NO inhalation reduces pulmonary arterial pressure but not hemorrhage in maximally exercising horses.

The research studied the impact of nitric oxide (NO) inhalation on exercise-induced pulmonary arterial pressure elevation and pulmonary hemorrhage in thoroughbred horses. Although NO inhalation resulted in a slight decrease in peak pulmonary arterial pressure, it did not reduce exercise-induced pulmonary hemorrhage and may have worsened its severity.

Study Design and Methodology

• The researchers selected five Thoroughbred horses for the study, which underwent trials on a treadmill until they voluntarily ceased effort (‘volitional fatigue’). The trials occurred under both control conditions (inhaling Nitrogen at the same flow rate as the NO trial) and under nitric oxide inhalation (80 ppm). Each trial was conducted two weeks apart with the order randomized.
• Before each trial, a pressure transducer was inserted in the pulmonary artery of each horse, 8 cm past the pulmonic valve, to monitor pulmonary arterial pressure (Ppa).
• Pulmonary hemorrhage severity was measured through a bronchoalveolar lavage (BAL), a procedure that washes out the airways and helps evaluate the presence of red blood cells, 30 minutes after each treadmill run.

Findings

• The exercise time was consistent between both NO and control conditions, indicating that inhalation of NO didn’t impact the horses’ exercise capacity.
• Inhalation of NO led to a small but statistically significant reduction in peak Ppa when compared to the control trial. The average Ppa in the control trial measured 102.3 +/- 4.4 mmHg while it was 98.6 +/- 4.3 mmHg in the NO trial.
• Despite the lowered Ppa, the severity of exercise-induced pulmonary hemorrhage was markedly higher when horses inhaled NO. BAL fluid showed a significant increase in red blood cells post-exercise in the NO trial (42.6 +/- 15.4 x 10(6) cells/ml) compared to control (22.4 +/- 6.8 x 10(6) cells/ml).

Conclusions and Implications

• The results suggest that very high Ppa could be indicative of arteriolar vasoconstriction – a narrowing of small arteries – functioning to protect the blood vessel network from very high Ppa during maximum exercise in Thoroughbred horses.
• Furthermore, while inhalation of NO reduced the maximal Ppa, it did not lower the incidence of exercise-induced pulmonary hemorrhage. In fact, it might exacerbate its severity, raising concerns about the prophylactic use of NO in horses during exercise.

In conclusion, the findings indicate a complex relationship between Ppa, inhalation of NO, and exercise-induced pulmonary hemorrhage in horses, requiring further study to unravel these interactions fully.