Materials and Methods

Animals and instrumentation—Seven healthy sexually intact domestic shorthair cats (4 males and 3 females) aged 12 to 17 months and weighing 2.8 to 4.6 kg were included in the study. Two weeks before beginning phase 1 of the study, each cat had been surgically implanted with a telemetry device that permitted the simultaneous and continuous monitoring of respiration, ECG data, arterial (femoral artery) blood pressure, and rectal temperature. The study protocols were approved by the institutional animal care and use committee of the study facility.

Experimental design—In phase 1, the effects of perzinfotel administration were determined. To do so, 6 cats underwent 7 treatments as follows: saline (0.9% NaCl) solution (0.2 mL/kg), then perzinfotel at 5 mg/kg, IM; 10 mg/kg, IM; 15 mg/kg, IM; 10 mg/kg, IV; and 10 mg/kg, SC. Each treatment was separated by a minimum washout period of 7 days. Saline solution was administered first to determine the baseline MAC (MAC0) and other variables during isoflurane anesthesia. The order of the 6 remaining treatments was determined via a Latin square crossover design. Each treatment was administered 30 minutes before anesthetic induction with isoflurane. The isoflurane MAC was determined twice for each treatment approximately 30 minutes after anesthesia onset (MAC1) and 2 hours later (MAC2).

In phase 2, the effects of IM administration of a low dose of perzinfotel and a combination of perzinfotel and butorphanol were determined. Because of dysfunction of one of the telemeters, 1 of the original 6 cats was replaced for this phase. As in phase 1, saline solution was first administered at a dose of 0.2 mL/kg to allow determination of the MAC0 and other variables in all cats to control for possible confounding effects (ie, lessening of anesthetic requirements) resulting from habituation to the laboratory environment, the noxious stimulation protocol, or the repeated anesthesia (ie, temporal factors). For the second treatment in all cats, the low-dose perzinfotel (2.5 mg/kg, IM) was administered 30 minutes before induction of anesthesia. For the first and second treatments, the isoflurane MAC was determined at MAC1 and MAC2. For the third treatment in all cats, butorphanol tartrate (0.2 mg/kg, IM) was administered 30 minutes before induction of anesthesia. Then, after the isoflurane MAC was first determined at MAC1, a higher dose of perzinfotel (5 mg/kg, IM) was administered. Two hours later, the second isoflurane MAC (MAC2) measurement was made.

Experimental procedures—Food was withheld from cats for 12 hours and water was withheld for 2 hours prior to preanesthetic treatment administration. After the aforementioned preanesthetic treatments were administered, anesthesia was induced with isoflurane^a in oxygen in an induction box and cats were orotracheally intubated and positioned in right lateral recumbency. Isoflurane in oxygen was used to maintain anesthesia through an out-of-circle, agent-specific vaporizer^b in a semiclosed anesthetic circle rebreathing system.^c The Petco₂ was maintained between 35 and 45 mm Hg^d by means of controlled breathing. The ECG, heart rate, various measures of blood pressure (SAP, DAP, and MAP), ET_ISO, Petco₂, Po₂, rectal temperature, and inspiration and expiration isoflurane concentrations were continuously monitored.^e,f Heating pads^g and hot water blankets were used during anesthesia to maintain rectal temperature between 37.5° and 38.5°C.

Determination of isoflurane MAC—Isoflurane MAC was determined by delivering a noxious supramaximal electrical stimulus to the buccal mucosa of each cat.¹⁸ Two 24-gauge, 10-mm insulated stimulating electrodes^h were inserted 1 cm apart into the buccal mucosa at a location dorsal and caudal to the incisors. The opposite ends of the electrodes were connected to an electrical stimulatorⁱ that delivered a predetermined stimulus of 50 V, 5 Hz, and 10-millisecond duration. Stimulation continued for 1 minute unless the cat had signs of gross purposeful movement (ie, lifting of the head and repeated movement of the limbs) before completion of the 1-minute stimulation. The ET_ISO was initially set at 1.5% for each cat's first control MAC determination and at 1.2 times each cat's control MAC value during subsequent days when experimental treatments were administered. When a cat did not respond to the stimulus, the ET_ISO was decreased by 20% and allowed to equilibrate for at least 15 minutes before the stimulus was reapplied. This process was continued until the cat responded with gross purposeful movement. The ET_ISO was then increased by increments of 10% until the cat failed to have gross purposeful movement. The MAC was considered to be the mean of the lowest ET_ISO value that did not result in gross purposeful movement and the highest ETISO value that did result in gross purposeful movement.¹⁸

Determination of BIS—The BIS value was derived by continuously monitoring EEG activity. An EEG was obtained via platinum subdermal needle electrodes by use of a 3-lead referential montage, arranged in a bifrontal configuration with the reference electrode positioned on the midline of the head rostral to the medial canthus of the eyes. The ground electrode was positioned on the midline in the atlantooccipital region.³⁰ The EEG and BIS values were continuously acquired and displayed by use of a proprietary BIS monitor,^j with the high-frequency filter set at 70 Hz and the low-frequency filter set at 2 Hz. The BIS number was automatically calculated and digitally displayed every 5 seconds and represented the EEG activity during the previous 60 seconds. Eight BIS values were recorded during a 2-minute period before and after buccal mucosal stimulation.

Interval to sternal recumbency—Interval to sternal recumbency was defined as the interval between extubation (laryngeal cough reflex) and attainment of sternal recumbency. Time was measured with a digital clock.

Statistical analysis—Values for MAC, BIS, and all cardiopulmonary variables (at MAC level) for each cat and treatment are reported as mean ± SD. For phase 1 of the study, control values for all variables were first compared with baseline values. Because no significant (P > 0.05) differences were detected, effects of all drug treatments included in the Latin square design were compared with each other and with those of the saline solution control treatment. For phase 2, effects of the drug treatments were compared with each other and with baseline values. Control MACs for the 2 phases were compared to verify whether equivalence existed, and a combined analysis was conducted for the 2 phases in which the effect of all drug treatments could be compared. Comparisons were made by means of ANOVA, with least squares means of groups compared with each other via a 2-sided Student t test.^k Values of P ≤ 0.05 were considered significant.

Results

MAC—Control MAC values remained stable throughout phase 1, in which the effects of perzinfotel administered at various doses were determined. The mean ± SD baseline MAC value for isoflurane was 1.54 ± 0.15, whereas the mean control MAC value during the trial was 1.50 ± 0.17 (Table 1). Intravenous, SC, and IM administration of perzinfotel 30 minutes before anesthetic induction significantly decreased the mean isoflurane MAC values by 58.0% to 68.0% for all doses of perzinfotel. The decrease after administration of all different doses and routes of administration of perzinfotel was equivalent (P > 0.05), and no evidence of dose dependency was detected.

In phase 2, in which the effects of IM administration of a low dose of perzinfotel and a combination of perzinfotel and butorphanol were determined, IM administration of 2.5 mg of perzinfotel/kg 30 minutes before anesthetic induction significantly reduced the isoflurane MAC by 43.3%, compared with the control MAC (Table 2). Intramuscular administration of 0.2 mg of butorphanol/kg 30 minutes before anesthetic induction provided a significant reduction of isoflurane MAC by 15.3%, whereas IM administration of 5 mg of perzinfotel/kg 30 minutes before MAC2 was measured resulted in a significant reduction in MAC of 58.0%, compared with baseline.

When the results of phase 1 and phase 2 were combined and analyzed, a dose dependency was evident. The dose of 2.5 mg of perzinfotel/kg resulted in a significantly higher isoflurane MAC than did the higher (5, 10, and 15 mg/kg) doses.

BIS—All BIS values were measured after the isoflurane MAC had been attained. In phase 1, BIS values significantly increased after all perzinfotel treatments, relative to BIS values for the saline solution control treatment (Table 1). However, all doses and routes of perzinfotel administration resulted in a similar, albeit insignificant, increase of approximately 15% relative to baseline. In phase 2, the BIS values for perzinfotel at 2.5 mg/kg, IM, and for the butorphanol-perzinfotel combination, also administered IM, were similar and were significantly higher than the control treatment value (Table 2). Administration of butorphanol alone resulted in a nonsignificant increase in BIS.

HR—All HRs were measured after the isoflurane MAC had been attained. During phase 1, mean HRs of cats when treated with perzinfotel were 6.4% to 10.0% less than the control treatment value; however, these differences between values were not significant (Table 1). During phase 2, mean HRs of cats when treated with perzinfotel were slightly lower (4.5% to 6.6%) than the control treatment value, whereas HR was somewhat higher (4.3%) after butorphanol was administered (Table 2). However, again these changes were not significant.

Blood pressure—All measures of blood pressure were made after the isoflurane MAC had been attained. During phase 1, treatment with perzinfotel at the various doses resulted in a consistent increase in DAP (11.8% to 16.4%), SAP (7.6% to 13.2%), and MAP (9.3% to 14.2%) relative to control values during isoflurane anesthesia (Table 3). Relative to control values, increases in DAP, SAP, and MAP were significant for perzinfotel at 5 mg/kg, IM; 10 mg/kg, IV; and 10 mg/kg, SC. Administration of perzinfotel at a dose of 10 mg/kg, IM, also yielded a significant increase in DAP. In phase 2, DAP, SAP, and MAP increased 9.7%, 8.2%, and 7.8%, respectively, after administration of perzinfotel at 2.5 mg/kg, IM. After administration of 0.2 mg of butorphanol/kg, IM, DAP, SAP, and MAP values were 11.3%, 7.9%, and 10.1% lower than control values, respectively. Intramuscular administration of 5 mg of perzinfotel/kg after IM administration of 0.2 mg of butorphanol/kg caused an increase in DAP, SAP, and MAP of 1.4%, 4.4%, and 2.4% relative to control values (Table 4). However, all changes in blood pressure were insignificant in phase 2.

Interval from sternal recumbency to extubation—Administration of perzinfotel alone yielded some sedative effects such as sleepiness and a decreased ability to react to external stimuli in cats during the 30 minutes prior to anesthetic induction, particularly at the higher (10 to 15 mg/kg, IM) doses. Dose-dependent increases were evident in the interval required for cats to achieve sternal recumbency after extubation (Table 5). In control conditions, the cats required a mean of 68 ± 35 seconds to reach a sternal position after isoflurane anesthesia in phase 1. Premedication with perzinfotel at the various doses (5 mg/kg, IM; 10 mg/kg, IM; 10 mg/kg, SC; and 10 mg/kg, IV) increased this interval; however, the treatment values did not differ significantly from the control value. Only administration of perzinfotel at 15 mg/kg, IM, led to a significant increase in the interval to sternal recumbency (724 ± 737 seconds). During phase 2, cats in the control conditions required 83 ± 44 seconds to reach a sternal position. This interval did not significantly change after IM treatment with perzinfotel at 2.5 mg/kg or with the butorphanol-perzinfotel combination (296 ± 374 seconds). No adverse reactions were observed during anesthesia or anesthetic recovery.

Discussion

Results of the present study provided evidence of the anesthetic-sparing effects of perzinfotel in healthy cats. Administration of all doses of perzinfotel resulted in lower isoflurane MAC values relative to control values, regardless of route of administration. Intramuscularly administered doses of 5, 10, and 15 mg of perzinfotel/kg did not cause a dose-dependent reduction in isoflurane MAC values (58.0% to 68% reduction relative to control values), indicating that a plateau might have been reached. However, perzinfotel when administered IM at a dose of 2.5 mg/kg lowered the isoflurane MAC less than it did at the other doses (43.3% reduction), signifying a dose-dependent effect when compared with the higher doses. The isoflurane MAC reduction of 15.3% after administration of butorphanol was similar to that reported in earlier studies.²⁴ Intramuscular administration of 5 mg of perzinfotel/kg to butorphanol-treated cats resulted in an isoflurane MAC reduction of 58.0%, which was equivalent to the effect of the same dose of perzinfotel administered alone.

The MAC of an inhaled anesthetic is used as a clinical index of drug potency and a guide to selection of the inhalant anesthetic concentration required for general anesthesia.¹⁸ The repeatability and stability over time of the control MAC values reported here indicated that the measured decrease in isoflurane MAC values is scientifically valid. The decrease in isoflurane MAC values was associated with significant increases in BIS values for all doses and routes of perzinfotel, suggesting a reduction in CNS-and anesthetic-associated depression but adequate analgesia to prevent gross purposeful movement in response to a noxious stimulus. A lack of change in BIS and hemodynamic values, among all treatment groups, when the isoflurane concentration was held constant at 1.5% suggested that the decrease in the isoflurane concentration was responsible for the changes detected.

Perzinfotel administration caused a nonsignificant (4.5% to 10%) decrease in HR, compared with control treatment administration in our study cats. In dogs, perzinfotel administration reportedly increases the HR, although not significantly.^22,23 The reason for this apparent discrepancy is unclear and might be species related.

The DAP, MAP, and SAP values in the anesthetized healthy cats in our study were higher than control values when they received perzinfotel and the butorphanol-perzinfotel combination as preanesthetic treatments. These increases were consistent for all doses of perzinfotel and were significant during phase 1 for doses of 5 mg/kg, IM, and 10 mg/kg, SC and IV; the dose of 10 mg/kg, IM, caused a significant increase in DAP. Hypotension as a function of cardiovascular management during anesthesia is an important contributor to anesthetic-related death in humans and other animals.³¹ The ability of perzinfotel to increase blood pressure could therefore potentially improve anesthetic safety. Such hemodynamic effects appeared to be secondary (dependent) effects in our study and were most likely attributable to a decrease in inhaled isoflurane concentrations.

Recovery from isoflurane anesthesia was significantly longer when cats were pretreated with perzinfotel only at the highest dose (15 mg/kg, IM). This finding suggested that perzinfotel administration may have resulted in some immobilizing activity when combined with inhalant anesthetics and supports findings of another study³² suggesting that NMDAR inhibition contributes part of the immobilizing activity of aromatic volatile anesthetics. Because the cats in our study had smooth and unremarkable recoveries, the longer recovery intervals would be of little or no practical importance at a therapeutic dose in a clinical setting.

Ketamine is an NMDAR antagonist approved for use in feline medicine for restraint (11 mg/kg, IM) and anesthesia (22 to 33 mg/kg, IM).³³ In the present study, perzinfotel was evaluated for a different use, as a premedication, which resulted in a significant isofluranesparing effect and an increase in arterial blood pressure, thereby enhancing cardiopulmonary safety during anesthesia. At a therapeutic dose of 5 mg/kg, perzinfotel administration had a negligible sedative effect. Unlike ketamine, perzinfotel can be administered IV, IM, or SC and, at the dose range used in our study, may provide a much greater anesthetic-sparing effect²¹ and cause less adverse effects (eg, dysphoria and difficult anesthetic recovery) than ketamine.

Findings of the present study were somewhat limited because of the small number of cats used. Increases in DAP, MAP, and SAP after perzinfotel administration, although a consistent finding throughout phase 2, were not significant but might have been had more cats been evaluated.