A horse in general surgery is an awkward sight. For the best access, the animals may be placed on their sides or even their backs, a position that puts considerable pressure on their internal organs, often leading to partial lung collapse. In spite of using oxygen-rich ventilation, blood oxygen levels can fall to dangerous levels during lengthy procedures.
Veterinarians have employed a variety of strategies to mitigate these respiratory challenges, including using oxygen delivered under high pressure to ensure the alveoli—the small sacs in the lungs where gas exchange takes place—are opened up, or “recruited.” Others had used a combination of helium and oxygen, “heliox,” with mixed results.
Researchers at the University of Pennsylvania wanted to get more clarity on the best approach for ventilating horses. A study led by Klaus Hopster, assistant professor of large animal anesthesiology at Penn’s School of Veterinary Medicine’s New Bolton Center, found that horses receiving heliox while under anesthesia did better than those receiving pure oxygen. Adjusting the pressure of the ventilation the horses received also improved overall oxygenation, the study found.
“Our paper showed that when you use high pressure ventilation using heliox, you’re able to achieve better results,” says Hopster. “The results are clear in healthy horses; what we’d like to do in the near future is see if we can extend this to other species or compromised animals.”
Hopster collaborated on the work, which appears in the American Journal of Veterinary Research, with Penn Vet’s Lauren Duffee, Charlotte Hopster-Iversen and Bernd Driessen.
Colic surgery is a common reason that horses may be put under general anesthesia in dorsal position, in order to easily access the abdomen. In this position, using gas under higher pressure to ventilate a patient can succeed in getting oxygen to the lungs’ alveoli, and overcome the heavy weight that makes lungs prone to collapse. But high pressures also put stress on the heart and blood vessels, in extreme cases leading to damage.
In colic procedures, which can be lengthy, prior research by Hopster and colleagues has shown that ensuring sufficient respiratory function while a patient is under anesthesia improves outcomes afterward, likely because the volatile anesthetic drugs must be exhaled.
“The ultimate goal is to use the best pressure for gas exchange and oxygenation and the lowest pressure possible, because that is ideal for cardiac function,” Hopster says. “However low pressures are not good for the lungs. It’s a balancing act.”
Earlier studies by other researchers have hinted that incorporating helium could be a way of striking this balance. Helium is less dense than oxygen, making it easier to flow through small vessels and reach the alveoli, improving ventilation and gas exchange. Studies in humans have seen beneficial effects.
To see if the same benefits were present in horses, the researchers examined several measures of good lung function and oxygenation in animals under anesthesia that were given either heliox or pure oxygen. They used six horses owned by Penn for the study. To ensure that the lung’s alveoli remained open while the horse was in a dorsal position, they performed what is known as an alveoli recruitment maneuver; in essence, slowly stepping up ventilation pressure, keeping the volume of gas constant.
Hopster and colleagues observed that horses ventilated with heliox rated better for two measures of respiratory mechanical function—low peak inspiratory pressure and higher lung compliance—compared to those breathing pure oxygen.
“With helium, you get a more unturbulent flow, so you need a lower pressure to the end of the lungs,” Hopster says. “You can get the same result in terms of pulmonary function with lower pressures.”
In future work, Hopster and colleagues would like to examine whether a mixture of oxygen and nitrogen might also be effective for horses, or whether this result is particular to the heliox mixture. They would also like to extend their studies to sheep, whose respiratory systems provide a good model for humans. That could help shed light on good strategies to ventilate people who already have compromised respiratory systems, such as individuals with asthma.
The research was supported by Penn Vet’s Department of Clinical Studies at New Bolton Center.