Determinants of the maximal change in pleural pressure during tidal breathing in COPD-affected horses

In six COPD-affected horses, we analysed the factors responsible for the changes in pleural pressure (delta Pplmax) that occur during tidal breathing. Four-hundred-and-sixty-eight measurements of each parameter: pulmonary resistance (RL), dynamic elastance (Edyn), air flow rates, and the timing of breathing, were gathered during a trial of the bronchodilator pirbuterol. Data were placed into seven ranks, based on the magnitude of delta Pplmax; rank 1: 5-15; rank 2: 15-25; rank 3: 25-35; rank 4: 35-45; rank 5: 45-55; rank 6: 55-65; and rank 7 > 65 cm H2O. Up to rank 4 (45 cm H2O), the increase in delta Pplmax was due to increases in RL, Edyn, and inspiratory and expiratory air flow rates. Further increases in delta Pplmax were due to continuing increases in Edyn and flow rates with little change in resistance. The increase in inspiratory flow rates was the result of a decrease in inspiratory time. The large increase in peak expiratory flow could not be explained by the small decrease in expiratory time and must therefore be due to a change in breathing strategy as delta Pplmax increased. Changes in air flow rates as well as changes in RL and Edyn must be considered as reasons for a change in delta Pplmax when evaluating horses with COPD.