Pulmonary artery, aortic and oesophageal pressure changes during high intensity treadmill exercise in the horse: a possible relation to exercise-induced pulmonary haemorrhage

Real-time three-dimensional (3D) echocardiographic characterisation of an atrial septal defect in a horse

Atrial septal defects have been well reported in humans and dogs and the principles of intervention have been well established. In contrast, there is very little information published about these congenital anomalies in horses. True ASDs are regarded as rare and little is known about the clinical significance of these defects in horses. An 11‐year‐old Thoroughbred gelding with a history of poor performance was diagnosed with an atrial septal defect, measuring approximately 2 cm in diameter, on 2D transthoracic echocardiography. Real‐time three‐dimensional (3D) echocardiography was used to map the structure of the defect and was useful in fully characterising the ASD in this case.

Complete upper airway collapse and apnoea during tethered swimming in horses

BACKGROUND

There is limited knowledge of the breathing strategy and impact on the patency of the upper respiratory tract (URT) in swimming horses.

OBJECTIVES

To describe the respiratory responses and endoscopic appearance of the URT during tethered swimming in horses.

STUDY DESIGN

Prospective descriptive study.

METHODS

Ten race-fit horses, with no history of URT obstruction, were examined during tethered swimming. Endoscopic examination, heart rate, sound recordings and above and below water video recordings were obtained. Plasma lactate concentration was measured before and 5 min after swimming and tracheal endoscopy was performed 30 min after exercise to assess for presence of blood or mucus. Four horses also underwent endoscopy during exercise on the track.

RESULTS

Mean (±s.d.) breathing frequency was 28 ± 5 breaths/min during swimming, with a brief inspiration (mean ± s.d. T_I  = 0.51 ± 0.08 s), followed by a period of apnoea (1.59 ± 0.53 s) and then a short, forced expiration (T_E  = 0.42 ± 0.5 s). During apnoea all horses exhibited complete collapse of the URT including closure of the external nares, nasopharynx and rima glottidis (with bilateral adduction of the arytenoid cartilages and vocal folds) and, in two horses, epiglottic retroversion. No horses had URT collapse during overground exercise. Locomotor-respiratory coupling was not observed during swimming. Median (IQR) plasma lactate post swim was 4.71 mmol/L (2.08-8.09 mmol/L) vs 0.68 mmol/L (0.65-0.71 mmol/L) preswim. Post swim endoscopy revealed grade 1 exercise-induced pulmonary haemorrhage (EIPH) in 2 horses. Median mucus grade was 1 (range 0-3).

MAIN LIMITATIONS

Overground endoscopy was not performed in all horses.

CONCLUSIONS

Horses experienced complete URT collapse associated with post inspiratory apnoea when swimming. The reason for this is unknown but may be to aid buoyancy or associated with the mammalian dive response - a survival reflex to preserve oxygen stores and prevent water entering the lungs.

The heart remains the core: cardiac causes of poor performance in horses compared to human athletes

Cardiac remodelling occurs in response to exercise and is generally beneficial for athletic performance due to the increase in cardiac output. However, this remodelling also may lead to an increased prevalence of cardiac murmurs and arrhythmias. In most cases, these are not considered to be significant. However, in some cases, there may be potentially deleterious consequences. Whilst sudden cardiac death (SCD) is a rare occurrence, the consequences are catastrophic for both the horse and potentially the rider or driver. Furthermore, the sudden death of a horse in the public arena has negative connotations in regards to public perception of welfare during equestrian sports. Prediction of which individuals might be susceptible to potential deleterious effects of exercise is a focus of interest in both human and equine athletes but remains a challenge because many athletes experience cardiac murmurs and exercise-induced arrhythmias that are clinically irrelevant. This review summarises the effects of exercise on cardiac remodelling in the horse and the potential effects on athletic performance and SCD. The use of biomarkers and their future potential in the management of athletic horses is also reviewed.

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.

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.

Acetazolamide attenuates transvascular fluid flux in equine lungs during intense exercise

  During intense exercise in horses the transvascular fluid flux in the pulmonary circulation (Jv-a) represents 4% of cardiac output (Q). This fluid flux has been attributed to an increase in pulmonary transmural hydrostatic forces, increases in perfused microvascular surface area, and reversible alterations in capillary permeability under conditions of high flow and pressure. Erythrocyte fluid efflux, however, accounts for a significant fraction of Jv-a. In the lung the Jacobs-Stewart cycle occurs with diffusion of CO2 into alveolar space with possible accompanying chloride (Cl-) and water movement from the erythrocyte directly into the pulmonary interstitium. We hypothesised that inhibition of carbonic anhydrase in erythrocytes inhibits the Jacobs-Stewart cycle and attenuates Jv-a. Five horses were exercised on a treadmill until fatigue without (control) and with acetazolamide treatment (30 mg kg(-1) 30 min before exercise). Erythrocyte fluid efflux, plasma fluid flux across the lung and Jv-a were calculated using haemoglobin, haematocrit, plasma protein and Q. Fluid fluxes were used to calculate erythrocyte, plasma and whole blood Cl- fluxes across the lung. Cardiac output was not different between control and acetazolamide treatment. During exercise erythrocyte fluid efflux and Jv-a increased in control (9.3±3.3 and 11.0±4.4 l min(-1), respectively) and was higher than after acetazolamide treatment (3.8±1.6 and 1.2±1.2 l min(-1), respectively) (P<0.05). Plasma fluid flux did not change from rest in control and decreased after acetazolamide treatment (-4.5±1.5 l min(-1)) (P<0.05). Erythrocyte Cl- flux increased during exercise in control and after acetazolamide treatment (P<0.05). During exercise plasma Cl- flux across the lung did not change in control; however, it increased with acetazolamide treatment (P=0.0001). During exercise whole blood Cl- flux increased across the lung in control (P<0.05) but not after acetazolamide treatment. The results indicate that Jv-a in the lung is dependent on the Jacobs-Stewart cycle and mostly independent of transmural hydrostatic forces. It also appears that Jv-a is mediated by Cl- and water egress from erythrocytes directly into the interstitium without transit through plasma.

Transvascular fluid flux from the pulmonary vasculature at rest and during exercise in horses

Exercise causes changes in pulmonary haemodynamics through redistribution of blood flow, increase in the pulmonary surface area, and increase in pulmonary vascular pressures. These changes contribute to the increase in fluid exchange across the alveolar-capillary barrier. To determine the extent of the fluid exchange across the alveolar-capillary barrier at rest and during exercise, six horses were exercised on a high-speed treadmill until fatigue. Arterial and mixed venous blood were sampled at rest and during exercise and recovery. Blood volume changes across the lung (DeltaBV; measured in percentage) were calculated from changes in plasma protein and haemoglobin concentration, and haematocrit. Cardiac output (Q) was calculated using the Fick equation. Fluid flux (J(V-A); measured in l min(-1)) across the alveolar-capillary barrier was then quantified based on Q and DeltaBV. At rest, no fluid movement occurred across the pulmonary vasculature (0.6 +/- 0.6 l min(-1)). During exercise, the amount of fluid moved from the pulmonary circulation was 8.3 +/- 1.3 l min(-1) at 1 min, 6.4 +/- 2.9 l min(-)(1) at 2 min, 10.1 +/- 1.0 l min(-1) at 3 min, 12.9 +/- 2.5 l min(-1) at 4 and 9.6 +/- 1.5 l min(-1) at fatigue (all P < 0.0001). Erythrocyte volume decreased by 6% (P < 0.01) across the lungs, which decreased the colloid osmotic gradient in the pulmonary vasculature. Decrease colloid osmotic gradient along with increased hydrostatic forces in the pulmonary vasculature would enhance displacement of fluid into the pulmonary interstitium. In conclusion, exercise caused large increases in transpulmonary fluid fluxes in horses. Here, we present a simple method to calculate transpulmonary fluid fluxes in different species, which can be used to elucidate mechanisms of lung fluid balance in vivo.

Nitric oxide and exercise in the horse

1. The effects of exercise on the production rate of nitric oxide (NO) in exhaled air (VNO) and the effects of inhaled NO (80 p.p.m.) on cardiovascular and respiratory parameters were investigated in five Throughbred horses. 2. The concentration of NO ([NO]) in exhaled air collected from within the nasal opening was lower when collected at a high flow rate of 80 l min-1 than at a low flow rate of 20 l min-1: when trotting at 3.7 m s-1 the values were 0.78 +/- 0.15 and 1.23 +/- 9.14 p.p.b., respectively, and when cantering at 9 m s-1 the values were 1.69 +/- 0.31 and 2.25 +/- 0.32 p.p.b., respectively. 3. Nebulized methoxamine (40 mg ml-1 for 60 s), an alpha 1-adrenergic agonist, further reduced [NO] during the 9 m s-1 canter to 1.05 +/- 0.14 and 1.99 +/- 0.41 p.p.b. when collected at 80 and 20 l min-1, respectively, and induced cyclical changes in the breathing pattern. 4. Exercise induced a linear increase in VNO with work intensity to a maximum (428.1 +/- 31.6 pmol min-1 kg-1) which coincided with the maximal oxygen uptake for the horses (138.3 +/- 11.7 ml min-1 kg-1), although a further increase in VNO (779.3 +/- 38.4 pmol min-1 kg-1) occurred immediately after exercise. The changes in VNO correlated well with the tidal volume (r = 0.968; P < 0.01) and the haematocrit (r = 0.855; P < 0.01). 5. In the first 2 min of high intensity exercise, inhaled NO (80 p.p.m.) significantly (P < 0.05) reduced the pulmonary artery pressure: during the first minute, pulmonary artery pressure was 83.1 +/- 7.6 mmHg compared with a control value of 94.4 +/- 6.3 mmHg, and during the second minute, 84.2 +/- 7.1 mmHg compared with a control value of 98.4 +/- 4.7 mmHg. There were no other significant changes in cardiovascular or respiratory indices, including cardiac output, measured during exercise between control and inhaled NO tests. 6. The results show that exhaled NO is released from the airways of the horse and may contribute to the regulation of pulmonary vascular tone during exercise.

Vulnerability of pulmonary capillaries in heart disease

The pulmonary blood-gas barrier presents a dilemma. It must be extremely thin for efficient gas exchange. However, it also needs to be immensely strong because the stresses in the pulmonary capillary wall become extremely high when the capillary pressure rises. Stress failure of the capillaries occurs in several pathological conditions. It causes high-permeability edema as in neurogenic pulmonary edema or high-altitude pulmonary edema; alveolar hemorrhage, which occurs in all galloping racehorses; or a combination of the two as in severe congestive heart failure. The vulnerability of the capillary wall to increased mechanical stress has not previously been sufficiently appreciated.