Effects of ketamine and lidocaine in combination on the sevoflurane minimum alveolar concentration in alpacas

Abstract

This study investigated the effects of ketamine and lidocaine in combination on the minimum alveolar concentration of sevoflurane (MAC_SEVO) in alpacas. Eight healthy, intact male, adult alpacas were studied on 2 separate occasions. Anesthesia was induced with SEVO, and baseline MAC (MAC_B) determination began 45 min after induction. After MAC_B determination, alpacas were randomly given either an intravenous (IV) loading dose (LD) and infusion of saline or a loading dose [ketamine = 0.5 mg/kg body weight (BW); lidocaine = 2 mg/kg BW] and an infusion of ketamine (25 μg/kg BW per minute) in combination with lidocaine (50 μg/kg BW per minute), and MAC_SEVO was re-determined (MAC_T). Quality of recovery, time-to-extubation, and time-to-standing, were also evaluated. Mean MAC_B was 1.88% ± 0.13% and 1.89% ± 0.14% for the saline and ketamine + lidocaine groups, respectively. Ketamine and lidocaine administration decreased (P < 0.05) MAC_B by 57% and mean MAC_T was 0.83% ± 0.10%. Saline administration did not change MAC_B. Time to determine MAC_B and MAC_T was not significantly different between the treatments. The quality of recovery, time-to-extubation, and time-to-standing, were not different between groups. The infusion of ketamine combined with lidocaine significantly decreased MAC_SEVO by 57% and did not adversely affect time-to-standing or quality of recovery.

Introduction

The use of multidrug regimens to maintain general anesthesia allows a decrease in the minimum alveolar concentration (MAC) of volatile anesthetics, and is generally associated with a lesser decrease in arterial blood pressure than when inhalational anesthesia alone is used (1). Ketamine and lidocaine infusions are commonly used in combination with inhalational anesthetics, such as sevoflurane (SEVO), for the maintenance of anesthesia.

Ketamine infusions decreased the MAC of isoflurane in alpacas (2), cats (3), dogs (4,5), and goats (6,7); the MAC of enflurane in dogs (8); the MAC_SEVO in dogs (9) and rats (10), and the MAC of halothane in horses (11). Intravenous (IV) infusions of lidocaine decreased the MAC of isoflurane in a variety of species, including dogs (4,12), cats (13), goats (7), and horses (14), and the MAC_SEVO in dogs (9,15) and horses (16). The hypnotic effects of lidocaine combined with ketamine were demonstrated to be synergistic in mice (17), and the combination had an additive effect in decreasing the MAC of isoflurane in goats (7), and the MAC_SEVO in dogs (9). Although ketamine decreased the MAC of isoflurane in alpacas when administered at 40 μg/kg body weight (BW) per minute (2), there is no information on the effect of either lidocaine alone or lidocaine in combination with ketamine on the MAC of volatile anesthetics in camelids.

The objective of this study was to investigate the effects of a constant rate infusion of lidocaine and ketamine in combination on the MAC_SEVO in alpacas. It was hypothesized that the combination would significantly decrease the MAC_SEVO.

Materials and methods

Animals

Eight healthy, intact male, adult (2.5- to 10-year-old) alpacas, weighing between 42.3 kg and 60.5 kg, were used in the study. Alpacas were determined to be healthy on the basis of history, physical examination, complete blood cell count, and serum biochemical profile. Food was withheld for 18 h prior to anesthesia but water was permitted. The study was approved by the Institutional Animal Care and Use Committee of the Louisiana State University School of Veterinary Medicine.

Experimental design

Alpacas were used in a randomized 8 × 2 crossover design (Wichman-Hill procedure for pseudo-random number generation). Briefly, each alpaca was studied on 2 different occasions, getting each treatment once, with a washout period of at least 10 d between treatments. Baseline MAC (MAC_B) and post-treatment MAC (MAC_T) for SEVO were measured for each anesthetic occasion. Investigators were aware of the selected treatments.

Anesthesia

Anesthesia was induced with SEVO at 6% (SevoFlo; Abbott Laboratories, Illinois, USA) in oxygen (5 L/min) delivered by face mask from a circle system attached to a small animal anesthetic machine (SurgiVet Small Animal Veterinary Anesthesia Machine; Smiths Medical, Ohio, USA). After endotracheal intubation, alpacas were placed in left lateral recumbency and anesthesia was maintained with SEVO in oxygen (3 L/min). Monitoring (Passport 2 Datascope Gas Module SE; Datascope, New Jersey, USA) consisted of electrocardiogram (ECG), indirect blood pressure, pulse oximetry, and continuous gas analysis for capnography (PE′CO₂), end-tidal oxygen (PE′O₂), and end-tidal sevoflurane (FE′SEVO), and body temperature. Gas sample was drawn from the proximal portion of the endotracheal tube at a rate of 150 mL/min. The gas analyzer was calibrated, according to the manufacturer’s instructions at the start of each experiment. Indirect blood pressure was monitored using an oscillometric technique, and a suitably sized cuff (width approximately 40% of limb circumference) placed between the elbow and carpal joint. Ventilation was controlled (SAV 2500 Anesthesia Ventilator; SurgiVet, Wisconsin, USA) to achieve PE′CO₂ between 30 and 45 mmHg. Body temperature was monitored continuously via an esophageal probe and maintained within normal limits (37.5°C to 38.6°C) using a forced warm air device (Bair Hugger; 3M, Minnesota, USA). Hemoglobin oxygen saturation was estimated using a probe placed on the tongue. A 16-gauge (51 mm) IV catheter (Abbocath-T; Venisystems, Illinois, USA) was placed into the right jugular vein for administration of a Normosol-R solution (Hospira, Illinois, USA) at 3 mL/kg BW per hour, and ketamine (Ketathesia; Butler Health Supply, Ohio, USA) and lidocaine (Lidocaine Injectable; Sparhawk Laboratories, Kansas, USA) administration. An 18-gauge (51 mm) IV catheter (Abbocath-T; Venisystems, Illinois, USA) was placed into the saphenous vein at the medial aspect of the left hind limb for blood sampling for drug analysis. Catheters were placed percutaneously and aseptically after animals were anesthetized and recumbent on the surgical table.

Determination of baseline minimum alveolar concentration

Approximately 45 min after induction of anesthesia, and with FE′SEVO held constant at 2.3 vol% for at least 20 min, determination of the MAC_B for SEVO was initiated. A noxious stimulus, which consisted of clamping a claw between the jaws of a 25.4 cm Vulsellum forceps, was delivered. The forceps were closed tightly to the first or second ratchet, depending on the claw size, just below the coronary band, and the claw was moved continuously for 1 min, or until purposeful movement occurred. Purposeful movement was defined as gross movement of the head or extremities. Coughing, stiffening of the neck or limbs, or chewing was not considered purposeful movement. If purposeful movement occurred, the FE′SEVO was increased by 0.1 vol%, otherwise it was decreased by 0.1 vol%, and the stimulus was re-applied after a 20-minute equilibration period. Claws were clamped in a rotating manner to prevent overuse of individual claws. The MAC_B for SEVO was defined as the mean of the highest concentration that resulted in gross purposeful movement and the lowest concentration that prevented that movement. The MAC_B was determined in duplicate for each anesthetic occasion and the mean of the mean values was used for statistical analysis. Time-to-MAC_B was recorded as the time from intubation to the completion of MAC_B determination, in duplicate.

Drug administration

After MAC_B determination, alpacas were randomly given one of the following 2 IV treatments as a loading dose (LD) and constant rate infusion (CRI), as follows:

• Saline: 0.9% NaCl solution (LD of 10 mL followed by a CRI at the volume of ketamine + lidocaine calculated for that specific animal)

• Ketamine + lidocaine: ketamine (LD of 0.5 mg/kg BW followed by CRI of 25 μg/kg BW per minute) combined with lidocaine (LD of 2 mg/kg BW followed by CRI of 50 μg/kg BW per minute)

Loading doses were made up to a final volume of 10 mL in normal saline (0.9% NaCl) and given over 3 min. The CRI was started immediately after the loading dose was administered. For each anesthetic occasion, the LD and CRI of ketamine + lidocaine were gently shaken several times into the syringes for homogeneous mixture of both drugs, ketamine and lidocaine. The LD and CRI were delivered using a syringe pump (Medfusion; Medex, Duluth, Georgia, USA).

Post-treatment MAC (MAC_T) determination began 30 min after the start of the LD with the FE′SEVO held constant for at least 20 min at each alpaca’s MAC_B value. MAC_T was determined in duplicate (MAC_T1 and MAC_T2) using the same methodology as described for MAC_B. After MAC_T determination, SEVO and ketamine + lidocaine infusion were discontinued and alpacas were allowed to recover. The time-to-MAC_T determination was defined as the time from starting the CRI of ketamine + lidocaine to the completion of MAC_T in duplicate. Approximately 6 mL of blood was collected from a saphenous catheter for analysis of ketamine and lidocaine plasma concentrations at the time of each MAC_T determination (MAC_T1 and MAC_T2). The blood was placed into lithium heparin tubes and the plasma was harvested and stored at −80°C until analysis.

Recovery evaluation

Alpacas were placed in sternal recumbency for recovery with their heads elevated on a pad. A quality recovery score system was made and used to subjectively evaluate (by the same investigator, PQW) alpacas’ recovery using a 3-point scale as follows:

• Score 1 = no drooping of eyelids; alert and able to hold head up and move head around while in sternal recumbency; no struggling or paddling; and standing on first attempt.

• Score 2 = mild drooping of eyelids; partially able to hold head up and to move head around while in sternal recumbency; mild struggling and paddling; more than 2 attempts to stand.

• Score 3 = moderate drooping of eyelids; unable to hold head up or to move head around while in sternal recumbency; moderate paddling movements and unwillingness to remain in sternal recumbency; more than 2 attempts to stand.

Extubation was performed after chewing motions were present. Time-to-extubation was defined as time (min), from discontinuing ketamine + lidocaine and SEVO to extubation. Time-to-standing (min), was defined as the time from discontinuing ketamine + lidocaine and SEVO to standing. Although animals were allowed to recovery freely, they were encouraged to stand up with noise stimulation (hand clapping and calling alpacas’ names).

Drug analysis

Samples were prepared for analysis by a protein precipitation method. A standard curve of ketamine, lidocaine, and lidocaine metabolites, monoethylglycinexylidide (MEGX), and glycinexylidide (GX) was prepared from 0, 50, 100, 500, 1000, 5000, and 10 000 ng/mL.

A TSQ Vantage triple quadrupole mass spectrometer with a Transend turboflow LC system (Thermo Scientific) was used for the positive ion electrospray (ESP+) analysis. An Eclipse Plus C₁₈ column, 3.0 × 100 mm; 3.5 μm particle size (Agilent) was used for the analysis with an injection size of 10 μL. The mobile phases used were: A — 0.1% formic acid in H₂O, and B — 0.1% formic acid in acetonitrile with a flow rate of 300 μL/min. The gradient was as follows: 0 to 1.5 min — 90% A: 10% B; 1.5 to 5 min — 2% A: 98% B; 5.5 to 10 min — 90% A: 10% B.

Multiple reaction monitoring analysis was performed for all compounds. Ion transitions used for quantization were 292 > 152 m/z for the internal standard (Morphine-d6); 235.16 > 86.15 m/z for lidocaine; 238.08 > 125.05 m/z for ketamine; 207.12 > 58.16 m/z for MEGX, and 179.13 > 122.15 m/z for GX. Dwell for each transition was 0.10 s and collision energy was optimized according to each compound. Mass spectrometric conditions were optimized for lidocaine by infusion of a pure standard. Although, the tube lens optimization was according to each compound.

Statistical analysis

Treatment effect on MAC_B, MAC_T, percent change in MAC, time to MAC_B, time to MAC_T, time-to-extubation, and time-to-standing were determined using the MIXED procedure of SAS (SAS system; SAS Institute, Cary, North Carolina, USA). Treatment (saline or ketamine + lidocaine), period (1 or 2) and sequence (1: saline first and ketamine + lidocaine second or 2: ketamine + lidocaine first and saline second) were included in the model as fixed effects. Plasma concentration of ketamine, lidocaine, and lidocaine metabolites (MEGX and GX) were analyzed including sequence and time of MAC_T determination (1 or 2) in the model as fixed effect. Alpaca within sequence was included as a random effect in all the above models. Recovery scores were analyzed using the GLIMMIX procedure of SAS using the model previously described for the non-plasma variables. The multinomial distribution with a cumulative logit link was implemented. Results are reported as least squares means ± SEM, unless stated otherwise. Significance was declared at P < 0.05.

Results

The least-squares mean of SEVO MAC_B was 1.88% ± 0.13% and 1.89% ± 0.14% for the saline and K + L groups, respectively, and did not significantly differ between treatments (Table I). Ketamine combined with lidocaine significantly (P = 0.0001) decreased MAC_B by 56.9% ± 4.6%. Saline did not significantly (P = 0.898) change MAC_B. Time to determine MAC_B and MAC_T was not significantly different between the treatments (Table I). Plasma concentrations (ng/mL) of lidocaine and its main metabolites, GX and MEGX, and ketamine are reported in the Table II. The mean plasma concentration was 1797 ng/mL for lidocaine, 748 ng/mL for ketamine, 217 ng/mL for GX, and 927 ng/mL for MEGX. A significant (P = 0.005) increase in GX plasma concentration was observed from MAC_T1 to MAC_T2 determination. Although no other significant difference was observed in plasma concentration of ketamine (P = 0.860), lidocaine (P = 0.905), and the MEGX metabolite (P = 0.093) between MAC_T1 and MAC_T2. The mean induction time was 11.7 min and mean arterial blood pressure in all alpacas was greater than 70 mmHg at all times. The time-to-extubation (P = 0.522) and time-to-standing (P = 0.737) did not significantly differ between treatment groups. The mean time-to-extubation was 18.13 ± 3.22 min for saline and 21.13 ± 3.22 min for ketamine + lidocaine. The mean time-to-standing was 45.25 ± 4.90 min for saline and 42.88 ± 4.90 min for ketamine + lidocaine. The median recovery score was 1 for saline treatment and 1.5 for K + L treatment. There was no significant difference (P = 0.404) between treatments for quality of recovery, although one alpaca in the K + L group received a score of 3.

Table I.

Effect of IV saline (0.9% NaCl) solution (control group) and ketamine (LD: 0.5 mg/kg BW IV; CRI: 25 μg/kg BW per minute) combined with lidocaine (LD: 2 mg/kg BW, IV; CRI: 50 μg/kg BW per minute) on sevoflurane MAC in alpacas (mean ± SEM)

Treatment	MACB	MACT	Change (%)a	Time MACBb	Time MACTb
Saline	1.88 ± 0.13c	1.84 ± 0.10a	−0.61 ± 4.6c	276 ± 31c	145 ± 25c
K + L	1.89 ± 0.13c	0.83 ± 0.10b	−56.9 ± 4.6d,e	230 ± 31c	169 ± 25c

^aPercentage change from baseline MAC = [(MAC_T erMAC_B)/MAC_B] × 100.

^bTime (min) to complete MAC_B and MAC_T determination in duplicate.

^c,dValues in the same columns with different letters are significantly different (P < 0.05).

^eValue significantly different from 0.

LD — loading dose; MAC — minimum alveolar concentration; MAC_B — baseline MAC; MAC_T — post-treatment MAC

Table II.

Plasma concentration of ketamine, lidocaine, and lidocaine metabolites, monoethylglycinexylidide (MEGX), and glycinexylidide (GX) at the time of post-treatment MAC determination (MAC_T1 and MAC_T2) in 8 alpacas given ketamine (LD: 0.5 mg/kg BW IV; CRI: 25 μg/kg BW per minute) combined with lidocaine (LD: 2 mg/kg BW IV; CRI: 50 μg/kg BW per minute), expressed as mean ± SEM

	MACT1	MACT2
GX	204 ± 69b	229 ± 69a
MEGX	884 ± 213a	971 ± 213a
Lidocaine	1803 ± 239a	1791 ± 241a
Ketamine	788 ± 303a	708 ± 325a

^a,bValues in the same row with different letters are significantly different (P < 0.05).

Discussion

The MAC method used in this study consisted of bracketing SEVO concentrations up and down by 0.1%. In the present study, the mean baseline MAC_SEVO was 1.89%, which is less than the MAC_SEVO (2.33%) reported in another study in alpacas (18). A variation in MAC values of up to 20% within species is well-accepted in MAC studies (19). Larger variations in MAC values due to different experimental designs (differences in type and intensity of noxious stimulus and the subjectivity of purposeful movement assessment), have been reported (20). Interestingly, in a study on dogs (21), it was speculated that a low number of subjects (low study power) could contribute to MAC variation among different species. The difference in MAC_SEVO values for alpacas between the present study and the study done by Grubb et al (18), could be explained by the different of type of noxious stimuli used (clamping a claw versus electrical stimulus, respectively) and the subjective assessment of the purposeful movement (different investigators). One could also speculate if the power of these studies contributed to the MAC variation (8 alpacas versus 6 alpacas, respectively).

The administration of ketamine + lidocaine was associated with a 57% decrease in the MAC_SEVO, and this is consistent with the reported effects of the combination on MAC_SEVO in dogs (9).

In a previous study in alpacas (2), an infusion of ketamine at 40 μg/kg BW per minute decreased the MAC_SEVO by 37%, but plasma concentrations of ketamine were not reported. In goats (6), a similar infusion rate of ketamine decreased the MAC of isoflurane by 28.7% at a plasma concentration of 592 ng/mL, which is less than the mean value of 782 ng/mL achieved herein. Thus, it is likely that in the present study, the contribution of ketamine to MAC reduction was greater than 28.7%. In dogs, ketamine infusion of 40 μg/kg BW per minute resulted in a plasma concentration of 824 ± 196 ng/mL with an isoflurane MAC decrease of 33%. Yet, in cats (3), a ketamine infusion of 23 μg/kg BW per minute resulted in mean plasma ketamine concentrations of 1750 ng/mL and was associated with an isoflurane MAC decrease of 45% ± 17%. A study in dogs (22) reported how ketamine pharmacokinetics can be greatly variable even within same group of species. Therefore, the discrepancies in the plasma ketamine concentrations among studies cited here are probably due to differences in the ketamine’s pharmacokinetics among species.

Lidocaine infusions of 50 μg/kg BW per minute are associated with decreases in MAC_SEVO between 22% in dogs (9) and 27% in horses (16) at plasma concentrations of approximately 1500 ng/mL and 2200 ng/mL, respectively. In goats, an infusion of 100 μg/kg BW per minute resulted in a plasma concentration of lidocaine comparable (1900 ng/mL) to the present study, and was associated with a decrease in the isoflurane MAC of 18% (7). Based on these studies, it is unlikely that lidocaine was associated with more than a 25% decrease in the MAC_SEVO herein. Lidocaine metabolism occurs mainly by oxidative dealkylation via cytochrome P-450 enzymes. Dealkylation produces MEGX and GX, the 2 main metabolites of lidocaine, with the MEGX being further metabolized to GX (23). After an IV administration of lidocaine, plasma concentrations of MEGX and, especially, GX increase with time while lidocaine rapidly decreases (24). Therefore, the difference in the GX plasma concentrations between MAC_T1 and MAC_T2 could be expected according to the time elapsing for the post-treatment MAC determinations.

A weakness in our study was that each drug’s effect on MAC was not assessed separately and therefore, the individual contribution of each drug to the decrease in MAC is unknown.

The interaction between ketamine and lidocaine on volatile anesthetic-induced immobility in other species was considered to be additive (7,9). However, it is difficult to compare studies because of differences in species and drug infusion rates; and, in addition, plasma concentrations of the drugs differed even when similar infusion rates were used (4,7,9,12,16). For example, higher infusion rates (100 μg/kg BW per minute) of ketamine and lidocaine in dogs decreased the MAC_SEVO by approximately 63% (9), which is comparable to the decrease of 57% in the MAC_SEVO in the present study. In goats (7), an infusion of ketamine at 50 μg/kg BW per minute and lidocaine at 100 μg/kg BW per minute decreased the MAC of isoflurane by 69%. In a clinical study in horses (1), ketamine at 50 μg/kg BW per minute combined with lidocaine at 50 μg/kg BW per minute reduced the MAC of isoflurane by 40%. On the contrary, lower infusion rates of ketamine at 10 μg/kg BW per minute combined with lidocaine at 20 μg/kg BW per minute in a clinical study in sheep (25) reduced the requirement for isoflurane by 23%. The doses of ketamine and lidocaine used in the present study were based on studies in alpacas (2) and goats (6,7).

In conclusion, the infusion of ketamine combined with lidocaine, at the doses used in this study, decreased the MAC_SEVO by 57% in alpacas, and did not affect the quality of recovery from anesthesia. Based on these results, IV administration of ketamine combined with lidocaine, during SEVO anesthesia, may be a safe source to achieve a balanced anesthesia in alpacas.