Supine fall in lung volumes in the assessment of diaphragmatic weakness inneuromuscular disorders

Objective: To determine whether diaphragmatic function can be determined by noninvasive respiratory indices in neuromuscular disease. Design: Vital capacity (VC) and mouth pressure generated during a maximal s tatic inspiratory effort (Pi max) were measured with patients in both sitti ng and supine positions. Setting: Rehabilitation hospital, Patients: Twenty-four patients with generalized neuromuscular disease. Main Outcome Measures: Changes in indices from sitting to supine position w ere compared with invasive diaphragmatic function indices consisting of tra nsdiaphragmatic pressures during maximal sniff (Pdi sniff) and the ratio of gastric pressure (Pga) increases over transdiaphragmatic pressure (Delta P ga/ Delta Pdi) during quiet breathing. Results: The fall in VC in the supine position was greater in the 15 patien ts who had spontaneous paradoxical diaphragmatic motion (Delta Pga/Delta Pd i < 0) than in the 9 patients who did not. Specificity and sensitivity of a greater than 25% supine fall in VC for the diagnosis of diaphragmatic weak ness (<Delta>Pga/ Delta Pdi < 0 and/or Pdi sniff < 30cmH(2)O) were 90% and 79%, respectively. Stepwise multiple regression analysis of Pdi sniff showe d that both the supine fall in VC and Pi max were associated with diaphragm atic weakness (R-2 =.66; p <.0001), These factors contributed 52% and 14% o f the Pdi sniff variance, respectively. Conclusions: Simple VC measurement in the sitting and supine positions may be helpful in detecting severe or predominant diaphragmatic weakness.