Alfaxalone is a neuroactive synthetic steroid that enhances the inhibitory action of endogenous Y-aminobutyric acid in the CNS and binds to γ-aminobutyric acid A receptors to produce its anesthetic effect.1,2 In cats, alfaxalone has a rapid onset and short duration of action, rapid redistribution, and short elimination half-life.3 Alfaxalone also produces dose-dependent depression of the respiratory system in cats, denoted by a decrease in Ve and Pao2.4,5 After healthy cats are administered a clinically relevant dose of 3 to 5 mg of alfaxalone/kg, IV, alfaxalone induces a mild decrease in systemic vascular resistance,5 systemic hypotension,3,6 a decrease in HR, hypoventilation, and apnea.5 The dose of alfaxalone is recommended to be titrated to effect to induce anesthesia in cats because of the dose-dependent cardiorespiratory depression.1
The autocoinduction or priming technique consists of administering a precalculated subhypnotic dose of an induction agent a few minutes prior to administering a second dose of the same agent to induce anesthesia, with the goal of decreasing the total dose of induction agent administered.7–9 A priming dose is often 20% to 25% of the calculated total induction dose.9,10 A priming dose of propofol administered to people reduces the total induction dose of propofol by 10% to 32%,7–9,11 and after induction, hemodynamic variables (eg, blood pressure and HR) are stable8,10 and apnea occurs less frequently.10 However, administration of a priming dose of propofol (4 mg/kg/min, IV) to healthy dogs in 1 study12 reportedly had no effect on the total required induction dose of the agent, although the fast administration rate of the priming dose may have obscured such an effect.
The objective of the study reported here was to investigate the effects of a priming dose of alfaxalone on the total dose of alfaxalone necessary to permit orotracheal intubation and on the cardiorespiratory function of sedated cats. We hypothesized that a priming dose of alfaxalone would significantly decrease the total induction dose of alfaxalone needed to allow orotracheal intubation and prevent apnea in cats.
Materials and Methods
Animals
Eight 5- to 12-year-old neutered (4 females and 4 males) mixed-breed healthy cats weighing 4.7 to 6.4 kg were selected from an institutional research colony of 27 cats. Health status of each cat was determined on the basis of physical examination, CBC, and serum biochemical analysis. For acclimatization, the 8 cats were relocated to a room separate from the other cats 1 week before the start of the study. Each cat was allocated to a separate cage, with free access to a litter box, fresh water, and a commercial feline diet. On the day of the study, food was withheld for 8 hours prior to sedation but water remained available until sedation. The study was approved by the Louisiana State University Institutional Animal Care and Use Committee (protocol No. 2016/16-100).
Study design
Cats were randomly assigned by use of a randomization programa to receive 1 of 2 priming agents first in a crossover design with an 8-day washout period before the alternate priming agent was administered. On each day of anesthetic induction, cats were first sedated with dexmedetomidineb (3 μg/kg) and methadonec (0.3 mg/kg) mixed in the same syringe and injected in the semitendinosus or semimembranosus muscle. Fifteen minutes after injection of the sedatives, the degree of sedation was subjectively assessed by 1 investigator (ALC) using a previously published13 rating scale. Then, a 22-gauge, 2.5-cm catheterd was aseptically placed into a medial saphenous vein. Noninvasive SAP, MAP, and DAP measurements were obtained with a multiparameter monitore that uses oscillometric technology. A pneumatic cuff of a size equivalent to 40% of the circumference of the measured portion of the forelimb was placed proximal to the carpus and connected to the monitor. Heart rate and rhythm were assessed through review of a continuous lead II ECG trace generated by the same monitor.
Cats were preoxygenated for 5 minutes with 100% oxygen administered at 2 L/min via a face mask connected to a Bain breathing circuit. Anesthetic induction was divided into 2 phases (Figure 1). In phase 1, a priming dose of alfaxalonef (0.25 mg/kg) or an equivalent volume (0.025 mL/kg) of saline (0.9% NaCl) solution was administered IV over 60 seconds. The priming dose of alfaxalone was 25% of the lowest total induction dose (1 mg/kg) reported14 for sedated cats. A 60-second interval was observed, and then phase 2 was initiated, in which alfaxalone was administered IV at a rate of 0.5 mg/kg/min to all cats until signs (lack of lateral and medial palpebral reflexes and swallowing, relaxed jaw tone, and the ability for an investigator [ALC] to grasp and pull the tongue out of the mouth without resistance) were observed that indicated orotracheal intubation could succeed. Alfaxalone and saline solution were administered IV with a precision syringe pumpg that was evaluated for accuracy prior to the beginning of the study.
Illustration of the study timeline. In a crossover study design, 8 healthy cats were sedated with dexmedetomidine (3 μg/kg, IM) and methadone (0.3 mg/kg, IM). Fifteen minutes later, the degree of sedation was evaluated (sedation score), and multiparameter monitors were readied and connected to each cat (instrumentation). Cats then received 1 of 2 priming agents first: alfaxalone (0.25 mg/kg, IV) or an equivalent volume (0.025 mL/kg) of saline (0.9% NaCl) solution, IV (P1), administered over 60 seconds. After 60 seconds, cats received 0.5 mg of alfaxalone/kg/min, IV, until orotracheal intubation was possible (P2). Cardiorespiratory variables were measured at baseline (immediately prior to priming agent administration), at the end of P1, at the end of P2, and every 2 minutes thereafter after orotracheal intubation (T2, T4, … Tn) until the cats were extubated. After an 8-day washout period, the procedures were repeated with the alternate priming agent.
Citation: American Journal of Veterinary Research 81, 11; 10.2460/ajvr.81.11.850
Once signs were observed, one investigator (PQW) immediately stopped the IV infusion of alfaxalone and another investigator (ALC) that was blinded to the administered priming agent attempted to intubate the cat with a cuffed endotracheal tubeh with an internal diameter of 4.5 mm and the aid of a laryngoscope. If orotracheal intubation was successful, the total administered dose of alfaxalone was recorded. If orotracheal intubation was unsuccessful, the IV infusion of alfaxalone was restarted at the same rate until recurrence of signs that suggested that orotracheal intubation was likely achievable. Then, orotracheal intubation was reattempted. Quality of induction, ease of orotracheal intubation, and quality of anesthetic recovery were assessed by 1 investigator (ALC) using previously published scoring systems.15
After successful intubation was achieved, the cuff of the orotracheal tube was inflated to an intracuff pressure of 20 cm H2O, as measured with an aneroid manometer.i A second multiparameter monitorj that included a spirometer measured Spo2, Petco2, RR, and Vt. The Ve was determined by the product of the RR and the Vt. Prior to beginning the study, the capnography function of the monitor was calibrated by use of a commercial gas mixturek (1% isoflurane in 5% carbon dioxide and 60% nitrous oxide) as recommended by the manufacturer.
Data collection
The HR, RR, SAP, MAP, and DAP were recorded immediately prior to (baseline) and after administration of the priming agent and immediately after orotracheal intubation (Figure 1). After successful orotracheal intubation and every 2 minutes thereafter, HR, RR, SAP, MAP, DAP, Petco2, Spo2, and Vt were recorded until the cats were extubated. If apnea (ie, no chest excursions noted for > 60 seconds) or an Spo2 < 90% was observed, intermittent positive-pressure ventilation was initiated at 12 breaths/min through the use of a Bain breathing circuit and administration of 100% oxygen at a rate of 2 L/min until the return of spontaneous breathing. If hypotension (ie, MAP < 60 mm Hg) developed, a constant rate infusion of dopaminel was initiated at 5 μg/kg/min.
Cats were extubated as soon as they regained lateral and medial palpebral reflexes, strong jaw tone, and the ability to swallow or were coughing and head and body movements were observed. Total anesthesia time, defined as the interval between administration of the priming agent and orotracheal extubation, was recorded in minutes. After extubation, the cats were disconnected from the monitors and pneumatic cuffs were removed. Then, each cat was placed into its own carrier and observed for a minimum of 1 hour until it was considered to be completely recovered from anesthesia (ie, cats appeared aware of their environment and did not appear to be sedate). If signs of dysphoria (ie, increased motor activity and excessive vocalization) were observed for a cat during anesthetic recovery, the cat received acepromazine maleatem (0.01 mg/kg, IV) and the dysphoric episode was recorded. After complete recovery, the catheter in the medial saphenous vein was removed.
Statistical analysis
Data recorded at baseline, after each phase of anesthetic induction, and at 2, 4, 6, and 8 minutes thereafter were included in the analyses. Values are reported as mean ± SD. One-way ANOVA with a mixed-effects model was used to compare values of ordinal (scores for degree of sedation, quality of induction, ease of orotracheal intubation, and quality of anesthetic recovery) and continuous (total administered dose of alfaxalone) variables between priming agents (alfaxalone or saline solution; treated as a fixed effect), controlling for cat (included as a random effect). Mean values for cardiorespiratory variables were compared via a 2-way ANOVA with a mixed-effects model. Priming agent, recording times of cardiorespiratory variables, and their interactions were included in the analysis as fixed effects, and cat was included as a random effect. Kenward-Roger degrees of freedom approximation was used to determine the degrees of freedom for each model. When a cardiorespiratory variable was determined to be significant (P < 0.05), post hoc analysis was performed with a Tukey test for pairwise comparisons of the effect on time and time-by-treatment interaction. The residuals from the ANOVA models were checked for normality with the Shapiro-Wilk test, and with the exception of a few outliers at both ends of the distributions, the residuals were normally distributed. Pearson correlation coefficients were determined for the relationships between the total administered dose of alfaxalone and Ve and between dose and MAP. Data were analyzed by use of commercial statistical software.n Values of P < 0.05 were considered significant.
Results
All cats were successfully intubated on the first attempt. Sedation, intubation, induction, and recovery scores were not significantly different between the 2 priming agents. All cats that received alfaxalone as the priming agent and 7 of 8 cats that received saline solution were extubated between 8 and 10 minutes; 1 cat that received saline solution was extubated between 16 and 18 minutes. Two cats were administered acepromazine because of signs of dysphoria, regardless of priming agent; however, these cats were fractious, and signs of dysphoria were instead attributed to their demeanor. The total induction dose of alfaxalone administered when alfaxalone was used as the priming agent (0.98 ± 0.28 mg/kg) was significantly (P = 0.04) lower than when saline solution was used (1.41 ± 0.17 mg/kg).
When saline solution was used as the priming agent, overall mean HR was significantly (P < 0.01) higher than when alfaxalone was used. Mean HR at phase 2 of anesthetic induction through 8 minutes afterward was also significantly (P < 0.01) higher when saline solution was used (Figure 2). Overall MAP was significantly (P < 0.01) lower when saline solution was administered as the priming agent. No cats developed systemic hypotension. A significant (P < 0.01) negative correlation was noted between total induction dose of alfaxalone and MAP at phase 2 of anesthetic induction and 2, 4, and 8 minutes afterward.
Least squares mean ± SEM values of HR (A) and MAP (B) for the cats of Figure 1 recorded before and after administration of a priming dose of alfaxalone (squares) or saline solution (circles). B = Baseline. See Figure 1 for remainder of key.
Citation: American Journal of Veterinary Research 81, 11; 10.2460/ajvr.81.11.850
Mean RR was not significantly different between priming agents at any time points. No cats had apnea or an Spo2 < 90% during the assessed period. Overall, Vt was significantly (P < 0.01) greater when saline solution was used, but no significant differences in Ve were noted (Figure 3).
Least squares mean ± SEM values of Vt (A) and Ve (B) for the cats of Figure 1 recorded at the time of and after administration of 0.5 mg of alfaxalone/kg/min, IV, for anesthetic induction. Squares = Priming dose of alfaxalone. Circles = Priming dose of saline solution. See Figure 1 for remainder of key.
Citation: American Journal of Veterinary Research 81, 11; 10.2460/ajvr.81.11.850
Discussion
The present study revealed that a priming dose of alfaxalone significantly reduced the total induction dose of alfaxalone by 30.5% for cats sedated with dexmedetomidine and methadone. Although the lower dose of alfaxalone did not impart any clinically important respiratory system benefits on the basis of the measured variables, higher MAP was observed, and therefore, the development of hypotension may have been prevented.
The priming dose of alfaxalone administered to the cats equaled 25% (0.25 mg/kg) of the lowest induction dose (1 mg/kg) reported14 for alfaxalone administration to sedated cats. The lowest induction dose was used as the reference dose from which the priming dose was determined so the phase 2 induction dose could be titrated to the desired effect. Different effects may have been observed, however, with a higher priming dose, and further studies are necessary to identify the optimal priming dose of alfaxalone for sedated cats. The priming dose was injected slowly over 60 seconds in an attempt to minimize the risk of rate-related adverse respiratory effects. The rate of 0.5 mg of alfaxalone/kg/min during phase 2 of anesthetic induction was decided on the basis of the results of a preliminary study,16 in which alfaxalone administered to healthy cats at 0.5 mg/kg/min, compared with 2 mg/kg/min, resulted in a lower total induction dose of the agent.
In people, a slow administration rate of propofol for anesthetic induction decreases the total induction dose because the slow rate allows time for propofol to equilibrate across the blood-brain barrier and exert its effect; subsequently, a lower total dose successfully induces unconsciousness.17 Although a similar study of alfaxalone in cats is lacking, the significant dose-sparing effect achieved with the administration of a priming dose of alfaxalone to the cats of the present study may also be similarly explained; with administration of a priming dose of alfaxalone and a 60-second delay before starting phase 2 of induction, time was permitted for alfaxalone to reach the brain to induce subhypnosis, and therefore, a lower dose of alfaxalone at phase 2 of induction was needed to induce unconsciousness.
The interval between administration of the priming dose and the dose for phase 2 of induction has not yet been established for people; however, many studies7–10,18,19 regarding the priming technique with propofol included a 1- or 2-minute interval. For the cats of the present study, both the time to administer the priming dose of alfaxalone and the interval between administration of the priming and induction doses were set at 60 seconds, the latter because of the aforementioned studies7–10,18,19 of people. Intervals > 60 seconds may further decrease the total induction dose, and studies that evaluate the effect of intervals > 60 seconds on the total anesthetic induction dose of alfaxalone for cats are needed.
In veterinary medicine, titration of an induction agent to a dose that produces the desired effect has been suggested to be the same as the priming technique.20 However, the priming technique includes at least a 60-second interval between administration of a subhypnotic and a hypnotic dose of an induction agent, and, with dose titration, often < 60 seconds elapses between administration of each dose.
In the cats of the present study, a significant negative correlation was observed between the total induction dose of alfaxalone and MAP, for both priming agents. Although hypotension (MAP < 60 mm Hg) was not evident in those cats, the development of hypotension in cats that do not receive a priming dose of alfaxalone remains concerning, especially for cats with unstable cardiovascular function, because of the need for a higher total induction dose. Additional studies of cats are needed to confirm the dose-sparing effect of the priming technique on systemic arterial blood pressure, possibly measured invasively, and other cardiovascular variables.
The higher mean HR over the recorded time period for cats that received saline solution as the priming agent may be explained by the single spike in HR at phase 1 of induction, during which an IV bolus of saline solution was administered over 60 seconds. The increase in HR for those cats may have occurred because they were physically stimulated during the injection (eg, the cold temperature of the saline solution). An increase in HR may not have been observed for cats when alfaxalone was used as the priming agent because alfaxalone may have already started to induce subhypnosis as it was being administered, such that those cats were not as physically stimulated.
Alfaxalone causes dose-dependent respiratory depression,4 often manifested in cats by apnea, hypoxemia, and hypoventilation.5 In 1 study,16 IV administration of 0.5 mg of alfaxalone/kg/min caused apnea in 2 of 6 cats. Although alfaxalone was administered at the same rate for phase 2 of induction in the present study, no cat became apneic. Apnea may have been observed for the cats in the previous study16 because alfaxalone (3 mg/kg, SC) was also administered as a sedative prior to administration of the induction dose; dexmedetomidine and methadone were instead administered as sedatives in the present study. Although the total induction dose of alfaxalone was significantly lower when alfaxalone was used as the priming agent in the present study, this dose reduction did not produce any clinically important respiratory benefits (eg, Ve was not significantly different between the 2 priming agents). The phase 2 induction rate may have prevented the development of apnea and any clinically important differences in the measured respiratory variables between the 2 priming agents.
The present study had limitations. A type II error could not be excluded because the number of cats was small. Therefore, a significant difference between priming agents in the measured respiratory variables other than Vt and an association between the degree of sedation and the total induction dose of alfaxalone, if present, were not detected. Second, the concurrent administration of dexmedetomidine may have influenced the time to attain the peak concentration of alfaxalone in the brain; α2-adrenoceptor agonists decrease the cardiac index21 and may increase the time for an induction agent to reach the brain.20 Yet, because the study was of a crossover design, any effect of dexmedetomidine on the total induction dose of alfaxalone was likely uniform within each cat and, therefore, would have uniformly affected the measured variables regardless of the priming agent.
The results of the study reported here indicated that the priming technique with alfaxalone successfully induced anesthesia in sedated healthy cats and had a dose-sparing effect. The measured respiratory variables were similar, irrespective of whether cats received a priming dose of alfaxalone. However, the priming technique attenuated the decrease in MAP that was observed when saline solution was used as the priming agent.
Acknowledgments
Funded by a Louisiana State University VCS Corp grant (PG007871).
The authors declare that there were no conflicts of interest.
The authors thank Dr. Jon Fletcher for the use of animals from his feline research colony and Dr. Sarah Keeton for assistance with data collection.
ABBREVIATIONS
| DAP | Diastolic arterial blood pressure |
| HR | Heart rate |
| MAP | Mean arterial blood pressure |
| Petco2 | End-tidal partial pressure of carbon dioxide |
| RR | Respiratory rate |
| SAP | Systolic arterial blood pressure |
| Spo2 | Arterial hemoglobin oxygen saturation |
| Ve | Expired volume per minute |
| Vt | Tidal volume |
Footnotes
Randomization.com. Available at: randomization.com. Accessed May 28, 2016.
Dexdomitor, Zoetis Inc, Parsippany, NJ.
Mylan, Rockford, Ill.
Sur-Vet Surflo ETFE, Terumo Medical Canada Inc, Vaughan, ON, Canada.
Vet Trends V, Tampa, Fla.
Alfaxan, Jurox Inc, North Kansas City, Mo.
BD CareFusion Alaris syringe 8110 series, Cardinal Health, San Diego, Calif.
Medline tubes, Medline Industries Inc, Northfield, Ill.
Inspiratory force meter 4102, Boehringer Ingelheim, Ingelheim, Germany.
Carescape monitor B850, GE Healthcare, Helsinki, Finland.
Airgas USA LLC, Plumsteadville, Pa.
Hospira Inc, Lake Forest, Ill.
VetOne MWI, Boise, Idaho.
SAS, version 9.4, SAS Institute Inc, Cary, NC.
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