Comparison of arterial blood pressure and quality of induction and recovery in horses anesthetized with diazepam or guaifenesin co-induction combined with xylazine and ketamine and maintained on isoflurane

Abstract

The objective of this prospective alternating crossover study was to compare quality of induction and recovery and cardiopulmonary function in horses administered guaifenesin or diazepam as co-induction drugs. Six healthy horses were anesthetized twice to undergo an MRI and surgical biopsies, with 5 wk between treatments. Horses were sedated intravenously (IV) with xylazine, 0.6 mg/kg BW, and IV induction started 4 min later. Group G received guaifenesin, 100 mg/kg BW, over 2 min and ketamine, 2 mg/kg BW, within 1 to 2 min of starting the guaifenesin. Group D received diazepam, 0.1 mg/kg BW, within 5 s and ketamine, 2 mg/kg BW, 0.5 to 1 min after and the quality of induction assessed. Horses were positioned on an MRI-compatible table, the trachea intubated, allowed to breathe spontaneously for the initial 10 min, then connected to a rebreathing circuit and anesthetic machine and mechanically ventilated under anesthesia [end-tidal isoflurane (F_E′Isoflurane): 1.3 to 1.5%] during the MRI. Direct arterial blood pressure, heart rate, end-tidal CO₂ (P_E′CO₂) and F_E′Isoflurane were recorded at 10, 20, 30, and 40 min post-induction. Requirements for dobutamine and/or phenylephrine for arterial blood pressure support were recorded. A 2-way analysis of variance (ANOVA) was used to compare arterial blood pressure and a Wilcoxon matched-pairs signed rank test for comparing sedation, induction, and recovery scores (significance was set at P < 0.05). Arterial pressures were not significantly different between groups; however, systolic (P = 0.004), diastolic (P < 0.0001), and mean (P = 0.004) arterial pressures decreased significantly at 20, 30, and 40 min in Group D. Dose of dobutamine and/or phenylephrine was similar between groups. Sedation, induction, and recovery scores were similar. Induction scores were good to excellent [both groups: 0.5 (0, 2); median (range)]. In conclusion, guaifenesin and diazepam at the doses used in this study produced similar effects on arterial blood pressure and on the quality of induction and recovery.

Introduction

Co-induction drugs are often used in equine anesthesia to improve the quality of induction by enhancing muscle relaxation, analgesia, and/or central nervous system depression. Inclusion of these drugs can also reduce the dose of sedative drugs used for premedication, as well as the dose of the induction drug (1–7).

Benzodiazepines, such as diazepam and midazolam, are commonly used intravenously (IV) as part of co-induction techniques in equine anesthesia because they have sedative and muscle relaxant properties that help smooth the induction. They can also prolong the anesthesia time and reduce the number of supplemental doses from anesthetic techniques with IV sedatives such as alpha-2 agonists, e.g., xylazine, dexmedetomidine, romifidine, detomidine, and induction with IV ketamine (1–4). Benzodiazepine co-induction with the same dose of either diazepam or midazolam provided faster inductions and better quality of induction and recovery than butorphanol co-induction in horses induced with romifidine and ketamine and maintained under sevoflurane anesthesia for 1 h (1).

Guaifenesin is also used intravenously as a co-induction drug due to its potent muscle relaxant properties (5–7). The first comparison between guaifenesin and diazepam as co-induction drugs (6) established that IV doses of 100 mg/kg body weight (BW) and 0.1 mg/kg BW, respectively, were equipotent in terms of degree of relaxation after induction, ease of intubation, transition to inhalant anesthesia, quality of anesthesia, and quality of recovery after 30 min of halothane anesthesia. However, there was significantly greater ataxia at induction with the use of guaifenesin (6).

An IV dose of 0.06 mg/kg BW of midazolam used as co-induction with ketamine in horses premedicated with acepromazine and xylazine and maintained on isoflurane for elective surgery resulted in a higher degree of ataxia in recovery when the anesthesia time was less than 75 min, compared to horses co-induced with guaifenesin (100 mg/kg BW) either with thiopental or ketamine, although the time to standing was similar in all 3 groups (7).

The objective of this study was to compare equipotent doses of diazepam and guaifenesin as co-induction drugs in horses anesthetized with xylazine and ketamine and maintained under isoflurane anesthesia for at least 2 h to complete a magnetic resonance imaging (MRI) scan, followed by surgical biopsies. The first hypothesis was that the quality of induction and recovery would be similar between both groups and the second hypothesis was that horses given guaifenesin would exhibit hypotension sooner during isoflurane anesthesia than horses given diazepam.

Materials and methods

Animals

Six adult research horses (4 Thoroughbreds, 2 Standardbreds; 4 females, 2 geldings; 498 ± 40 kg; 11.6 ± 2.4 y old) were used in this study. Horses were in good health (status I) according to the American Society of Anesthesiologists (ASA) physical status classification, based on physical examination, venous blood analysis for acid-base and electrolytes, total protein, and hematocrit.

The horses were part of a research project designed to investigate the effects of mesenchymal stromal cell-based treatment for experimentally induced forelimb bilateral superficial digital flexor tendon injuries at the level of the mid-metacarpus. The study was approved by the Animal Care Committee of the University of Guelph (Animal Utilization Protocol, AUP #5202) and followed the guidelines of the Consolidated Standards of Reporting Trials (CONSORT) (8).

The desired sample size was calculated based on 4 different variables: i) mean arterial blood pressure (MAP); ii) time to dobutamine and/or phenylephrine support for MAP and dose of dobutamine and/or phenylephrine; iii) sedation and induction score; and iv) recovery score.

Using an alternative hypothesis that MAP would be 25% lower in the guaifenesin group at 10 min after induction, based on a previous study that demonstrated a 24% decrease from conscious values with guaifenesin (from 119 ± 14 to 96 ± 7 mmHg) and a 6% increase with diazepam (from 113 ± 16 to 120 ± 13 mmHg) at 5 min after induction, while administered halothane (6), and using a power of 90% and an alpha of 0.05, it was determined that a sample size of 5 horses would be required.

It was also speculated that, given guaifenesin’s vasodilatory effects and the combined effects with isoflurane anesthesia, horses given guaifenesin would require dobutamine and/or phenylephrine support sooner after induction and a higher total dose throughout anesthesia than horses given diazepam. For the first assumption, horses given guaifenesin would require dobutamine and/or phenylephrine 20 min after induction, while already under isoflurane anesthesia, and horses given diazepam would require either drug at 30 min after induction. With a coefficient variation of 20% of the mean, a power of 90%, and an alpha of 0.05, this would require a total sample of 6 horses.

The second assumption was that the total dose required during anesthesia in horses given guaifenesin would be at least 60% higher, using a coefficient variation of 30% of the mean, with a power of 90% and an alpha of 0.05, which corresponded to a sample of 6 horses.

For the degree of sedation and quality of induction score, using an alternative hypothesis that there is a difference of 1 point in the respective scores, e.g., 0 versus 1 or 1 versus 2, with a standard deviation of 0.5, and using a power of 90% and alpha of 0.05, a sample of 5 horses would be required.

Finally, for quality of recovery score, based on previous research (9) that used an alternative hypothesis to demonstrate a significant effect of ataxia in a descriptive score (maximum score of 6), a difference of 1, e.g., 1 versus 2 or 2 versus 3, with a standard deviation of 0.7, or in a numerical score (maximum score of 31 with 5 attempts to complete standing), a difference of 5 points, e.g., 8 versus 13 or 11 versus 16, with a standard deviation of 3, using a power of 90% and an alpha of 0.05, a total sample size of 6 horses would be required with either score.

Anesthesia and instrumentation

Horses were fasted from food, but not water, for 8 h before anesthesia. A 14-gauge, 13.36-cm catheter (BD Insyte-W; Becton Dickinson Infusion Therapy Systems, Sandy, Utah, USA) was placed in the right jugular vein for subsequent drug delivery. Flunixin (Flunazine; Vétoquinol NA, Quebec City, Quebec), 1.0 mg/kg BW, IV was given preoperatively, 5 to 10 min before induction of general anesthesia. All the anesthesias were administered by the same investigator (AV).

Sedation was completed outside a dual induction/recovery room and consisted of xylazine (Xylamax; Bimeda, Cambridge, Ontario), 0.6 mg/kg BW, IV, followed in 4 min by the assigned induction treatment. The depth of sedation at 3 min post-xylazine was video-recorded for later assessment by a blinded investigator (TM) using a sedation scoring system of 0 to 3, which was developed for this study, with 3 indicating the highest depth of sedation (Appendix A).

Appendix A.

Scoring system for depth of sedation.

Sedation depth	Description
0	No sedation. Alert, normal posture, responsive to environment and physical stimulation.
1	Mild sedation. Mild lowering of the head, reduced responsiveness to environment, but arousable with physical stimulation.
2	Moderate sedation. Lowering of the head and neck, ptosis, droopiness of ears and lips. Marked decrease in responsiveness to environment and stimulation.
3	Marked sedation. As per moderate sedation, but no response to stimulation, remains immobile, and shows ataxia if attempting to move.

Horses were then placed in the induction room, which had a swing door that was kept open during administration of the induction drugs while the horse stood next to the padded wall. When signs of ataxia appeared, the door was pressed against the horse until recumbency was completed. For induction, horses were randomly allocated to each treatment in an alternating crossover design and given each IV treatment 4 wk apart: i) guaifenesin (Rhodia Canada, Mississauga, Ontario) (Group G), reconstituted with 500 mL of dextrose 5% (D5W; Baxter Corporation, Mississauga, Ontario) to a 10% solution and dosed at 100 mg/kg BW; or ii) diazepam (Diazepam; Sandoz, Boucherville, Quebec) (Group D) dosed at 0.1 mg/kg BW. The guaifenesin and diazepam doses have been deemed to be equipotent (6).

Horses in Group G were administered guaifenesin using a pressure bag (Vital Signs; Vyaire Medical, Mettawa, Illinois, USA) pressurized between 200 to 300 mmHg and ketamine (Narketan; Vétoquinol, Lavaltrie, Quebec), 2 mg/kg BW, IV 1 to 2 min of when signs of muscle relaxation and swaying were present and/or subjectively when at least 67% of the calculated volume had been infused.

For Group D horses, diazepam was administered as a fast bolus (< 5 s) and IV ketamine was given when signs of muscle relaxation and swaying were present and/or 30 s had elapsed since the diazepam injection. The quality of induction was videorecorded for later assessment using an induction scoring system from 0 to 3, which was developed for this study, with 0 indicating the smoothest induction (Appendix B). Since the sequence of co-induction and induction drugs and the onset of ataxia were different for each treatment, the blinded investigator (TM) was able to detect from the videos which treatment was used.

Appendix B.

Scoring system for quality of induction.

Quality of induction	Description
0	Excellent. Calm and controlled smooth induction. Horse leans gently on the lateral and caudal wall to a sitting position before assuming sternal. Horse relaxed and unconscious when positioned in lateral recumbency.
1	Good. Calm and controlled induction, but the horse shows some degree of ataxia and limb mispositioning during the drop, requiring support from handler. Horse relaxed and unconscious when positioned in lateral recumbency.
2	Satisfactory. Controlled induction with some resistance to recumbency, manifested by forward motion that prolongs the time to sternal and lateral recumbency. Horse mildly responsive to positioning in lateral recumbency.
3	Poor. A phase of excitement and confusion is present, with attempts to move out of the induction space. Horse shows excessive muscular activity and resistance when assuming sternal recumbency and maintains muscle tone and signs of awareness when positioned in lateral recumbency.

Once induced, the trachea was intubated and horses were hoisted by their limbs at the pastern and transferred onto a compatible MRI table in the pre-MRI room located 3 m away, positioned in left lateral recumbency. A 20- or 22-gauge, 4.78-cm catheter (Becton Dickinson Infusion Therapy Systems) was placed in the facial or transverse facial artery and attached with noncompliant fluid-filled extension tubing to an electronic pressure transducer positioned and zeroed at the height of the mid-sternum for continuous display and monitoring of MAP, systolic (SAP) and diastolic (DAP) arterial pressure and heart rate (Invivo Expression MRI Monitor; Philips Medical Systems, Orlando, Florida, USA). The transducer was previously verified for accuracy at 100 mmHg against a mercury manometer (Mercurial Sphygmomanometer; Acoma Medical Industry Co., Tokyo, Japan) before each use. An adaptor for end-tidal CO₂ (P_E′CO₂) and end-tidal concentration of isoflurane (F_E′Isoflurane) was connected to the endotracheal tube and monitored using a sidestream infrared gas analyzer (Invivo Expression; Philips Medical Systems) calibrated weekly (755571-HEL, Calibration Gas Mixture; GE Healthcare, Helsinki, Finland). Heart rate, arterial blood pressures, and P_E′CO₂ and F_E′Isoflurane were recorded at 10 min after induction, before connecting the horse to the rebreathing circuit and anesthetic ventilator (Mallard 2800 C; Mallard Medical, Redding, California, USA), and continued to be recorded while the horse was connected to the anesthetic machine and under isoflurane anesthesia (Aerrane; Baxter Corporation) at 20, 30, and 40 min once they were transferred to the MRI room (4 m away) for the imaging.

The anesthesia machine was kept outside the MRI room and the rebreathing circuit consisted of 5 sets of hoses (each 205 cm long) connected to each other in order to reach the horse. Using mechanical ventilation, isoflurane was delivered at a rate of 7 breaths/min, a tidal volume of 10 to 15 mL/kg BW to maintain a P_E′CO₂ of 35 to 45 mmHg (4.7 to 6 kPa), and an oxygen flow rate of 10 L/min for the first 15 min and 5 L/min thereafter, to reach an F_E′Isoflurane of approximately 1.3 to 1.5%, according to depth requirements for the imaging. These included a slow-to-absent palpebral reflex, the nondependent eye positioned craniomedially, absence of nystagmus, and prevention of movement by maintaining the horse at that depth.

For the anesthetic management of each horse, heart rate, arterial blood pressures, P_E′CO₂ and F_E′Isoflurane were monitored and recorded in the anesthetic record every 5 min after induction, but these data were not used in the statistical analysis. A urinary catheter was placed for passive urine collection throughout anesthesia.

Lactated Ringer’s solution (Baxter Corporation) was infused at a rate of 3 mL/kg BW per hour. If MAP was ≤ 60 mmHg for longer than 5 min, dobutamine (Hospira, Lake Forest, Illinois, USA) was administered at a constant rate infusion (CRI) of 1 μg/min per kg BW until MAP was maintained at ≥ 70 mmHg for at least 5 min. The CRI was then decreased by 50% and the process was continued in the same fashion.

If MAP decreased again, then the CRI was increased by 100%. If the response to 1 μg/min per kg BW of dobutamine was not adequate within 5 min, then 1 mg IV of phenylephrine (Phenylephrine HCl; Marcan Pharmaceuticals, Ottawa, Ontario) was administered and the dobutamine CRI kept at the same rate until the expected response was achieved and maintained.

When the MRI was completed, horses were moved back to the pre-MRI room where they underwent surgery to remove biopsies from the superficial digital flexor tendons of both forelimbs. Isoflurane administration was discontinued approximately 10 to 15 min before the end of the surgery and the horses remained connected to the anesthetic machine and still on mechanical ventilation.

During this time, the depth of anesthesia was maintained with intermittent IV injections approximately every 5 min of xylazine, 33 mg mixed with ketamine, 67 mg. These were given as required until the end of surgery, based on clinical signs of anesthetic depth until surgery was completed, which included stronger palpebral reflex, spontaneous blinking, and more frequent rotation of the eye.

Once the procedure was completed, horses were disconnected from the anesthetic machine and phenylephrine (Phenylephrine hydrochloride; Marcan Pharmaceuticals), 5 mg diluted into 5 mL of Lactated Ringer’s solution, was administered into each nasal passage, all instrumentation removed, and the horse was hoisted back to the recovery room. The recovery room was 4.11 m deep × 3.81 m wide (15.7 m²) (13.6 × 12.6 ft; 171 ft²).

Post-anesthetic sedation and recovery

Horses were placed in left lateral recumbency in the recovery stall and immediately given xylazine, 0.2 mg/kg BW, IV, the upper eye covered with a towel, the endotracheal tube secured in place, and oxygen insufflated at 15 L/min for at least 10 min through the endotracheal tube, and horses were allowed to recover freely.

The quality of recovery was assessed using a recovery scoring system (9; Appendix C), consisting of first completing a numerical score that assesses all phases of anesthesia and the horse’s behavior from move to sternal until the horse resumes standing. The total score was then matched to an overall descriptive score that ranks the recovery on a scale of 1 to 6, according to the quality of recovery, with a score of 1 indicating the best recovery (Appendix D).

Appendix C.

Numerical recovery scoring system.

A. Overall Attitude Calm Calm/determined Confused, dizzy Frantic

B. Move to Sternal Smooth, methodical Incomplete attempts, but controlled Crashing, flopping over

C. Sternal Phase An organized pause Non-existing Multiple, with struggle

D. Move to Stand Methodical An organized scramble Used walls for support Ricocheting off walls

E. Strength Near full Mildly rubbery Dog sitting before standing Repeated attempts due to weakness

F. Number of Attempts to Stand (score = #)

G. Balance and Coordination Solid Moderate dancing Reflex saves Careening Falls back down

H. Knuckling None Hind limbs only All four limbs

TOTAL: _________

Appendix D.

Descriptive recovery score.

1	Smooth, calm, and easy recovery in 1 attempt, no ataxia or mild ataxia of less than 5 min duration.
	A (1), B (1), C (1 to 2), D (1), E (1), F (1), G (1), H (1) = 8 to 9
2	Calm recovery with some difficulty or weakness, 1 to 3 attempts, mild ataxia of more than 5 but less than 10 min duration.
	A (2), B (1), C (1 to 2), D (1 to 2), E (1 to 2), F (1 to 3), G (2), H (1) = 10 to 15
3	Same as 2 but ataxia of more than 10 min duration.
	A (2), B (1), C (1 to 2), D (1 to 3), E (2 to 3), F (1 to 3), G (2), H (1) = 11 to 17
4	Uncoordinated recovery with notable difficulty and weakness, more than 3 attempts to stand, marked ataxia, stumbling, ataxia duration of more than 10 but less than 20 min.
	A (3), B (1 to 2), C (2 to 3), D (3 to 4), E (3 to 4), F (3 to 5), G (3), H (2) = 20 to 26
5	Difficult recovery with disorderly efforts and unable to stand in the first 5 attempts. Marked ataxia that can last for up to 20 min or more.
	A (4), B (2 to 3), C (2 to 3), D (3 to 4), E (4), F (5 or more), G (3 to 5), H (2 to 3) = 25 to 31
6	Accident.

The following events were also recorded: time from IV xylazine for sedation to start of induction with administration of co-induction drugs (guaifenesin or diazepam); time from co-induction with guaifenesin or diazepam to ketamine administration; time from induction to transfer to MRI room; duration of MRI; time from induction to start of dobutamine CRI; dobutamine administration time in proportion to duration of anesthesia; number of horses given dobutamine at 10, 20, 30, and 40 min and total dose of dobutamine infused throughout anesthesia; number of horses given phenylephrine at 10, 20, 30, and 40 min and total number of horses receiving phenylephrine doses throughout anesthesia; duration of anesthesia (from induction to transfer to the recovery room); time from discontinuation of isoflurane until transported to the recovery room; F_E′Isoflurane at the time of transport to the recovery room; and time from entering the recovery room until sternal recumbency, extubation (due to chewing on tube or constant swallowing), and standing.

Statistical analysis

Tests of normality included the Shapiro-Wilk and Kolmogorov-Smirnov tests and QQ plots to detect unequal variances, outliers, and other nonrandom patterns within the data (GraphPad, Version 10.6.1 for Windows; GraphPad Software, Boston, Massachusetts, USA). Variables not included in the power analysis for sample size were handled with descriptive statistics.

A 2-way ANOVA was used for comparisons within and between treatments for arterial blood pressure and the number of horses receiving dobutamine and/or phenylephrine during the first 40 min of anesthesia. A Tukey’s multiple comparison test with individual variances was used to correct for each comparison.

Scores for sedation, induction, and recovery were compared using a Wilcoxon matched-pairs signed rank test. Results were considered statistically significant if the value of P was < 0.05. Data are expressed as mean ± standard deviation for normally distributed variables and as median and range for non-normally distributed variables.

Results

All horses completed the study uneventfully. Arterial blood pressures were statistically similar between the 2 groups at all time intervals, although values in Group D tended to be higher during the first measurement at 10 min. Subsequent measurements under isoflurane anesthesia showed a greater decrease in Group D, which resulted in significant differences with the first measurement, whereas arterial pressures did not change significantly throughout the first 40 min of anesthesia in Group G (Table I).

Table I.

Cardiopulmonary variables in 6 horses co-induced with intravenous (IV) guaifenesin (100 mg/kg BW) or diazepam (0.1 mg/kg BW) on 2 alternate occasions.

Variable	Group	Time after induction (minutes)				P-value
		10	20	30	40
Systolic arterial pressure (mmHg)	G	113 ± 28 (84 to 143)	92 ± 13 (78 to 106)	89 ± 6 (83 to 96)	95 ± 17 (77 to 113)	0.173
	D	138 ± 18 (119 to 156)	105 ± 9* (95 to 115) P = 0.026	94 ± 9* (84 to 103) P = 0.0009	94 ± 18* (75 to 113) P = 0.001	0.004
Diastolic arterial pressure (mmHg)	G	80 ± 22 (57 to 103)	64 ± 13 (50 to 77)	57 ± 10 (47 to 67)	63 ± 22 (40 to 87)	0.082
	D	98 ± 16 (81 to 114)	68 ± 11* (57 to 79) P = 0.01	57 ± 6* (51 to 63) P = 0.0002	53 ± 3* (50 to 56) P < 0.0001	< 0.0001
Mean arterial pressure (mmHg)	G	93 ± 24 (68 to 118)	74 ± 11 (62 to 86)	68 ± 9 (58 to 77)	73 ± 21 (51 to 96)	0.079
	D	112 ± 19 (93 to 132)	82 ± 9* (72 to 91) P = 0.01	69 ± 6* (63 to 76) P = 0.0001	66 ± 5* (61 to 71) P < 0.0001	0.0004
Heart rate (beats/min)	G	35 ± 3	36 ± 3	35 ± 4	37 ± 4	ND
	D	36 ± 3	37 ± 4	38 ± 3	38 ± 4
End-tidal CO2 (mmHg)	G	41 ± 5	40 ± 3	41 ± 3	43 ± 4	ND
	D	39 ± 4	38 ± 5	38 ± 4	40 ± 4
End-tidal isoflurane (%)	G	0	1.33 ± 0.20	1.38 ± 0.08	1.4 ± 0.06	ND
	D	0	1.42 ± 0.08	1.42 ± 0.04	1.38 ± 0.04

Data expressed as mean ± standard deviation. Values in parenthesis are 95% confidence intervals of the mean.

^*Significantly different from 10 min within the same group.

ND — Not determined.

The time from induction to the start dobutamine and the number of horses requiring it in the first 40 min of anesthesia were similar between groups (Tables II and III). The number of horses requiring dobutamine was increased significantly at 40 min when compared to that at 10 min in both groups (Table III). After this time, all horses in both groups received dobutamine, except for 1 horse in Group D. The total dose of dobutamine and the percentage of time the horses received dobutamine throughout anesthesia were similar in both groups (Table II). The number of horses requiring phenylephrine was similar between groups in the first 40 min of anesthesia (Table III). After that, 1 horse in Group G required 1 dose of phenylephrine and another horse required 3 doses. In Group D, 1 horse required 5 doses of phenylephrine.

Table II.

Times between administration of induction drugs, anesthesia time, and doses used for cardiovascular support in 6 horses co-induced with intravenous (IV) guaifenesin (100 mg/kg BW) or diazepam (0.1 mg/kg BW) on 2 alternate occasions.

	Guaifenesin	Diazepam	P-value
Time from xylazine sedation to start of induction (minutes)	4.1 ± 0.2 (4.0, 4.5)	4.1 ± 0.1 (4.0, 4.3)	ND
Time from co-induction drug to ketamine administration (seconds)	85 ± 30 (60, 120)	33 ± 0.1 (30, 45)	ND
Time from induction to transfer to MRI room (minutes)	15 ± 3 (12, 19)	17 ± 5 (12, 25)	ND
Duration of MRI (minutes)	52 ± 8 (42, 65)	53 ± 9 (41, 65)	ND
Duration of anesthesia (minutes)	137 ± 14 (120, 154)	142 ± 16 (118, 162)	ND
Time from induction to start of dobutamine infusion (minutes)	26 ± 8 (17, 40)	38 ± 11 (30, 57)	0.238
Total dose of dobutamine infused (mg)	32.3 ± 20.9 (8.0, 61.5)	24.3 ± 20.3 (0, 60.7)	0.286
Percentage of anesthesia time receiving dobutamine (%)	70.9 ± 14.8 (45.1, 88.1)	56.4 ± 28.3 (0, 74.1)	0.323

Data expressed as mean ± standard deviation and (range).

ND — Not determined.

Table III.

Number of horses receiving arterial blood pressure support with dobutamine and/or phenylephrine during general anesthesia after co-induction with intravenous (IV) guaifenesin (100 mg/kg BW) or diazepam (0.1 mg/kg BW) on 2 alternate occasions.

Variable	Group	Time after induction (minutes)				P-value
		10	20	30	40
Number of horses given IV dobutamine	G	0	2	3	5* P = 0.042	0.08
	D	0	0	1	4* P = 0.042
Number of horses given IV phenylephrine (1 mg)	G	0	1	2	1	0.092
	D	0	0	0	0

^*Significantly different from 10 min within the same group.

ND — Not determined.

Degree of sedation was similar in both groups (P = 0.5) and was assessed as marked sedation [median and (range); Group G: 3.0 (1, 3) and Group D: 3.0 (3, 3)] and the quality of induction was also similar (P > 0.999) and assessed as good to excellent [both groups: 0.5 (0, 2)]. Anesthetic management during the MRI and in preparation for recovery, as well as times of anesthesia and recovery, were similar in both groups (Table IV). There was no significant difference in recovery scores between groups (Table V).

Table IV.

Recovery preparation and times to recovery in 6 horses co-induced with intravenous (IV) guaifenesin (100 mg/kg BW) or diazepam (0.1 mg/kg BW) on 2 alternate occasions.

	Guaifenesin	Diazepam
Time off isoflurane before transfer to recovery (minutes)	13 ± 1.0 (12 to 15)	13 ± 1.3 (11 to 15)
End-tidal isoflurane before the beginning of recovery (%)	0.73 ± 0.05 (0.7 to 0.8)	0.68 ± 0.04 (0.6 to 0.7)
Time to sternal (minutes)	27 ± 6 (19 to 36)	34 ± 11 (22 to 50)
Time to standing (minutes)	36 ± 9 (25 to 46)	41 ± 7 (33 to 53)
Time to extubation (minutes)	36 ± 9 (25 to 47)	39 ± 8 (30 to 55)

Data expressed as mean ± standard deviation and (range).

Table V.

Recovery scores in in 6 horses initially co-induced with intravenous (IV) guaifenesin (100 mg/kg BW) or diazepam (0.1 mg/kg BW) on 2 alternate occasions.

	Guaifenesin	Diazepam	P-value
Overall attitude	2.0 (1, 3)	1.5 1 (1, 33)	0.5
Move to sternal	1.0 (1, 3)	1.5 (1, 3)	0.5
Sternal phase	1.0 (1, 1)	1.0 (1, 1)	1.0
Move to standing	2.0 (1, 3)	1.5 (1, 3)	0.75
Strength	2.0 (1, 2)	2.0 (1, 4)	0.5
Number of attempts	1.0 (1, 2)	1.5 (1, 3)	0.5
Balance and coordination	3.0 (1, 3)	3.0 (2, 3)	0.75
Knuckling	1.0 (1, 2)	1.0 (1, 2)	> 0.99
Total score	14.0 (8, 15)	13.5 (9, 21)	0.625
Descriptive score	2.5 (1, 3)	1.5 (1, 4)	0.625

Data expressed as median and (range).

Discussion

In this study, guaifenesin and diazepam at the doses administered did not result in significantly different effects on arterial blood pressure and quality of induction and recovery in horses in either group. Our first hypothesis that the quality of induction and recovery would be similar with co-induction with guaifenesin or diazepam was therefore accepted. However, horses given guaifenesin did not exhibit a significantly higher degree of hypotension than those given diazepam, which rejects our second hypothesis.

Arterial blood pressure measurements at 10 min post-induction were not statistically different between groups, despite being higher by 20 to 23% in Group D. Similar findings were reported by Brock and Hildebrand in 1990 (6) using the same anesthetic protocols and doses. In the current study, horses in both groups were already at similar F_E′Isoflurane by 20 min and onwards and, despite smaller differences in arterial blood pressures between groups, the percentage decrease in arterial blood pressure since 10 min post-induction was higher in Group D (24% for SAP, 31% for DAP, 27% for MAP), which resulted in a statistical difference in all subsequent values in this group, compared to Group G (approximately 20%) for all arterial pressures. There was probably a higher vascular resistance in horses in Group D during the first 10 min post-induction because diazepam has minimal effects on vascular tone and does not affect arterial blood pressure in conscious horses at doses of 0.05 to 0.4 mg/kg BW, IV (10).

Horses were connected to the anesthetic machine and isoflurane was not delivered until 10 min after induction, which means that the impact of isoflurane on decreasing systemic vascular resistance and arterial blood pressure (11) was probably greater in Group D than in Group G. Horses in Group G had lower arterial blood pressures and, although not statistically significant, lower vascular resistance and arterial blood pressures have been reported in horses 15 min after being given guaifenesin at doses of 88 to 134 mg/kg BW, IV to induce recumbency (5).

Arterial blood pressure support with dobutamine and/or phenylephrine was statistically similar in both groups despite the higher values for dobutamine’s total dose, longer time of administration, and sooner requirement for dobutamine and/or phenylephrine in Group G. However, these variables did not reach significance based on the parameters used for the a priori power analysis.

It has been shown that the inclusion of guaifenesin in the induction protocols during anesthesia has resulted in more horses requiring dobutamine and for longer periods of time (7). In that study, horses sedated with acepromazine and xylazine and then anesthetized with co-induction of midazolam and ketamine, guaifenesin and ketamine, or guaifenesin and thiopental and maintained with isoflurane had lower MAP 30 to 90 min after induction with guaifenesin and ketamine than with the 2 other induction protocols (7).

Both diazepam and guaifenesin are centrally acting muscle relaxants that can induce muscle relaxation by interrupting motor interneurons (internuncial neurons or second order neurons) located in the brain and spinal cord and preventing them from facilitating the communication between first order upper motor neurons and third order lower motor neurons, thereby suppressing motor firing and causing muscle relaxation (12–14). Diazepam enhances the inhibitory effects of gamma-aminobutyric acid (GABA) on these interneurons, which inhibits polysynaptic reflexes and overall skeletal muscle tone, without peripheral activity on skeletal muscle (12,13).

Diazepam also causes sedation through modulation of the inhibitory neurotransmitter GABA actions on GABA_A receptors in the brain opposing any excitatory effects (14), which also contributes to muscle relaxation. In horses, sedation with diazepam is best achieved with doses of < 0.05 mg/kg BW, IV; higher doses are avoided because they may result in transient restlessness, marked ataxia, and even recumbency (10,15).

Guaifenesin actions are extrapolated from mephenesin and methocarbamol, which are also propanediol derivatives that are more extensively used for their muscle relaxation properties (12,13,16). Their effects as muscle relaxants are also related to inhibition of polysynaptic reflexes via inhibition of NMDA-receptors, which are normally activated by excitatory amino acids (EAA) (16,17).

To promote a smooth induction with any of these co-induction drugs, they must be administered when marked sedation from the preanesthetic drug(s) is present and the induction drug (ketamine) must be administered in a timely fashion to avoid excessive ataxia before the horse assumes recumbency. It is our practice to administer the co-induction drug separately from the induction drug. For the combination of guaifenesin and ketamine, the ketamine must be injected within 1 to 2 min of starting the infusion of guaifenesin from a pressurized bag, as previously described. This is usually when signs of muscle relaxation and swaying are present and at least 50 to 67% of the guaifenesin has already been injected.

Previous studies using similar combinations do not specify whether the guaifenesin was infused as free flow or under pressurized conditions. In one study, ketamine was induced when 50% of the guaifenesin was injected; these horses were restrained and induced using a hydraulic table that was turned when the remaining 50% of the guaifenesin injection was completed and ataxia was assessed to be mild (6). In another study, there is no mention of how rapidly guaifenesin was administered and it is indicated that ketamine was injected when the horse’s forelimbs started to bend; the quality of induction was not assessed, although it was deemed that this protocol was not as safe and smooth as that used in a group of horses induced with midazolam and ketamine (7).

For the combination of diazepam and ketamine, in one study the diazepam was administered immediately before the ketamine and the horses were kept standing with a hydraulic table until the administration of 250 mL of lactated Ringer’s solution (placebo) was completed before turning the table (6). Although the authors did not observe ataxia with this combination, the immediate induction with ketamine and the use of restraint and the hydraulic table did not permit an objective assessment. In another study, midazolam (0.06 mg/kg BW) was combined with ketamine and given as a single bolus for induction; this protocol was subjectively assessed as smooth and safe (7).

The combination of ketamine and diazepam or ketamine and midazolam and administration as a single bolus or the ketamine injected immediately after the benzodiazepine is common practice for induction and usually includes doses of either 0.05 to 0.1 mg/kg BW of benzodiazepine (1–4,7), which likely makes the individual effects of benzodiazepines go undetected. In the present study, diazepam was administered as a fast bolus (< 5 s) and its effects were assessed for a maximum of 30 s or less before the ketamine was administered in order to detect its individual effects on signs of relaxation and ataxia.

Despite the widespread use of these combinations, to the authors’ knowledge, this is the first study that objectively describes the timing of administration of each drug and assesses the resulting quality of induction. Previous studies have not focussed on the individual effects of the drugs due to their design and methodology, which includes facilitating the induction with the use of a hydraulic table, not assessing the induction phase and degree of ataxia, and giving the drugs simultaneously or immediately, one before the other.

In our study, a blinded observer assessed the unedited video recordings of sedation and induction. Based on the results of this assessment, all horses were induced at the same time after sedation and under the same degree of sedation. Each co-induction drug yielded a similar quality of induction, despite the differences in timing of administration, which was determined by the properties of each co-induction drug and the restrictions for administering a large volume of injection, i.e., guaifenesin.

The information gathered from the use of midazolam instead of diazepam in other studies is useful. Although doses of diazepam and midazolam for co-induction in horses are usually interchangeable (1,3), in humans they are considered to have different potencies: midazolam is 1.5 to 3.4 times more potent as a sedative or co-induction drug than diazepam in humans and the doses are therefore adjusted for equipotency (18–21).

This has not been thoroughly investigated in veterinary anesthesia of small or large animals. When these benzodiazepines have been compared at the same doses, there were no major differences in ease of endotracheal intubation, cardiopulmonary variables after induction, quality of induction and recovery, and recovery times in horses maintained for 60 min with inhalational anesthesia (1) and horses undergoing castration under field conditions (3).

Several other studies have assessed the effects of individual benzodiazepines as co-induction drugs. One study, using 0.06 mg/kg BW, IV of midazolam in horses that were later maintained on isoflurane for mean duration times of 90 to 108 min for elective surgery, noted a higher degree of ataxia during recovery when the anesthesia time was less than 75 min, compared to horses co-induced with guaifenesin (100 mg/kg BW, IV) and thiopental or guaifenesin and ketamine, despite similar times to standing among the 3 groups (7). When that same dose of diazepam and midazolam was compared in research horses maintained with sevoflurane for 60 min without surgery, the recovery was assessed to be of better quality than when horses were given butorphanol (0.1 mg/kg BW) as the co-induction drug (1).

In horses receiving benzodiazepine at 0.06 mg/kg BW, IV for anesthesia and castration under field conditions, ataxia was minimal and similar between groups during recovery, with similar time to standing, but not when compared to a control group (3). In another study in horses also undergoing castration under field conditions, midazolam was given at that same dose and compared to a group of horses not receiving it, with better quality of induction and degree of relaxation noted during surgery and with longer time to sternal recumbency and standing, but similar quality of recovery (4).

In our study, although not blinded to the treatments for the anesthetic maintenance and preparation for recovery, including the discontinuation of isoflurane and maintenance with the supplemental doses of xylazine and ketamine, anesthesia was carried out in the same way in both groups without complications. Horses did not require supplemental doses of anesthetic drugs to complete cardiovascular measurements at 10 min post-induction and the transfer to isoflurane anesthesia was smooth during the MRI. Duration of anesthesia and recovery were similar in both groups and recovery scores described the horses as calm, but with some ataxia for most horses in each group. These are considered good recoveries for the duration of anesthesia in this study (range: 118 to 162 min) since it has been shown that anesthetic risk usually increases with anesthesia time beyond 60 to 90 min (22–26).

Guaifenesin induced recumbency in non-premedicated horses at an IV dose of 134 mg/kg BW and a dose of 88 mg/kg BW in horses premedicated with a full dose of xylazine (1.1 mg/kg BW) 5 min before infusion (5). Horses were able to stand in 23.8 ± 6.5 min without premedication and slightly longer with premedication (27.3 ± 5.9 min) despite the lower dose, but the quality of recovery was not reported (5). In that study, the reported terminal plasma half-life was 1.32 ± 0.22 h and 1.26 ± 0.29 h for the higher and lower dose, respectively (5) and even longer (1.77 ± 0.32 h) in horses given a higher dose (211 mg/kg BW, IV), which also resulted in longer time to standing (36 ± 12 min in 2 ± 1 attempts), with no mention of the quality of recovery (27).

Diazepam at single IV doses of 0.17, 0.18, or 0.19 mg/kg BW in 3 different horses had a variable terminal half-life of 21.6 h, 9.7 h, and 6.94 h, respectively, in each of those horses (10). The following are the clinical effects and duration, which are dose-dependent: IV doses of 0.05 mg/kg BW caused a fixed gaze and muscle fasciculations of head, neck, and trunk for no longer than 5 min; 0.1 mg/kg BW caused the same signs and ataxia for 20 to 50 min; 0.2 mg/kg BW increased the degree of the same signs and even caused recumbency in 1 of 7 horses for a few minutes, with signs lasting at least 50 min; and 0.4 mg/kg BW caused collapse and recumbency in 4 of 7 horses for up to 21 min and signs of ataxia for up to 2 h (10).

Based on the different terminal half-lives of guaifenesin and diazepam, the latter remains longer in the body, but this does not allow for a clear comparison between the drugs for the duration of their clinical effects of sedation and/or relaxation since these effects depend on the individual drug-effective plasma concentrations and not strictly on the elimination half-life of the drug. In general, the higher the dose, the higher the plasma concentration and the longer the clinical effect, i.e., dose-dependent effect, although this is also influenced by the presence of other drugs and the health status of the individual and how these factors affect the biotransformation and clearance of the drug (28).

This study used an alternating treatment design in which one treatment was given to the first horse, the next horse received the other treatment, and so on until each of the 6 horses had received one treatment. The block of treatments was completed by giving the other treatment to each horse using the reverse-alternating sequence. This type of design has been shown to be an adequate randomization test for Type-I and Type-II error rates (29). The sample size used in this study resulted in 6 pairs of repetitions of the alternating sequence, which meets the standard criteria of at least 5 pairs (29,30).

One of the limitations of this study is that the blinded investigator could distinguish between treatments when scoring the quality of induction, based on the presence of the pressurized guaifenesin and the different ketamine administration times across treatments. In the original study by Brock and Hildebrand in 1990 (6), precautions were taken to effectively blind the investigator who was assessing the induction phase by administering the guaifenesin or a placebo (isotonic saline) and then 2 small volumes that represented diazepam or its placebo (lactated Ringer’s solution) and ketamine, with the blinded investigator to complete the same treatments used in this study, i.e., guaifenesin and ketamine or diazepam and ketamine. In that study, ataxia was absent in their Group D and mild in their Group G, but no other differences were noted, which is similar to our findings in this study.

Another limitation of this study was that recovery scores were completed by the same person who administered the anesthesia (non-blinded) and was done live. This assessment process has also been used in other studies (31–33), however, and has shown good agreement with scores done with blinding (31).

In conclusion, this study showed that there were no significant differences in arterial blood pressure and the quality of induction and recovery using equipotent doses of guaifenesin and diazepam for co-induction of anesthesia in horses. It was also determined that all horses had the same quality of recovery for the approximately 2-hour duration of anesthesia.

Funding Statement

This study was funded by a grant from the Equine Guelph Research Program.