Ventilation estimated from efferent phrenic nerve activity in the paralysed cat

The purpose of this study is to develop a reliable method for obtaining information about "spontaneous respiration" in paralysed cats. Therefore action potentials from one of the phrenic nerves are recorded. In a spontaneously breathing animal, a CO2 rebreathing experiment is performed in order to obtain a relationship between phrenic nerve activity and tidal volume. This phrenic nerve activity is corrected for the noise measured during expiration and quantified proportional to the square root of the mean impulse rate of the whole nerve bundle. Thus, high correlation coefficients (0.95 or more) between phrenic nerve activity and tidal volume can be obtained. After paralysing the cat this relationship can be used to estimate "spontaneous tidal volume" from the phrenic nerve activity. It appears to be necessary to perform unilateral phrenicotomy on the nerve from which recordings are taken, because there is a condiserable amount of afferent signals in the phrenic nerve which is dependent on the stroke volume of the respirator, on the alveolar PCO2 and somewhat on the alveolar PO2. It is concluded that after vagotomy and phrenicotomy and if suitably quantified, the electrical activity in the phrenic nerve gives accurate information on "spontaneous ventilation" in a paralysed cat.