Cardiac output but not high pulmonary artery pressure varies with FIO2 in exercising horses

Measuring V̇O2 in hypoxic and hyperoxic conditions using dynamic gas mixing with a flow-through indirect calorimeter

Measurements of gas exchange while breathing gases of different O₂ concentrations are useful in respiratory and exercise physiology. High bias flows required in flow-through indirect calorimetry systems for large animals like exercising horses necessitate the use of inconveniently large reservoirs of mixed gases for making such measurements and can limit the amount of equilibration time that is adequate for steady-state measurements. We obviated the need to use a pre-mixed reservoir of gas in a semi-open flow-through indirect calorimeter by dynamically mixing gases and verified the theoretical accuracy and utility of making such measurements using the mass-balance N₂-dilution method. We evaluated the accuracy of the technique at different inspired oxygen fractions by measuring exercising oxygen consumption (V̇O₂) at two fully aerobic submaximal exercise intensities in Thoroughbred horses. Horses exercised at 24% and 50% maximum oxygen consumption (V̇O₂ max) of each horse while breathing different O₂ concentrations (19.5%, 21% and 25% O₂). The N₂-dilution technique was used to calculate V̇O₂. Repeated-measures ANOVA was used to tested for differences in V̇O₂ between different inspired O₂ concentrations. The specific V̇O₂ of the horses trotting at 24%V̇O₂max and cantering at 50%V̇O₂max were not significantly different among the three different inspired oxygen fractions. These findings demonstrate that reliable measurements of V̇O₂ can be obtained at various inspired oxygen fractions using dynamic gas mixing and the N₂-dilution technique to calibrate semi-open-circuit gas flow systems.

Inclined running increases pulmonary haemorrhage in the Thoroughbred horse

REASONS FOR PERFORMING STUDY

Capillary stress failure-induced (exercise-induced) pulmonary haemorrhage (EIPH) during intense running in horses is thought to involve both intravascular (i.e. mean pulmonary arterial pressure [Ppa] > 100 mmHg) and extravascular (e.g. negative inspiratory pressure swings) mechanisms.

HYPOTHESIS

That inclined running would reduce breathing frequency (coupled to stride frequency) and increase tidal volume thus increasing lung volume changes and intrapleural pressure swings resulting in more pronounced EIPH.

METHODS

Six Thoroughbred horses were run to volitional fatigue (incremental step test) on a level (L) and inclined (I; 10%) treadmill in random order. Pulmonary minute ventilation, arterial blood gases and mean Ppa were obtained during each run while EIPH severity was quantified via bronchoalveolar lavage (BAL) 30 mins post run.

RESULTS

Time to fatigue did not differ between trials (P > 0.05). At end-exercise, breathing frequency was reduced (L, 127.8 +/- 3.0; I, 122.6 +/- 2.1 breaths/min; P < 0.05) and tidal volume increased (L, 11.5 +/- 0.6; I, 13.1 +/- 0.5 L; P < 0.05) during inclined running. No differences existed in end-exercise plasma [lactate] between trials (L, 24.5 +/- 2.9; I, 26.2 +/- 3.4 mmol/l, P > 0.05); however, the mean peak Ppa was reduced during the inclined run (L, 105+5; I, 96 +/- 4 mmHg, P < 0.05). In the face of reduced Ppa, EIPH severity was increased significantly (P < 0.05) during the inclined vs. level run (L, 37.0 +/- 11.7; I, 49.6 +/- 17.0 x 10(6) red blood cells/ml BAL fluid).

CONCLUSIONS

Although inclined running lowered peak Ppa, EIPH severity was increased. It is likely that this effect resulted, in part, from an altered ventilatory pattern (i.e. increased tidal volumes and associated intrapleural pressure changes).

POTENTIAL RELEVANCE

This conclusion supports an important role for extravascular factors in the aetiology of EIPH.

Effects of a specific endothelin-1A antagonist on exercise-induced pulmonary haemorrhage (EIPH) in thoroughbred horses

REASONS FOR PERFORMING STUDY

During high intensity exercise, the very high pulmonary artery pressure (Ppa) experienced by Thoroughbred horses is considered a major factor in the aetiology of exercise-induced pulmonary haemorrhage (EIPH). Recently, endothelin-1 (ET-1), a potent vasoconstrictive hormone, has been found to increase Ppa in horses at rest via binding to its ET-1A receptor subtype. In addition, plasma concentrations of ET-1 are increased in horses during and after high intensity exercise.

HYPOTHESIS

If ET-1 increases Ppa during exercise in the horse, administration of a specific ET-1A antagonist would decrease Ppa and therefore EIPH.

METHODS

Saline (CON) or an ET-1A receptor antagonist, TBC3214 (3 mg/kg bwt i.v.; ANTAG) was administered to horses 1 h prior to maximal incremental exercise on a high-speed treadmill. Gas exchange measurements were made breath-by-breath and blood samples collected during each 1 min stage to determine blood gases, acid-base status and cardiac output. EIPH was determined via bronchoalveolar lavage (BAL) approximately 30 min after exercise.

RESULTS

The time to fatigue, gas exchange and cardiovascular responses were not different between groups (P>0.05). Resting and peak Ppa did not differ significantly between treatments. Most importantly, ANTAG did not decrease EIPH.

CONCLUSIONS

These results do not support a deterministic role for ET-1 in the increased Ppa and therefore EIPH, during maximal exercise in the equine athlete.

POTENTIAL RELEVANCE

Treatment with an ET-1A receptor antagonist does not appear to be a viable therapeutic intervention in the prevention of EIPH.

NO inhalation reduces pulmonary arterial pressure but not hemorrhage in maximally exercising horses

In horses, the exercise-induced elevation of pulmonary arterial pressure (Ppa) is thought to play a deterministic role in exercise-induced pulmonary hemorrhage (EIPH), and thus treatment designed to lower Ppa might reasonably be expected to reduce EIPH. Five Thoroughbred horses were run on a treadmill to volitional fatigue (incremental step test) under nitric oxide (NO; inhaled 80 ppm) and control (N(2), same flow rate as per NO run) conditions (2 wk between trials; order randomized) to test the hypothesis that NO inhalation would reduce maximal Ppa but that this reduction may not necessarily reduce EIPH. Before each investigation, a microtipped pressure transducer was placed in the pulmonary artery 8 cm past the pulmonic valve to monitor Ppa. EIPH severity was assessed via bronchoalveolar lavage (BAL) 30 min postrun. Exercise time did not differ between the two trials (P > 0.05). NO administration resulted in a small but consistent and significant reduction in peak Ppa (N(2), 102.3 +/- 4.4; NO, 98.6 +/- 4.3 mmHg, P < 0.05). In the face of lowered Ppa, EIPH severity was significantly higher in the NO trial (N(2), 22.4 +/- 6.8; NO, 42.6 +/- 15.4 x 10(6) red blood cells/ml BAL fluid, P < 0.05). These findings support the notion that extremely high Ppa may reflect, in part, an arteriolar vasoconstriction that serves to protect the capillary bed from the extraordinarily high Ppa evoked during maximal exercise in the Thoroughbred horse. Furthermore, these data suggest that exogenous NO treatment during exercise in horses may not only be poor prophylaxis but may actually exacerbate the severity of EIPH.

Regional differences in endothelial function in horse lungs: possible role in blood flow distribution?

We investigated regional differences of in vitro responses of pulmonary arteries (6-mm OD) from the dorsocaudal (top) and cranioventral (bottom) lung regions to endothelium-dependent vasodilators (methacholine, bradykinin, and calcium ionophore A-23187). Methacholine relaxed endothelium-intact top vessels; however, in bottom vessels, a small relaxation preceded a profound contraction. In top vessels, removal of endothelial cells converted relaxation to contraction, and in bottom vessels it abolished relaxation and enhanced contraction. Bradykinin and A-23187 were more potent and caused greater endothelium-mediated relaxation in top than in bottom arteries. The endothelium-independent vasodilator sodium nitroprusside caused similar relaxations in all rings. Nomega-nitro-L-arginine and NG-monomethyl-L-arginine and methylene blue abolished relaxation of top and bottom arteries to methacholine; meclofenamate had little effect. We conclude that regional differences in endothelium-mediated relaxation are caused by differences in the magnitude of the endothelial release of nitric oxide. Similar differences in endothelium-dependent flow-mediated vasodilation and endothelial nitric oxide release may result in preferential perfusion of caudodorsal lung regions.

Force, speed, and oxygen consumption in thoroughbred and draft horses

Thoroughbred (TB) and draft horses (DH) have long been selected for tasks of very different intensities and force-speed relationships. To study their adaptations, we measured O2 consumption and related variables in three TB and four DH during progressive exercise tests on a level treadmill. The horses exerted a draft force of 0, 5, 10, 15, or 20% of their body weight at speeds that increased by 2 m/s every 3 min until they could not maintain that speed. We found that TB could exert the same draft forces as DH and, at each force, TB achieved about twice the speed, twice the external power, and twice the O2 consumption as DH; thus the two breeds had the same gross efficiencies. We also found maximal O2 consumption of TB to be about twice that of DH (134 vs. 72 ml . kg-1 . min-1, respectively), suggesting adaptations to high-intensity exercise. Peak efficiency was reached at lower speeds in DH than in TB, suggesting adaptations to high-force, low-speed exercise. These differences between TB and DH in force-speed and aerobic capacities and in speed for peak efficiency likely reflect different contraction velocities in locomotor muscles.