MAXIMUM INSPIRATORY MUSCLE ENDURANCE CAPACITY DURING RESISTIVE LOADING IN CHRONIC OBSTRUCTIVE PULMONARY-DISEASE

In 10 patients with stable severe chronic obstructive pulmonary disease (COPD) we evaluated the relationship between the degree of airway obstruction and hyperinflation, and the maximum inspiratory muscle endurance capacity during added inspiratory resistive loading. We measured the ratio of forced expiratory volume in 1 s to forced vital capacity (FEV1/FVC) and airway resistance (R(aw)) as indices of airway obstruction, and the ratio of functional residual capacity to total lung capacity (FRC/TLC) as an index of hyperinflation. The mean resting transdiaphragmatic pressure to its maximum (P(di)/P(dimax)), the tension time index of the diaphragm, and the maximum transdiaphragmatic pressure (P(dimax)) were also determined. Following 15 min of resting breathing, the patients breathed through added inspiratory resistances which were progressively increased every 3 min until exhaustion. Maximum endurance capacity (EC(max)) was defined as the product of the esophageal pressure - time integral and frequency at the maximum load sustainable for 3 min. EC(max) correlated significantly with R(aw) (r = -0.67, p < 0.04). The addition of FRC /TLC to the analysis resulted in a significant increase in the correlation coefficient (r = 0.86, p < 0.01). EC(max) did not correlate with FEV1/FVC. Both resting P(di)/P(dimax) and P(dimax) independently influenced EC(max). In addition, P(dimax) correlated significantly with FRC/TLC, and resting P(di)/P(dimax) with R(aw). We concluded that in stable patients with severe COPD, both airway obstruction and hyperinflation affect maximum inspiratory muscle endurance capacity during inspiratory resistive loading.