The diaphragmatic compound muscle action potentials (DCMAPs) at different lung volumes--a prospective...

Phrenic nerve conduction study and measurements of static lung volumes such as functional residual capacity (FRC), total lung capacity (TLC) and residual volumes (RV) using body plethysmography were carried out in 31 normal healthy male subjects (31 +/- 8 years). The objective was to correlate changes in latency, amplitude, duration and area of diaphragmatic compound muscle action potential (DCMAPs) with lung volumes and changes in them with changes in postures. The mean phrenic nerve latency did not show any significant change with lung volumes or postures, but the latency and height of the subject had a significant correlationship (r = 0.68). The peak to peak amplitude of DCMAP showed a significant relationship with the quantitative lung volumes (r = 0.65). The amplitude was significantly higher and duration reduced (p < 0.001) at TLC as compared with those at measured FRC and RV in both sitting and supine postures. The area under the curve did not change significantly. Similarly, amplitudes and latency did not show any significant relation with other anthropometric parameters. The study evaluates a new quantitative relationship between DCMAPs amplitude and lung volumes. This may be carefully used along with other clinical parameters in critically ill patients for an early weaning from ventilator. Posture exerts minimal influence on DCMAPs amplitude; since DCMAPs amplitudes depends on the position of the diaphragm which in turn depends on dipolic potential, its electromagnetic fields and its moving angle subtends at the recording electrode, one can confidently use it to predict lung volume in respiratory failure due to neuromuscular diseases where subjective lung function assessment is impossible. However, a further study is in process in the critical care unit to confirm its utility.