKM treatment.

CONCLUSIONS AND CLINICAL RELEVANCE

The DKM and isoflurane anesthetic protocols did not have any significant effect on echocardiographic measurements in the BTPDs. Increases in plasma cTnI concentration during the second anesthetic episode were evident when BTPDs underwent the DKM anesthetic protocol as the first of the 2 treatments, suggestive of potential myocardial injury associated with that anesthetic protocol. Clinicians should consider these findings, especially when evaluating BTPDs with known or suspected cardiac disease.

The use of injectable anesthetic drugs such as α2-adrenoceptor agonists, ketamine hydrochloride, and other agents has many potential benefits in rodents.1–3 However, very few studies have assessed the use and effects of injectable anesthetic drugs in prairie dogs.4 For this species, chemical immobilization is often required for examination of individuals because of their fractious nature.3,5,6 Traditionally, inhalation anesthesia with isoflurane, which requires administration via a face mask or in an induction chamber, has been used in prairie dogs.3,5,6 As an alternative, injection of anesthetic agents can reduce the exposure of veterinary staff to waste anesthetic gas, allow for quicker induction of anesthesia, and provide an anesthetic option in a field setting where inhalant anesthetic agents may not be available.1,2,7 For the present study, the effects of injection of a DKM combination were chosen for comparison with the effects of isoflurane inhalation anesthesia in prairie dogs. The DKM combination is a balanced injectable anesthetic protocol that is partially reversible; that partial reversibility of anesthesia may be an additional benefit in certain clinical circumstances.

Echocardiographic variables in black-tailed prairie dogs (Cynomys ludovicianus) that underwent anesthesia via inhalation of isoflurane were evaluated in a previous study.6 However, information regarding echocardiographic findings for healthy animals of this and other rodent species is limited.8–11 To the authors’ knowledge, there are no reports of studies that have evaluated the effects of injectable anesthetic drugs on echocardiographic variables in prairie dogs.

Cardiac troponin I is an inhibitory subunit of cardiac troponin and is used as a cardiac-specific marker for myocardial injury in many species, including rats, dogs, horses, and humans.12–15 Assessment of circulating cTnI concentration has been used for diagnosis of ischemic heart disease (eg, myocardial infarction) in humans, but abnormally high concentrations can be detected as a result of other causes, which include primary cardiac disease, infectious disease, and episodes of reduced myocardial blood flow such as those that may occur during anesthesia.14–17 Although the effect of different anesthetic protocols on circulating cTnI concentration has been investigated in other species, little information is available for prairie dogs. In a previous study6 that evaluated plasma cTnI in 17 apparently healthy black-tailed prairie dogs anesthetized with isoflurane, all but 1 animal had a plasma cTnI concentration near 0 ng/mL.

The objective of the study reported here was to compare changes in echocardiographic variables and plasma cTnI concentrations associated with administration of an injectable anesthetic protocol with a DKM combination and with isoflurane inhalation anesthesia in black-tailed prairie dogs. The hypothesis was that there would be no significant differences in echocardiographic values and plasma cTnI concentrations at the predetermined time points between the 2 anesthetic protocols.

Materials and Methods

Animals

The present study was reviewed and approved by the Institutional Animal Care and Use Committee at Kansas State University (IACUC No. 3952.3). Nine 6-month-old sexually intact male black-tailed prairie dogs from a zoological collection were included.a The mean weight for the prairie dogs was 0.72 g, median weight was 0.71 g, and SD of the weights was 0.09 g. This same group of prairie dogs was used in a previous study,4 and the data for anesthetic findings have been reported. To ensure that all prairie dogs were healthy, each animal was anesthetized via inhalation of isoflurane 1 week prior to the first experimental treatment and underwent a physical examination; PCV and plasma total solids concentration (measured by refractometry) were assessed, and a plasma biochemical profile was performed. No notable abnormalities were detected by use of those evaluations. During this initial examination, each prairie dog was individually color marked and implanted with an identification microchip.

Experimental treatments

The study had a randomized crossover design, and each prairie dog underwent anesthesia on 2 occasions. Randomization of the first anesthetic protocol assignment for each prairie dog was accomplished with an online automated randomization tool.b Each prairie dog underwent a second anesthetic episode with the alternative protocol 3 days after the first anesthetic episode.

The DKM anesthetic protocol involved IM administration of a combination of dexmedetomidinec (0.25 mg/kg), ketamine hydrochlorided (40 mg/kg), and midazolame (1.5 mg/kg). The doses used had been determined during preliminary testing 1 week earlier, the results of which indicated that the dose combination resulted in a deep surgical plane of anesthesia.4 This drug combination was injected (time point designated as 0 minutes) in the epaxial musculature by use of a single insulin syringe with a 28-gauge needle. When a prairie dog underwent the DKM anesthetic protocol, it was allowed to spontaneously breathe room air throughout the period of anesthesia and no supplemental oxygen was provided. Flumazenilf (0.05 mg/kg) and atipamezoleg (0.15 mg/kg) were administered separately IM in the epaxial musculature after 45 minutes of anesthesia (time point designated as 45 minutes). If the animal did not regain a bright and alert mentation or was unable to ambulate within 1 hour after administration of the first doses of reversal agents, a second dose of flumazenil was injected.

For the isoflurane anesthetic protocol, each prairie dog was placed in a standard medium-sized induction chamber (71 × 25 × 20 cm) into which isoflurane in 100% oxygen (5 L/min) was delivered. Once the prairie dog was observed to be fully anesthetized (time point designated as 0 minutes), a tight-fitting face mask was placed on the prairie dog and connected to a nonrebreathing circuit; delivery of isoflurane was maintained at 2% in oxygen (2 L/min) for 45 minutes; at the 45-minute time point, isoflurane administration was discontinued and oxygen was delivered via the face mask until the righting reflex returned.

For each prairie dog, various clinical variables were measured every 5 minutes during each anesthetic episode. Heart rate and respiratory rate were determined by use of a stethoscope, Spo2 was measured with a commercial monitoring systemh applied to the right hind foot pad, and indirect mean arterial blood pressure was assessed by oscillometry with a size 2 cuff applied to the left hind limb.4 Rectal temperature was monitored manually with a digital thermometer. A 40-mL bolus of lactated Ringer solution was immediately administered SC at the end of the 45-minute period of anesthesia. Each prairie dog was placed in a crate during recovery from anesthesia and was provided with heat support by use of a forced-air patient warming system.i

Assessment of plasma cTnI concentration

During each anesthetic protocol, a blood sample was collected from each prairie dog within 5 minutes after commencement of the 45-minute period of anesthesia (ie, between the 0- and 5-minute time points) and just prior to reversal of anesthesia with atipamezole and flumazenil or cessation of isoflurane administration at the 45-minute time point. A 1.0-mL syringe with a 25-gauge needle was used to collect the blood sample from the cranial vena cava; the sample was placed in a 0.5-mL tube containing lithium heparin. Immediately after collection, plasma cTnI concentration was determined with a single-assay cTnI cartridge in a blood analyzer.j The cTnI assay used in this study was the same type as that used in another study6 in which plasma cTnI concentrations of healthy prairie dogs anesthetized with isoflurane were measured.

Echocardiography

Each prairie dog underwent 2-D, M-mode, spectral Doppler, and color flow Doppler echocardiographic examinations within 5 minutes after commencement of the 45-minute period of anesthesia (ie, between the 0- and 5-minute time points) and just prior to reversal of anesthesia with atipamezole and flumazenil or cessation of isoflurane administration at the 45-minute time point. Echocardiographic examinations were performed by use of an ultrasound machinek that was equipped with a 10-MHz (7.5- to 10-MHz) phasedarray transducer. Images were stored for analysis at a later time. With each animal positioned in right lateral recumbency, left ventricular M-mode measurements were obtained from the right parasternal long-axis view and included interventricular septal wall thickness in end diastole, interventricular septal wall thickness in end systole, left ventricular posterior wall thickness in diastole, left ventricular posterior wall thickness in systole, LVIDd, LVIDs, and left atrial diameter in long axis. Fractional shortening was calculated as follows:

article image

Pulsed-wave Doppler assessment of both the right and left ventricular outflow tracts and color flow Doppler interrogation of all 4 cardiac valves were also obtained when each prairie dog was positioned in left lateral recumbency. The same board-certified veterinary cardiologist (JDT) performed all echocardiographic examinations.

Statistical analysis

Commercial softwarel was used for all statistical analyses. For each outcome variable (plasma cTnI concentration and echocardiographic data), longitudinal data analysis was performed with linear mixed modeling. The fixed effects were time (0 to 5 minutes vs 45 minutes), anesthetic protocol (DKM treatment vs isoflurane treatment), and interaction effect, and the random effect was prairie dog. Data linearity, normality, and outliers and homogeneity of variances were assessed by use of residual plots. Quantile plots were performed on the residuals by treatment for data normality assessment. The autocorrelation function method was used for autocorrelation of the residuals over time. Variance on the fixed effects was analyzed, and a Tukey adjustment was used for post hoc comparisons. Residuals were evaluated graphically. Significance was defined as an α of 0.05.

Results

The condition of each prairie dog was stable throughout each perianesthetic period. There were no complications or adverse effects related to anesthesia during the anesthetic episode or following the conclusion of the study; each prairie dog was monitored for 1 week after the second anesthesia episode. Of the 9 prairie dogs, 4 underwent the DKM protocol as the first treatment and 5 underwent the isoflurane anesthetic protocol as the first treatment. All 9 prairie dogs underwent 2 anesthetic episodes, once with each protocol. However, during the second anesthetic episode, 1 prairie dog was removed from the study shortly after the induction of anesthesia with the DKM combination because of difficulties during blood sample collection that disrupted the planned course of the experimental procedure; data for this prairie dog were not included in the analyses.

Plasma cTnI concentration

For assessment of plasma cTnI concentration, a blood sample was collected from each prairie dog at 4 time points; 1 sample was collected within 5 minutes after induction of anesthesia (between the 0- and 5-minute time points) for either protocol, and 1 sample was collected just prior to cessation of isoflurane administration or reversal of anesthesia with atipamezole and flumazenil at the 45-minute time point. The mean ± SD plasma cTnI concentration when the prairie dogs underwent the isoflurane anesthetic protocol was 0.48 ± 0.93 μg/L within 5 minutes after induction of anesthesia and 0.01 ± 1.17 μg/L at the 45-minute time point. The mean plasma cTnI concentration when the prairie dogs underwent the DKM anesthetic protocol was 0.63 ± 0.21 μg/L within 5 minutes after induction of anesthesia and 0.01 ± 0.21 μg/L at the 45-minute time point. No influence of time was evident; for either anesthetic protocol, there was no significant (P = 0.62) difference between the mean plasma cTnI concentration at 0 to 5 minutes and that at 45 minutes. It was initially noted that plasma cTnI concentration was significantly (P = 0.002) higher when prairie dogs underwent the isoflurane anesthetic protocol, compared with findings when they underwent the DKM anesthetic protocol; the mean ± SE of the difference was 0.51 ± 0.15 μg/L. This was likely attributable to treatment order interaction, meaning that the order in which the prairie dogs underwent the 2 anesthetic protocols had an effect. Because variances were significantly (P < 0.001) heterogeneous between orders of treatment (compared with a homoscedastic model), a heteroscedastic model was used for analysis of plasma cTnI concentrations. This model revealed that there was no significant (P = 0.71) difference in plasma cTnI concentration (irrespective of time point) when prairie dogs underwent either anesthetic protocol as the first treatment. However, when prairie dogs underwent the isoflurane anesthetic protocol as the second treatment, prairie dogs had a higher plasma cTnI concentration, compared with values obtained for either anesthetic protocol as the first treatment. In addition, plasma cTnI concentration was significantly (P = 0.01) increased when prairie dogs underwent the isoflurane anesthetic protocol as the second treatment, compared with findings when prairie dogs underwent the DKM anesthetic protocol as the second treatment (Figure 1). This indicated the likely presence of a carryover effect for plasma cTnI concentrations from the initial treatment, given that the DKM anesthetic protocol was the first treatment when the prairie dogs underwent the isoflurane anesthetic protocol as the second treatment.

Figure 1—
Figure 1—

Mean ± SEM plasma cTnI concentration in 9 black-tailed prairie dogs anesthetized by an IM injection of dexmedetomidine (0.25 mg/kg), ketamine (40 mg/kg), and midazolam (1.5 mg/kg [black circles and solid line]) or via inhalation of 2% isoflurane and oxygen (white circles and dashed line) in a randomized, crossover design study. Three days after the first anesthetic episode, each prairie dog underwent the alternative anesthetic protocol. Blood samples (for assessment of plasma cTnI concentration) were collected within 5 minutes after initiation and just prior to cessation of each anesthetic protocol (duration of anesthesia, 45 minutes). There was no significant (P = 0.71) difference in mean cTnI concentration when prairie dogs underwent the DKM or isoflurane anesthetic protocol as the first treatment. Regarding the second treatment, the mean cTnI concentration when prairie dogs were anesthetized via inhalation of isoflurane was significantly (P = 0.01) increased, compared with the value when prairie dogs were anesthetized by injection of the DKM combination. During the second anesthetic episode, 1 prairie dog was removed from the study shortly after the induction of anesthesia with the DKM combination because of difficulties during blood sample collection that disrupted the planned course of the experimental procedure; data for this prairie dog were not included in the analysis.

Citation: American Journal of Veterinary Research 80, 12; 10.2460/ajvr.80.12.1114

Echocardiographic data

The results for the echocardiographic data were summarized (Tables 1 and 2). There was no significant (all P > 0.13) effect of time on echocardiographic variables within each anesthetic protocol or between the 2 anesthetic protocols. Subjectively, systolic function appeared normal in all prairie dogs, and no notable chamber enlargement, vessel dilation, or valve regurgitation was detected.

Table 1—

Echocardiographic values of left atrial diameter (LAD), LVIDd, LVIDs, and FS obtained at the start and conclusion of anesthesia in 9 healthy black-tailed prairie dogs anesthetized by an IM injection of dexmedetomidine (0.25 mg/kg), ketamine (40 mg/kg), and midazolam (1.5 mg/kg) or via inhalation of 2% isoflurane and 100% oxygen in a randomized, crossover design study (2 treatments/prairie dog).

TreatmentProtocolTime point (min)ValueLAD (mm)LVIDd (mm)LVIDs (mm)FS (%)
1DKM (n = 4)0–5Mean ± SD6.00 ± 1.418.00 ± 2.165.50 ± 1.7333 ± 11.3
  0–5Median (range)5.5 (5–8)7.5 (6–11)5 (4–8)33 (23–44)
  45Mean ± SD6.00 ± 0.828.75 ± 1.716.00 ± 2.5833 ± 17.1
  45Median (range)6 (5–7)8.5 (7–11)6 (3–9)31.5 (15–54)
 Isoflurane (n = 5)0–5Mean ± SD5.80 ± 0.847.60 ± 1.144.60 ± 1.5239 ± 13.9
  0–5Median (range)6 (5–7)8 (6–9)5 (2–6)38 (26–61)
  45Mean ± SD6.00 ± 0.717.00 ± 0.714.60 ± 0.8936 ± 8.4
  45Median (range)6 (5–7)7 (6–8)5 (3–5)33 (30–51)
2DKM (n = 4)0–5Mean ± SD5.50 ± 0.585.75 ± 1.232.75 ± 0.9644 ± 10.1
  0–5Median (range)5.5 (5–6)6 (4–7)2.5 (2–4)42.5 (33–57)
  45Mean ± SD5.25 ± 0.55.00 ± 1.413.00 ± 1.4144 ± 15.3
  45Median (range)5 (5–6)4.5 (4–72.5 (2–5)45.5 (26–59)
 Isoflurane (n = 4)0–5Mean ± SD6.00 ± 08.00 ± 0.825.00 ± 037 ± 10.1
  0–5Median (range)6 (6–6)8 (7–9)5 (5–5)35 (23–40)
  45Mean ± SD6.25 ± 2.067.75 ± 0.965.00 ± 037 ± 7.3
  45Median (range)6.5 (4–8)7.5 (7–9)5 (4–6)39.5 (26–44)

Anesthetic protocol assignment for each prairie dog was randomized for the first anesthetic episode (treatment 1). Each prairie dog underwent a second anesthetic episode (treatment 2) with the alternate protocol 3 days after the first anesthetic episode. The DKM anesthetic protocol involved injection of the drug combination (0 minutes) in the epaxial musculature; each prairie dog was allowed to spontaneously breathe room air throughout the period of anesthesia, and no supplemental oxygen was provided. For the isoflurane anesthetic protocol, anesthesia was induced (0 minutes) and maintained with 2% isoflurane in oxygen for 45 minutes. At the 45-minute time point, isoflurane administration was discontinued and oxygen was delivered via the face mask until the righting reflex returned. Echocardiographic data were obtained within 5 minutes after initiation (ie, between the 0- and 5-minute time points) and just prior to cessation (45 minutes) of each anesthetic protocol (duration of anesthesia, 45 minutes). One prairie dog was removed from the study on the second day of the testing shortly after the induction of anesthesia with the DKM combination because of difficulties during blood sample collection that disrupted the planned course of the experimental procedure; data for this prairie dog were not included in the analyses.

Table 2—

Additional echocardiographic data obtained at the start and conclusion of each of 2 anesthetic episodes from the 9 healthy black-tailed prairie dogs in Table 1.

TreatmentProtocolTime point (min)ValueIVSd (mm)IVSs (mm)LVPWd (mm)LVPWs (mm)
1DKM (n = 4)0–5Mean ± SD1.88 ± 0.853.00 ± 02.50 ± 0.583.88 ± 0.85
  0–5Median (range)1.75 (1–3)3 (3–3)2.5 (2–3)3.75 (3–5)
  45Mean ± SD1.75 ± 0.52.75 ± 0.963.00 ± 0.824.25 ± 1.26
  45Median (range)2 (1–2)2.5 (2–4)3 (2–4)4 (3–6)
 Isoflurane (n =5)0–5Mean ± SD1.60 ± 0.552.60 ± 0.552.80 ± 0.454.00 ± 0.71
  0–5Median (range)2 (1–2)3 (2–3)3 (2–3)4 (3–5)
  45Mean ± SD2.00 ± 02.70 ± 0.842.60 ± 0.553.90 ± 0.55
  45Median (range)2 (2–2)2.5 (2–4)3 (2–3)4 (3–1.5)
2DKM (n = 4)0–5Mean ± SD1.38 ± 0.482.39 ± 0.482.75 ± 0.54.13 ± 0.25
  0–5Median (range)1.25 (1–2)2.25 (2–3)3 (2–3)4 (4–4.5)
  45Mean ± SD2.00 ± 02.75 ± 0.53.50 ± 0.585.00 ± 0.82
  45Median (range)2 (2–2)3 (2–3)3.5 (3–4)5 (4–6)
 Isoflurane (n = 4)0–5Mean ± SD2.00 ± 02.75± 0.53.00 ± 0.824.50 ± 0.58
  0–5Median (range)2 (2–2)3 (2–3)3 (2–4)4.5 (4–5)
  45Mean ± SD2.00 ± 03.25 ± 0.52.75 ± 0.54.25 ± 0.96
  45Median (range)2 (2–2)3 (3–4)3 (2–3)4.5 (3–5)

IVSd = Interventricular septal wall thickness at end diastole. IVSs = Interventricular septal wall thickness at end systole. LVPWd = Left ventricular posterior wall thickness at end diastole. LVPWs = Left ventricular posterior wall thickness at end systole.

See Table 1 for key.

Pulse oximetry data

Measurements of Spo2 were obtained from the prairie dogs at 5-minute intervals from 0 to 45 minutes and were analyzed to determine whether there was a significant change in the prairie dogs’ oxygen saturation status during anesthesia (Table 3). During the 5- to 25-minute period, there was significantly (all P < 0.038) lower Spo2 when the prairie dogs underwent the DKM anesthetic protocol, compared with findings when the prairie dogs underwent the isoflurane anesthetic protocol. With the DKM anesthetic protocol, Spo2 was significantly (all P < 0.006) increased after the 35-, 40-, and 45-minute time points, compared with the lowest value obtained at the 5-minute time point. Values of Spo2 were high and constant when prairie dogs underwent the isoflurane anesthetic protocol.

Table 3—

Mean ± SD SpO2 (%) assessed at 5-minute intervals in the 9 prairie dogs in Tables 1 and 2 when they underwent the DKM or isoflurane anesthetic protocol, irrespective of treatment order.

 Time point (min)
Protocol051015202530354045
DKM (n = 8)94.0 ± 6.089 ± 8.191 ± 4.593 ± 4.793 ± 2.893 ± 4.294 ± 4.995 ± 3.295 ± 2.396 ± 2.7
Isoflurane (n = 9)98 ± 1.499 ± 1.599 ± 1.299 ± 1.098 ± 2.198 ± 1.198 ± 0.899 ± 0.799 ± 0.999 ± 0.8

See Table 1 for key.

Discussion

In the present randomized, complete crossover design study, 9 black-tailed prairie dogs were anesthetized twice, once via inhalation of isoflurane and once via IM administration of a DKM combination. Isoflurane causes decreased blood pressure and heart rate secondary to a reduction in sympathetic tone, which results in diminished cardiac output and peripheral resistance.18–20 Dexmedetomidine is an α2-adrenoceptor agonist that provides analgesia, sedation, and muscle relaxation following injection.20,21 It has potent cardiovascular effects through stimulation of arterioles, causing systemic vasoconstriction, coronary vasoconstriction, and increases in systemic resistance.21,22 Ketamine is a centrally acting N-methyl-d-aspartate receptor antagonist that provides sedation and analgesia. Ketamine causes activation of the CNS, leading to increases in heart rate and cardiac output. In rodents and rabbits, injection of ketamine and α2-adrenoceptor agonist combinations provides effective anesthesia for a variety of procedures and is useful in field conditions.1,23 In the present study, midazolam was added to the combination of ketamine and dexmedetomidine because injection of those 2 drugs alone did not result in an adequate plane of anesthesia in the same group of black-tailed prairie dogs during an initial experiment performed 1 week earlier.4 Midazolam is a benzodiazepine that provides anxiolysis and muscle relaxation and that has sedative effects; it is virtually free of cardiovascular effects in conscious animals and causes only slight decreases in cardiac performance in anesthetized dogs.24,25 In the present study, both the DKM and isoflurane anesthetic protocols enabled full cardiologic examination of the evaluated prairie dogs.

Standard echocardiographic measurements were obtained for all prairie dogs in the present study. The echocardiographic values obtained when the prairie dogs underwent the DKM or isoflurane anesthetic protocol did not differ with time during each anesthetic episode or between anesthetic protocols. Currently, there are no reported echocardiographic reference ranges for nonanesthetized prairie dogs. However, the echocardiographic values obtained during each anesthetic episode were similar to findings for a group of prairie dogs anesthetized with isoflurane and examined by similar methods.6 The FS values for the prairie dogs of the present study were similar to values reported for mice, rats, chinchillas, and guinea pigs that underwent echocardiography during anesthesia with a combination of ketamine and an α2-adrenoceptor agonist.9–11

In a study9 that compared the effects of a dexmedetomidine-ketamine anesthetic protocol in a group of 8 client-owned adult chinchillas, the dexmedetomidine-ketamine combination had significant effects on echocardiographic variables, compared with findings for those animals when they were conscious. These changes included reduced FS, reduced cardiac output, and larger left ventricular volume. In chinchillas, the effects of anesthesia with isoflurane are similar to those of anesthesia with dexmedetomidine and ketamine, although there is a greater decrease in FS associated with the latter protocol.9 In dogs, dexmedetomidine sedation led to increased E-point septal separation, increased LVIDd:LVIDs ratio, decreased FS, and false-positive diagnosis of valvular regurgitation.21 In the present study, no significant differences in echocardiographic values were detected when the prairie dogs underwent the DKM anesthetic protocol and isoflurane inhalation anesthesia. Because there are no established echocardiographic reference ranges for conscious prairie dogs, it is possible that the echocardiographic values recorded for both anesthetic protocols did not accurately represent values for those animals when they were not anesthetized. This may be one explanation for the finding that FS in anesthetized black-tailed prairie dogs is slightly lower than that reported for conscious hamsters.6,8

To the authors’ knowledge, comparison of plasma cTnI concentrations in captive black-tailed prairie dogs during anesthesia achieved with different anesthetic protocols has not been reported. Cardiac troponin I is a marker for acute myocardial injury.15 Plasma cTnI concentration is near 0 ng/mL in laboratory animals (including dogs, mice, and rats) that have no evidence of cardiac disease.12–14 In a previous study6 of 17 prairie dogs anesthetized with isoflurane alone, plasma cTnI concentrations were also near 0 ng/mL, although 1 prairie dog had a comparatively high plasma cTnI concentration (1.5 ng/mL) as a result of an unidentified cause. In that study,6 mean plasma cTnI concentrations in the other 16 animals were approximately 0.2 ng/mL, suggesting that this may be the baseline value for healthy prairie dogs. However, it is possible that near-0 concentrations of cTnI may reflect the inadequate sensitivity of this assay to detect small changes in low plasma cTnI concentrations.

In the present study, there was a significant increase in plasma cTnI concentration when the prairie dogs underwent isoflurane inhalation anesthesia second in the treatment order, which was likely secondary to a carryover effect from administration of the DKM combination as the first treatment. Cardiac troponin I is released from cardiac myocytes after myocardial cell damage and has biphasic release kinetics.14,15 In humans, circulating cTnI concentration increases initially at 4 to 6 hours after myocardial cell damage and peaks at 12 to 24 hours after the myocardial injury.15 This is a result of its initial release from the cytosolic pool after myocardial injury. At 2 to 4 days after the initial injury, structural protein release leads to a second increase in circulating cTnI concentration.15 The biphasic release kinetics likely explains the increase in plasma cTnI concentration 3 days after the prairie dogs were administered DKM as their first anesthetic protocol.

In 1 study,22 serum cTnI concentrations in healthy dogs undergoing sedation with medetomidine (a racemic mixture of dexmedetomidine and levomedotomidine) and in dogs premedicated with medetomidine and anesthetized with sevoflurane and propofol were compared. Dogs receiving medetomidine alone had higher serum cTnI concentrations 4 days after sedation, compared with presedation baseline values. Similar changes at day 4 were not evident in dogs that received the same dose of medetomidine followed by anesthesia with sevoflurane and propofol.22 It has been shown that dogs sedated with an α2-adrenoceptor agonist and breathing room air can develop variable degrees of hypoxemia and that this can be exacerbated by additional administration of opioids.24,26,27 For dogs receiving medetomidine alone, it was suggested that the increase in serum cTnI concentration was attributable to development of hypoxemia during sedation. For the dogs that were treated with medetomidine followed by propofol and sevoflurane, no significant increase in serum cTnI concentration was evident, likely because those dogs received supplemental oxygen during anesthesia.22

Results of the present study suggested that when the prairie dogs were treated with DKM initially, sufficient myocardial injury occurred, causing release of structural cTnI and a peak of plasma cTnI concentration 2 to 4 days after the initial insult.15,22 It is possible that the increased plasma cTnI concentrations in prairie dogs following administration of DKM initially may have been secondary to development of myocardial hypoxia during this anesthetic episode. When prairie dogs underwent the DKM anesthetic protocol in the present study, Spo2 measurements at the 5- to 25-minute time points were significantly lower than those recorded when the animals underwent the isoflurane anesthetic protocol. It is important to consider that Spo2 readings can be decreased when local tissue perfusion is poor, such as during vasoconstriction. Vasoconstriction in the prairie dogs of the present study may have falsely decreased these Spo2 measurements; arterial blood gas analysis would have been required to more accurately assess oxygenation status.28 However, for the DKM anesthetic protocol, the Spo2 measurements were consistently decreased throughout the anesthetic episode. This degree of hypoxemia may have been an important contributing factor for the carryover effect that led to significantly increased plasma cTnI concentrations during isoflurane inhalation anesthesia in prairie dogs that had previously received the DKM anesthetic protocol as the first treatment. Further studies to assess the correlation of the degree of hypoxemia with the degree of circulating cTnI concentration increases could be considered. On the basis of the findings of the present study and of research projects on other species, it is recommended to provide supplemental oxygen, when possible, to animals undergoing an injectable anesthetic protocol.22,26,27,29

Increases in circulating cTnI concentration are associated with many types of myocardial disease processes in multiple species.14–17,30 In humans, dogs, and cats, primary myocardial injury (including acute myocardial infarction, primary cardiac disease, infections, and cardiac trauma) can cause a considerable increase in circulating cTnI concentration.30 Secondary myocardial injury, which can be a result of sepsis, anemia, fever, treatment with cardiotoxic drugs, and hypoxemia, also leads to increases in circulating cTnI concentration.30 It is important to note that an increase in circulating cTnI concentration reflects myocardial injury and is not indicative of the underlying cause. Persistent myocardial injury in humans, dogs, and cats with critical disease can result in myocardial dysfunction characterized by ventricular dilation, reduced contractility, and decreased ventricular compliance.30

Increases in circulating cTnI concentration have also been found to provide important prognostic information. Among dogs with underlying cardiac disease, those with persistent concentrations of cTnI > 1.0 ng/mL over the course of 2 months had a worse prognosis than did those in which cTnI concentrations normalized within the same period.31 In addition, in critically ill humans and humans who underwent noncardiac surgery, high cTnI concentration was associated with greater mortality rates.32,33 In the authors’ experience, plasma cTnI concentrations > 1 ng/mL (equivalent to 1 μg/L) in dogs and cats are indicative of severe myocardial insult, which warrants further diagnostic investigation, especially in the face of systolic dysfunction. That cutoff value is similar to the plasma cTnI concentrations during isoflurane inhalation anesthesia in the prairie dogs that had previously undergone the DKM anesthetic protocol as their first treatment in the present study. It is not known whether circulating cTnI concentrations of this magnitude would cause long-term adverse clinical effects. Among the prairie dogs of the present study, there were no notable echocardiographic changes in the animals that had higher plasma cTnI concentrations. However, if cardiovascular disease is a concern for an individual prairie dog, it may be advisable to avoid use of a DKM anesthetic protocol to prevent additional myocardial injury. In addition, it is advisable to provide supplemental oxygen to all black-tailed prairie dogs undergoing an injectable anesthetic protocol with a DKM combination. If a prairie dog undergoes the DKM anesthetic protocol, a longer washout interval (> 3 days) might also be required before the animal is anesthetized again.

One limitation of the present study was the small sample size, which can lead to type II errors in statistical analyses. The influence of sex on the study results was not evaluated because only male prairie dogs were used. Because the injectable anesthetic protocol involved > 1 drug, it is possible that there was a cumulative effect of the 3 medications on both echocardiographic data and measured plasma cTnI concentrations. A correlation between plasma cTnI concentrations and order of treatment was identified in this study, but underlying risk factors that contributed to increases in plasma concentrations (eg, transient hypotension, hypoxemia, and preexisting occult cardiac disease) could not be determined. Future research should evaluate the potential effects of sex and individual anesthesia drugs on the duration of increased circulating plasma cTnI concentration after anesthesia in prairie dogs.

The results of the study reported here supported the hypothesis that there were no significant differences in echocardiographic variables for black-tailed prairie dogs anesthetized with DKM injection or via inhalation of isoflurane. However, the significant increase in plasma cTnI concentration identified during isoflurane inhalation anesthesia when prairie dogs had previously undergone the DKM anesthetic protocol as their first treatment suggested that there may be an increased risk for myocardial injury associated with the DKM anesthetic protocol in this species. Clinicians should be aware that provision of supplemental oxygen may help avoid potential development of myocardial hypoxia in prairie dogs undergoing a DKM anesthetic protocol.

Acknowledgments

Funded by an Association of Exotic Mammal Veterinarians Research Grant and the Department of Clinical Sciences, College of Veterinary Medicine, Kansas State University

The authors thank Abaxis Incorporated for providing the plasma cTnI assays used in this study. The authors also thank Jessie Roberts and Kirk Nemechek at the Sunset Zoo and Sarah Ostrom and Christy Zimmer for technical assistance.

ABBREVIATIONS

cTnl

Cardiac troponin 1

DKM

Dexmedetomidine-ketamine-midazolam

FS

Fractional shortening

LVIDd

Left ventricular internal dimension at end diastole

LVIDs

Left ventricular internal dimension at end systole

Spo2

Oxygen saturation as measured by pulse oximetry

Footnotes

a.

Sunset Zoo, Manhattan, Kan.

b.

Research Randomizer, Urbaniak GC, Plous S. Available at: randomizer.org. Accessed Aug 20, 2017.

c.

Dexdomitor, Zoetis, New York, NY.

d.

Ketaset, Fort Dodge Laboratories Inc, Fort Dodge, Iowa.

e.

Versed, West-Ward Pharmaceutical Corp, Eatontown, NJ.

f.

Hikma Farmaceutica, Terrugem, Portugal, Spain.

g.

Antisedan, Zoetis, Parsippany, NJ.

h.

DRE Waveline Touch, DRE Veterinary, Louisville, Ky.

i.

Bair Hugger; Augustine Medical Inc, Eden Prairie, Minn.

j.

i-STAT blood analyzer, Abaxis Veterinary Diagnostics, Union City, Calif.

k.

Vivid 7, GE Medical Systems, Horten, Norway.

l.

R package, version 3.1-121, R Foundation for Statistical Computing, Vienna, Austria.

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Contributor Notes

Address correspondence to Dr. Ross (esross24@vet.k-state.edu).