Upper airway obstruction (partial asphyxia) as the possible cause of exercise-induced pulmonary hemorrhage in the horse: An hypothesis

Pulmonary bleeding in racehorses: A gross, histologic, and ultrastructural comparison of exercise-induced pulmonary hemorrhage and exercise-associated fatal pulmonary hemorrhage

Exercise-induced pulmonary hemorrhage (EIPH) is a common condition of Thoroughbred racehorses that is usually responsible for reduced performance, while exercise-associated fatal pulmonary hemorrhage (EAFPH) is characterized by severe pulmonary bleeding of unknown pathogenesis resulting in sudden death during strenuous exercise. The aim of the study was to characterize and compare anamnestic data together with pulmonary gross, histologic, and ultrastructural findings in racehorses with EIPH (n = 10), EAFPH (n = 10), and control horses (n = 5). No differences in anamnesis were identified between the 3 groups. Grossly cranial lobe reddening and edema scores were significantly more prevalent and severe in the EAFPH group compared with the EIPH and control groups. Histologically, hemorrhage scores were higher in the EAFPH group, while hemosiderophages, iron encrustations of collagen and elastin fibers, and vascular remodeling scores were significantly higher in EIPH group compared with the EAFPH and control groups. In all groups, caudal lung locations exhibited a significantly higher score for vascular remodeling, hemosiderophage accumulation, iron encrustation, and type II pneumocyte hyperplasia when compared with cranial, dorsal, and ventral locations. Ultrastructural analysis of perivascular collagen showed fibrils with significantly larger diameters in the EAFPH group compared with the EIPH group but not compared with the control group. This study demonstrates that lungs of horses that experienced EAFPH show significantly less vascular remodeling and other long-term pulmonary abnormalities that characterize horses with EIPH.

Upper and Lower Airways Evaluation and Its Relationship with Dynamic Upper Airway Obstruction in Racehorses

Dynamic upper airway obstructions (DUAO) are common in racehorses, but their pathogenetic mechanisms have not been completely clarified yet. Multiple studies suggest that alterations of the pharyngo-laryngeal region visible at resting endoscopy may be predictive of the onset of DUAO, and the development of DUAO may be associated with pharyngeal lymphoid hyperplasia (PLH), lower airway inflammation (LAI) and exercise-induced pulmonary hemorrhage (EIPH). The present study aims to investigate the possible relationship between the findings of a complete resting evaluation of the upper and lower airways and DUAO. In this retrospective study, 360 racehorses (Standardbreds and Thoroughbreds) referred for poor performance or abnormal respiratory noises were enrolled and underwent a diagnostic protocol including resting and high-speed treadmill endoscopy, cytological examination of the bronchoalveolar lavage fluid and radiographic assessment of the epiglottis length. In this population, epiglottis flaccidity was associated with dorsal displacement of the soft palate, while no relationship was detected between DUAO and epiglottis length. No associations were detected between DUAO and PLH, LAI or EIPH. In conclusion, it is likely that epiglottis plays a role in upper airway stability, while airways inflammation does not seem to be involved in the pathogenesis of DUAO.

Equine Welfare during Exercise: An Evaluation of Breathing, Breathlessness and Bridles

Simple Summary

Horses have superior athletic capabilities due largely to their exceptional cardiorespiratory responses during exercise. This has particular relevance to horses’ potential to experience breathlessness, especially when their athletic performance is reduced by impaired respiratory function. Breathlessness, incorporating three types of unpleasant experiences, has been noted as of significant animal welfare concern in other mammals. However, the potential for breathlessness to occur in horses as usually ridden wearing bitted bridles has not yet been evaluated in detail. Accordingly, key physiological responses to exercise and the consequences of impaired respiratory function are outlined. Then the physiological control of breathing and the generation of the aversive experiences of breathlessness are explained. Finally, the potential for horses with unimpaired and impaired respiratory function to experience the different types of breathlessness is evaluated. This information provides a basis for considering the circumstances in which breathlessness may have significant negative welfare impacts on horses as currently ridden wearing bitted bridles. Potential beneficial impacts on respiratory function of using bitless bridles are then discussed with emphasis on the underlying mechanisms and their relevance to breathlessness. It is noted that direct comparisons of cardiorespiratory responses to exercise in horses wearing bitless and bitted bridles are not available and it is recommended that such studies be undertaken.

Abstract

Horses engaged in strenuous exercise display physiological responses that approach the upper functional limits of key organ systems, in particular their cardiorespiratory systems. Maximum athletic performance is therefore vulnerable to factors that diminish these functional capacities, and such impairment might also lead to horses experiencing unpleasant respiratory sensations, i.e., breathlessness. The aim of this review is to use existing literature on equine cardiorespiratory physiology and athletic performance to evaluate the potential for various types of breathlessness to occur in exercising horses. In addition, we investigate the influence of management factors such as rein and bit use and of respiratory pathology on the likelihood and intensity of equine breathlessness occurring during exercise. In ridden horses, rein use that reduces the jowl angle, sometimes markedly, and conditions that partially obstruct the nasopharynx and/or larynx, impair airflow in the upper respiratory tract and lead to increased flow resistance. The associated upper airway pressure changes, transmitted to the lower airways, may have pathophysiological sequelae in the alveolae, which, in their turn, may increase airflow resistance in the lower airways and impede respiratory gas exchange. Other sequelae include decreases in respiratory minute volume and worsening of the hypoxaemia, hypercapnia and acidaemia commonly observed in healthy horses during strenuous exercise. These and other factors are implicated in the potential for ridden horses to experience three forms of breathlessness—”unpleasant respiratory effort”, “air hunger” and “chest tightness”—which arise when there is a mismatch between a heightened ventilatory drive and the adequacy of the respiratory response. It is not known to what extent, if at all, such mismatches would occur in strenuously exercising horses unhampered by low jowl angles or by pathophysiological changes at any level of the respiratory tract. However, different combinations of the three types of breathlessness seem much more likely to occur when pathophysiological conditions significantly reduce maximal athletic performance. Finally, most horses exhibit clear behavioural evidence of aversion to a bit in their mouths, varying from the bit being a mild irritant to very painful. This in itself is a significant animal welfare issue that should be addressed. A further major point is the potential for bits to disrupt the maintenance of negative pressure in the oropharynx, which apparently acts to prevent the soft palate from rising and obstructing the nasopharynx. The untoward respiratory outcomes and poor athletic performance due to this and other obstructions are well established, and suggest the potential for affected animals to experience significant intensities of breathlessness. Bitless bridle use may reduce or eliminate such effects. However, direct comparisons of the cardiorespiratory dynamics and the extent of any respiratory pathophysiology in horses wearing bitted and bitless bridles have not been conducted. Such studies would be helpful in confirming, or otherwise, the claimed potential benefits of bitless bridle use.

Exercise-induced pulmonary haemorrhage in horses – review

Exercise-induced pulmonary haemorrhage is a major cause of poor performance in the equine athlete. It is an important cause of exercise intolerance and results from strenuous exercise and pathophysiological changes in the equine lung and possibly in the airways. Endoscopic surveys of the respiratory tracts of horses after competitive events have shown that many horses experience exercise-induced pulmonary haemorrhage. The reported incidence of exercise-induced pulmonary haemorrhage in different breeds varies between 40–85%. The cause of bleeding in exercising horses has fostered considerable debate over the past three centuries, but currently, the most accepted hypothesis is that the source of haemorrhage is disruption of the pulmonary capillaries during exercise. Furosemide is the medication used most widely for the treatment and prevention of exercise-induced pulmonary haemorrhage. This review provides an update on the aetiology, clinical signs, physiopathology, diagnosis and treatment of exercise-induced pulmonary haemorrhage.

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.

Cohort study examining long-term respiratory health, career duration and racing performance in racehorses that undergo left-sided prosthetic laryngoplasty and ventriculocordectomy surgery for treatment of left-sided laryngeal hemiplegia

REASONS FOR PERFORMING STUDY

The risk of respiratory conditions, such as inflammatory airway disease (IAD) and exercise-induced pulmonary haemorrhage (EIPH), are thought to be higher in racehorses that undergo prosthetic laryngoplasty with ventriculocordectomy (PLVC) surgery to treat left-sided laryngeal hemiplegia (LLH) than in racehorses with normal laryngeal function. However, this has not been investigated formally owing to the difficulty of obtaining reliable follow-up data.

OBJECTIVES

To determine the incidence of respiratory conditions (IAD and EIPH), duration of racing career, number of starts and number of starts for which stakes money was earned in racehorses that had undergone PLVC surgery to treat LLH, compared with racehorses that did not have LLH or undergo any laryngeal surgery.

METHODS

A retrospective cohort study design was used, with surgical, clinical and race data of Thoroughbred racehorses obtained from the time of importation until retirement. The surgical cohort consisted of racehorses that had undergone PLVC for LLH and met specific inclusion criteria. Every surgical case was matched, according to trainer, year of import into Hong Kong and pre-import international handicap rating, to 2 unexposed racehorses.

RESULTS

Respiratory conditions, such as excessive tracheal mucus and epistaxis due to severe EIPH, were significantly increased in the surgical cohort, compared with the matched unexposed cohort (P values <0.001 and <0.004, respectively). Racing career duration in the surgical cohort was significantly shorter than in the unexposed cohort, which was primarily due to retirement because of epistaxis. The number of race starts was fewer in the surgical than in the unexposed cohort after surgery/matching, but the number of starts for which stakes money was earned was not significantly different.

CONCLUSIONS AND POTENTIAL RELEVANCE

Owners and trainers should be advised that racehorses with LLH that undergo PLVC surgery are at an increased risk of respiratory conditions (IAD and severe EIPH), which is likely to shorten their racing career compared to racehorses with normal laryngeal function. Racing performance in terms of race starts was significantly less in racehorses that had undergone PLVC surgery; however, the number of starts for which stakes money was earned was similar to those racehorses that were unexposed.

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.

Platelet function in the racing thoroughbred: implication for exercise-induced pulmonary hemorrhage

Platelet function was evaluated in horses with exercise-induced pulmonary hemorrhage (bleeder) and in control horses (nonbleeder). Platelet aggregation, secretion, and adhesion to rabbit aortic subendothelium were similar for bleeders and nonbleeders. Platelets readily aggregated in response to ADP, thrombin, collagen, and arachidonic acid, but platelet secretion occurred only with high concentrations of thrombin. Platelets readily adhered to rabbit aortic subendothelium and tended to form large thrombi rather than platelet monolayers or aggregates. These data suggest that horses may be predisposed to thrombus formation and subsequent microvascular obstruction.

Evidence of an association between inflammatory airway disease and EIPH in young Thoroughbreds during training

In an epidemiological study of risk factors for exercise-induced pulmonary haemorrhage (EIPH) in young Thoroughbreds in the UK, in which 148 horses contributed 1614 horse-months of data, there were 64 (4%) episodes of endoscopically visible tracheal bleeding and 824 (51%) episodes of increased quantities of haemosiderophages in tracheal washes. There were increases in prevalence and risk of EIPH by both definitions with age from < or = 2- > or = 4 years, season of sampling from winter (Nov-Jan) to autumn (Aug-Oct) and several different measures of airway inflammation, including tracheal mucus, neutrophil proportion, inflammation score and fungal material in tracheal washes. There was considerable variability in the prevalence of EIPH between trainers. EIPH in the preceding month significantly increased the risk of the condition the following month. There was no evidence that EIPH was associated with infection of the airways with even large numbers of Streptococcus zooepidemicus or Pasteurella-like spp., which are significantly associated with airway inflammation in younger racehorses. Multiple logistic regression modelling that took account of random variability between horses and the effects of each trainer and an episode the preceding month, confirmed that after controlling for the other risk factors, EIPH was still significantly associated with increasing age, different seasons, airway inflammation and evidence of airway fungal material.

Effects of an external nasal strip and frusemide on pulmonary haemorrhage in Thoroughbreds following high-intensity exercise

The purpose of this study was to examine the effects of an external nasal strip (NS), frusemide (FR) and a combination of the 2 treatments (NS + FR) on exercise-induced pulmonary haemorrhage (EIPH) in Thoroughbred horses. It was hypothesised that both the NS and FR would attenuate EIPH as assessed by red blood cell count in bronchoalveolar lavage fluid. In random order, 8 horses completed each of 4 sprint exercise tests on a treadmill: 1) NS; 2) FR (0.5 mg/kg bwt i.v., 4 h pre-exercise); 3) NS + FR; and 4) control (C; no treatment). After a 5 min warm-up (4.5 m/s), horses completed 2 min running at 120% maximum oxygen consumption (VO2max) with the treadmill set at 3 degrees incline. Mean +/- s.d. running speed was 14.2+/-0.2 m/s. In the FR and NS + FR trials, horses carried weight equal to that lost as a result of frusemide administration. During exercise at 120% Vo2max, oxygen consumption (Vo2) and carbon dioxide production (Vco2) were measured at 15 s intervals. Plasma lactate concentration was measured in samples collected before exercise, at the end of the sprint and after 5 min cool-down at the trot. Thirty minutes after the run, bronchoalveolar lavage (BAL) was performed and the red cell count in the fluid quantified. Vo2 and Vco2 were significantly lower in NS and NS + FR trials than in the C and FR trials at the end of the sprint exercise protocol. However, plasma lactate concentrations did not differ among treatments. Compared with the C trial (61.1+/-30.5 x 10(6) red blood cells/ml BAL fluid), pulmonary haemorrhage was significantly (P<0.05) decreased in both the NS (15.9+/-4.0 x 106 RBC/ml) and FR (12.2+/-5.8 x 10(6) RBC/ml) trials. EIPH in the NS + FR trial (7.9+/-1.0 x 10(6) RBC/ml) was further diminished (P<0.05) compared to the NS trial, but not different from the FR trial. We conclude that both the external nasal strip and frusemide attenuate pulmonary haemorrhage in Thoroughbred horses during high-speed sprint exercise. The external nasal strip appears to lower the metabolic cost of supramaximal exertion in horses. Given the purported ergogenic effects of frusemide, the external nasal strip is a valuable alternative for the attenuation of EIPH.

Efficacy of nasal strip and furosemide in mitigating EIPH in Thoroughbred horses

The purpose of this investigation was to study the effects of an equine nasal strip (NS), furosemide (Fur), and a combination of both (NS + Fur) on exercise-induced pulmonary hemorrhage (EIPH) at speeds corresponding to near-maximal effort. Five Thoroughbreds (526 +/- 25 kg) were run on a flat treadmill from 7 to 14 m/s in 1 m x s(-1) x min(-)1 increments every 2 wk (treatment order randomized) under control (Con), Fur (1 mg/kg iv 4 h prior), NS, or NS + Fur conditions. During each run, pulmonary arterial (Ppa) and esophageal (Pes) pressures were measured. Severity of EIPH was quantified via bronchoalveolar lavage (BAL) 30 min postrun. Furosemide (Fur and NS + Fur trials) reduced peak Ppa approximately 7 mmHg compared with Con (P < 0.05) whereas NS had no effect (P > 0.05). Maximal Pes swings were not different among groups (P > 0.05). NS significantly diminished EIPH compared with the Con trial [Con, 55.0 +/- 36.2; NS, 30.8 +/- 21.8 x 10(6) red blood cells (RBC)/ml BAL fluid; P < 0.05]. Fur reduced EIPH to a greater extent than NS (5.2 +/- 3.0 x 10(6) RBC/ml BAL; P < 0.05 vs. Con and NS) with no additional benefit from NS + Fur (8.5 +/- 4.2 x 10(6) RBC/ml BAL; P > 0.05 vs. Fur, P < 0.05 vs. Con and NS). In conclusion, although both modalities (NS and Fur) were successful in mitigating EIPH, neither abolished EIPH fully as evaluated via BAL. Fur was more effective than NS in constraining the severity of EIPH. The simultaneous use of both interventions appears to offer no further gain with respect to reducing EIPH.

Pulmonary capillary pressure in horses undergoing alteration of pleural pressure by imposition of various upper airway resistive loads

We hypothesized that changes in pleural pressure induced by resistive breathing would affect transmural pulmonary artery, pulmonary capillary, and pulmonary wedge pressures. Seven horses were assigned to exercise with each of 4 upper respiratory resistive loads in random order at intervals of at least 2 days: 1) control--no added resistive loads; 2) inspiratory resistive load (Iobst)--left laryngeal hemiplegia; 3) expiratory resistive load (Eobst)--one-way valve in the right nostril; and 4) combined inspiratory and expiratory resistive loads (CIEobst)--left nostril occlusion. On each occasion, the horses performed an incremental exercise protocol consisting of exercise episodes of 3 min duration at 75, 90, and 100% of maximal heart rate (HRmax). Pulmonary artery and oesophageal pressures were recorded continuously. Subsequent analysis was carried out on the pulmonary arterial pressure signal with the oesophageal pressure signal subtracted, hence the pulmonary vascular pressures in this paper approximate transmural pressures. Pulmonary vascular pressures, heart rate, and arterial blood gas tensions were measured at each level of exercise. Pulmonary capillary and pulmonary wedge pressures were determined from the pulmonary artery waveform after dynamic occlusion of a branch of the pulmonary artery. During exercise, peak expiratory oesophageal pressure was more positive in horses with Eobst and CIEobst (adjusted means = 43, and 39 mmHg, respectively) compared with control (adjusted mean = 23 mmHg) (P = 0.0001). Peak inspiratory oesophageal pressure was more negative in horses at exercise with Iobst and CIEobst (adjusted means = -42 and -39 mmHg, respectively) compared with control (adjusted mean = -26 mmHg) (P = 0.0012). Eobst was associated with an increase in mean oesophageal pressure while Iobst was associated with a decrease in mean oesophageal pressure. There were significant increases in mean pulmonary artery pressure in horses with CIEobst (adjusted means = 82 mmHg) and in pulmonary wedge pressure in horses with CIEobst and Iobst (adjusted means = 51, and 55 mmHg, respectively) when compared to control (73 and 42 mmHg, respectively) (P = 0.0001). Pulmonary capillary pressure was significantly increased in horses with CIEobst or Iobst (adjusted means = 61 mmHg, 63 mmHg, respectively) when compared to control (adjusted mean = 50 mmHg)(P = 0.0001). At maximal exercise intensity with inspiratory obstruction, the mean oesophageal (pleural) pressure was -17 mmHg while the mean pulmonary capillary pressure was 77 mmHg. The latter exceeds the reported 75 mmHg threshold for capillary failure in horses. We conclude that inspiratory resistive breathing can lead to a significant increase in transmural pulmonary capillary pressure which may contribute to loss of capillary integrity and rupture.

Quantification of exercise-induced pulmonary haemorrhage with bronchoalveolar lavage

Exercise-induced pulmonary haemorrhage (EIPH) causes serious economic losses in the horse racing industry. Endoscopic examination indicates that 40-90% of horses exhibit EIPH following sprint exercise, but the limitations of the endoscope prevent diagnosis in many horses. Bronchoalveolar lavage (BAL) was utilised to detect red blood cells (RBCs) in the terminal airways in 6 horses. Two lavages were performed at weekly intervals prior to exercise, one within 90 min after exercise, and 5 at weekly intervals after exercise. The horses were exercised strenuously at 12.5-14.6 m/s on a treadmill (3 degree incline). Heart rates ranged from 192-207 beats/min, and mean pulmonary arterial pressures (mPAP) ranged from 80-102 mmHg. Neither epistaxis nor endoscopic evidence of EIPH was seen in any of the 6 horses following exercise. However, the number of RBCs in the lavage fluid increased significantly over control values immediately after exercise in all horses but returned to control values by one week after exercise. Haemosiderophages in the BAL fluid did not increase until one week after exercise and remained elevated for 3 weeks after exercise. Twenty per cent of the total population of alveolar macrophages contained haemosiderin. A positive relationship occurred between the number of RBCs in the lavage fluid and mPAP; the amount of haemorrhage increased as the mPAP exceeded 80 to 90 mmHg. The results with BAL used as the diagnostic tool, suggest that all strenuously exercised horses may exhibit EIPH; the amount of haemorrhage appears to be associated with the magnitude of the high pulmonary arterial pressure.

Stress failure of pulmonary capillaries as a mechanism for exercise induced pulmonary haemorrhage in the horse

Exercise induced pulmonary haemorrhage (EIPH) is a serious problem in the Thoroughbred industry. The condition apparently occurs essentially in all Thoroughbreds in training but the mechanism has proved elusive. There is now strong evidence that the condition is caused by mechanical failure of the walls of the pulmonary capillaries when the pressure inside them rises to very high levels. It is well known that pulmonary capillaries have extremely thin walls to allow rapid exchange of respiratory gases across them. Recently we have shown that the wall stresses are very large when the capillary transmural pressure is raised, and in anesthetised rabbits, ultrastructural damage to the walls is seen at pressures of 40 mmHg and above. The incidence of stress failure is greatly increased at high lung volumes; and many of the ultrastructural changes are rapidly reversible when the capillary pressure is reduced. The principal forces acting on the capillary have been analysed. The strength of the thin part of the capillary wall can be attributed to the Type IV collagen in the extracellular matrix. The pulmonary vascular pressures of galloping Thoroughbreds reach very high levels. Mean pulmonary artery and left atrial pressures of up to 120 and 70 mmHg respectively have been directly measured with indwelling catheters. The reason for the high pulmonary vascular pressures is that these animals have been selectively bred over hundreds of years to run at great speeds over short distances and their maximal oxygen consumptions are very high. As a consequence, cardiac outputs are substantial, and the left ventricle needs very high filling pressures.(ABSTRACT TRUNCATED AT 250 WORDS)

Exercise-induced pulmonary hemorrhage

This article addresses many aspects of exercise-induced pulmonary hemorrhage (EIPH). Reports of the prevalence, effect on performance, and the clinical signs and means of diagnosis of EIPH are included. Radiologic and scintigraphic findings in horses with EIPH are reported. Pathogenesis and treatment are discussed.