Comparison of the efficacy of 2 sedative protocols in pediatric dogs undergoing brainstem auditory-evoked response testing

Abstract

This study compared the quality of sedation with dexmedetomidine or alfaxalone during brainstem auditory-evoked response (BAER) tests in 6- to 17-week-old dogs. This was a prospective, randomized clinical study involving 19 client-owned pediatric dogs of breeds with reported congenital deafness. Group A (G_A) received alfaxalone, 2 mg/kg body weight (BW) (n = 9) and group D (G_D) dexmedetomidine, 0.005 mg/kg BW, and postprocedure antagonism with atipamezole (n = 10) intramuscularly. Time from injection to sedation, duration of sedation, sedation scores, need for re-dosing, rectal temperature, pulse and respiratory rate were recorded at baseline, before and after the BAER test, and once recovered from sedation. Pulse rate was significantly lower in G_D (P = 0.004) and the number of re-dosing was significantly higher in G_A (P = 0.011). Both sedation protocols allowed good quality BAER test recordings in pediatric dogs. Sedation with dexmedetomidine required less re-dosing, whereas alfaxalone maintained more physiological pulse rates.

Introduction

Breeders are advised to test parents and offspring of dogs belonging to breeds with breed-associated sensorineural deafness (1,2) to try to avoid this congenital defect. Auditory function in the dog can be evaluated using electrodiagnostic tests such as brainstem auditory-evoked response (BAER) (3). Normal amplitudes and latencies of BAER recordings in dogs are reached at 30 and 40 d of age, respectively, according to some authors (4); however, others reported that normal BAER test waveforms could not be obtained until 6 to 8 wk of age (5). The BAER test is a non-invasive, safe, and economic diagnostic tool (5) that can be performed in conscious, sedated, or anesthetized patients, as it is independent of the level of arousal and minimally affected by anesthetic drugs (4–6). Some animals will not cooperate enough to record the BAER for a reasonable length of time (7) leading to artefacts due to myographic motion (8); therefore, sedation is sometimes necessary to obtain reliable results.

Sedation protocols described for performance of the BAER test in adult dogs include acepromazine alone or in combination with an opioid (9,10) and medetomidine alone (11–14), or in combination with butorphanol (15,16). The sedative effects of alpha-2 agonists in dogs younger than 16 wk of age has not been described; however, judicious use of low doses may be considered in pediatric dogs with a healthy cardiovascular system (17). The use of IM medetomidine for sedation during the BAER test has no apparent side effects in neonatal and pediatric dogs of breeds predisposed to congenital deafness (11,16,18). There is only 1 published report on the use of dexmedetomidine followed by either alfaxalone or sevoflurane anesthesia for BAER testing in adult cats (19).

Alfaxalone is a progesterone derivative with anesthetic properties as a neuroactive steroid. This molecule is insoluble in water and was initially formulated as alfaxalone-alphadolone acetate (Saffan; Glaxovet, London, UK). Intraperitoneal Saffan during BAER tests in laboratory ferrets provided 10 min of anesthesia but caused muscle tremors in some animals (20). Later on, Saffan was withdrawn from the market due to adverse reactions after its administration in dogs and cats. Nowadays alfaxalone is formulated with the solubilizing agent 2-hydroxypropyl-beta-cyclodextrin, licensed for use in rabbits, cats, and dogs as Alfaxan (Dechra, Barcelona, Spain). Unlike the previous formulation, this preparation does not cause histamine release. The Alfaxan leaflet states that the safety of the product in animals less than 12 wk of age has not been demonstrated, but there is 1 published report that assessed its efficacy as an anesthetic induction agent for this age group with good quality of induction and recovery (21). Alfaxan has been used as a sedative agent when administered IM to adult dogs, providing a dose-dependent sedation (22). During BAER testing, Alfaxan has been used as an induction agent in dogs (12), and for induction and maintenance of anesthesia in adult cats (19).

To the authors’ knowledge, the sedative effects of IM alfaxalone or dexmedetomidine in pediatric dogs from 6 to 17 wk of age have not been compared. The secondary aim of this study was to evaluate the feasibility of BAER test in dogs sedated with alfaxalone or dexmedetomidine and its post-procedure antagonism with atipamezole, and to report the possible side effects observed during or after sedation in pediatric dogs.

Materials and methods

Study population

Client-owned dogs from 6 to 17 wk of age admitted to the Small Animal Teaching Hospital of the Catholic University of Valencia for BAER testing were enrolled in the study. Informed owner’s consent was obtained before all procedures and for data collection. The Animal Care Committee of the Catholic University of Valencia (UCV/2017-2018/99) approved this project.

The pre-sedation assessment consisted of patient’s history (date of birth, deworming and vaccination status, current diet and gastrointestinal or respiratory abnormalities), complete physical examination, and accurate body weight. Exclusion criteria were dogs younger than 6 wk of age, and dogs with abnormalities found on the pre-sedation assessment such as vomiting, diarrhea, cough, nasal discharge, abnormal cardiopulmonary auscultation, or fever.

Sedation protocol

Owners were advised to withhold food from the dogs for 4 h before hospital admission. Animals were randomized (www.random.org/sequences/) to receive 1 of the 2 treatments: group A (G_A) was administered alfaxalone (Alfaxan; Dechra), 2 mg/kg body weight (BW), IM, and group D (G_D) was administered dexmedetomidine (Dexdomitor 0.5; Ecuphar, Barcelona, Spain), 0.005 mg/kg BW, IM. Dexmedetomidine or alfaxalone was injected into the epaxial lumbar muscles using a 25-G needle. The dogs were then placed in a quiet room for 10 to 15 min. After this period, the observers interacted with the dogs to evaluate the degree of sedation and monitor their vital parameters. Using an adaptation of a previously validated sedation scoring system for dogs (23,24), the posture, behavior, and degree of muscle relaxation were evaluated to assess the level of sedation (Table 1). Clicker sound was standardized to the maximum level of sound volume using a commercial clicker for dog training (CLIX CX Multiclicker; Royal Pet Kingdom Europe, London, UK) applied 20 cm away from the head.

Table 1.

Sedation Scoring System adapted from Grint et al (23). Clicker sound was tested in all puppies at all time points.

Spontaneous posture:	Response to clicker sound:
0 = Standing	0 = Normal startle reaction (head turn towards noise/cringe)
1 = Tired but standing	1 = Reduced startle reaction (reduced head turn/minimal cringe)
2 = Lying but able to rise	2 = Minimal startle reaction
3 = Lying but difficulty rising	3 = Absent reaction
4 = Unable to rise
Palpebral reflex:	Resistance when laid into lateral recumbency:
0 = Brisk	0 = Much struggling, perhaps not allowing this position
1 = Slow but with full corneal sweep	1 = Some struggling, but allowing this position
2 = Slow but with only partial corneal sweep	2 = Minimal struggling/permissive
3 = Absent	3 = No struggling
Eye position:	General appearance/attitude
0 = Central	0 = Excitable
1 = Rotated forwards/downwards but not obscured by third eyelid	1 = Awake and normal
2 = Rotated forwards/downwards and obscured by third eyelid	2 = Tranquil
	3 = Stuporous
Jaw and tongue relaxation:	Total score:
0 = Normal jaw tone, strong gag reflex	T0 = Before premedication
1 = Reduced tone, but still moderate gag reflex	TPRE = Before BAER test
2 = Much reduced tone, slight gag reflex	TPOST = After BAER test
3 = Loss of jaw tone and no gag reflex	TREC = Recovery

Vital parameters recorded were pulse rate (PR) by femoral pulse palpation, respiratory rate ( f_R) determined visually counting the respiratory thoracic movements, and rectal temperature (T) measured with a thermometer. Blood pressure measurements were attempted using an oscillometric monitor (B40 Patient Monitor; General Electric Healthcare, Madrid, Spain) but abandoned due to electronic interference with BAER equipment. All parameters were evaluated before injection as baseline (t0), before BAER testing (tPRE), immediately after BAER testing (tPOST), and when the dogs recovered from sedation (tREC). Subcutaneous needle electrode placement for BAER testing started when patients had reached a sedation score ≥ 14/21, or 15 min after injection if the dogs had not yet reached this score. The person performing the BAER test was blinded to the sedation protocol used in each case. If needle electrode placement or internal earphone insertion was not tolerated after 2 to 3 attempts, this blinded observer indicated the need for re-dosing, which consisted of half of the initial dose injected into the contralateral epaxial lumbar musculature. The number of re-dosing injections needed to perform the BAER test was recorded. During sedation, patients were supplemented with oxygen, placed over absorbent pads to avoid direct contact with cold surfaces, and carefully covered with a blanket (passive warming methods). Onset time for sedation (ti) was considered from IM injection until the patient had reached a score ≥ 14/21 or the BAER test started. The duration of the sedation (td) was the time from IM injection to recovery, which was considered when the sedation score was ≤ 4. Dogs in G_D received atipamezole (Antisedan; Ecuphar) injected into the epaxial lumbar muscles, after completion of the BAER test. Pain during IM injection and post-sedation swelling at the local sites were annotated. All patients were monitored continuously until recovery and if any adverse event (prolonged recovery, excitation, hypothermia, or gastrointestinal side effects) was noticed throughout the procedure it was treated and recorded. Before hospital discharge all dogs were normothermic, drinking and eating normally. The owner, who was unaware of the sedation protocol administered, was contacted by telephone the day after discharge to obtain and record follow-up information on any adverse events (vomiting, diarrhea, dullness, and hyporexia).

The BAER test and equipment

Electrophysiological studies were performed in a quiet setting, using the same electrodiagnostic equipment for all dogs (Micromed; Myoquick SystemPlus, Treviso, Italy). Before the BAER tests, an otoscopic examination was carried out and the ears were cleaned if necessary. Stainless steel needle electrodes (Disposable Sterile Subdermal Needle Electrodes; Xi’an, Shaanxi, China) were placed subcutaneously with the active positive electrode placed at the vertex (dorso-caudal part of the skull), the reference negative electrode at the mastoid area (rostrally to the ear base), and the ground electrode in the dorsal part of the neck.

The BAER test was elicited by applying multiple click stimuli of 0.1 ms duration and intensity of 95 decibels (dB). Sound waves were delivered at a stimulation rate of 20 Hz using an alternating polarity. An average of 1000 click stimuli were delivered to the ear via internal earphones (Nihon Kohdem Europe GmbH, Rosbach vor der Höhe, Germany), and the response from the first 10 ms was averaged until the waveform remained stable. A white masking noise at 30 dB less than the stimulus was delivered to the contralateral (nonstimulated) ear.

Latency was expressed in ms and was defined as the time between the beginning of the stimulus to the peak of the wave. Amplitude was expressed in microvolts (μV) and was measured from the peak of the wave to the lowest point of the following negative trough.

Statistics

Data were assessed for normality by evaluation of descriptive statistics using histograms and the Kolmogorov-Smirnov test. Variables were summarized as frequency (percentage) for categorical variables; mean ± standard deviation (SD) for continuous, normally distributed variables or median (range) for skewed data.

The Mann-Whitney U-test was used to compare age, weight, temperature (T), time from injection to sedation (ti), and time from injection to recovery (td) between groups. Chi-Squared test was used to compare the number of re-dosing injections between the 2 study groups. Paired samples t-test was used to compare PR and Wilcoxon Rank test was used to compare T, at t0 with tPRE, tPOST, and tREC, respectively. Using repeated measures analysis of variance (ANOVA), PR and sedation scores were compared, and the Kruskal-Wallis test was used to compare f_R and T between groups at the different time points. Statistical significance was set at P < 0.05. All analyses were performed with SPSS Statistics version 23 (SPSS Statistics; IBM, Madrid, Spain).

Results

Pediatric dogs (N = 20) were recruited for the study. Breed distribution was: Dalmatians (n = 8), Jack Russell terriers (n = 5), Bichon Maltese (n = 2), Labrador retrievers (n = 2), boxers (n = 2), and French bulldog (n = 1). One boxer was excluded from the study, due to increased respiratory noises on auscultation and purulent nasal discharge.

A total of 19 dogs were included in the study: 10 dogs in G_D and 9 in G_A. Gender distribution was 52.6% male and 47.4% female. Median age was 7 wk (range: 6 to 17 wk) and median body weight was 4.8 kg (range: 1.3 to 6.15 kg). Discomfort was observed in all cases during IM injection but only 1 dog in G_D moved during drug administration. No statistically significant differences were found in body weight (P = 0.400), f_R (P = 0.656) or T (P = 0.370) between groups at t0. There were no significant differences at ti (P = 0.325) or td (P = 0.390) between groups: in G_A median ti was 19 min (5 to 53 min), median td was 35 min (20 to 110 min) and in G_D median ti was 12.5 min (5 to 25 min) and median td was 42 min (22 to 102 min). Mean sedation scores at the different time points are presented in Table 2.

Table 2.

Sedation scores obtained at the different time points in the 2 study groups.

	ti (minutes)	Sedation score tPRE	Re-dosing	Sedation score tPOST	Sedation score tREC	td (minutes)
Group Alfaxalone (n = 9)	19 (5–53)	10.6 ± 5.5	5/9	8.6 ± 6.9	1.9 ± 2.4	35 (20–110)
Puppy 1	10	8	Yes	16	1	110
Puppy 5	19	12	No	7	7	37
Puppy 6	40	8	No	7	4	70
Puppy 7	19	15	No	10	3	34
Puppy 10	21	2	Yes	0	0	45
Puppy 11	5	16	Yes	17	0	20
Puppy 14	19	5	Yes	0	0	29
Puppy 16	9	10	No	3	1	35
Puppy 19	53	19	Yes	17	1	31
Group Dexmedetomidine (n = 10)	12.5 (5–25)	8.2 ± 5.8	0/10	10.5 ± 6.6	0.5 ± 2.4	42 (22–102)
Puppy 2	12	12	No	18	3	102
Puppy 3	9	6	No	13	0	35
Puppy 4	25	11	No	20	0	57
Puppy 8	17	10	No	10	0	43
Puppy 9	15	3	No	8	0	35
Puppy 12	10	3	No	5	0	37
Puppy 13	10	2	No	3	0	22
Puppy 15	13	14	No	9	1	41
Puppy 17	24	2	No	1	0	42
Puppy 18	5	19	No	18	1	55

ti — Time from injection to sedation; tPRE — time before brainstem auditory response (BAER) test; tPOST — immediately after BAER test; tREC — sedation recovery; td — time from injection to recovery. Re-dosing — administration of half of the initial dose of the sedative used, after attempting electrode placement for BAER test.

Sedation scores showed a median onset of sedation of approximately 15 min (range: 5 to 53 min), and a return to normal behavior in 37 min (range: 20 to 110 min) post-injection in both groups.

The PR was significantly lower in G_D than in G_A (P = 0.004). In G_D the PR at t0 was significantly higher compared with tPRE (P = 0.001) and tPOST (P = 0.001). In G_A the PR initially increased slightly but this was not statistically significant. No differences in T were found between groups at the different time points; however, there was a significant difference over time, being lower at tPRE (P = 0.015) and tREC (P = 0.005) compared with t0 despite the use of passive warming throughout the procedure.

The results from the ANOVA indicated that there was no statistically significant difference between groups on sedation scores at tPRE (P = 0.187), tPOST (P = 0.318) and tREC (P = 0.323). The number of re-dosing injections was significantly higher in G_A (need of redosing 5/9) than in G_D (need of redosing 0/10) (P = 0.011). Both sedation protocols allowed good quality BAER test recordings, with no side effects encountered during sedation, recovery or reported in the follow-up questionnaire. In 3 dogs, 2 female Dalmatians and 1 male French bulldog, the BAER test detected unilateral sensorineural deafness, while the other patients showed normal bilateral BAER recordings.

Discussion

The BAER test is an objective electrodiagnostic test that is generally unaffected by anesthetic drugs (4–6). There is little information in the veterinary literature regarding sedation protocols in pediatric animals; however, younger patients sometimes need sedation for the performance of a reliable BAER test, as movement can produce interference (8). In the present study, alfaxalone and dexmedetomidine provided good quality sedation for the BAER test in pediatric dogs, with no evident side effects. The BAER test revealed unilateral sensorineural deafness in 2 Dalmatians and 1 French bulldog.

The degree of sedation in dogs is difficult to assess objectively. The scale used in this study has been previously validated in adult dogs (24), ensuring minimal variability in sedation scores obtained by different investigators; however, our study population consisted of pediatric dogs, for which there are no published sedation scoring systems. Despite some animals being potentially deaf, a clicker was included in the sedation scoring system to ensure standardization, as auditory stimuli could affect the level of arousal. There were no significant differences in this parameter between unilaterally deaf and normally hearing dogs in our study; however, differences may be found with a bigger sample size.

The endpoint for an adequate degree of sedation in this study was the ability to perform a good quality BAER test; if electrode placement was not tolerated due to stress or insufficient sedation, the dogs were administered an additional half of the initial dose. We found that the need for re-dosing was statistically higher in the G_A group, potentially indicating that the quality of sedation with alfaxalone is less reliable than with dexmedetomidine. Although SC needle electrode placement is, generally, not considered an invasive procedure, the analgesia provided by alpha-2 agonists could also explain the differences found.

One of the dogs moved during IM injection, potentially misplacing the drug into SC tissues; however, this patient belonged to the G_D group and did not require re-dosing.

Another factor that may have influenced the need for re-dosing was the difference in volume of injection: at 2 mg/kg BW, the injection volume for alfaxalone was 0.2 mL/kg BW, and at 0.005 mg/kg BW, the injection volume for dexmedetomidine was 0.01 mL/kg BW. For both drugs, the reported absorption of after IM administration in dogs is relatively rapid, achieving sedation with dexmedetomidine in 5 to 10 min (25) and alfaxalone in 4 to 10 min (22).

Dexmedetomidine could have been diluted in sterile saline or water for injection to be the same volume of injection as in the G_A group, but in order to avoid modifications in the absorption of the drug by changing the pH of the solution this was not attempted (26). Additionally, pain and discomfort have been associated with larger volumes of injection (27).

Alfaxalone in cyclodextrine solution has a pH of 6.05 to 7.0, while dexmedetomidine can be more acidic, with a pH between 4.5 and 7.0. The pH of a drug can influence the pain during its administration, but there were no differences between groups in the reaction during injection compared to simple handling of young dogs.

Sedation and recovery quality were good in all patients regardless of the protocol used. There is published evidence regarding poor recovery quality with excitation after alfaxalone sedation in adult dogs. The reported side effects are transient staggering gait, ataxia, auditory hyperesthesia, visual disturbances, muscle tremors, paddling, hypersalivation, and nystagmus (22,28). A possible reason for the differences in sedation and recovery quality is the dose of alfaxalone needed to achieve sedation in our study population. In pediatric dogs the blood-brain barrier may still be immature and this could have resulted in deeper sedation with a small dose of alfaxalone (2 mg/kg BW). Furthermore, there are important differences in drug metabolism between pediatric and adult dogs due to immaturity in renal and hepatic systems (17), and this factor could influence the recovery quality and time.

All patients in our study had a decrease in rectal temperature from t0 to tREC. Mean temperature remained within the reference range and the changes were not considered clinically significant, although, based on our results, we recommend application of active warming during sedation in pediatric dogs.

Physiologically, neonatal and pediatric dogs maintain their blood pressure with an increased heart rate (29). Alfaxalone at 2 mg/kg BW, IM, provided good quality sedation in pediatric dogs which maintained PR and f_R, consistent with previous reports in which alfaxalone at 2.5 mg/kg BW, IM, caused minimal cardiorespiratory effects in adult dogs (28). Dexmedetomidine causes a biphasic response in blood pressure with initial vasoconstriction followed by vasodilation and significant decrease in PR and cardiac output (17); however, f_R is usually maintained. Clinically, we observed similar effects with a decrease in PR but maintained f_R in patients in G_D; however, an important limitation of our study is that systemic blood pressure was not measured during sedation. This was a clinical study, and oscillometric systemic blood pressure measuring was attempted; however, it was difficult to obtain reliable results due to interference with the BAER test, the small size of the dogs, and the peripheral vasoconstriction of patients in G_D.

One limitation of this study is that the scale used to score the sedation has been validated for adult but not pediatric dogs. Although minimal variability between observers is ensured, our study population consisted of pediatric dogs and it is unknown how age differences affect sedation scores.

The study design has 2 important limitations: firstly, the anesthetists were not blinded to the treatments; however, the person placing the SC electrodes, who was assessing whether the sedation was adequate or not for BAER test performance, was unaware of the sedation protocol used. The other major limitation of the study design was the post-procedure atipamezole injection in G_D, acting as a confounding factor for measurement of td in this study group. The rationale for atipamezole administration was to minimize the bradycardia and residual sedation secondary to dexmedetomidine administration during the recovery period in pediatric dogs (29). This fact needs to be taken into account when interpreting the td of this clinical study. Alfaxalone doesn’t have an antidote, however, its sedative effects are short-lived (22,28).

Lastly, differences between individuals’ demeanors could influence the level of sedation required to perform the BAER test, as some patients may tolerate handling and physical immobilization better than others. Including this parameter (demeanor) and a larger sample size would avoid confounding factors and facilitate interpretation of the results obtained in the statistical analyses.

In conclusion, both sedation protocols tested in this study allowed good quality BAER test recordings in dogs aged 6 to 17 wk. Sedation with IM dexmedetomidine was more reliable as it required less re-dosing; however, sedation with IM alfaxalone maintained a more physiological heart rate in pediatric dogs.