Quantification of electrical activity in the phrenic nerve in the study of ventilatory control.

T he reason for wishing to use electrical signals to represent the output of the respiratory control system is that the traditional measurement, ventilation, is often unsatisfactory for this purpose. The basic problem is that ventilation is too far removed from the central controller, by a number of variable transformation steps, to satisfactorily represent its output. Abnormalities in neuromuscular transmission or muscular function will affect the control output-ventilation relationship. Changes in the mechanical properties of lungs, airways or chest wall will change respiration for a given respiratory drive.'-' Even an alteration in absolute lung volume (ie, end-expiratory level) will, by changing the precontraction length of the inspiratory muscles and the radius of curvature of the diaphragm, affect the resultant tidal volumes even though the same respiratory drive e x i ~ t s . ~ Another problem in studying respiratory control is that the system is a closed servo-loop. When one studies stimuli other than carbon dioxide in a ventilating subject, the change in ventilation causes a change in C0,H+ which is negatively fed back through the chemoreceptors to the input of the system. This prevents an accurate analysis of the effects of such a non-CO, stimulus. Although it is possible to maintain more or less isocapnic conditions in the steady-state despite increasing ventilation, the technique is not possible during hypoventilation and does nothing to solve the problem of mechanical or lung volume alterations. If the recording of electrical activity of respiratory muscle or nerve could represent respiratory output, it would entirely avoid the mechanical linkage to ventilation and in the presence of muscle paralysis would allow the chemical servo-loop to be opened. This report concerns studies in which various processings of electrical activities of phrenic nerve or external intercostal muscles (ICM) were correlated with tidal volumes or airway occlusion pressures. It shows that with the proper type of processing, one can obtain a satisfactory electrical index which appears to be related to changes in respiratory output.