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Laryngeal function in normal dogs administered isoflurane following partial clearance of alfaxalone or propofol

Published:March 31, 2021DOI:https://doi.org/10.1016/j.vaa.2021.03.009

      Abstract

      Objective

      To assess laryngeal function in normal dogs administered isoflurane following partial clearance of alfaxalone or propofol.

      Study design

      Randomized experimental crossover study.

      Animals

      A group of 12 purpose-bred, male Beagle dogs.

      Methods

      Dogs were randomly assigned to one of two treatments: alfaxalone–isoflurane (ALF-ISO) or propofol–isoflurane (PRO-ISO) and anesthetized for three video laryngoscopy examinations. The alternate treatment occurred after ≥ 14 days interval. Examinations were performed after induction of anesthesia (LS-A), after 20 minutes of breathing isoflurane via a facemask (LS-B) and after a further 20 minutes of isoflurane (LS-C). Parameters of objective laryngeal function included inspiratory rima glottidis surface area (RGSA-I), expiratory rima glottidis surface area (RGSA-E) and % RGSA increase, calculated from three consecutive respiratory cycles in the final 15 seconds of each video laryngoscopy examination. The % RGSA increase was calculated using [(RGSA-I – RGSA-E)/RGSA-E] × 100. Subjective laryngeal function was evaluated independently by two experienced surgeons blinded to treatment.

      Results

      The % RGSA increase within each treatment was greater for LS-B and LS-C than for LS-A (ALF-ISO: p = 0.03, PRO-ISO: p = < 0.001). There was no difference within each treatment from LS-B compared with LS-C. RGSA-I increased within each treatment from LS-A to both LS-B and LS-C (ALF-ISO: p = 0.002) and to LS-C (PRO-ISO: p = 0.006). Subjective laryngeal function scores improved from LS-A to LS-C.

      Conclusions and clinical relevance

      Laryngeal function improved from postinduction examination following either 20 or 40 minutes of anesthesia with isoflurane via facemask. This study demonstrates that isoflurane may have a lesser effect on arytenoid abduction activity compared with more commonly used intravenous induction anesthetics (alfaxalone and propofol).

      Keywords

      Introduction

      Canine laryngeal disease can potentially be life-threatening by altering laryngeal function and resulting in upper airway obstruction (
      • MacPhail C.
      Laryngeal disease in dogs and cats: an update.
      ). Laryngeal paralysis is a common cause of altered laryngeal function in dogs and may be either hereditary or acquired. Diagnosis requires consideration of clinical history, presenting problems and upper airway examination including evaluation of arytenoid cartilage abduction during inspiration (
      • MacPhail C.
      Laryngeal disease in dogs and cats: an update.
      ). Dogs suspected of laryngeal paralysis presented to our institution often have laryngeal function evaluated during laryngoscopy under a light plane of anesthesia, preceding surgical intervention such as arytenoid lateralization.
      The effects of anesthetics on laryngeal motion and ventilatory drive may affect accurate evaluation of laryngeal function (
      • MacPhail C.
      Laryngeal disease in dogs and cats: an update.
      ;
      • Ranninger E.
      • Kantyka M.
      • Bektas R.N.
      The influence of anesthetic drugs on laryngeal motion in dogs: a systemic review.
      ). Observation of at least some degree of laryngeal paresis was reported as high as 25% in a population of 250 dogs undergoing general anesthesia for various indications (
      • Broome C.
      • Burbidge H.M.
      • Pfeiffer D.U.
      Prevalence of laryngeal paresis in dogs undergoing general anaesthesia.
      ). Since that study, the influences of the induction anesthetic, dose and administration rates, use of premedicants and use of chemical or mechanical stimulants as part of a laryngeal function examination have been investigated (
      • Gross M.E.
      • Dodam J.R.
      • Pope E.R.
      • Jones B.D.
      A comparison of thiopental, propofol, and diazepam–ketamine anesthesia for evaluation of laryngeal function in dogs premedicated with butorphanol–glycopyrrolate.
      ;
      • Miller C.J.
      • McKiernan B.C.
      • Pace J.
      • Fettman M.J.
      The effects of doxapram hydrochloride (Dopram–V) on laryngeal function in healthy dogs.
      ;
      • Jackson A.M.
      • Tobias K.
      • Long C.
      • et al.
      Effects of various anesthetic agents on laryngeal motion during laryngoscopy in normal dogs.
      ;
      • Tobias K.M.
      • Jackson A.M.
      • Harvey R.C.
      Effects of doxapram HCl on laryngeal function of normal dogs and dogs with naturally occurring laryngeal paralysis.
      ;
      • McKeirnan K.L.
      • Gross M.E.
      • Rochat M.
      • Payton M.
      Comparison of propofol and propofol/ketamine anesthesia for evaluation of laryngeal function in healthy dogs.
      ;
      • Smalle T.M.
      • Hartman M.J.
      • Bester L.
      • et al.
      Effects of thiopentone, propofol and alfaxalone on laryngeal motion during oral laryngoscopy in healthy dogs.
      ;
      • Ambros B.
      • Gaunt M.C.
      • Duke-Novakovski T.
      • Taylor S.M.
      Effects of alfaxalone, thiopental, or propofol and diazepam on laryngeal motion in healthy dogs.
      ;
      • Norgate D.
      • Ter Haar G.
      • Kulendra N.
      • Veres-Nyéki K.O.
      A comparison of the effect of propofol and alfaxalone on laryngeal motion in nonbrachycephalic and brachycephalic dogs.
      ;
      • Radkey D.I.
      • Hardie R.J.
      • Smith L.J.
      Comparison of the effects of alfaxalone and propofol with acepromazine, butorphanol and/or doxapram on laryngeal motion and quality of examination in dogs.
      ;
      • Brown M.B.
      • Dugat D.R.
      • Lyon S.D.
      • et al.
      Comparison of methohexital and propofol as induction agents for evaluation of laryngeal function in healthy dogs.
      ;
      • Labuscagne S.
      • Zeiler G.E.
      • Dzikiti B.T.
      Effects of chemical and mechanical stimulation on laryngeal motion during alfaxalone, thiopentone or propofol anaesthesia in healthy dogs.
      ;
      • DeGroot W.D.
      • Tobias K.M.
      • Browning D.C.
      • Zhu X.
      Examination of laryngeal function of healthy dogs by using sedation protocols with dexmedetomidine.
      ;
      • Ranninger E.
      • Kantyka M.
      • Bektas R.N.
      The influence of anesthetic drugs on laryngeal motion in dogs: a systemic review.
      ). To date, only drugs administered via intramuscular and intravenous (IV) routes have been investigated for laryngeal function examination. Isoflurane has been suggested as an option for mask induction in dogs premedicated with acepromazine, with or without an opioid, which required additional anesthetic restraint to facilitate laryngoscopy (
      • Jackson A.M.
      • Tobias K.
      • Long C.
      • et al.
      Effects of various anesthetic agents on laryngeal motion during laryngoscopy in normal dogs.
      ). The effect or utility of inhalant anesthetics for laryngeal examinations is largely unknown.
      Propofol and alfaxalone are commonly used induction anesthetics for ease in titratability and fast recovery from anesthesia (
      • Smalle T.M.
      • Hartman M.J.
      • Bester L.
      • et al.
      Effects of thiopentone, propofol and alfaxalone on laryngeal motion during oral laryngoscopy in healthy dogs.
      ;
      • Ambros B.
      • Gaunt M.C.
      • Duke-Novakovski T.
      • Taylor S.M.
      Effects of alfaxalone, thiopental, or propofol and diazepam on laryngeal motion in healthy dogs.
      ;
      • Norgate D.
      • Ter Haar G.
      • Kulendra N.
      • Veres-Nyéki K.O.
      A comparison of the effect of propofol and alfaxalone on laryngeal motion in nonbrachycephalic and brachycephalic dogs.
      ;
      • Labuscagne S.
      • Zeiler G.E.
      • Dzikiti B.T.
      Effects of chemical and mechanical stimulation on laryngeal motion during alfaxalone, thiopentone or propofol anaesthesia in healthy dogs.
      ;
      • DeGroot W.D.
      • Tobias K.M.
      • Browning D.C.
      • Zhu X.
      Examination of laryngeal function of healthy dogs by using sedation protocols with dexmedetomidine.
      ). In a subset of the studies evaluating propofol or alfaxalone as induction anesthetics, weak or absent laryngeal motion, paradoxical motion or apnea have been reported during laryngeal function examinations in apparently healthy dogs (
      • Smalle T.M.
      • Hartman M.J.
      • Bester L.
      • et al.
      Effects of thiopentone, propofol and alfaxalone on laryngeal motion during oral laryngoscopy in healthy dogs.
      ;
      • Ambros B.
      • Gaunt M.C.
      • Duke-Novakovski T.
      • Taylor S.M.
      Effects of alfaxalone, thiopental, or propofol and diazepam on laryngeal motion in healthy dogs.
      ;
      • Norgate D.
      • Ter Haar G.
      • Kulendra N.
      • Veres-Nyéki K.O.
      A comparison of the effect of propofol and alfaxalone on laryngeal motion in nonbrachycephalic and brachycephalic dogs.
      ;
      • Radkey D.I.
      • Hardie R.J.
      • Smith L.J.
      Comparison of the effects of alfaxalone and propofol with acepromazine, butorphanol and/or doxapram on laryngeal motion and quality of examination in dogs.
      ;
      • DeGroot W.D.
      • Tobias K.M.
      • Browning D.C.
      • Zhu X.
      Examination of laryngeal function of healthy dogs by using sedation protocols with dexmedetomidine.
      ). Doxapram has been advocated for increasing laryngeal motion during laryngeal examination (
      • Miller C.J.
      • McKiernan B.C.
      • Pace J.
      • Fettman M.J.
      The effects of doxapram hydrochloride (Dopram–V) on laryngeal function in healthy dogs.
      ;
      • Tobias K.M.
      • Jackson A.M.
      • Harvey R.C.
      Effects of doxapram HCl on laryngeal function of normal dogs and dogs with naturally occurring laryngeal paralysis.
      ). However, in one study doxapram failed to improve laryngeal motion in a subset of normal dogs where alfaxalone or propofol was used (
      • Radkey D.I.
      • Hardie R.J.
      • Smith L.J.
      Comparison of the effects of alfaxalone and propofol with acepromazine, butorphanol and/or doxapram on laryngeal motion and quality of examination in dogs.
      ). The use of doxapram may also complicate diagnosis (e.g., paradoxical arytenoid adduction on inspiration) or lead to upper airway obstruction, requiring emergency intubation (
      • Tobias K.M.
      • Jackson A.M.
      • Harvey R.C.
      Effects of doxapram HCl on laryngeal function of normal dogs and dogs with naturally occurring laryngeal paralysis.
      ;
      • Radkey D.I.
      • Hardie R.J.
      • Smith L.J.
      Comparison of the effects of alfaxalone and propofol with acepromazine, butorphanol and/or doxapram on laryngeal motion and quality of examination in dogs.
      ). Situations exist postinduction or post doxapram administration where laryngeal function either cannot be evaluated or where there is a risk of false-positive diagnosis of laryngeal paralysis.
      The objectives of this study were to compare the objective and subjective measures of laryngeal function 1) after induction of anesthesia with alfaxalone or propofol; and 2) to reassess laryngeal function after 20 and 40 minutes of isoflurane administered by facemask. The hypothesis was that objective measurements including inspiratory rima glottidis surface area (RGSA-I) and the percent increase in RGSA along with subjective evaluation would reveal less depression of laryngeal movement following 20 and 40 minutes of isoflurane administration, compared with postinduction evaluation.

      Materials and methods

       Protocol approval and development

      The study was performed in compliance with and approval from the Institutional Animal Care and Use Committee at Kansas State University College of Veterinary Medicine. The Animals in Research Reporting in vivo Experiments (ARRIVE) guidelines were followed throughout the study protocol (
      • Kilkenny C.
      • Browne W.
      • Cuthill I.C.
      • et al.
      Animal research: reporting in vivo experiments: the ARRIVE guidelines.
      ).

       Animals

      Purpose-bred male Beagle dogs (n = 12), aged 5–6 years and weighing 9.6–15.0 kg, were acquired from a university research colony for use in this study. The dogs were intermittently used for diet trials. However, the dogs were not enrolled in other research protocols for at least 2 weeks prior to the present study. The dogs were assessed as healthy based on physical examination, complete blood count and blood chemistries performed prior to enrollment in the study. Exclusion criteria included any dog with a history of exercise intolerance, dysphagia, coughing or gagging while eating, cervical trauma, the presence of any masses involving the cervical or laryngeal region, or any physical examination or laboratory diagnostic abnormality that would preclude anesthesia. Dogs were individually housed, fed twice daily and provided water ad libitum.

       Study design

      The dogs were anesthetized in a crossover design on two occasions separated by 2–8 weeks. They underwent the second anesthetic event when they could be scheduled for a complete oral health assessment and treatment following completion of the study. The dogs were randomly assigned (Excel; Microsoft Corporation, WA, USA) to one treatment for the first anesthetic event, six dogs per treatment. Anesthesia was induced with alfaxalone (Alfaxan; Jurox Animal Health, MO, USA; treatment ALF-ISO) or propofol (Propofol Injectable Emulsion USP; Sagent Pharmaceuticals, IL, USA; treatment PRO-ISO) and anesthesia was maintained with isoflurane (Isoflurane; Akorn Animal Health, IL, USA) delivered in 4 L minute–1 of oxygen using a nonrebreathing circuit (Bain modification of a Mapleson D circuit). Three separate laryngoscopy examinations were performed. The initial examination (LS-A) was performed immediately after induction of anesthesia, the second examination (LS-B) was performed after 20 minutes of breathing isoflurane and the third (LS-C) after an additional 20 minutes of isoflurane.
      Food, but not water, was withheld for 12 hours before anesthesia. On the day of assessment, a 20 gauge, 3.2 cm catheter (Excel International Inc., FL, USA) was placed into the right cephalic vein. With the dog lightly restrained in sternal recumbency, alfaxalone (1.5 mg kg–1) or propofol (4 mg kg–1) was administered via the IV catheter over 30 seconds. A board-certified anesthesiologist (DH or RM) evaluated anesthetic depth by assessing eye position, palpebral reflex, jaw muscle tone and response to laryngoscope blade placement (swallowing, gagging). If anesthetic depth was inadequate for laryngoscope placement, supplemental alfaxalone (0.3 mg kg–1) or propofol (1 mg kg–1) was administered over 10 seconds. Anesthetic depth was re-evaluated 30 seconds after each supplemental dose. This was repeated until an appropriate anesthetic plane (defined by ease of opening the jaws, absence of swallowing or gagging with insertion of laryngoscope) was achieved. The total dose of alfaxalone or propofol administered was recorded and the first laryngoscopy examination LS-A was started.
      With the dog in sternal recumbency, a handheld sling was placed around the maxilla to support the head and neck in an extended position. The mandible was distracted ventrally, and the tongue gently pulled rostrally. The distal tip of a 15.7 cm Cranwall Miller laryngoscope blade (SunMed Healthcare, MI, USA) was placed lightly on the surface of the epiglottis, exposing the rima glottidis (RG). A metal probe was placed in the same location relative to RG during each video recording and used to both elevate the soft palate and to standardize and scale each video (Fig. 1). The custom-made probe was 25 cm long with a 3 mm diameter, 1.5 cm long cylindrical end (measurement portion) mounted at a right angle to the probe. Once anesthetic depth was adequate and the measurement probe and laryngoscope were in place, a continuous video recording was made of the RG on an iPhone (iPhone 6S; Apple Corporation, CA, USA). A custom apparatus was designed and constructed to hold the iPhone in the same position and maintain a standard depth of field for each examination. The time from beginning video laryngoscopy to the end of the examination was recorded. Each examination was terminated when anesthetic depth was no longer adequate.
      Figure 1
      Figure 1Image during (a) maximal inspiration and (b) expiration of a single respiratory cycle in an anesthetized Beagle dog, demonstrating tracing of the rima glottidis surface area (RGSA). The black square on the end of the 3 mm diameter measurement probe provides a 3 mm2 area to scale the RGSA.
      Once LS-A was completed, the dog was administered isoflurane in 4 L minute–1 oxygen for 20 minutes via a nonrebreathing Mapleson D circuit (Bain modification) and a tight-fitting facemask. A calibrated Ohio vaporizer was used. Prior to beginning the study, the vaporizer output was confirmed by infrared gas analysis using a Cardiocap/5 (Datex-Ohmeda, GE Healthcare Finland Oy, Finland). Dial settings were identified which resulted in isoflurane output of 2%, 3%, 4% or 5% using 4 L minute–1 of oxygen. The isoflurane dial setting was adjusted between the four identified concentration outputs to maintain each dog at a light plane of anesthesia, as assessed by the anesthesiologist (DH or RM). Vaporizer dial settings were recorded and reported as the weighted mean for each 20 minute period of isoflurane administration. At the end of each 20 minute period, laryngeal examinations LS-B and LS-C, respectively, were performed as described above. Following the first treatment, each dog was allowed to recover from anesthesia. Following the second treatment, each dog underwent dental prophylaxis prior to recovery.
      Immediately after LS-A, each dog was instrumented with a pulse oximeter on the tongue (Rad-5; Masimo, CA, US), a 3-lead electrocardiogram, and a Doppler (Model 801-B; Parks Medical Electronics, OR, USA) flow crystal and blood pressure cuff on the tail. Variables recorded throughout the two 20 minute periods of isoflurane administration and subsequent laryngeal examinations included hemoglobin saturation with oxygen, respiratory rate (fR), heart rate and rhythm and systolic blood pressure. Variables were recorded every 5 minutes until completion of LS-C.

       Subjective laryngeal assessment

      Evaluators (EK and KB) of laryngeal function were not present at the time of video laryngoscopy and blinded to treatment and examination time (i.e., LS-A/B/C). Each evaluator independently assessed recordings based on previously defined grading criteria (
      • Smalle T.M.
      • Hartman M.J.
      • Bester L.
      • et al.
      Effects of thiopentone, propofol and alfaxalone on laryngeal motion during oral laryngoscopy in healthy dogs.
      ; Appendix A). Video recordings had audio during which one of the anesthesiologists (DH or RM) called the phase of respiration (inspiration or expiration).

       Objective laryngeal assessment

      Three consecutive respiratory cycles were evaluated during the last 15 seconds of each video and images recorded at maximal inspiration and expiration. The number of respiratory cycles that occurred in the final 15 seconds of each recording was multiplied by four, producing fR in breaths minute–1. Images were obtained using a high-resolution display (2880 × 1800 native resolution with 87 pixels cm–1) and stored on a laptop computer (IdeaPad Flex 5; Lenovo, NC, USA). The RGSA measurements were made in Photoshop (Creative Cloud Version 20.0.7; Adobe Inc., CA, USA). The RG border was traced to determine area in pixels. A square section of the measuring probe was traced to determine the pixels per 3 mm2 area. The RGSA in mm2 was determined by dividing the RGSA tracing by 3 mm2 area (Fig. 1a & b). The three inspiratory and expiratory RGSA measurements from each video recording were averaged to determine the mean inspiratory (RGSA-I) and expiratory area (RGSA-E), respectively.
      Objective assessment of laryngeal function included measuring of RGSA-I and calculating % RGSA increase from maximal expiration to maximal inspiration. The % RGSA increase was calculated as previously described:
      [(RGSA-I – RGSA-E)/RGSA-E] × 100 = % RGSA increase (
      • Radkey D.I.
      • Hardie R.J.
      • Smith L.J.
      Comparison of the effects of alfaxalone and propofol with acepromazine, butorphanol and/or doxapram on laryngeal motion and quality of examination in dogs.
      )


       Statistical analysis

      A power analysis (alpha = 0.05 and 1-beta = 0.80) performed prior to this study, based on a previously published study (
      • Tobias K.M.
      • Jackson A.M.
      • Harvey R.C.
      Effects of doxapram HCl on laryngeal function of normal dogs and dogs with naturally occurring laryngeal paralysis.
      ), determined that six examinations would detect a significant difference between RGSA-E, and 56 examinations were required to detect a difference of RGSA-I. Continuous variables were assessed for normality by Anderson–Darling analysis. Variables of weight, video laryngoscopy duration, fR, RGSA-I, RGSA-E and % RGSA increase were compared between treatments by a paired t test. The length of time at each vaporizer dial setting was recorded and a weighted mean of the dial setting calculated for each 20 minute period of isoflurane administration. Comparison of weighted mean vaporizer dial settings within and between treatments were made using a paired t test. Video laryngoscopy duration, fR, RGSA-I, RGSA-E and % RGSA increase were compared within treatments by repeated measures analysis of variance with post hoc testing by Newman–Keuls multiple comparisons.
      Observer categorical assignments between treatments were compared by Chi-square analysis. Inter-rater agreement between the two observers (EK and KB) for the category (0–3) were tested by calculation of a Kappa coefficient (κ). A weighted κ was reported for the ordinal category scores.
      Analyses were performed using WINKS SDA Version 7.0.9 (TexaSoft Mission Technologies, TX, USA; www.texasoft.com) and on QuickCalcs (www.graphpad.com). A p < 0.05 was considered significant for statistical comparisons.

      Results

      All 12 dogs completed the study. Body weight prior to each treatment, isoflurane dial setting and examination duration were not significantly different between treatments (Table 1). The fR during the final 15 seconds of each examination did not differ within or between treatments (Table 1). The duration of LS-A, following either alfaxalone (p = 0.031) or propofol (p = < 0.001) induction, was longer than LS-B or LS-C (Table 1).
      Table 1Variables for 12 dogs anesthetized with alfaxalone and anesthesia maintained with isoflurane (treatment ALF-ISO) or anesthetized with propofol and anesthesia maintained with isoflurane (treatment PRO-ISO). Video laryngoscopy was performed after induction of anesthesia (time point LS-A), after breathing isoflurane for 20 minutes (isoflurane discontinued for laryngoscopy at time point LS-B) and after breathing isoflurane for a further 20 minutes (isoflurane discontinued for laryngoscopy at time point LS-C). Isoflurane was administered through a tight-fitting facemask from a nonrebreathing (Bain) delivery system. Data are presented as mean ± standard deviation (range)
      VariableTreatment
      ALF-ISOPRO-ISO
      Weight (kg)12.7 ± 1.7 (9.6–14.8)12.7 ± 1.7 (10.5–15.0)
      Induction dose (mg kg–1)2.4 ± 0.4 (1.5–3.0)6.8 ± 2.2 (4.0–11.0)
      LS-A
       Duration (seconds)108 ± 99∗148 ± 63∗
      fR (breaths minute–1)32 ± 15 (20–72)28 ± 13 (12–52)
      Isoflurane vaporizer setting for the first 20 minutes (%)3.9 ± 0.63.8 ± 0.5
      LS-B
       Duration (seconds)48 ± 4128 ± 15
      fR (breaths minute–1)28 ± 13 (16–64)34 ± 18 (16–72)
      Isoflurane vaporizer setting for the second 20 minutes (%)3.7 ± 0.73.8 ± 0.5
      LS-C
       Duration (seconds)38 ± 2527 ± 15
      fR (breaths minute –1)27 ± 12 (12–60)35 ± 21 (12–80)
      fR = respiratory rate.
      ∗Significant difference between treatments for laryngoscopy time at LS-A (p < 0.05).
      The RGSA-I and RGSA-E did not differ between treatments (Table 2). In treatment ALF-ISO, RGSA-I was lower at LS-A (p = 0.002) than at LS-B or LS-C (Table 2). RGSA-E was not different within treatment ALF-ISO. In treatment PRO-ISO, RGSA-I was lower at LS-A than at LS-C (p = 0.006), but not from LS-A to LS-B or from LS-B to LS-C (Table 2). RGSA-E was higher (p = < 0.001) at LS-A than at LS-B or LS-C in treatment PRO-ISO (Table 2).
      Table 2Mean ± standard deviation of objective laryngeal motion variables in 12 dogs anesthetized with alfaxalone and anesthesia maintained with isoflurane (treatment ALF-ISO) or anesthetized with propofol and anesthesia maintained with isoflurane (treatment PRO-ISO). Video laryngoscopy was performed after induction of anesthesia (time point LS-A), after breathing isoflurane for 20 minutes (isoflurane discontinued for laryngoscopy at time point LS-B) and after breathing isoflurane for a further 20 minutes (isoflurane discontinued for laryngoscopy at time point LS-C). The mean rima glottidis surface area (RGSA) at maximal inspiration (RGSA-I), at maximal expiration (RGSA-E) and % increase in RGSA from maximal expiration to inspiration were measured. Data are the average of three consecutive respiratory cycles obtained from the final 15 seconds of each video laryngoscopic examination at each time point
      VariablesTreatment and time points
      ALF-ISOPRO-ISO
      LS-ALS-BLS-CLS-ALS-BLS-C
      RGSA-I (mm2)13.7 ± 2.317.2 ± 4.9∗19.2 ± 4.1∗14.6 ± 3.617.7 ± 6.020.1 ± 5.0∗
      RGSA-E (mm2)13.0 ± 2.810.3 ± 5.09.8 ± 4.313.6 ± 3.37.3 ± 3.9∗7.2 ± 2.8∗
      RGSA (% increase)7.4 ± 10.6129.3 ± 114.0∗132.8 ± 103.2∗,7.4 ± 8.9186.1 ± 118.0∗224.3 ± 143.3∗
      ∗Significantly different from LS-A within the treatment (p < 0.05).
      Significant difference between treatments (p < 0.05).
      The % RGSA compared between treatments was different for LS-C only (p = 0.03), with a greater % RGSA increase in treatment PRO-ISO (Table 2). The % RGSA increase was higher within each treatment for LS-A than for LS-B or LS-C in both treatments PRO-ISO (p = < 0.001) and ALF-ISO (p = 0.03) (Table 2).
      The LS-A was assigned a lower category score (p = 0.038) in treatment ALF-ISO than in treatment PRO-ISO. There was moderate inter-rater agreement for the category scores assigned for LS-A (κ = 0.52), LS-B (κ = 0.565) and LS-C (κ = 0.579). The category score assigned, independent of treatment, improved over time (Table 3).
      Table 3Frequency (%) of scores for laryngeal motion assigned during 72 video laryngoscopy examinations performed at three time points in 12 Beagle dogs anesthetized twice. Video laryngoscopy was performed after induction of anesthesia with either alfaxalone or propofol (time point LS-A), after breathing isoflurane for 20 minutes (isoflurane discontinued for laryngoscopy at time point LS-B) and after breathing isoflurane for a further 20 minutes (isoflurane discontinued for laryngoscopy at time point LS-C). Isoflurane was administered through a tight-fitting facemask from a Bain delivery system between time points
      Score (%)Time points
      LS-ALS-BLS-C
      0060.379.2
      14.318.810.3
      218.714.66.3
      3776.34.2
      Score 0, full arytenoid cartilage abduction; Score 3, arytenoid cartilages immobile (Appendix A).
      In treatment ALF-ISO one dog had a dysphoric recovery that resolved without intervention. Hypotension (Doppler recording < 90 mmHg) was recorded in three of 12 dogs in each of the treatments. Isoflurane administration via mask allowed anesthesia to be maintained at the desired depth in all dogs. None of the dogs required tracheal intubation.

      Discussion

      Determination of normal laryngeal function relies on the ability to identify abduction of the arytenoid cartilages during inspiration. Objective measures used to evaluate arytenoid abduction by function of the recurrent laryngeal nerves and cricoarytenoideus dorsalis (CAD) muscles in the present study were measurements of RGSA-I and % RGSA increase.
      Alfaxalone and propofol may alter laryngeal function after administration resulting in weak or absent laryngeal motion, paradoxical motion or apnea (
      • Smalle T.M.
      • Hartman M.J.
      • Bester L.
      • et al.
      Effects of thiopentone, propofol and alfaxalone on laryngeal motion during oral laryngoscopy in healthy dogs.
      ;
      • Ambros B.
      • Gaunt M.C.
      • Duke-Novakovski T.
      • Taylor S.M.
      Effects of alfaxalone, thiopental, or propofol and diazepam on laryngeal motion in healthy dogs.
      ;
      • Norgate D.
      • Ter Haar G.
      • Kulendra N.
      • Veres-Nyéki K.O.
      A comparison of the effect of propofol and alfaxalone on laryngeal motion in nonbrachycephalic and brachycephalic dogs.
      ;
      • Radkey D.I.
      • Hardie R.J.
      • Smith L.J.
      Comparison of the effects of alfaxalone and propofol with acepromazine, butorphanol and/or doxapram on laryngeal motion and quality of examination in dogs.
      ;
      • DeGroot W.D.
      • Tobias K.M.
      • Browning D.C.
      • Zhu X.
      Examination of laryngeal function of healthy dogs by using sedation protocols with dexmedetomidine.
      ). How laryngeal function may change as time elapses postinduction using these agents has not been published to date. Objective measures of function improved from LS-A to both LS-B and LS-C within each treatment. Subjective scores assigned to each examination time also improved after LS-A, with moderate inter-rater agreement between examiners for category assignments during each examination time. Interpretation of κ values is as follows for slight (0.00–0.20), fair (0.21–0.40), moderate (0.41–0.60), substantial (0.61–0.80) and almost perfect agreement (0.81–1.00) (
      • McHugh M.L.
      Interrater reliability: the kappa statistic.
      ).
      • Radkey D.I.
      • Hardie R.J.
      • Smith L.J.
      Comparison of the effects of alfaxalone and propofol with acepromazine, butorphanol and/or doxapram on laryngeal motion and quality of examination in dogs.
      suggested that an examiner may not be sensitive to laryngeal motion when the % RGSA increase is less than 20%. In the present study, the % RGSA increase was < 10% during LS-A for each treatment, potentially explaining the poor subjective grades assigned for LS-A and subsequent improvement in grade assignments during LS-B and LS-C. As there was no improvement in objective variables from LS-B to LS-C in either treatment, there may be no benefit to the second period of isoflurane; however, type II error cannot be ruled out.
      Laryngeal motion variables after both 20 minute periods of isoflurane administration were significantly improved compared with postinduction. Plasma half-lives of single induction doses of alfaxalone and propofol are reported as 24 minutes and 7–10 minutes, respectively (
      • Zoran D.L.
      • Riedesel D.
      • Dyer D.C.
      Pharmacokinetics of propofol in mixed-breed dogs and greyhounds.
      ;
      • Ferré P.J.
      • Pasloske K.
      • Whittem T.
      • et al.
      Plasma pharmacokinetics of alfaxalone in dogs after an intravenous bolus of Alfaxan-CD RTU.
      ;
      • McIntosh M.P.
      • Rajewski R.A.
      Comparative canine pharmacokinetics-pharmacodynamics of fospropofol disodium injection, propofol emulsion, and cyclodextrin-enabled propofol solution following bolus parenteral administration.
      ). Therefore, it is likely that at the time of LS-B and LS-C, the effects of alfaxalone and propofol on laryngeal function would have diminished. Plasma concentrations of alfaxalone and propofol were not measured and potential residual effects on laryngeal function during LS-B and LS-C could not be determined. Assessing laryngeal function in these dogs during anesthesia with isoflurane only may have elucidated potential residual effects of alfaxalone and propofol.
      The influence of inhalant anesthetics on laryngeal motion in healthy dogs is largely unknown. There is evidence of reduced afferent information from laryngeal drive receptors, via the cranial laryngeal nerve, within 1 minute of exposure to isoflurane in healthy dogs (
      • Mutoh T.
      • Kanamaru A.
      • Kojima K.
      • et al.
      Effects of volatile anesthetics on the activity of laryngeal ‘drive’ receptors in anesthetized dogs.
      ). However, how this may influence ventilatory drive or respiratory pattern and subsequently affect laryngeal motion is unknown (
      • Mutoh T.
      • Kanamaru A.
      • Kojima K.
      • et al.
      Effects of volatile anesthetics on the activity of laryngeal ‘drive’ receptors in anesthetized dogs.
      ). Interestingly, there is controversy over ideal anesthetics for airway endoscopy in humans (
      • Lerman J.
      • Jöhr M.
      Inhalational anesthesia vs total intravenous anesthesia (TIVA) for pediatric anesthesia.
      ;
      • Malherbe S.
      • Whyte S.
      • Singh P.
      • et al.
      Total intravenous anesthesia and spontaneous respiration for airway endoscopy in children — a prospective evaluation.
      ;
      • Gazzaz M.
      • Saini J.
      • Pagliardini S.
      • et al.
      Comparison of inhaled versus intravenous anesthesia for laryngoscopy and laryngeal electromyography in a rat model.
      ). Laryngeal electromyography (LEMG) is used to facilitate the diagnosis of laryngeal mobility disorders in humans (
      • Berkowitz R.G.
      • Ryan M.M.
      • Pilowsky P.M.
      Respiration-related laryngeal electromyography in children with bilateral vocal fold paralysis.
      ;
      • Lerman J.
      • Jöhr M.
      Inhalational anesthesia vs total intravenous anesthesia (TIVA) for pediatric anesthesia.
      ;
      • Malherbe S.
      • Whyte S.
      • Singh P.
      • et al.
      Total intravenous anesthesia and spontaneous respiration for airway endoscopy in children — a prospective evaluation.
      ;
      • Gazzaz M.
      • Saini J.
      • Pagliardini S.
      • et al.
      Comparison of inhaled versus intravenous anesthesia for laryngoscopy and laryngeal electromyography in a rat model.
      ). In a rat model for video laryngoscopy, during administration of propofol, the CAD LEMG amplitudes were only 66.9% of the amplitudes obtained during isoflurane anesthesia (
      • Gazzaz M.
      • Saini J.
      • Pagliardini S.
      • et al.
      Comparison of inhaled versus intravenous anesthesia for laryngoscopy and laryngeal electromyography in a rat model.
      ), suggesting that inhalation anesthetics preserve CAD muscle function.
      The % RGSA increase is influenced by both an increase in RGSA-I and changes in RGSA-E. Active adduction of laryngeal cartilages may occur during activation of upper airway reflexes, such as laryngospasm, or during phonation (
      • MacPhail C.
      Laryngeal disease in dogs and cats: an update.
      ). Expiration is normally passive with no active adduction. RGSA-E in treatment PRO-ISO was significantly decreased during LS-B and LS-C compared with LS-A. A possible explanation for this change may be related to the shorter plasma half-life of propofol compared with alfaxalone, resulting in less residual propofol during LS-B and LS-C. Despite being at an equivocal plane of anesthesia compared with dogs in treatment ALF-ISO, the dogs in treatment PRO-ISO may have been at a lighter plane of anesthesia during LS-B and LS-C and thus experienced less depression of airway reflexes allowing more active adduction on expiration. Examination of laryngeal function during IV anesthesia should be performed with the animal as lightly anesthetized as possible (
      • Jackson A.M.
      • Tobias K.
      • Long C.
      • et al.
      Effects of various anesthetic agents on laryngeal motion during laryngoscopy in normal dogs.
      ;
      • Labuscagne S.
      • Zeiler G.E.
      • Dzikiti B.T.
      Effects of chemical and mechanical stimulation on laryngeal motion during alfaxalone, thiopentone or propofol anaesthesia in healthy dogs.
      ). This approach may be unnecessary for laryngeal function assessment in isoflurane-anesthetized dogs.
      It was not possible to measure an accurate end-tidal partial pressure of carbon dioxide (Pe′CO2) while the dogs were administered isoflurane via facemask. Despite fR being similar between examination times, fR contributes no objective information about minute ventilation. Immediately following intubation prior to performing dental prophylaxis, hypoventilation (i.e., Pe′CO2 60–80 mmHg; 8.0–10.6 kPa) was noted.
      • Cheetham J.
      • Jones A.
      • Martin-Flores M.
      Effect of inhaled carbon dioxide on laryngeal abduction.
      demonstrated that stepwise increases in severity of hypercapnia increase laryngeal motion in normal dogs anesthetized with isoflurane. In their study, laryngeal motion was assessed using the normalized glottal gap area (NGGA), which is obtained by dividing RGSA by glottic length squared. Glottic length, RGSA and NGGA increased with increasing Pe′CO2 but plateaued at Pe′CO2 75 mmHg (10 kPa) and 80 mmHg (10.6 kPa) for NGGA and RSGA, respectively (
      • Cheetham J.
      • Jones A.
      • Martin-Flores M.
      Effect of inhaled carbon dioxide on laryngeal abduction.
      ). The NGGA method has been described as a common method for laryngeal function assessment in dogs (
      • Miller C.J.
      • McKiernan B.C.
      • Pace J.
      • Fettman M.J.
      The effects of doxapram hydrochloride (Dopram–V) on laryngeal function in healthy dogs.
      ;
      • Jackson A.M.
      • Tobias K.
      • Long C.
      • et al.
      Effects of various anesthetic agents on laryngeal motion during laryngoscopy in normal dogs.
      ;
      • Tobias K.M.
      • Jackson A.M.
      • Harvey R.C.
      Effects of doxapram HCl on laryngeal function of normal dogs and dogs with naturally occurring laryngeal paralysis.
      ;
      • Ambros B.
      • Gaunt M.C.
      • Duke-Novakovski T.
      • Taylor S.M.
      Effects of alfaxalone, thiopental, or propofol and diazepam on laryngeal motion in healthy dogs.
      ;
      • DeGroot W.D.
      • Tobias K.M.
      • Browning D.C.
      • Zhu X.
      Examination of laryngeal function of healthy dogs by using sedation protocols with dexmedetomidine.
      ), however, the validity of the NGGA method has also been questioned in dogs that may be hypercarbic (
      • Cheetham J.
      • Jones A.
      • Martin-Flores M.
      Effect of inhaled carbon dioxide on laryngeal abduction.
      ). Because dogs in the present study were at times anesthetized with both injectable anesthetics and isoflurane, when hypoventilation was possible, NGGA was not considered.
      Objective and subjective methods of assessing laryngeal function were used in the present study. Laryngeal motion, measured objectively using RGSA-I and % RGSA increase, improved from LS-A to LS-B and LS-C. Real-time subjective assessment of laryngeal function is a valid component of the diagnosis of laryngeal paralysis; however, depression of laryngeal motion by anesthetics may hinder the ability to identify normal laryngeal abduction.
      The tight-fitting facemask increased circuit dead space and may have contributed to an inspired isoflurane concentration lower than the vaporizer dial settings. Administration of isoflurane via facemask was selected for the present study to avoid disturbing the larynx by endotracheal intubation. In clinical practice, animals suspected of having a compromised airway should not be administered inhalation agents via a facemask without consideration of potential adverse consequences. Tracheal intubation to facilitate isoflurane administration then extubation prior to re-examination should be investigated in future studies for impact on laryngeal function.
      The induction dosages of alfaxalone and propofol produced comparable depths of anesthesia. Despite the longer plasma half-life of alfaxalone, there was no difference in the duration of LS-A between treatments. By contrast, the durations of LS-B and LS-C were shorter. Possibly, the dogs were at a lighter plane of anesthesia during LS-B and LS-C than during LS-A, which may also have contributed to the improved laryngeal motion variables compared with those obtained during LS-A. The shorter available time for LS-B and LS-C did not compromise the ability to perform assessment and recording of laryngeal function.
      Observations from the present study may apply to dogs where, after induction, show no-to-minimal arytenoid motion and when doxapram is either contraindicated or administered with little to no improvement (
      • Yost C.S.
      A new look at the respiratory stimulant doxapram.
      ). Physiologic effects of doxapram administered perioperatively are undefined in dogs (
      • Sullivan G.M.
      • Apergis J.
      • Gorman J.M.
      • LeDoux J.E.
      Rodent doxapram model of panic: behavioral effects and c-Fos immunoreactivity in the amygdala.
      ;
      • Gutman D.A.
      • Coplan J.
      • Papp L.
      • et al.
      Doxapram-induced panic attacks and cortisol elevation.
      ;
      • Preter M.
      • Klein D.F.
      Panic, suffocation false alarms, separation anxiety and endogenous opioids.
      ). A more practical period of 5–10 minutes on isoflurane may lead to similarly improved laryngeal motion; however, this is unknown.
      There were several limitations in this study. First, the protocol used relied on placement of the laryngoscope blade lightly on the epiglottis, exposing the RG for video laryngoscopy. It is possible that mechanical stimulation of the epiglottis by the laryngoscope blade reduced laryngeal motion in this study (
      • Mutoh T.
      • Kanamaru A.
      • Kojima K.
      • et al.
      Effects of volatile anesthetics on the activity of laryngeal ‘drive’ receptors in anesthetized dogs.
      ;
      • Labuscagne S.
      • Zeiler G.E.
      • Dzikiti B.T.
      Effects of chemical and mechanical stimulation on laryngeal motion during alfaxalone, thiopentone or propofol anaesthesia in healthy dogs.
      ). Second, the measurement portion of the probe was not exactly in plane with the RG, an estimated 0.5 cm rostral to the RG. This may have led to some magnification of RSGA-I and RSGA-E, but it is unlikely that this would have significantly influenced the results of this study. The % RGSA increase would not have been affected. Third, a treatment where dogs were only exposed to isoflurane (i.e., induction of anesthesia with isoflurane) was not included. This protocol was not pursued because it was unlikely to be used for an animal with suspected dysfunctional laryngeal function. Lastly, the influence of hypoventilation on laryngeal function could not be assessed because neither Pe′CO2 nor blood gases were measured.

      Conclusions

      Laryngeal function both objectively (RGSA-I and % RGSA increase) and subjectively improved from the first examination after induction of anesthesia with alfaxalone or propofol to following either 20 or 40 minutes of maintenance of anesthesia with isoflurane via facemask. Improved laryngeal function is probably multifactorial and partly the result of redistribution and clearance of the injectable anesthetics lessening respiratory system depression. Isoflurane may have less inhibitory effects on laryngeal function compared with either alfaxalone or propofol.

      Acknowledgements

      The authors thank Mal Hoover, certified medical illustrator, Veterinary Medical Library at Kansas State University, College of Veterinary Medicine, for her contributions in performing image measurements instrumental in preparing this manuscript. This research received no specific grant from funding agencies in the public, commercial or not-for-profit sectors.

      Authors’ contributions

      NK: study design, data collection and management, data interpretation, preparation of manuscript. RM: study design, data collection and interpretation, preparation of manuscript. DH: study design, data collection, preparation of manuscript. JR: statistical analysis, data interpretation, preparation of manuscript. KB and EK: data interpretation, preparation of manuscript. All authors read and approved the final version of the manuscript.

      Conflict of interest statement

      The authors declare no conflict of interest.

      Appendix A. Scoring arytenoid cartilage movements.

      Tabled 1
      ScoreDescription
      0All arytenoid cartilage movements are synchronous, staccato, and symmetrical and full arytenoid cartilage abduction can be achieved and maintained.
      1All arytenoid cartilage movements are synchronous and symmetrical. Full abduction of the arytenoid cartilages is not achieved.
      2Arytenoid cartilage movements are asynchronous and/or larynx is asymmetrical at times, but full arytenoid cartilages abduction can be achieved and maintained.
      3Complete immobility of the arytenoid cartilages and vocal fold.
      Reprinted from
      • Smalle T.M.
      • Hartman M.J.
      • Bester L.
      • et al.
      Effects of thiopentone, propofol and alfaxalone on laryngeal motion during oral laryngoscopy in healthy dogs.
      with permission. © Association of Veterinary Anaesthetists and American College of Veterinary Anesthesia and Analgesia. Published by Elsevier Ltd.

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