Bleeding into the lungs during exercise—exercise-induced pulmonary hemorrhage, or EIPH—has long been a source of questions for equine researchers. Why does it happen? How common is it? How does it impact horse health and performance?

Researchers have made great strides in understanding condition, which is common in horses that work at high speeds. And, almost ironically, they’ve learned that “in essence, the physiologic factors that enable horses to be such wonderful athletes are also at the heart of the phenomenon we call EIPH,” said Warwick Bayly, BVSc, MS, PhD, Dipl. ACVIM. “The things that enable these animals to perform at such an elite level are probably also responsible for the development of EIPH.”

At the 2021 American Association of Equine Practitioners’ Convention, held Dec. 4-8 in Nashville, Tennessee, Bayly, a professor of equine medicine at the Washington State University College of Veterinary Medicine, in Pullman, reviewed what researchers know and what they’re learning about EIPH.

The Basics

Simply put, EIPH is bleeding in the lungs during exercise that can result in blood accumulation in the pulmonary interstitium (a collection of support tissues in the lungs) and airways.

“It’s been documented, based on endoscopy, that there’s very a high prevalence amongst horses that engage in high-speed exercise,” said Bayly.

He said EIPH in horses is scored on a four-point scale. Grade 0 indicates no blood is present on an endoscopic exam, while Grade 4 represents multiple, coalescing streams of blood covering more than 90% of the tracheal surface with blood pooling at the thoracic inlet, where the esophagus enters the chest at the end of the neck.

Blood in the airways and pulmonary interstitial tissue can cause inflammation, especially after many repeated episodes. Occasionally, the inflammation progresses over time, exacerbates subsequent EIPH episodes, and results in pathology that could be career-ending or, rarely, life-threatening.

Bayly said tracheal endoscopy has long been the gold standard for diagnosing EIPH. But another test—post-exercise bronchoalveolar lavage (BAL)—appears to be a more sensitive diagnostic option.

Researchers recently found that 41 of 102 horses were negative for EIPH on tracheal endoscopy but had evidence of EIPH on BAL fluid analysis following a simulated race. Meanwhile, 99 of the 102 horses were positive for EIPH based on a red blood cell count of more than 992 cells/µl in recovered BAL fluid and/or tracheal endoscopy findings.

EIPH Pathophysiology

“The reason that the horses are such great athletes relates to their cardiovascular system and their ability to increase the circulating blood volume associated with the exercise-related activities in which they engage,” said Bayly.

He said it’s long been accepted that a main cause of EIPH is pulmonary capillary stress failure—when the horse’s normally low blood pressure in the lungs rises during exercise, the pulmonary capillaries have the potential to rupture, resulting in EIPH.

Especially in research from the past 20 years, “there’s a lot of reference to (EIPH) being related to pulmonary arterial hypertension, because there is no question that the blood pressure in the pulmonary artery gets extremely high during maximal exercise in horses,” he said.

However, researchers have shown that arterial pressures have a very small impact on capillary pressure. Rather, “the veins and the pressures in the veins are much more influential with regard to controlling capillary pressure in any part of the body, not just the lungs,” he said. “So that led us to ask the question, ‘Well, could the source of EIPH really be differences or changes in the left side of the heart or the left side of the circulation between the pulmonary capillaries and the heart?’

“It occurred to us that hypervolemia (increased blood volume) associated with splenic contraction might be the root cause for why there would be changes or alterations in left-sided blood pressures that would contribute to an elevation in the pulmonary capillary pressure.”

Among other functions, the horse’s spleen stores red blood cells and, during exercise, contracts to release red blood cells into the circulation and increase the horse’s oxygen transportation capacity. Splenic contraction can increase horses’ resting blood volumes by 40% to 50% during exercise, Bayly said.

As a result of the increased blood volume, “the heart rate jumps five- or sixfold,” he said. “Imagine, if you will, what happens when, basically, a container fills with fluid and you keep trying to put more fluid in. The sides may expand to some extent, especially if they’re reasonably malleable, but after a while they cannot expand anymore. Keep putting fluid in and, inevitably, the pressure increases and, after a while, … you get some back pressure.”

Because maximum filling pressure within the heart occurs at the end of diastole (the heartbeat phase when the muscle relaxes and allows the chambers to fill with blood) and diastolic pressure increases, the back pressure also increases. These increases likely impact pulmonary capillary pressure and, so, could result in EIPH.

“This is what we propose is happening in horses,” Bayly said.

EIPH in Performance Horses

Researchers have evaluated the effects of EIPH on performance in numerous studies, and Bayly reviewed several key findings:

• Horses with Grade 3 or Grade 4 EIPH (which tends to be less than 10% of horses) have a greater chance of experiencing reduced performance than horses with Grades 0, 1, or 2, he said.
• In one study researchers found that horses with more severe EIPH tended to run faster in the early and middle stages of races, but then they slow down near the end, he said. Horses with EIPH scores of 2 or less, however, tended to accelerate or run faster over the last three-eighths of a mile than in the early stages of the race.
• In ongoing research scientists have demonstrated that EIPH severity gradually increases with the number of race starts. “This is work we’ve done on about 150 stakes horses this year in the U.S.,” he added. “This (continues) work from a study from Western Australia that demonstrated after about 50 starts EIPH tended to increase or get more severe. We found that, probably, the cutoff is more like 25 to 30 starts, although there are not a lot of stakes horses who have that many starts if they’re really, really good.”
• Finally, researchers have found that the score of one EIPH episode isn’t necessarily predictive of the next, but there tends to be an association. “If you were a 0 last time, you’re probably going to be, at worst, a 1 or a 2. If you’re a 4 last time, you’re probably going to bleed again; there’s not much chance it’ll be a 0,” Bayly said. “But there is variation. A Grade 0 might be a Grade 2 (this time). A Grade 3 last time could be a 0 this time. We just have to bear that in mind. It reflects what I would call the vagaries of EIPH.”

Questions Remain

While researchers have made great strides in understanding EIPH and how it impacts high-performance horses, there are still questions to be answered, Bayly said.

“How do we quantify … how much blood is actually in the interstitium, and is this causing an inflammatory problem?” he said. “Does bronchial circulation play a role? Why do some horses develop these massive fibrotic pulmonary veins, whereas the majority don’t? How much does upper airway obstructive disease (such as recurrent laryngeal neuropathy) contribute to the severity of EIPH?”

Researchers are continuing work to understand these complexities.

Take-Home Message

In closing, Bayly said EIPH is unavoidable in horses performing maximally and at high speeds. But, he added, “the critical question is not really—or should not be—did the horse bleed, but how severe was the hemorrhage? If you see (nothing) in the trachea with your endoscope, my response would be, ‘Well, the horse probably bled a little but not enough to really be concerned about.’ ”

While EIPH can cause decreased performance in a small population of horses—particularly those with 30 or more starts on the track—“most of the lesions caused by EIPH are microscopic or submicroscopic,” he said.

“And, finally,” he noted, “even though people get very concerned about it, in horses that have suffered EIPH, sometimes severely, it rarely—or never—leads to respiratory disease once they reach .”