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

Racetrack exercise vs treadmill exercise with respect to exercise-induced pulmonary haemorrhage (EIPH): implications for studies of putative treatments of EIPH

Multiple treadmill-based studies using low numbers of horses have evaluated potential prophylactic treatments for exercise-induced pulmonary haemorrhage (EIPH) and found no effect. However, the relevance of these findings to racing is unclear. Because severity of EIPH incurred on treadmills has not been compared to that following high-speed racetrack exercise in the same horses, we retrospectively performed this comparison using bronchoalveolar lavage fluid red cell numbers (BALFRBC) due to the relative insensitivity of tracheobronchoscopy. Six race-fit Thoroughbreds with recent tracheobronchoscopic EIPH scores ≥2 were exercised to fatigue on a treadmill at 115% V̇O2max (5% incline, 12.3-14.2 m/s), and maximally on a racetrack over 800 m and 1,100 m with average speeds ranging from 16.4-16.7 and 15.5-16.6 m/s, respectively. Run order varied but was not randomised. Bronchoalveolar lavage (BAL) was performed blindly using Bivona tubes 45-60 mins post-exercise. BALFRBC were determined using a haemocytometer. Data were expressed as median and interquartile range, and analysed using RM ANOVA with significance set at P<0.05. BALFRBC were greater after both racetrack runs than after treadmill exercise (P<0.05; treadmill: 10,305/μl (3,871-26,079); 800m: 25,000/μl (17,175-73,400); 1,100m: 19,500/μl (8,962-800,600). Treadmill exercise resulted in lower numbers and a narrower range in BALFRBC than racetrack exercise. Thus, when a small number of horses is used to study EIPH treatments on a treadmill, a lower BALFRBC would be anticipated following the baseline run than with a similar study using racetrack exercise, and might reduce the likelihood of demonstrating significant treatment effects. Results of this retrospective study raise concern regarding the advisability of extrapolating conclusions regarding efficacy of EIPH treatments from treadmill studies to racetrack scenarios.

Exercise-induced pulmonary hemorrhage: where are we now?

As the Thoroughbreds race for the final stretch, 44 hooves flash and thunder creating a cacophony of tortured air and turf. Orchestrated by selective breeding for physiology and biomechanics, expressed as speed, the millennia-old symphony of man and beast reaches its climax. At nearly 73 kilometers per hour (45 mph) over half a ton of flesh and bone dwarfs its limpet-like jockey as, eyes wild and nostrils flaring, their necks stretch for glory. Beneath each resplendent livery-adorned, latherin-splattered coat hides a monstrous heart trilling at 4 beats per second, and each minute, driving over 400 L (105 gallons) of oxygen-rich blood from lungs to muscles. Matching breath to stride frequency, those lungs will inhale 16 L (4 gallons) of air each stride moving >1,000 L/min in and out of each nostril - and yet failing. Engorged with blood and stretched to breaking point, those lungs can no longer redden the arterial blood but leave it dusky and cyanotic. Their exquisitely thin blood-gas barrier, a mere 10.5 μm thick (1/50,000 of an inch), ruptures, and red cells invade the lungs. After the race is won and lost, long after the frenetic crowd has quieted and gone, that blood will clog and inflame the airways. For a few horses, those who bleed extensively, it will overflow their lungs and spray from their nostrils incarnadining the walls of their stall: a horrifically poignant canvas that strikes at horse racing's very core. That exercise-induced pulmonary hemorrhage (EIPH) occurs is a medical and physiological reality. That every reasonable exigency is not taken to reduce/prevent it would be a travesty. This review is not intended to provide an exhaustive coverage of EIPH for which the reader is referred to recent reviews, rather, after a brief reminder of its physiologic and pathologic bases, focus is brought on the latest developments in EIPH discovery as this informs state-of-the-art knowledge, the implementation of that knowledge and recommendations for future research and treatment.

Exercise induced pulmonary hemorrhage in horses: American College of Veterinary Internal Medicine consensus statement

Background

Published studies of exercise‐induced pulmonary hemorrhage (EIPH), when assessed individually, often provide equivocal or conflicting results. Systematic reviews aggregate evidence from individual studies to provide a global assessment of the quality of evidence and to inform recommendations.

Objectives

Evaluate evidence to determine: if EIPH adversely affects the health, welfare or both of horses; if EIPH affects the athletic capacity of horses; the efficacy of prophylactic interventions for EIPH; and if furosemide affects the athletic capacity of horses.

Animals

None.

Materials and Methods

Systematic review. A panel of 7 experts was formed to assess evidence in the peer reviewed literature addressing each of the 4 objectives. Methodology followed that of the Grading of Recommendations, Assessment, Development, and Evaluation (GRADE). Publications were assessed for quality of evidence by working groups of the panel, and a summary of findings was presented in tables. Recommendations were based on quality of evidence and were determined by a vote of the panel.

Results

Much of the evidence was of low to very low quality. Experimental studies frequently lacked adequate statistical power. There was moderate to high quality evidence that EIPH is progressive, is associated with lung lesions, that it adversely affects racing performance, that severe EIPH (Grade 4) is associated with a shorter career duration, that furosemide is efficacious in decreasing the incidence and severity of EIPH, and that administration of furosemide is associated with superior race performance.

Conclusions and clinical significance

Strong recommendation that EIPH be considered a disease and a weak recommendation for use of furosemide in management of racehorses with EIPH.

Howard H. Erickson: contributions to equine exercise physiology and veterinary medicine

For over four and a half decades Howard Erickson has been at the forefront of scientific discovery. As a veterinary cardiovascular specialist with the United States Air Force he made fundamental progress to developing a working artificial heart. Subsequently as a retired US Air Force Colonel and Professor at Kansas State University College of Veterinary Medicine Erickson detected the immensely high pulmonary vascular pressures in the horse during exercise. These observations were essential to resolving the mechanistic bases for exercise-induced arterial hypoxemia and pulmonary haemorrhage (EIPH) that afflict all racehorses. Subsequently, Erickson pioneered the scientific proof-of-concept of the equine Nasal Strip™ which reduces lung damage and epistaxis, and constitutes the only effective non-pharmaceutical treatment for EIPH available today. We owe much of our understanding of equine cardiorespiratory physiology to this remarkable scientist.

Regional heterogeneity in the reactivity of equine small pulmonary blood vessels

Regional differences in large equine pulmonary artery reactivity exist. It is not known if this heterogeneity extends into small vessels. The hypothesis that there is regional heterogeneity in small pulmonary artery and vein reactivity to sympathomimetics (phenylephrine and isoproterenol) and a parasympathomimetic (methacholine) was tested using wire myography on small vessels from caudodorsal (CD) and cranioventral (CV) lung of 12 horses [9 mares, 3 geldings, 8.67 ± 0.81 (age ± SE) yr, of various breeds that had never raced]. To study relaxation, vessels were precontracted with U46619 (10(-6) M). Methacholine mechanism of action was investigated using L-nitroarginine methylester (L-NAME, 100 μM) and indomethacin (10 μM). Phenylephrine did not contract any vessels. Isoproterenol relaxed CD arteries more than CV arteries (maximum relaxation 28.18% and 48.67%; Log IC50 ± SE -7.975 ± 0.1327 and -8.033 ± 0.1635 for CD and CV, respectively, P < 0.0001), but not veins. Methacholine caused contraction of CD arteries (maximum contraction 245.4%, Log EC50 ± SE -6.475 ± 0.3341), and relaxation of CV arteries (maximum relaxation 40.14%, Log IC50 ± SE -6.791 ± 0.1954) and all veins (maximum relaxation 50.62%, Log IC50 ± SE -6.932 ± 0.1986) in a nonregion-dependent manner. L-NAME (n = 8, P < 0.0001) and indomethacin (n = 7, P < 0.0001) inhibited methacholine-induced relaxation of CV arteries, whereas indomethacin augmented CD artery contraction (n = 8, P < 0.0001). Our data demonstrate significant regional heterogeneity in small blood vessel reactivity when comparing the CD to the CV region of the equine lung.

Highly athletic terrestrial mammals: horses and dogs

Evolutionary forces drive beneficial adaptations in response to a complex array of environmental conditions. In contrast, over several millennia, humans have been so enamored by the running/athletic prowess of horses and dogs that they have sculpted their anatomy and physiology based solely upon running speed. Thus, through hundreds of generations, those structural and functional traits crucial for running fast have been optimized. Central among these traits is the capacity to uptake, transport and utilize oxygen at spectacular rates. Moreover, the coupling of the key systems--pulmonary-cardiovascular-muscular is so exquisitely tuned in horses and dogs that oxygen uptake response kinetics evidence little inertia as the animal transitions from rest to exercise. These fast oxygen uptake kinetics minimize Intramyocyte perturbations that can limit exercise tolerance. For the physiologist, study of horses and dogs allows investigation not only of a broader range of oxidative function than available in humans, but explores the very limits of mammalian biological adaptability. Specifically, the unparalleled equine cardiovascular and muscular systems can transport and utilize more oxygen than the lungs can supply. Two consequences of this situation, particularly in the horse, are profound exercise-induced arterial hypoxemia and hypercapnia as well as structural failure of the delicate blood-gas barrier causing pulmonary hemorrhage and, in the extreme, overt epistaxis. This chapter compares and contrasts horses and dogs with humans with respect to the structural and functional features that enable these extraordinary mammals to support their prodigious oxidative and therefore athletic capabilities.

The effect of sildenafil citrate administration on selected physiological parameters of exercising Thoroughbred horses

REASONS FOR PERFORMING STUDY

Sildenafil, a phosphodiesterase-5 inhibitor vasodilator, increases cGMP concentrations by inhibiting enzymatic degradation. Marketed to treat erectile dysfunction in men, it also reduces pulmonary arterial pressure (PAP). Because it reduces PAP, sildenafil may enhance performance and/or prevent exercise induced-pulmonary haemorrhage (EIPH).

OBJECTIVE

To determine if sildenafil citrate administration altered commonly measured indices of performance or reduced EIPH in exercised horses.

METHODS

Thirteen athletically conditioned Thoroughbred horses (2 mares and 11 geldings, age 3-12 years) were administered sildenafil citrate or placebo in 2 crossover design exercise testing studies. In a step-wise test to exhaustion, inspired/expired gas analysis, blood lactate, heart rate, runtime and bronchoalveolar lavage (BAL) cytology were measured. In a 13 m/s test to exhaustion, blood lactate, heart rate, runtime, BAL cytology and pulmonary arterial pressure were measured. Data were analysed with paired and unpaired t tests, one-way ANOVA and Tukey's pair-wise multiple comparison and Friedman repeated measure analysis of variance on ranks.

RESULTS

The administration of sildenafil did not alter mean inspired/expired gas measurements, plasma lactate concentrations or acute pulmonary haemorrhage in either exercise test or pulmonary arterial pressure measurement in the 13 m/s trial. Heart rates in both stress tests were significantly different at submaximal speeds and during the early recovery period. Run time was not affected by sildenafil administration in the step-wise trial (P = 0.622) or in the 13 m/s trial (P = 0.059).

CONCLUSIONS

Sildenafil did not alleviate pulmonary haemorrhage or enhance performance-related indices in these trials. Sildenafil administration altered cardiovascular adaptation to intense exercise as evidenced by altered heart rates at submaximal speeds and post exercise. The effect of these alterations on other performance perimeters was not evident.

Exercise-induced pulmonary haemorrhage during submaximal exercise

REASONS FOR PERFORMING STUDY

Maximally exercising horses achieve mean pulmonary artery pressures (Ppa(mean)) that exceed the minimum threshold (75 mmHg) estimated for pulmonary capillary rupture and exercise-induced pulmonary haemorrhage (EIPH). EIPH is not expected to occur during moderate submaximal exercise (i.e. 40-60% VO2max) since Ppa(mean) remains well below this threshold.

HYPOTHESIS

Prolonged submaximal exercise (trotting) would precipitate locomotory respiratory uncoupling and cause EIPH. This would be present as a result of the most negative intrapleural pressures (as estimated by the minimum oesophageal pressure; Poes(min)) occurring simultaneously with the most positive Ppa (Ppa(peak)) to produce estimated maximal pulmonary artery transmural pressures (PATMPmax) that surpass the EIPH threshold.

METHODS

Five Thoroughbred horses trotted to fatigue (approximately 25 min) at 5 m/sec on a 10% incline. Ventilation (V(E)), Poes, and Ppa were measured at 5 min intervals, and bronchoalveolar lavage (BAL) red blood cells (RBCs) were quantified 45 min post exercise.

RESULTS

BAL revealed an increased EIPH (rest: 2.0 +/- 1 x 10(5), exercise: 17 +/- 10 x 10(5) RBCs/ml BALF; P<0.05), despite the highest Ppamean reaching only mean +/- s.e. 55 +/- 3 mmHg, while V(E), tidal volume and Poes(min) approached 70-80% of the values achieved at maximal running speeds (10% incline: 12-13 m/sec) by these same horses. The resulting PATMPmax was well above the level considered causative of EIPH.

CONCLUSIONS

The finding of significant EIPH during submaximal exercise broadens the spectrum of performance horses susceptible to EIPH and supports studies that suggest that extravascular factors are of primary importance in the aetiology of EIPH.

POTENTIAL RELEVANCE

Consideration of strategies such as the equine nasal strip for reducing negative extravascular pressures is warranted even for exercise at moderate intensities.

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.

Regional distribution of collagen and haemosiderin in the lungs of horses with exercise-induced pulmonary haemorrhage

REASONS FOR PERFORMING STUDY

Regional veno-occlusive remodelling of pulmonary veins in EIPH-affected horses, suggests that pulmonary veins may be central to pathogenesis. The current study quantified site-specific changes in vein walls, collagen and haemosiderin accumulation, and pleural vascular profiles in the lungs of horses suffering EIPH.

HYPOTHESIS

In the caudodorsal lung regions of EIPH-affected horses, there is veno-occlusive remodelling with haemosiderosis, angiogenesis and fibrosis of the interstitium, interlobular septa and pleura.

METHODS

Morphometric methods were used to analyse the distribution and accumulation of pulmonary collagen and haemosiderin, and to count pleural vascular profiles in the lungs of 5 EIPH-affected and 2 control horses.

RESULTS

Vein wall thickness was greatest in the dorsocaudal lung and significantly correlated with haemosiderin accumulation. Increased venous, interstitial, pleural and septal collagen; lung haemosiderin; and pleural vascular profiles occurred together and changes were most pronounced in the dorsocaudal lung. Further, haemosiderin accumulation colocalised with decreased pulmonary vein lumen size. Vein wall thickening, haemosiderin accumulation and histological score were highly correlated and these changes occurred only in the caudodorsal part of the lung.

CONCLUSION

The colocalisation of these changes suggests that regional (caudodorsal) venous remodelling plays an important role in the pathogenesis of EIPH.

POTENTIAL RELEVANCE

The results support the hypothesis that repeated bouts of venous hypertension during strenuous exercise cause regional vein wall remodelling and collagen accumulation, venous occlusion and pulmonary capillary hypertension. Subjected to these high pressures, there is capillary stress failure, bleeding, haemosiderin accumulation and, subsequently, lung fibrosis.

The oxidant/antioxidant equilibrium in horses

Since "free radical research" started in 1954, understanding the role of oxidants and antioxidants in physiological and pathological conditions has increased continuously. Oxidants are essentially generated by metabolic enzymes, inflammatory cells and mitochondrial electron leakage; they are indispensable for the cellular redox regulation and may, under certain conditions, have a pro-inflammatory stimulatory role. Endogenous and exogenous antioxidants counterbalance the oxidative processes and so maintain the oxidant/antioxidant equilibrium. Excessive oxidant generation or antioxidant insufficiency can lead to oxidative stress. The aims of this review are: (1) to provide an insight into the concept of the oxidant/antioxidant equilibrium by briefly introducing the oxidant and the antioxidant systems; (2) to describe how the oxidant/antioxidant equilibrium or oxidative stress can be evaluated in horses, and (3) to summarise current knowledge about oxidative stress in equine medicine and equine exercise physiology.

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.

Oxidants and antioxidants in disease: oxidative stress in farm animals

Important infectious diseases in farm animals, such as pneumonia and enteritis, are thought to be associated with the so-called oxidative stress, i.e. a chemical phenomenon involving an imbalance in the redox status of the individual animal. The specifics of oxidative stress and how it may result in disease or be prevented are complex questions with no simple answers. However, the considerable literature on the subject suggests that many researchers consider oxidative stress-related mechanisms to be important early events in disease development. A particularly intriguing aspect is that, at least theoretically, oxidative stress should be easily prevented with antioxidants yet the use of antioxidants as therapy remains controversial. The present knowledge on oxidative stress in farm animals is the topic of this review.

Extra corporeal membrane oxygenation and plasmapheresis for pulmonary hemorrhage in microscopic polyangiitis

Early initiation of extracorporeal membrane oxygenation to treat acute hypoxemic respiratory failure secondary to massive pulmonary hemorrhage in microscopic polyangiitis in children can be life-saving while awaiting control of the autoimmune disease process by plasmapheresis and immunosuppression.

Chronic fatigue syndrome: intracellular immune deregulations as a possible etiology for abnormal exercise response

The exacerbation of symptoms after exercise differentiates Chronic fatigue syndrome (CFS) from several other fatigue-associated disorders. Research data point to an abnormal response to exercise in patients with CFS compared to healthy sedentary controls, and to an increasing amount of evidence pointing to severe intracellular immune deregulations in CFS patients. This manuscript explores the hypothetical interactions between these two separately reported observations. First, it is explained that the deregulation of the 2-5A synthetase/RNase L pathway may be related to a channelopathy, capable of initiating both intracellular hypomagnesaemia in skeletal muscles and transient hypoglycemia. This might explain muscle weakness and the reduction of maximal oxygen uptake, as typically seen in CFS patients. Second, the activation of the protein kinase R enzyme, a characteristic feature in atleast subsets of CFS patients, might account for the observed excessive nitric oxide (NO) production in patients with CFS. Elevated NO is known to induce vasidilation, which may limit CFS patients to increase blood flow during exercise, and may even cause and enhanced postexercise hypotension. Finally, it is explored how several types of infections, frequently identified in CFS patients, fit into these hypothetical pathophysiological interactions.

Exercise-induced pulmonary hemorrhage

EIPH is a condition affecting virtually all horses during intense exercise worldwide. The hemorrhage originates from the pulmonary vasculature and is distributed predominantly bilaterally in the dorsocaudal lung lobes. As the condition progresses, the lung abnormalities extend cranially along the dorsal portions of the lung. An inflammatory response occurs in association with the hemorrhage and may contribute to the chronic sequela. Although conflicting opinions exist as to its affect on performance, it is a syndrome that is thought to increase in severity with age. The most commonly performed method to diagnose EIPH at the present time is endoscopy of the upper airway alone or in combination with tracheal wash analysis for the presence of erythrocytes and hemosiderophages. Because horses may not bleed to the same extent every time and the bleeding may originate from slightly different locations, these diagnostic procedures may not be extremely sensitive or quantitative. At this time, there is no treatment that is considered a panacea, and the currently allowed treatments have not proven to be effective in preventing EIPH. Future directions for therapeutic intervention may need to include limiting inflammatory responses to blood remaining within the lungs after EIPH.