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Clearing the airway
Studies show equine nasal strips reduce airway resistance and mitigate exercise-induced pulmonary hemorrhage.


DVM360 MAGAZINE


Further investigation

David Poole, PhD, DSc, a professor of physiology at Kansas State University, says he wasn't ready to buy into the idea of an equine nose strip. "When first presented with the strips I was skeptical about their ability to modify upper airway structure and function," he says. "Over several years we'd been measuring the metabolic rate of horses running on the treadmill, and our studies used a facemask to measure pulmonary gas exchange. When we took the facemask off, we paid close attention to the rostral incisive notch region and noticed a partial collapse of the nasal passages.

"Some horses show a pronounced nasal collapse," Poole continues. "We believe that the extent depends on their exact geometry, but there is often an area or region of 3 to 4 centimeters where narrowing occurs, as several researchers have shown."


Diagram: A line drawing of the nasal system in horses.
In fact, 70 percent to 75 percent of breathing resistance in a running horse comes from the nasal passageways. Horses are obligate nasal breathers, their upper airway extending from the opening of the nostrils to the larynx (see Diagram). The nasal passages are continuous, a hollow muscular tube from the nose to the nasopharynx. For exercising horses, this is the primary site of resistance.

"Even a small narrowing of the passages is significant, as the resistance to flow increases to the fourth power of any reduction in airway radius," Poole says. "So I came from the side of being extremely skeptical to understanding the benefit of nasal strips."

When Poole and his colleagues looked at various measurements, including breath-to-breath evaluation of ventilation, breathing frequency and inspired and expired flow profiles, they found clear differences in gas exchange induced by the nasal strip.2 During high-intensity running, the nasal strip could actually decrease the oxygen cost of the exercise.

"The horse's respiratory ability is really enormous," Poole says. "For example, a strong human athlete can reach a maximal oxygen uptake of close to 5 L/min. The estimated oxygen uptake of the respiratory muscles alone in a quality horse during a sprint is close to this value."

Research in human athletes has found that the respiratory muscles will steal cardiac output from the rest of the body. After the heart, they are the first to benefit from cardiac output. When the work of breathing is decreased (e.g., by using a ventilator-assist device in humans), that extra cardiac output that would have gone to the respiratory muscles now goes to the locomotory muscles and improves human performance.3

"We suspect that this may be happening in the running horse when wearing the nasal strip," says Poole. "If so, it can account for some of the improvements in exercise performance noted in equine athletes wearing the nasal strip."

The strips' application in reducing EIPH

The first study that Erickson and his team conducted at Kansas State demonstrated a significant decrease in oxygen consumption when the horses were wearing the nasal strip.4 "Though it doesn't seem like much, it lowers the work of breathing," Erickson says. "By reducing the large swings in airway pressure, the nasal strip reduces pulmonary vessel damage and bleeding into the lungs."

This condition is called exercise-induced pulmonary hemorrhage (EIPH), and in extreme cases, blood can pour from the horse's nostrils. If you can reduce the vascular pressures and decrease the large swings in airway pressures that occur during peak exercise, the vascular "transmural" pressure is reduced, according to Erickson. That's the physiological basis behind the nasal strip's reducing EIPH and potentially improving pulmonary gas exchange.

Thus, the nasal strip has at least two important physiological actions: it decreases EIPH and, as described above, by lowering the oxygen cost of breathing, it may allow more cardiac output to perfuse the exercising locomotory muscles. These effects reduce fatigue and improve exercise tolerance (i.e., increase the time to fatigue).

"We've compared the use of furosemide and the use of the nasal strip, and they both decrease the amount of bleeding (EIPH) to essentially the same extent," Erickson says. "Neither one completely eliminates it."

Whether the nasal strip is better than furosemide or visa versa is equivocal. "In one study we observed that furosemide was better, but in that study the horses were not run to maximum intensity. In another study, when the horses were running at maximum intensity, the results were similar," Erickson says.

A study by McDonough et al. concluded that "two strategies designed to combat both the intravascular (furosemide) and extravascular (nasal strip) components associated with EIPH were demonstrated to be equally efficacious in reducing, although not abolishing, EIPH. Furthermore these treatments enhanced exercise tolerance. Thus, this investigation demonstrates that both airway and vascular pressures contribute to EIPH during maximal exercise in the horse and that both can be 'treated' in a manner that can reduce the severity of EIPH in horses run at high speed to fatigue."5

An additional study by another independent research group showed similar results between furosemide and the nasal strip.6 This study compared the effects of both the nasal strip and furosemide during strenuous exercise and showed that both treatments decreased the severity of pulmonary hemorrhage as compared with a control group. There was further improvement from the nasal strip alone when the treatments were combined. However, no significant difference was seen compared with furosemide alone.

The authors concluded that "both the nasal strip and furosemide attenuate pulmonary hemorrhage in Thoroughbred horses during high-speed sprint exercise. Moreover, the external nasal strip appears to lower the metabolic cost (i.e., oxygen consumption) of supramaximal exertion in horses. Given the purported ergogenic effects of furosemide, the external nasal strip is a valuable alternative for the attenuation of EIPH."6

A study from the University of California-Davis showed that among racehorses exhibiting severe EIPH, the nasal strip was even more beneficial.7 The most severe bleeders had the greatest benefit.

"We noted this finding on the treadmill as well," says Erickson.

According to the UC Davis study, "The mean number of red blood cells in bronchoalveolar lavage fluid was close to statistically significantly (P = 0.054) higher when horses raced without the nasal dilator strip (84.6 cells/l) than when they raced with it (41.7 cells/l). In contrast, when severe bleeders raced without the nasal strip (271.0 cells/l), EIPH was significantly (P = 0.05) higher than when horses raced with the strip (93.8 cells/l)."

All horses in the UC Davis study were also premedicated with furosemide before racing. Keep in mind that furosemide will reduce the vascular pressures only about 10 percent. But as a diuretic, it also reduces the horse's weight, so the animal is more efficient when it's running.

"I think that enters into the improved exercise tolerance," says Erickson. "Horses will undergo diuresis and lose 20 to 30 pounds with furosemide." It was found that horses lost 13 kg four hours after receiving 0.5 mg/kg of furosemide and 17.2 kg four hours after receiving 1 mg/kg.8


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Source: DVM360 MAGAZINE,
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