FACTORS INFLUENCING VENTILATORY MUSCLE RECRUITMENT IN PATIENTS WITH CHRONIC AIR-FLOW OBSTRUCTION

Patients with chronic airflow obstruction (CAO) frequently develop abnormal thoraco-abdominal excursion, but the patterns described are inconsistent and the factors that relate to their development remain unknown. We studied 45 stable patients with FEV1 ranging from 0.36 to 2.1 L. A pattern of ventilatory muscle recruitment (VMR) was established by simultaneously measuring gastric (Pg) and pleural (Ppl) pressures and rib cage (Vrc) and abdominal (Vab) volume displacement with inductance plethysmography. From these tracings, Pg-Ppl plots were constructed and the ΔPg/ΔPpl values were calculated. The ΔPg/ΔPpl was validated in 15 patients with simultaneous analysis of VAb-Pg plots. Pearson's test and multiple regression analyses were used to correlate ΔPg/ΔPpl to factors thought to influence respiratory muscle function such as age, sex, nutritional status (weight/height, albumin), hyperinflation, airflow obstruction, and arterial blood gases. We found a direct correlation between a more positive ΔPg/ΔPpl value and increasing hyperinflation (r = 0.69, p < 0.0001), increasing airflow obstruction (r = -0.55, p < 0.001), and decreasing diaphragmantic strength (r2 = 0.32, p < 0.001). We also found that expiratory Ppl became more positive with decreasing FEV1 (r2 = 0.33, p < 0.001). This change in VMR was independent of age, sex, nutritional status, and arterial blood gas determinations. The results of this study show that with increasing CAO there is a change in the pattern of VMR from one where most of the effective ventilatory pressure is generated by the diaphragm to one where most of the pressure is generated by the rib cage inspiratory muscles, with a significant contribution by the respiratory muscles of expiration. This altered pattern of VMR must be considered when designing therapeutic strategies directed at improving ventilatory muscle function.