Pulmonary gas exchange correlated to clinical signs and lung pathology in horses with chronic bronchiolitis.

The study analyzed the gas exchange process in the lungs of horses suffering from chronic obstructive pulmonary disease (COPD), their clinical symptoms, and lung morphology. It found that horses with COPD had heightened minute ventilation, significant morphologic alterations in the lungs, and a notable increase in pulmonary artery pressure along with a correlation between respiratory rate and the ventilation of normal lung regions.

Study Design and Participants

• The research involved eight horses with an average weight of 438 kg, all of whom were suffering from chronic obstructive pulmonary disease (COPD), a condition marked by protracted airway obstruction.
• As part of the study, the horses were analyzed for their clinical symptoms, the relationship of ventilation to perfusion (VA/Q), and lung morphology.
• At the end of the study, four of the horses were euthanized and autopsied.

Key Findings

• The minute ventilation in the COPD-afflicted horses was discovered to be nearly double the usual rate, indicative of abnormal lung function. Furthermore, their PaO2 levels (a measure of the oxygen dissolved in the blood) were significantly lower than average.
• Despite these abnormalities, the cardiac output of the horses remained normal. However, the pulmonary artery pressure and the pulmonary vascular resistance were both significantly elevated.
• The VA/Q distribution in the horses was irregular, with an increased variety in the ratios. In one horse, an upward shift in shunt (a measure of blood flow bypassing the lungs), indicating ventilation defect, was observed.
• Horses also exhibited an increase in the ventilation of unproductive areas known as dead space and of high VA/Q regions, to the extent that these comprised 74 per cent of their minute ventilation.
• All horses exhibited chronic bronchiolitis, a chronic inflammatory condition of the smallest airways in the lungs, as indicated by lung biopsy. Diffuse acinar hyperinflation, another indicator of chronic sickness, was evident at autopsy.

Insights and Correlations

• A significant reciprocal correlation was discovered between the horses’ respiratory rate and the fractional ventilation of normal VA/Q regions; as the former increased, the latter decreased.
• There was substantial agreement between the level of bronchiolar epithelial hyperplasia – abnormal cell growth – observed in lung autopsies and the degree of ventilation of high VA/Q regions and dead space.
• The study proposes that the common cause of the observed correlation between increased ventilation of high VA/Q regions, dead space ventilation, and elevated pulmonary artery pressure in horses suffering from COPD may be hyperinflation of the lungs due to obstructed airways.