Failure of subtotal arytenoidectomy to improve upper airway flow mechanics in exercising standardbreds with induced laryngeal hemiplegia

Upper airway flow mechanics and arterial blood gas measurements were used to assess the efficacy of subtotal arytenoidectomy for treatment of induced left laryngeal hemiplegia in horses. Measurements were collected with the horses at rest, and trotting or pacing on a treadmill (6.38 degrees incline) at speeds of 4.2 and 7.0 m/s. Experimental protocols were performed after right common carotid artery exteriorization (baseline), after left recurrent laryngeal neurectomy (LRLN), and after left subtotal arytenoidectomy. At baseline, increasing treadmill speed progressively increased peak inspiratory and expiratory flow (VImax and VEmax, respectively), peak inspiratory and expiratory transupper airway pressure (PuI and PuE, respectively), respiratory frequency (f), tidal volume (VT), minute volume (VE), and heart rate. Inspiratory and expiratory times (TI and TE, respectively) and arterial oxygen tension (PaO2) decreased with increased treadmill speed; inspiratory and expiratory impedance (ZI and ZE, respectively) did not change. After LRLN, VImax, f, and PaO2 significantly (P less than 0.05) decreased at exercise, whereas PuI, TI, and ZI significantly increased. Minute volume decreased at exercise after LRLN, but the changes were not significant; LRLN had no effect on VEmax, PuE, ZE, heart rate, arterial carbon dioxide tension (PaCO2), or VT. Subtotal arytenoidectomy did not improve upper airway flow mechanics or blood gas measurements impaired by laryngeal hemiplegia.     

Effect of frusemide on transvascular fluid fluxes across the lung in exercising horses

Reasons for performing study: Frusemide (Fru) is widely prescribed for management of racehorses experiencing EIPH. The effect of Fru in the lung appears to be a reduction in transcapillary pressures and inhibition of the erythrocyte anion exchange, which may lead to attenuation of transpulmonary fluid fluxes during exercise. Hypothesis: Treatment with Fru will attenuate transpulmonary fluid fluxes in horses during high intensity exercise. Methods: In a crossover study, 6 race‐fit Standardbred horses were treated with 250 mg of Fru i.v. (FruTr) or placebo (Con) 4 h before exercise on a high speed treadmill until fatigue. Arterial and central mixed venous blood, as well as CO₂ elimination and O₂ uptake, were sampled. Volume changes across the lung and transvascular fluid fluxes were calculated from changes in haemoglobin, packed cell volume, plasma protein and cardiac output (Q). Results: During exercise, Q increased in both Con and FruTr, with Q being significantly lower in FruTr (mean ± s.e. 301.8 ± 8.5 l/min at fatigue) compared to Con (336.5 ± 15.6 l/min) (P<0.01). At rest frusemide had no effect on erythrocyte (J_ER) and transvascular (J_V‐A) fluid fluxes across the lung. Exercise had a significant effect on J_ER and J_V‐A (P≤0.02). During exercise, J_ER (at fatigue 14.6 ± 2.3 l/min and 11.6 ± 2.2 l/min in Con and FruTr, respectively) and J_V‐A (at fatigue14.9 ± 2.3 l/min and 12.0 ± 2.2 l/min in Con and FruTr, respectively) were not significantly different between Con and FruTr (P = 0.6 and P = 0.8 for J_ER and J_V‐A, respectively). Conclusions and clinical importance: Fru does not have a measurable effect on J_ER and J_V‐A. Cardiac output was reduced in FruTr, suggesting that there were also smaller changes in the capillary recruitment and transvascular transmural hydrostatic pressures; however, this did not effect J_V‐A. Therefore, Fru at the dose of 250 mg does not appear to be an effective treatment for regulating pulmonary transvascular forces during exercise in horses.     

Treatment of left laryngeal hemiplegia in standardbreds, using a nerve muscle pedicle graft

The efficacy of a nerve muscle pedicle (NMP) graft in restoring upper airway function was evaluated in exercising horses with induced left laryngeal hemiplegia. The NMP graft was created from the first cervical nerve and the omohyoideus muscle and transplanted into the left cricoarytenoideus dorsalis muscle. Seven adult Standardbreds were trained to exercise on a treadmill inclined at 6.38 degrees. With the horses at rest and exercising at 4.2 and 7.0 m/s, the following variables were recorded: peak inspiratory and expiratory transupper airway pressures (defined as the pressure difference between a lateral tracheal catheter and a mask catheter), peak inspiratory and expiratory air flow, inspiratory and expiratory impedance, tidal volume, minute ventilation, heart rate, and respiratory frequency. Measurements were made before left recurrent laryngeal neurectomy (LRLN), 28 days after LRLN, and 12, 24, and 52 weeks after the NMP graft (n = 5) or sham operation (n = 2). Before LRLN, exercise increased inspiratory and expiratory air flow and transupper airway pressure, whereas the impedance was unchanged. After LRLN, transupper airway inspiratory pressure and impedance were significantly greater and inspiratory air flow was significantly less than baseline values at 7.0 m/s. The sham operation did not improve airway function. Twelve weeks after insertion of the NMP graft, inspiratory impedance and inspiratory air flow were significantly different (improved) from LRLN values. Twenty-four weeks after insertion of the NMP graft, inspiratory impedance was not significantly different from LRLN values.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of ventriculectomy, prosthetic laryngoplasty, and exercise on upper airway function in horses with induced left laryngeal hemiplegia

Effects of ventriculectomy and prosthetic laryngoplasty on upper airway flow mechanics and blood gas tensions in exercising horses with induced left laryngeal hemiplegia were assessed. Five adult horses were trained to stand, trot (4.5 m/s), and gallop (7.2 m/s) on a treadmill (6.38 degrees incline). Inspiratory and expiratory airflows (VImax, VEmax, respectively) were measured using a 15.2-cm diameter pneumotachograph in a face mask. Inspiratory and expiratory transupper airway pressures (PuI, PuE, respectively) were determined as pressure differences between barometric pressure and lateral tracheal pressure. Blood collected from exteriorized carotid arteries was analyzed for PaO2, PaCO2, pH, hemoglobin (Hb) content, and HCO3- values. Heart rate (HR) was determined with an HR monitor. Measurements were made with horses standing, trotting, and galloping before left recurrent laryngeal neurectomy (LRLN; base line), 14 days after LRLN, 30 days after ventriculectomy (44 days after LRLN), and 14 days after prosthetic laryngoplasty (58 days after LRLN). Before LRLN (base line), increasing treadmill speed for horses from standing to the trot and gallop progressively increased HR, respiratory frequency, VImax, VEmax, PuI, PuE, Hb, and PaCO2 values and decreased PaO2, pH, and HCO3- values; inspiratory and expiratory impedances were unchanged. After LRLN, inspiratory impedance and PuI were significantly (P less than 0.05) increased in horses at the trot and gallop, and PaCO2 was significantly increased in horses at the gallop. The VImax and respiratory frequency were significantly (P less than 0.05) decreased in horses at the gallop. Left recurrent laryngeal neurectomy had no effect on PuE, VEmax, HR, PaO2, pH, Hb, or expiratory impedance values.(ABSTRACT TRUNCATED AT 250 WORDS)     

Pulsed delivery of inhaled nitric oxide counteracts hypoxaemia during 2.5 hours of inhalation anaesthesia in dorsally recumbent horses

Objective The study aimed to investigate the effect of varying pulse lengths of inhaled nitric oxide (iNO), and 2.5 hours of continuous pulse‐delivered iNO on pulmonary gas exchange in anaesthetized horses. Study Design Experimental study. Animals Six Standardbred horses. Methods Horses received acepromazine, detomidine, guaifenesin, thiopentone and isoflurane in oxygen, were positioned in dorsal recumbency and were breathing spontaneously. iNO was on average pulsed during the first 20, 30, 43 or 73% of the inspiration in 15 minute steps. The pulse length that corresponded to the highest (peak) partial pressure of arterial oxygen (PaO₂) in the individual horses was determined and delivered for a further 1.5 hours. Data measured or calculated included arterial and mixed venous partial pressures of O₂ and CO₂, heart rate, respiratory rate, expired minute ventilation, pulmonary and systemic arterial mean pressures, cardiac output and venous admixture. Data (mean ± SD) was analysed using anova with p < 0.05 considered significant. Results Although the pulse length of iNO that corresponded to peak PaO₂ varied between horses, administration of all pulse lengths of iNO increased PaO₂ compared to baseline. The shortest pulse lengths that resulted in the peak PaO₂ were 30 and 43% of the inspiration. Administration of iNO increased PaO₂ (12.6 ± 4.1 kPa [95 ± 31 mmHg] at baseline to a range of 23.0 ± 8.4 to 25.3 ± 9.0 kPa [173 to 190 mmHg]) and PaCO₂ (8.5 ± 1.2 kPa [64 ± 9 mmHg] to 9.8 ± 1.5 kPa [73 ± 11 mmHg]) and decreased venous admixture from 32 ± 6% to 25 ± 6%. The increase in PaO₂ and decrease in venous admixture was sustained for the entire 2.5 hours of iNO delivery. Conclusions The improvement in arterial oxygenation during pulsed delivery of iNO was significant and sustained throughout 2.5 hours of anaesthesia. Clinical relevance Pulsed iNO potentially could be used clinically to counteract hypoxemia in anaesthetized horses.     

Comparative ergoespirometric adaptations to a treadmill exercise test in untrained show Andalusian and Arabian horses

Significant differences exist in the respiratory adaptation to exercise in different equine breeds. This research describes the ergoespirometric response to exercise of Andalusian (AN) and Arabian (A) horses, both selected according to morphological criteria. Thirteen untrained male horses (6 AN and 7 A) performed a treadmill exercise test (TET) with a slope of 6%, with workloads starting from 5 m/s and increasing 1 m/s every 3 min until the horses were not able to keep the required velocity. Tidal volume (TV), respiratory rate, minute ventilation (VE), oxygen uptake (VO2), carbon dioxide production, peak oxygen uptake (VO2peak), respiratory exchange ratio (RER), exercise time to fatigue (ETF) and respiratory aerobic threshold (RAT) were determined. AN horses presented higher TV and VE, whereas respiratory rate, VO2 and VCO2 were lower at the same velocities. RER was similar between breeds. ETF was longer in A horses (556.7 ± 66.5 in AN vs. 607.1 ± 71.1 s in A) and no significant differences were found in RAT (5.50 ± 0.50 in AN vs. 5.86 ± 1.07 m/s in A). In summary, despite the more intense ventilatory response to exercise at the same velocity, AN horses had lower VO2. The AN horse develops a more intense ventilatory response to fixed velocities than the A horse and it could be interesting to clarify the role of the locomotion characteristics in this response.     

Effect of Recovery Periods during Conditioning of Horses on Fitness Parameters

The effect of recovery from training has not been studied in horses. Therefore, the effect of recovery was examined with exercise of known effect within a conditioning period (CP). A standardized exercise test was performed at the beginning of CP to determine v₄, v₁₀, and v₁₈₀ (horse’s speed, which produced a blood lactate concentration of 4 and 10 mmol/L and a heart rate of 180 beats/min). Six horses were conditioned for three periods of 2 weeks, 5 times per fortnight at their individual v₁₀ for two bouts of 5 minutes on a treadmill. Every 2 weeks of conditioning was followed by 1 week with reduced workload. Standardized exercise test was repeated after each 2 weeks of conditioning and 2 weeks after finishing CP. Exercise speed was individually adapted to the new v₁₀ for every 2 weeks of conditioning. In addition, peak oxygen consumption before, after 3 weeks of conditioning, and at the end of the CP was measured. The mean v₄ increased steadily during CP. v₁₈₀ did not change, whereas peak oxygen consumption increased between the beginning and after 3 weeks of conditioning and leveled off thereafter. In conclusion, reducing the workload for 1 week after 2 weeks of conditioning 5 times per fortnight at v₁₀ for two bouts of 5 minutes allowed for a continuous increase of v₄, but the extent of the increase was smaller than in another study with a similar conditioning program but for the recovery week. The effect of recovery from training needs further studies.     

Pulmonary artery wedge pressure measurement in healthy warmblood horses and in warmblood horses with mitral valve insufficiencies of various degrees during standardised treadmill exercise

In 12 healthy warmblood horses and 10 horses with mitral valve insufficiencies (MVI) of various degrees heart rate and pulmonary artery wedge pressure (PWP) was measured at rest and during standardised exercise on a high speed treadmill. There was a significant increase in PWP with each change in speed of the treadmill (p < 0.01). The PWP of horses with mild mitral valve regurgitation under working conditions was not significantly different compared to the healthy horses. The horses with moderate mitral valve regurgitation showed a significant higher pulmonary artery wedge pressure at rest and during exercise compared to the healthy horses (p < 0.01) at rest and during treadmill velocity. The tendencies were seen that mild mitral valve regurgitation results only in mild hemodynamic changes during exercise, while moderate MVI have an important influence on haemodynamics.     

Effects of external nasal support on pulmonary gas exchange and EIPH in the horse

In the horse during high-speed running, partial collapse of the unsupported nasal airways may contribute to elevated inspiratory resistance. This effect would be expected to increase respiratory muscle work and augment negative intrapulmonary pressure swings which in turn might exacerbate exercise-induced pulmonary hemorrhage (EIPH). To investigate this issue, six Thoroughbreds and one Quarter Horse were evaluated while running at high speed (12±1 m/s) under control conditions (C) and wearing an external nasal dilator (ND) in individual, randomly ordered trials two weeks apart. Whole-body gas exchange (oxygen uptake, V̇O₂, carbon dioxide output, V̇CO₂), arterial blood gases, acid-base and blood temperature were measured. Compared with C, ND significantly reduced V̇O₂ (C, 59.9±5.3; ND, 56.4±5.0 L/min, P < 0.05) and V̇CO₂. However, neither arterial blood gases, acid-base, blood temperature nor plasma lactate were changed significantly. Bronchoalveolar lavage (BAL) revealed a 33% (P < 0.05) reduction in EIPH (quantified as red blood cells/ml BAL fluid) in the ND trial. These data demonstrate that nasal dilation can lower whole body V̇O₂ and reduce EIPH. It is possible that these effects are secondary to a decreased inspiratory resistance, lowered inspiratory muscle work and altered intrapulmonary pressures.     

Effects of two field continuous incremental tests on cardiorespiratory responses in Standardbred trotters

In humans, cardiorespiratory responses are widely evaluated from field incremental exercise tests. On the contrary, equine exercise physiology faces a huge lack of oxygen consumption measurements (VO₂) in field conditions due to technical concerns. The aim of this study was to test the effects of two incremental continuous field tests on cardiorespiratory responses in Standardbred trotters. The two protocols were realized at trot and ended when horses galloped. The tests started at 4.2 m/s (T1) and 6.4 m/s (T2), with speed increments of 1.4 m/s every 3 min for T1 and 0.8 m/s every 2 min for T2. Velocity (v), heart rate (HR) and gas exchanges were recorded continuously, and blood lactate concentration [La^?] was measured before and after tests. Values recorded at the end of the tests were considered as peak values. The v_peak values were 10.6 ± 0.3 and 10.7 ± 0.7 m/s for T1 and T2 respectively. Horses reached higher VO_2peak (T1: 116.6 ± 11.5 ml/min/kg; T2: 88.9 ± 10.2 ml/min/kg; p < 0.05) and HR_peak (T1: 217 ± 5 bpm; T2: 209 ± 3 bpm; p < 0.05) during T1 compared with T2. T1 was significantly longer than T2 (17.5 ± 1.9 vs. 12.9 ± 1.6 min respectively, p < 0.01), and the number of steps entirely ran tended to be different (T1: 5.6 ± 0.6; T2: 6.2 ± 0.8, p = 0.07). Compared to T2, the design of T1 appeared easier to implement and allowed higher cardiorespiratory responses. The relationship between HR‐VO₂ obtained through T1 gave a better correlation between the two variables than T2. These findings suggest that T1 might be better than T2 for evaluating cardiorespiratory adaptations to exercise and for estimating aerobic energy expenditure in exercising trotters.     

Evaluation of partial arytenoidectomy as a treatment for equine laryngeal hemiplegia

The efficacy of partial arytenoidectomy was assessed in 6 Standardbred horses, with surgically induced laryngeal hemiplegia, at rest (Period A) and during exercise at speeds corresponding to maximum heart rate (Period C) and 75% of maximum heart rate (Period B). Peak expiratory and inspiratory airflow rate (PEF and PIF), and expiratory and inspiratory transupper airway pressure (P_UE and P_UI) were measured and expiratory and inspiratory impedance (Z_E and Z_I) were calculated. Simultaneously, tidal breathing flow-volume loops (TBFVL) were acquired using a respiratory function computer. Indices derived from TBFVL included airflow rates at 50 and 25% of tidal volume (EF₅₀, IF₅₀, EF₂₅. and IF₂₅) and the ratios of expiratory to inspiratory flows. Measurements were made before left recurrent laryngeal neurectomy (baseline), 2 weeks after left recurrent laryngeal neurectomy (LRLN) and 16 weeks after left partial arytenoidectomy coupled with bilateral ventriculectomy (ARYT). After LRLN, during exercise Periods B and C, Z₁ and the ratio of EF₅₀/IF₅₀ significantly increased and PIF, IF₅₀ and IF₂₅ significantly decreased from baseline values. At 16 weeks after ARYT, Z₁ returned to baseline values during Periods B and C. Although PIF, IF₅₀, IF₂₅, PEF/PIF, and EF₅₀/IF₅₀ returned to baseline values during Period B, these indices remained significantly different from baseline measurements during Period C. After ARYT, TBFVL shapes from horses during Period C approached that seen at the baseline evaluation. Partial arytenoidectomy improved upper airway function in exercising horses with surgically induced left laryngeal hemiplegia, although qualitative and quantitative evaluation of TBFVLs suggested that some flow limitation remains at near maximal airflow rates. These results indicate that, although the procedure does not completely restore the upper airway to normal, partial arytenoidectomy is a viable treatment option for failed laryngoplasty and arytenoid chondropathy in the horse.     

Echocardiographic analysis of segmental left ventricular wall motion at rest and after exercise in horses with and without heart disease

An echocardiography was performed on 23 healthy warmblood horses and on 12 warmblood horses with cardiac diseases at rest and after treadmill or lungeing exercise. The B-mode technique was used, and the left ventricular wall motion was analyzed visually. The left ventricle was divided into 6 equally sized myocardial segments, 3 of them in the region of the interventricular septum, and the other 3 in the region of the left ventricular rear wall (“6-segment model”). The kinetic of each segment was determined using a kinetic score system. A normokinetic wall motion was defined with a kinetic index of 1. To the authors' knowledge, the current study is the first to present a visual qualitative method for assessment of the healthy equine heart with regard to the myocardial left ventricular wall motion at rest and after exercise under standardized conditions. First, reference values for the left ventricular myocardial kinetics were established in healthy horses. Subsequently, horses with cardiac diseases of differing severity were examined, and the diagnostic value of the method was evaluated. In contrast with human cardiology, the healthy equine heart is characterized by several deviating wall motion patterns. The majority of the healthy horses (52%) showed a normokinetic wall motion of all 6 segments at rest and a normokinetic concentrically increased hypertrophy after exercise. In 9 healthy horses, however, the middle septal segment (at the papillary muscle level) was characterized by a hypokinesia (kinetics index = 2) at rest. In 5 of these horses, the hypokinesia was not observed after exercise. The kinetic wall motion score was 1.1 ± 0.2 at rest in the healthy horses. In the horses with cardiac disorders, the score was 1.5 ± 0.4 at rest. In all healthy horses, the middle septal segment (at the papillary muscle level) was characterized by a hypokinesia (kinetics index = 2; n = 5) or akinesia (kinetics index = 3; n = 3) at rest.After standardized treadmill exercise, the wall motion score of the healthy horses was 1.12 ± 0.16 and the score of the horses with cardiac disorders was 1.52 ± 0.4. In contrast to human cardiology, the mid-septal segment showed a hypokinesia (kinetics index: 2) and an akinesia (kinetics index: 3) in two healthy horses after exercise. In all horses with low-grade and moderate cardiac disorder, an atypical wall motion pattern was observed after exercise. In all horses with high-grade cardiac disorder, an atypical motion pattern was documented in 5 out of 6 segments. An exercise test was not performed on the latter group.     

Comparison of some responses to exercise on the track and the treadmill in French trotters: determination of the optimal treadmill incline

The purpose of this study was to determine the optimal treadmill slope for trotters to produce the same heart rate and blood lactate responses as on the track during a standardized exercise test. Nine 2-year-old French trotters performed exercise tests on a training track and on a treadmill set at an incline of 0, 2 or 4%. For all horses, track testing was performed on day 1 and then on the treadmill according to a Latin-square design. The track test utilized three steps each of 3 min at speeds of 470, 530, 590 m/min and the same speeds were used on the treadmill. Derived physiological variables such as the speed at a HR of 200 bpm (V(200)) and the speed inducing blood lactate concentrations of 4 mmol/L (V(4)) were calculated. There were significant differences for V(200)and V(4)(P<0.05) between the track and the treadmill data when the treadmill was set at inclines of 0 and 4%, but no significant differences with the treadmill set at a 2% incline. The optimal treadmill incline to reproduce similar responses to those on the track was determined by regression analysis, and was found to be 2.4% for the two most often utilized derived physiological variables, V(4)and V(200).     

Pharmacokinetics of detomidine administered to horses at rest and after maximal exercise

Reason for performing study: Increased doses of detomidine are required to produce sedation in horses after maximal exercise compared to calm or resting horses. Objectives: To determine if the pharmacokinetics of detomidine in Thoroughbred horses are different when the drug is given during recuperation from a brief period of maximal exercise compared to administration at rest. Methods: Six Thoroughbred horses were preconditioned by exercising them on a treadmill. Each horse ran a simulated race at a treadmill speed that caused it to exercise at 120% of its maximal oxygen consumption. One minute after the end of exercise, horses were treated with detomidine. Each horse was treated with the same dose of detomidine on a second occasion a minimum of 14 days later while standing in a stocks. Samples of heparinised blood were obtained at various time points on both occasions. Plasma detomidine concentrations were determined by liquid chromatographymass spectrometry. The plasma concentration vs. time data were analysed by nonlinear regression analysis. Results: Median back‐extrapolated time zero plasma concentration was significantly lower and median plasma half‐life and median mean residence time were significantly longer when detomidine was administered after exercise compared to administration at rest. Median volume of distribution was significantly higher after exercise but median plasma clearance was not different between the 2 administrations. Conclusions and potential relevance: Detomidine i.v. is more widely distributed when administered to horses immediately after exercise compared to administration at rest resulting in lower peak plasma concentrations and a slower rate of elimination. The dose requirement to produce an equivalent effect may be higher in horses after exercise than in resting horses and less frequent subsequent doses may be required to produce a sustained effect.     

CEREBRAL RESPONSES IN HORSES TO HALOTHANE AND ISOFLURANE ANAESTHESIA: EEG POWER SPECTRUM ANALYSIS AND DIFFERENCES IN ARTERIOVENOUS OXYGEN CONTENT.

In a prospective study we compared the EEG variables total amplitude (TA), 80% spectral edge frequency (SEF-80), the ratio of fractional amplitudes distributed into the BETA and DELTA frequency band (BETA/DELTA-ratio), and differences in arteriovenous oxygen content (AVD0₂), obtained from horses anaesthetized with either halothane (H; n=4) or isoflurane (I; n=4) in oxygen. All horses underwent orthopaedic procedures. After premedication with xylazine (0.88 mg/kg IV), anaesthesia was induced with diazepam (0.033 mg/kg IV) and ketamine (2.2 mg/kg IV). During anaesthesia horses were ventilated using IPPV. EEG variables and AVD0₂ were recorded at equal levels of surgical anaesthesia (stage III/1–2), as determined by clinical signs and a dominant delta activity in the EEG power spectrum. PaC0₂was kept between 35 mmHg and 45 mmHg, PaO₂above 100 mmHg, and mean arterial blood pressure (MAP) was adjusted to at least 80 mmHg. The average body temperature was 35.4 ± 1.1°C (H) and 35.6 ± 0.7°C (I), respectively. In horses anaesthetized with I, TA was significantly higher (P<01) (I: 3533 ± 70 γV; H: 235.9 ± 63.4 γV), whereas SEF-80 (I: 10.7 ± 0.7 Hz; H: 12.4 ± 0.7 Hz) and BETA/DELTA-ratio (I: 035 ± 0.06; H: 0.53 ± 0.12) were significantly lower (P<01) compared with H. We also observed significantly lower (P<05) AVD02 values with I (1.5 ± 0.5 Vol%) than with H (2.0 ± 1.2 Vol%). Since a depression in cerebral activity during anaesthesia is characterized by a decrease in EEG frequency content and a concomitant increase in EEG amplitude, the authors conclude that at equal levels of surgical anaesthesia, isoflurane exerts a more pronounced depression in cerebral electrical and metabolic activity in horses.     

Pharmacokinetics of furosemide administered 4 and 24 hours prior to high‐speed exercise in horses

Furosemide is a diuretic agent used commonly in racehorses to attenuate the bleeding associated with exercise‐induced pulmonary hemorrhage (EIPH). The current study describes serum and urine concentrations and the pharmacokinetics of furosemide following administration at 4 and 24 hrs prior to maximal exercise. Eight exercised adult Thoroughbred horses received a single IV administration of 250 mg of furosemide at 4 and 24 hrs prior to maximal exercise on a high‐speed treadmill. Blood and urine samples were collected at time 0 and at various times for up to 72 hrs and furosemide concentrations determined using liquid chromatography–tandem mass spectrometry. Serum furosemide concentrations remained above the LOQ (0.05 ng/ml) for 36 hrs in 3/8 and 1/8 horses in the 4‐ and 24‐hrs groups, respectively. Serum concentration data were best fit by a two‐compartment model. There was not a significant difference in the volume of distribution at steady‐state (0.594 ± 0.178 [4 hrs] and 0.648 ± 0.147 [24 hrs] L/kg) or systemic clearance (0.541 ± 0.094 [4 hrs] and 0.617 ± 0.114 [24 hrs] L/hrs/kg) between horses that were exercised at 4‐ and 24 hrs postdrug administration. The mean ± SD elimination half‐life was 3.12 ± 0.387 and 3.23 ± 0.407 hrs following administration at 4 and 24 hrs prior to exercise, respectively.     

Quantitative Analysis of Stress Echocardiograms in Healthy Horses with 2‐Dimensional (2D) Echocardiography,Anatomical M‐Mode,Tissue Doppler Imaging,and 2D Speckle Tracking

Background: Stress echocardiography is used to diagnose myocardial dysfunction in horses, but current methods are not well standardized. The influence of heart rate (HR) on measurements is largely unknown. Objectives: To investigate the use of 2‐dimensional echocardiography (2DE), anatomical M‐mode (AMM), tissue Doppler imaging (TDI), and 2D speckle tracking (2DST) at rest and after exercise for quantification of regional and global left‐ventricular (LV) function. Animals: Five athletic Warmblood horses; 11.6 ± 3.6 years; 529 ± 48 kg. Methods: Prospective study. Three separate echocardiographic examinations were performed before (baseline) and over 5 minutes after treadmill exercise with 2DE (1st, short‐axis view; 2nd, long‐axis view) and pulsed‐wave TDI (3rd examination). Offline analyses were performed at baseline and after exercise at HR 120, 110, 100, 90, and 80 minute^?1. Global and segmental measurements were compared by analysis of variance. Results: Quantitative analyses of stress echocardiograms were feasible in all horses. None of the AMM indices changed significantly after exercise. Stroke volume and ejection fraction by 2DE and strain by 2DST decreased, whereas strain rate by 2DST increased significantly at HR > 100 minute^?1. TDI analyses were technically difficult and provided little additional information. Conclusions and Clinical Importance: Volumetric indices by 2DE and strain and strain rate by 2DST are applicable for quantitative assessment of stress echocardiograms. In healthy horses, they are significantly altered at a HR > 100 minute^?1 and need to be evaluated in view of the instantaneous HR. Further investigations are needed to define the clinical value of stress echocardiography in horses with cardiac disease.     

Blood lactate disappearance after maximal exercise in trained and detrained horses

The influence of training on blood lactate concentrations during treadmill exercise and a 40-minute inactive recovery period was examined in seven trained and seven detrained thorough-bred horses. Lactate concentrations were measured in venous blood collected at the end of each exercise state, and at intervals for 40 minutes afterwards. Measurements were made of maximum oxygen uptake (V̇O_2max, ml kg⁻¹ min⁻¹), VLA4 (velocity at which blood lactate concentration was 4 mmol litre⁻¹); LA8 (lactate concentration [mmol litre⁻¹] during exercise at 8 m sec⁻¹), peak lactate (highest lactate concentration after exercise), LA40 (lactate concentration 40 minutes after exercise), the time of peak lactate concentration (minutes after exercise) and the rate of disappearance of blood lactate (Rt_d). The trained horses had a significantly lower LA8 (2·1 ± 0·1 vs 6·5 ± 1 mmol litre⁻¹, P<0·01), higher VLA4 (9·8 ± 0·2 vs 5·8 ± 0·6 m sec⁻¹, P<0·01) and higher V̇0_2max (156·3 ± 3·8 vs 107·1 ± 3·9 ml kg⁻¹ min⁻¹, P<0·001). The value of Rt_d and the time of peak lactate concentration were not significantly different.     

Venous Blood Acid-Base Status in Show Jumper Horses Subjected to Different Physical Exercises

The aim of this study was to assess whether acid-base profile exhibits changes in regularly trained show jumping horses undergoing increasing exercise workloads. Seven female Italian saddle horses were subjected to three different physical exercise trials of increasing workload identified as three exercise phases (EPs). During EP_I horses were subjected to a standardized exercise test consisting of 15 minutes of treadmill, during EP_II horses were subjected to a show jumping test (height, 0.9–1.1 m; course length, 300 m), during EP_III horses underwent two jumping sessions carried out over two consecutive days. Blood samples were collected at rest (T_PRE), after exercise (T_POST), and 30 minutes after the end of exercise (T_POST30). The values of pH, partial pressure of carbon dioxide (Pco₂), partial pressure of oxygen (Po₂), bicarbonate level (HCO₃⁻), hemoglobin (Hb), and hematocrit (Hct) were measured. A significant effect of exercise workload and time (P < .001) on Po₂, Pco₂, HCO₃⁻, Hb, and Hct values was found. The variation in the studied parameters resulted mostly reversible within T_POST30 in horses when subjected to EP_I and EP_II, whereas Po₂, Hb, and Hct remained higher at T_POST30 than T_PRE in horses when subjected to the second day of jumping section (EP_III) indicating a failure to recover. The results suggest that jumping sessions carried out over two consecutive days represent extra workload for horses, and this should be taken into account by veterinarian to prevent acid-base imbalance and for the maintenance of health and performance in equine athletes.     

Effect of Conditioning Horses Once,Twice, or Thrice a Week with High-Intensity Intermittent Exercise on v4

Intensive short intervals of exercise are used to condition horses for racing. In this study, the effect of exercising horses one, two, or three times a week during 6 weeks using two intervals with near-maximal speed over 100 m on v₄ (speed at which, under defined conditions, the blood lactate concentration reaches 4 mmol/L) and muscle thickness (MT) of supraspinatus and extensor carpi radialis was examined. Thoroughbreds (4-5 years of age) were exercised twice at near-maximal speed over 100 m, separated by a 10-minute period at walk, on dirt track during conditioning periods (CP) of 6-week duration. This exercise was undertaken once (six horses), twice (six horses), or thrice (five horses) a week during a CP. Before, every 2 weeks during, and 2 weeks after the CP, horses were subjected to a standardized exercise test to determine their v₄. Before and after CP, the MT of the extensor carpi radialis and the supraspinatus was examined by ultrasonography. There was no differential effect of the number of weekly exercises on v₄. Pooling the data of all horses, a decrease of v₄ was found. The decrease became evident after the sixth week of conditioning. MT did not change. The results indicate that the examined exercise protocols could negatively impact racing performance of horses.