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. 2025 Jul 21;58(3):747–757. doi: 10.1111/evj.14557

Inspiratory and expiratory tracheal pressures during high‐intensity exercise in harness racehorses

Hanna Vermedal 1,✉, Ingunn Risnes Hellings 1, Zoe Louise Fretheim‐Kelly 1, Constanze Fintl 1, Hanna Margrethe Berg Olsen 1, Eric Strand 1
PMCID: PMC13041598  PMID: 40686083

Abstract

Background

Exercise‐related upper respiratory tract (URT) disorders are common in racehorses. Objective assessment of URT mechanics is essential to quantify degrees of obstruction caused by URT disorders identified upon dynamic endoscopy.

Objectives

To establish reference values for inspiratory and expiratory tracheal pressures (cmH2O) during high‐speed treadmill endoscopy in harness racehorses with clinically normal URTs.

Study Design

Prospective observational study.

Methods

Tracheal pressures were measured in harness racehorses (Standardbreds and Norwegian‐Swedish coldblooded trotters) in which no URT abnormalities were detected. Peak inspiratory and expiratory tracheal pressures were determined for each minute (phase) of a standardised treadmill test, which alternated between trotting with free head carriage (phases 1, 3, 5 and 7) and with poll flexion (phases 2, 4 and 6). Linear mixed‐effects models assessed changes in tracheal pressures across the exercise test, and effects of breed, racing experience, respiratory rate and head–neck position (free vs. poll flexion).

Results

Seventy‐six horses were included. Mean (SD) peak inspiratory tracheal pressures ranged from −21.8 (5.0) cmH2O in phase 1 to −34.9 (5.3) and −34.3 (5.7) cmH2O in phases 6 and 7. Inspiratory pressures became significantly more negative across phases (p < 0.001) and were −3.5 cmH2O (95% CI: −4.0 to −3.0, p < 0.001) lower during poll flexion versus free head carriage. Expiratory tracheal pressures remained stable across exercise phases (11.5 [2.8] to 12.5 [2.6] cmH2O) with no significant changes. There were no significant differences between the two breeds. Respiratory rate ranged from 79.8 (12.6) to 90.8 (15.0) breaths/min and remained stable between 89.6 and 90.8 breaths/min through phases 3–7.

Main Limitations

Simultaneous airflow measurements were not performed.

Conclusions

Inspiratory pressures became more negative with exercise progression and were significantly lower during poll flexion versus free head carriage. Reference values for tracheal pressures from clinically normal horses provide an objective tool for evaluating URT function during exercise.

Keywords: airway pressures, horse, treadmill endoscopy, upper respiratory tract

1. INTRODUCTION

Horses are unique in their cardiovascular capacities and respiratory anatomy, which contributes to making the respiratory system, as opposed to the cardiovascular system, the limiting factor to high‐intensity exercise. 1 , 2 , 3 Exercise‐related upper respiratory tract (URT) disorders are a common cause of poor performance in athletically active horses such as racehorses. 4 , 5 , 6 , 7 Diagnosis of exercise‐related URT disorders/abnormalities requires overground or high‐speed treadmill endoscopy (HSTE). 8 , 9 , 10 , 11 Use of HSTE allows for standardised exercise protocols which makes the examination replicable, decreases influence from outside factors and ensures a state of fatigue where exercise‐related URT disorders become apparent. 4 , 6 , 7 , 12 Traditionally HSTE has been performed with horses exercising with a natural or free head carriage. 4 , 6 , 13 In harness racing, the driver uses long reins to regulate the horse's speed, helping to prevent premature fatigue or breaking into a gallop during a race. When tension is applied to the reins and transferred through the bit, the horse is said to be driven ‘on the bit’. This action typically results in varying degrees of poll flexion, characterised by a reduced angle between the head and neck. 14 As several exercise‐related URT disorders in these horses are directly associated with poll flexion 15 , 16 , 17 and the prevalence of complex URT disorders occurring during poll flexion is high, 7 , 18 , 19 , 20 HSTE in harness racehorses should include periods with poll flexion. This is achieved by actively driving the horses on the treadmill using normal racing tack including bit and long reins.

Exercise‐related URT disorders may (or may not) cause an increase in resistance to airflow and thereby may (or may not) cause clinical signs such as poor performance and/or abnormal respiratory noise. 11 , 21 , 22 While HSTE allows visual diagnosis and subjective grading of URT disorders occurring during exercise, it does not allow for quantification of the respiratory impact or degree of obstruction caused by any given URT abnormality. Objective measurements of URT mechanics are necessary to quantify this, and measuring upper airway pressures is one such method. 21 , 22 , 23 Tracheal and/or nasopharyngeal pressures can be measured simultaneously with HSTE, which offers valuable information regarding the degree and inspiratory or expiratory characteristics of any obstruction present. 22 , 24 , 25 , 26 Measuring tracheal pressures via nasotracheal placement of pressure sensors is a minimally invasive, simple and reliable method of evaluating a horse's upper airway function in a clinical setting. 21 , 25 , 27 , 28 , 29 Upper airway pressure measurements also provide an objective scale to which the effect of any conservative and/or surgical intervention can be compared. 25 , 27 Values for inspiratory and expiratory tracheal pressures in exercising horses have been reported in experimental studies; however, direct comparison of these values is complicated by lack of standardisation. 21 , 27 , 28 , 30 , 31

The primary aim of this study was to establish reference values for inspiratory and expiratory tracheal pressures in harness racehorses (Standardbreds [STBs] and Norwegian‐Swedish coldblooded trotters [NSCTs]) with clinically normal URTs during high‐intensity exercise, both during periods with free head carriage and with poll flexion. Secondary aims were to explore whether tracheal pressures differ between the two breeds, between different levels of racing experience, and whether respiratory rate or head–neck position affected the tracheal pressures measured.

2. MATERIALS AND METHODS

2.1. Study population

The study population consisted of a convenience sample of harness racehorses (STBs and NSCTs) undergoing a standardised HSTE protocol 7 at the Norwegian University of Life Sciences between 2018 and 2024. These client‐owned horses presented either for racing fitness evaluation or evaluation of poor performance and/or suspected URT abnormalities. Harness racehorses were eligible for inclusion in the study if they had no URT abnormalities detected during HSTE examination. Exclusion criteria included previous URT surgeries, presence of obvious lameness during the examination, failure to follow the treadmill protocol (i.e., galloping) and presence of clinically significant electrocardiogram (ECG) abnormalities during or immediately following the HSTE examination. All ECG recordings from the exercise test and the immediate post‐exercise period were reviewed by board‐certified internal medicine specialists on an individual basis. Horses were also excluded from the study if there was equipment malfunction.

2.2. High‐speed treadmill exercise protocol

All horses were exercised on a high‐speed treadmill using a standardised exercise protocol developed specifically for URT evaluation of harness racehorses. 7 Figure 1 illustrates the layout of the treadmill protocol. Prior to the exercise test, horses were equipped with a light harness, standard snaffle bit, bridle, and conventional overcheck, as well as an ECG telemetry system (Televet, Engel Engineering Services). Horses were then warmed up on the treadmill with incremental increases in speed for approximately 2500–3000 m (about 5–6 min). Horses were not acclimatised to the treadmill prior to the warm‐up period. Following the warm‐up period, the treadmill was stopped, and long reins were equipped to the bridle and harness. A videoendoscope (Karl Stortz Endoscopes AS) was passed via the right ventral meatus and secured to the bridle to ensure visualisation of the larynx and caudal nasopharynx. A tracheal pressure sensor was then passed, also via the right ventral meatus (see more detailed description below). Videoendoscopic recordings were evaluated in real time throughout the exercise test and reviewed in slow motion post‐exercise.

FIGURE 1.

FIGURE 1

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Layout of the standardised treadmill protocol/exercise test used to evaluate the upper respiratory tract of harness racehorses. The protocol consists of a warm‐up phase, an instrumentation period, and the 7‐min exercise test. Treadmill incline (red line, left y‐axis) increases to 3% before the start of the exercise test and remains constant thereafter. The blue dashed line illustrates a typical heart rate trend (right y‐axis). The exercise test consists of consecutive 1‐min phases where the alternating background colours indicate head–neck position during each phase, which alternates between free head carriage (green) and poll flexion (orange).

Following instrumentation, increasing the treadmill incline to 3% and placing an experienced driver behind the horse to handle the long reins, the single step high‐speed exercise test would start. The STBs were exercised at 9 m/s and the NSCTs at 8.5 m/s. The full 7‐min exercise test consisted of consecutive 1‐min phases, alternating between 1 min with free head carriage (phases 1, 3, 5 and 7) and 1 min with poll flexion (phases 2, 4 and 6). Poll flexion was achieved by the driver applying tension on the long reins and bit while encouraging the horse forward. These 1‐min phases alternated until the horse fatigued and could no longer maintain a steady trotting gait, at which point the exercise test was stopped. Most harness racehorses undergoing this treadmill protocol at our facility fatigue before the full 7 min of the exercise test. Horses typically achieve a HR >200 bpm within the first minute and reach HRmax after 3–4 min.

2.3. Tracheal pressure measurements

A pressure sensor (Millar Mikro‐Cath™, Millar Inc.) was inserted into a 110 cm long, 4 mm OD polyethylene tubing (Baxter International Inc.) with a sealed end and 10 small, perforated holes in the distal 8 cm. The pressure sensor was secured to the tubing with the sensor tip level with the fourth hole from the sealed end. Using videoendoscopic guidance, the tubing containing the pressure sensor was passed nasotracheally and secured to the bridle with the sensor tip approximately 30 cm into the proximal trachea. The pressure sensor was connected via a control unit (PCU‐2000 Pressure Control Unit, Millar Inc.) to a recording unit (PowerLab 8/35, ADInstruments). The recording unit was connected to a laptop computer using LabChart Pro v8 software (ADInstruments) to continuously record and analyse the pressure tracings. A two‐point calibration was performed immediately prior to measurement in all cases by first zeroing the sensor to atmospheric pressure (0 cmH2O), followed by calibration at 100 mmHg (equivalent to 136 cmH2O). After data collection, the baseline pressure reading was re‐checked to confirm that the sensor remained properly zeroed at atmospheric pressure. This post‐measurement check ensured that no pressure drift occurred over the course of the recording and that pressure tracings remained stable and accurate.

Peak inspiratory and expiratory tracheal pressures were determined for each consecutive 1‐min phase of the exercise test by averaging peak pressure tracings of 10 consecutive breaths during the last 15 s of the phase (Peak Analysis LabChart Pro Module, ADInstruments). Respiratory rate (breaths/min) was determined for each phase of the exercise test using pressure tracings from the last 15 s of the phase.

2.4. Data analysis

Data were analysed using linear mixed‐effects models. Two models were created, one using inspiratory tracheal pressures and one using expiratory tracheal pressures as the outcome variables. In each model, an ordinal categorical variable for ‘phase number’ (phase 1–7) was included as a fixed effect to allow for non‐linear trends across phases. ‘Breed’ (STB or NSCT), ‘respiratory rate’ (breaths/min, centred) as a continuous covariate and ‘experience’ as a categorical variable were also included as fixed effects. Horses were classified into one of three ‘experience’ groups according to racing experience: ‘unraced’ (horses that had never started an official race), ‘raced’ (horses with racing experience) and ‘performing well’ (horses that had raced and achieved racing speed marks <1:15 min/km for STBs or <1:30 min/km for NSCTs). As the outcome variables involved repeated measures within individual horses, variability in baseline pressures was accounted for by including horse as a random effect. Random slopes for phase number (as a numeric variable) were also included to account for individual variability in pressure responses across phases. To investigate the specific effect of head–neck position (free or poll flexion) on inspiratory and expiratory tracheal pressures, a categorical variable ‘head–neck position’ was created with two levels: ‘free’ (phases 1, 3, 5 and 7) and ‘flexed’ (phases 2, 4 and 6). Due to collinearity between ‘phase number’ and ‘head–neck position’ (which alternated across phases), an alternative model was fit using ‘head–neck position’ and phase number as a numeric variable representing the continuous progression of phases.

The distribution of continuous outcome and predictor variables was assessed using histograms. All variables were evaluated in a univariable analysis using the same random effects structure as in the main models. Scatter plots were generated between each predictor and outcome variable to assess the shape of their relationships. Variables showing a non‐linear relationship were also modelled as a quadratic term, and the form (linear vs. quadratic) which provided the best model fit was retained. Models were fit using restricted maximum likelihood (REML) estimation. Model selection was guided by the Akaike Information Criterion (AIC) and likelihood ratio tests. Lower AIC values indicate better model fit while accounting for model complexity. Nested models were compared using likelihood ratio tests with maximum likelihood (ML) estimation to evaluate the significance of additional parameters and two‐way interactions. Model assumptions were checked using Q‐Q plots of residuals and plots of residuals versus fitted values. All analyses were performed using R Statistical Software (v4.4.2; R Core Team 2024) 32 and the lme4, 33 lmerTest, 34 dplyr, 35 ggplot2, 36 ggpubr 37 and broom.mixed 38 packages. Model performance was evaluated using marginal R2 (variance explained by fixed effects) and conditional R2 (variance explained by both fixed and random effects), calculated using the MuMIn 39 package. Fixed effect estimates were reported as regression coefficients with corresponding 95% confidence intervals (95% CI). Pairwise comparisons were conducted on estimated marginal means obtained using the emmeans 40 package, with adjustments for multiple comparisons made using Tukey's method. Statistical significance was set at p < 0.05.

3. RESULTS

3.1. Descriptive results

A total of 333 harness racehorses underwent the standardised HSTE exercise test during the study period. Of these, 123 horses had no URT abnormalities detected during examination and were eligible for inclusion in the study. A flow chart of case inclusion/exclusion is displayed in Figure 2. Forty‐seven horses were excluded due to clinically significant ECG abnormalities (i.e., atrial fibrillation, complex ventricular arrythmias), equipment malfunction (i.e., drifting of pressure tracings), obvious lameness, not following protocol (i.e., galloping/pacing, requiring slower speeds) or previous URT surgeries (i.e., laryngoplasty, laser procedures, alar fold resections). The remaining 76 horses were included in the study: 32 STBs and 44 NSCTs.

FIGURE 2.

FIGURE 2

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Flow chart of the study population detailing exclusion of horses from the study. NSCT, Norwegian‐Swedish Coldblooded trotter; STB, Standardbred; URT, upper respiratory tract.

All horses included in the study tolerated the tracheal pressure measurements well. Mean (SD) age for the study population at time of examination was 5.0 (2.1) years (STBs: 4.6 [1.5] years, NSCTs: 5.3 [2.4] years). According to previous racing experience, 31.6% (n = 24) were classified as unraced (STBs: 25% [n = 8], NSCTs: 36.4% [n = 16]), 25% (n = 19) as having some racing experience (STBs: 21.9% [n = 7], NSCTs: 27.3% [n = 12]) and 43.4% (n = 33) racehorses were classified as performing well (STBs: 53.1% [n = 17], NSCTs: 36.4% [n = 16]).

All 76 horses were able to complete the first 4 phases/min of the exercise test. Seventy horses (92.1%) completed 5 phases (STBs: 29/32, NSCTs: 41/44), 51 horses (67.1%) completed 6 phases (STBs: 24/32, NSCTs: 27/44) and 30 horses (39.5%) completed all 7 phases/min of the exercise test (STBs: 17/32, NSCTs: 13/44).

Table 1 summarises overall mean (SD) peak inspiratory and expiratory tracheal pressures along with respiratory rate for each phase/min of the exercise test. Values stratified by breed and racing experience are displayed in Table S1. Peak inspiratory tracheal pressures ranged from −21.8 (5.0) cmH2O in phase 1 to −34.9 (5.3) and −34.3 (5.7) cmH2O in phase 6 and phase 7, respectively. Peak expiratory tracheal pressures ranged from 11.5 (2.8) cmH2O in phase 1 to 12.5 (2.6) and 12.4 (3.0) cmH2O in phase 6 and phase 7, respectively. Mean respiratory rate ranged from 79.8 (12.6) to 90.8 (15.0) breaths/min and remained stable between 89.6 and 90.8 breaths/min through phases 3 to 7.

TABLE 1.

Mean (SD) peak inspiratory and expiratory tracheal pressures (cmH2O) and mean (SD) respiratory rate (breaths/min) for each phase/min of the treadmill exercise test.

Phase 1 Phase 2 Phase 3 Phase 4 Phase 5 Phase 6 Phase 7
Free Flexed Free Flexed Free Flexed Free
Nr included n = 76 n = 76 n = 76 n = 76 n = 70 n = 51 n = 30
Inspiratory pressures −21.8 (5.0) −28.3 (5.6) −27.7 (5.5) −32.4 (5.3) −31.1 (5.4) −34.9 (5.3) −34.3 (5.7)
Expiratory pressures 11.5 (2.8) 11.7 (2.5) 11.6 (2.6) 12.2 (2.4) 11.8 (2.5) 12.5 (2.6) 12.4 (3.0)
Respiratory rate 79.8 (12.6) 85.9 (14.6) 89.8 (15.5) 89.9 (15.1) 90.6 (15.1) 90.8 (15.0) 89.6 (15.3)
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Note: Head–neck position alternated between free head carriage (phases 1, 3, 5 and 7) and poll flexion (phases 2, 4 and 6).

3.2. Linear mixed‐effects models

The results of the linear mixed‐effects models for inspiratory and expiratory tracheal pressures are displayed in Tables 2 and 3, respectively. The univariable results are displayed in Table S2.

TABLE 2.

Fixed effect estimates of the linear mixed model analysis with inspiratory tracheal pressures as the outcome variable.

Estimate SE 95% CI lower 95% CI upper p‐value
Intercept −21.3 1.83 −24.75 −17.81 <0.001
Phase nr Phase 1 Referent
Phase 2 −6.0 1.07 −7.97 −3.97 <0.001
Phase 3 −4.6 1.22 −6.91 −2.36 <0.001
Phase 4 −9.2 1.40 −11.84 −6.60 <0.001
Phase 5 −6.9 1.74 −10.17 −3.64 <0.001
Phase 6 −12.4 2.08 −16.30 −8.46 <0.001
Phase 7 −5.1 3.26 −11.17 1.10 0.1
Breed STB Referent
NSCT 1.9 2.24 −2.36 6.16 0.4
Experience Unraced Referent
Raced 1.2 2.68 −3.92 6.25 0.66
Performing well −2.8 2.22 −6.97 1.45 0.22
Resp. rate 0.05 0.02 0.01 0.09 0.02
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Note: Number of observations: 455. The model included a random intercept for horse (variance ± SD: 26.4 ± 5.1) and a random slope for phase number (0.7 ± 0.8), with a correlation of −0.5 between the random intercept and slope.

Abbreviations: NSCT, Norwegian‐Swedish Coldblooded trotter; STB, Standardbred.

TABLE 3.

Fixed effect estimates of the linear mixed model analysis with expiratory tracheal pressures as the outcome variable.

Estimate SE 95% CI lower 95% CI upper p‐value
Intercept 11.4 0.81 9.86 12.98 <0.001
Phase nr Phase 1 Referent
Phase 2 0.1 0.23 −0.32 0.58 0.6
Phase 3 −0.1 0.26 −0.59 0.44 0.8
Phase 4 0.5 0.29 −0.07 1.06 0.1
Phase 5 0.1 0.33 −0.53 0.77 0.8
Phase 6 0.6 0.39 −0.16 1.35 0.1
Phase 7 0.5 0.47 −0.43 1.43 0.31
Breed STB Referent
NSCT −0.4 0.96 −2.21 1.52 0.7
Experience Unraced Referent
Raced −0.1 1.15 −2.28 2.08 0.93
Performing well 0.7 0.95 −1.18 2.49 0.49
Resp. rate 0.02 0.01 −0.01 0.04 0.12
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Note: Number of observations: 455. The model included a random intercept for horse (variance ± SD: 7.3 ± 2.7) and a random slope for phase number (0.2 ± 0.4), with a correlation of −0.63 between the random intercept and slope.

Abbreviations: NSCT, Norwegian‐Swedish Coldblooded trotter; STB, Standardbred.

Inspiratory tracheal pressures became significantly more negative during phases 2 through 6, relative to phase 1 (Table 2). This decrease in inspiratory pressures was of greatest magnitude in phase 4 with a −9.2 cmH2O change (95% CI: −11.8 to −6.6, p < 0.001) and phase 6 with a −12.4 cmH2O change (95% CI: −16.3 to −8.5, p < 0.001). In both these phases, along with phase 2, horses were exercising with poll flexion. Estimated marginal means of inspiratory tracheal pressures across the exercise test phases are shown in Figure 3A. Neither breed (NSCT vs. STB) nor racing experience (raced or performing well vs. unraced) had significant main effects on inspiratory tracheal pressures. For respiratory rate, a small but statistically significant increase (less negative) in inspiratory tracheal pressure of 0.05 cmH2O (95% CI: 0.01 to 0.09, p = 0.02) was associated with each additional breath recorded.

FIGURE 3.

FIGURE 3

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Estimated tracheal pressures (cmH2O) across the phases of the exercise test for inspiratory tracheal pressures (A) and expiratory tracheal pressures (B). Values represent estimated marginal means with 95% confidence intervals (error bars). Head–neck position alternated between free head carriage (phases 1, 3, 5 and 7) and poll flexion (phases 2, 4 and 6).

Including random slopes for individual horses significantly improved model fit for inspiratory tracheal pressures (∆AIC = 64; X 2 [2] = 75.3, p < 0.001). The random effect's structure revealed considerable between‐patient variability in baseline inspiratory tracheal pressures (SD = 5.1 cmH2O) (Table 2), as well as variability in how inspiratory tracheal pressure changes across phases (SD = 0.8 cmH2O). The inspiratory model explained 91.5% of the variance when including random effects (R2c = 0.915) and 44.3% of the variance through fixed effects alone (R2m = 0.443).

For expiratory tracheal pressures, the change in pressure across exercise phases was less substantial (0.1 cmH2O [95% CI: −0.3 to 0.6] to 0.6 cmH2O [95% CI: −0.2 to 1.4] difference relative to phase 1) compared with inspiratory tracheal pressures, and neither phase number, breed, racing experience, nor respiratory rate significantly influenced expiratory tracheal pressures (Table 3). Estimated marginal means of expiratory tracheal pressures across the exercise test phases are shown in Figure 3B.

Including random slopes for individual horses significantly improved the expiratory model fit (∆AIC = 44; X 2 [2] = 48.5, p < 0.001). The model for expiratory tracheal pressures explained 4.8% of the variance through fixed effects alone (R2m = 0.048) and 76.2% of the variance when including random effects (R2c = 0.762), indicating that the predictor variables had little effect on how expiratory tracheal pressures change during the exercise test, and that the small changes in expiratory pressures across phases were mostly due to individual variation between horses. This is reflected by the random effects indicating substantial individual variability in baseline expiratory pressures (SD = 2.7 cmH2O) (Table 3), and moderate variability in how expiratory pressures change across phases (SD = 0.4 cmH2O).

The alternative model investigating the specific effects of head–neck position (free or poll flexion) on inspiratory and expiratory tracheal pressures allowed for independent estimation of both the general trend in tracheal pressures across the phases of the exercise test and the deviation from that trend associated with head–neck position. Table 4 displays the Type III ANOVA results from the alternative models. There was a significant main effect of head–neck position on inspiratory tracheal pressures (F[1, 297.3] = 69.0, p < 0.001), but not on expiratory tracheal pressures (F[1, 305.1] = 0.03, p = 0.87). Likewise, phase number had a strong effect on inspiratory tracheal pressures but not on expiratory pressures. Breed and respiratory rate showed no significant effects in either model, while racing experience had a marginal main effect on inspiratory tracheal pressures. Post hoc pairwise comparisons (Table 5) confirmed that horses consistently exhibited more negative inspiratory tracheal pressures during poll flexion compared with free head carriage, both overall and across all racing experience groups. On average, inspiratory tracheal pressures were −3.5 cmH2O lower (more negative) during phases with poll flexion (95% CI: −4.0 to −3.0, p < 0.001) compared with free head carriage. This effect remained significant within each racing experience group: unraced (−3.3 cmH2O), raced (−3.9 cmH2O), and performing well (−3.3 cmH2O), all p < 0.001.

TABLE 4.

ANOVA table (Type III sums of squares) of the linear mixed‐effect models evaluating the effect of head–neck position (free vs. poll flexion) on inspiratory and expiratory tracheal pressures (cmH2O).

Inspiratory pressures Expiratory pressures
F‐value df p‐value F‐value df p‐value
Phase Nr 188.3 1, 91.1 <0.001 0.6 1, 91.5 0.4
Head–neck position 69.0 1, 297.3 <0.001 0.03 1, 305.1 0.9
Breed 0.2 1, 75.2 0.62 0.7 1, 77.1 0.4
Experience 3.0 2, 74.3 0.05 0.05 2, 76.1 >0.9
Respiratory rate 0.03 1, 401.2 0.86 1.8 1, 352.7 0.2
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Note: Number of observations: 455. ANOVA results are based on models fit with REML and Type III sums of squares with Satterthwaite's approximation for degrees of freedom.

TABLE 5.

Pairwise comparisons of the effect of head–neck position (free vs. poll flexion) on inspiratory tracheal pressures (cmH2O).

Head–neck position Contrast Estimate [95% CI] SE df t‐value p‐value
Overall Poll flexion—Free −3.5 [−4.0, −3.0] 0.24 299 −14.68 <0.001
Experience Unraced Poll flexion—Free −3.3 [−4.2, −2.4] 0.46 303 −7.15 <0.001
Raced Poll flexion—Free −3.9 [−4.8, −3.1] 0.45 297 −8.77 <0.001
Performing well Poll flexion—Free −3.3 [−3.9, −2.7] 0.32 296 −10.26 <0.001
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Note: Results are averaged over levels of breed and racing experience for the overall comparison and over breed for the stratified analyses. Degrees of freedom were calculated using Kenward‐Roger approximation.

4. DISCUSSION

This study reports reference values for inspiratory and expiratory tracheal pressures during high‐intensity exercise in clinically normal harness racehorses. This includes reference values for each min/phase of a 7‐min‐long standardised treadmill test, including periods with and without poll flexion.

Expiratory tracheal pressures remained relatively stable throughout the exercise test, and the slight variation seen across exercise phases was mostly due to individual differences between horses. Inspiratory tracheal pressures became significantly more negative across the exercise test phases relative to phase 1. This decline was non‐linear, with the steepest decreases observed during the first half of the exercise test and during phases with poll flexion. There were no significant differences between the two breeds in inspiratory or expiratory tracheal pressures during the exercise test. Most horses undergoing the current exercise test reach HRmax after 3–4 min and remain around HRmax until they fatigue and no longer can maintain a steady trotting gait on the inclined treadmill. When reviewing both raw means and estimated marginal means, inspiratory tracheal pressures tend to plateau between phases 4 and 7 of the exercise test. This suggests that inspiratory tracheal pressures stabilise over time in clinically normal harness racehorses without URT abnormalities, findings which have also been reported in top‐performing NSCTs in a different study. 28 Ducharme and colleagues have demonstrated using both high‐speed and incremental exercise tests on galloping horses that inspiratory pressures become more negative at higher speeds. 21 Directly comparing these results to our population of trotting horses with a single step high‐speed treadmill test is difficult; however, the time factor is similar; those results were obtained after 3–4 min of treadmill exercise similar to how we see the greatest decreases in inspiratory tracheal pressures in the first 3–4 phases/min of our treadmill test.

The majority of horses in the current study were able to complete the first 4–5 phases/min of the exercise test; however, 60% of the horses fatigued before the full 7 min of the test. A higher proportion of horses classified as ‘performing well’ were able to complete all 7 phases, and ‘unraced’ horses fatigued quickest (Table S1). There was a higher proportion of STBs classified as ‘performing well’ compared with NSCTs, which reflects the general population of harness racehorses at out facility. In official trotting races, STBs performing well generally have racing speeds between 1:10 and 1:15 min/km (<2:00 min/mile) and NSCTs performing well have racing speeds between 1:20 and 1:30 min/km (<2:25 min/mile) over distances of 1600–2100 m. According to previous racing experience, racehorses in the current study having achieved racing speeds <1:15 min/km (STBs) or <1:30 min/km (NSCTs) were classified as ‘performing well’.

Poll flexion in horses during exercise is known to alter the conformation/diameter of the URT 41 , 42 , 43 and to cause a mild increase in resistance to airflow, as demonstrated in two other studies by inspiratory tracheal pressures becoming more negative during poll flexion compared with free head carriage. 14 , 28 This finding was confirmed in the current study, where inspiratory tracheal pressures were significantly more negative in phases with poll flexion (phases 2, 4 and 6). The degree of poll flexion was assessed subjectively during the exercise test and varies naturally between horses. Some horses adopt a substantial and relatively ‘high’ poll flexion, whereas others show smaller changes in head–neck angle when being driven ‘on the bit’. This individual variation in degree of poll flexion will likely be reflected by the inspiratory tracheal pressures measured; horses with greater degrees of flexion will likely have greater changes in pressure between phases, while horses with lesser degrees of poll flexion likely will have smaller changes in pressure between phases. As the exercise protocol used in the current study was designed to evaluate the URT of clinical patients, including being able to diagnose URT disorders only evident during poll flexion, it was important that horses were examined with the degree of poll flexion they would naturally attain while being driven ‘on the bit’ during racing and training. Therefore, standardisation of poll flexion angle was not performed. Moreover, capturing this natural variation in our reference values make them more appropriate to use when comparing them to values from horses with different URT disorders (that inevitable also will adopt differing angles of poll flexion).

Previous studies on upper airway pressures in horses have included a variety of different exercise protocols/speeds, different breeds/gaits and different experimental designs which complicate comparison with values obtained in the current study. 14 , 21 , 26 , 30 , 44 , 45 Examples are simultaneous use of facemasks for airflow measurements and the use of research horses that are trained on the treadmill in a period immediately prior to study execution, as opposed to race‐trained or race‐active horses. The majority of those studies also included horses galloping on the treadmill as opposed to trotting.

To accommodate the increased demands for gas exchange during exercise, minute ventilation (product of respiratory rate and tidal volume) increases exponentially in horses from less than 100 L/min at rest to over 1800 L/min at high‐intensity exercise. 3 , 44 , 46 , 47 At slower paces, it is assumed that increased respiratory rate is the major contributor to increased minute ventilation, while tidal volume is the major contributor to increased minute ventilation during canter and gallop, as horses couple their respiration to stride frequency in a 1:1 fashion. 2 , 3 , 46 , 48 In trotting horses, this locomotion to respiration coupling (LRC) is not obligatory, and studies have shown that good performers have significantly lower respiratory rates and higher LRC ratios compared with poor performers. 49 In the current study, overall respiratory rate ranged from 80 to 91 breaths/min across all phases and remained fairly stable around 90 breaths/min from phase 3 to phase 7. When stratified by racing experience, unraced horses had higher respiratory rates than horses in the raced or performing well groups (Table S1), but the significance of this observation was not investigated as it extended beyond the scope of this study. Respiratory rates were also higher overall for STBs than NSCTs (Table S1), which could be explained by a ratio of LRC combined with the faster speed for STBs. Although respiratory rate showed a statistically significant association with inspiratory tracheal pressures, the estimated effect was small in magnitude. For every one‐unit increase in respiratory rate, inspiratory tracheal pressures increased by 0.05 cmH2O. A more substantial increase in respiratory rate of 10 breaths/min would result in an increase in inspiratory pressures of 0.5 cmH2O, which is a small effect compared with the larger pressure differences observed across exercise phases or during poll flexion versus free head carriage.

Some URT disorders occurring during exercise, such as laryngeal hemiplegia, cause significant impairment of ventilation as they substantially increase the upper airway resistance to airflow. 27 , 44 , 50 With the wide range of exercise‐related URT disorders seen in horses, 6 , 7 , 8 , 18 some disorders may cause lower or insignificant increases in URT resistance and thereby may not need to be addressed as a poor performance issue. Upper airway resistance is a measure of the opposition to airflow through the URT, typically defined as the ratio of peak upper airway pressure difference across the URT to peak airflow rates during respiration. 29 Changes in resistance to airflow can thereby be calculated by measuring upper airway pressures and airflow rates. 14 , 44 Measurement of airflow rates requires the use of facemasks around the horse's nares and mouth, and calculating changes in resistance to airflow during exercise is time‐consuming, making airflow measurements clinically inconvenient and more suitable in experimental settings. 2 , 21 , 22 , 44 As tracheal pressure measurements can be obtained non‐invasively in horses undergoing HSTE on an out‐patient basis, it represents a simple and feasible measure of upper airway function in horses presented for evaluation of suspected URT problems. 21 , 25 , 28 , 29 Nevertheless, upper airway pressures can be influenced by airflow rates, patterns and changes in respiratory rate and tidal volume, in addition to airway resistance, and this must be considered a limitation when interpreting tracheal pressures in exercising horses without simultaneous airflow measurements. 22 , 29 , 44 , 51

Other limitations of the current study include the sample size, which was limited to clinically normal harness racehorses undergoing client‐paid treadmill endoscopy examinations at our facility, that routine bronchoalveolar lavage was not conducted, and that not all horses completed the full 7 min of the exercise test. The latter is also a strength of the standardised treadmill protocol, as it ensures a level of fatigue where exercise‐related URT disorders of racehorses become apparent.

In conclusion, we successfully established reference values for inspiratory and expiratory tracheal pressures in active harness racehorses with clinically normal URTs. As there were no significant differences in inspiratory or expiratory tracheal pressures between the two breeds, we established unified reference values encompassing both breeds. Reference values are presented for each phase/min of the exercise test as inspiratory tracheal pressures are significantly affected by time/phase number (exercise duration) and head–neck position (free versus poll flexion). Using tracheal pressure measurements as part of a standardised exercise protocol provides an objective scale that may help decide whether an URT abnormality warrants intervention (surgical or conservative) on an individual basis. It may also allow for an objective, numerical evaluation of the success of any surgical/conservative intervention, as opposed to evaluation of postoperative racing records which have been the historical standard in racehorses. 25 , 52 , 53 , 54 , 55 Future studies on tracheal pressure measurements in harness racehorses should investigate pressure values obtained in horses with different exercise‐related URT disorders. That will allow us to grade different disorders according to severity of inspiratory or expiratory obstruction, relative to the reference values obtained in the current study.

FUNDING INFORMATION

This study was supported by the Norwegian‐Swedish Foundation of Equine Research and the Norwegian Research Council.

CONFLICT OF INTEREST STATEMENT

The authors declare no conflicts of interest.

AUTHOR CONTRIBUTIONS

Hanna Vermedal: Investigation; writing – original draft; methodology; visualization; writing – review and editing; software; formal analysis; conceptualization; data curation; resources. Ingunn Risnes Hellings: Writing – review and editing; investigation. Zoe Louise Fretheim‐Kelly: Conceptualization; investigation; data curation; validation; methodology; funding acquisition; resources. Constanze Fintl: Investigation; resources. Hanna Margrethe Berg Olsen: Investigation. Eric Strand: Conceptualization; investigation; funding acquisition; writing – review and editing; methodology; project administration; supervision; validation; resources.

DATA INTEGRITY STATEMENT

Hanna Vermedal and Eric Strand had full access to all the data in the study and take responsibility for the integrity of the data and the accuracy of the data analysis.

ETHICAL ANIMAL RESEARCH

Research ethics committee oversight not currently required by this journal: procedures were performed as part of clinical investigation.

INFORMED CONSENT

Owners/trainers were made aware that case information may be used for research in general.

Supporting information

Table S1. Mean (SD) peak inspiratory and expiratory tracheal pressures (cmH2O) and mean (SD) respiratory rate (breaths/min) for each phase/min of the exercise test, stratified by breed and racing experience.

EVJ-58-747-s001.pdf (196.9KB, pdf)

Table S2. Univariable results for inspiratory and expiratory tracheal pressure mixed models.

EVJ-58-747-s002.pdf (168.2KB, pdf)

ACKNOWLEDGEMENTS

We thank the equine treadmill team at the Norwegian University of Life Sciences for help driving the horses during the treadmill protocol: Marius Holm, Jessica Gunnulfsen, Diana Olsson and Karoline Solberg.

DATA AVAILABILITY STATEMENT

The data that support the findings of this study are openly available at https://data.mendeley.com/datasets/bftzzrftst/1.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

Table S1. Mean (SD) peak inspiratory and expiratory tracheal pressures (cmH2O) and mean (SD) respiratory rate (breaths/min) for each phase/min of the exercise test, stratified by breed and racing experience.

EVJ-58-747-s001.pdf (196.9KB, pdf)

Table S2. Univariable results for inspiratory and expiratory tracheal pressure mixed models.

EVJ-58-747-s002.pdf (168.2KB, pdf)

Data Availability Statement

The data that support the findings of this study are openly available at https://data.mendeley.com/datasets/bftzzrftst/1.

Articles from Equine Veterinary Journal are provided here courtesy of Wiley

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