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 ch anges in pleural pressure (Delta Pp1(max)) that occur during tidal breathin g. Four-hundred-and-sixty-eight measurements of each parameter: pulmonary r esistance (R-L), dynamic elastance (E-dyn), air flow rates, and the timing of breathing, were gather ed during a trial of the bronchodilator pirbutero l. Data were placed into seven ranks, based on the magnitude of Delta Ppl(m ax), 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 in crease in Delta Ppl(max) was due to increases in R-L, E-dyn, and inspirator y and expiratory air flow rates. Further increases in Delta Ppl(max) were d ue to continuing increases in E-dyn and flow rates with little change in re sistance. The increase in inspiratory flow rates was the result of a decrea se in inspiratory time. The large increase in peak expiratory now could not be explained by the small decrease in expiratory time and must therefore b e due to a change in breathing strategy as Delta Ppl(max) increased. Change s in air flow rates as well as changes in R-L and E-dyn must be considered as reasons for a change in Delta Ppl(max) when evaluating horses with COPD.