Use of a single pair of magnetometer coils to monitor breathing patterns in an intensive care unit

The use of a single pair of magnetometer coils placed centrally on the trunk to record tidal volumes and breath times has been evaluated in five normal subjects in five positions. Breath times were accurate in all positions tested but tidal volumes were only reliable with the subjects lying and the backrest raised to 45° or in the supine position only for volumes up to 1000 ml. Using this system, recordings have been made in seven patients with barbiturate overdose, five with salicylate overdose, and five undergoing anaesthesia with thiopentone. The single pair of coils was satisfactory for measuring changes in the pattern of breathing in these patients. In barbiturate overdose in five patients there was initially an increase in respiratory frequency with a decrease in tidal volume. As these patients recovered the tidal volume increased and the respiratory rate slowed. In two patients who had a raised PaCO₂ there was initially reduction of both tidal volume and respiratory rate, both increasing as the patients recovered. Monitoring respiratory rate appears to be a useful guide in barbiturate overdose; a slowing of the rate without recovery of consciousness warrants further investigation to assess the need for ventilatory support. The single pair of coils were also satisfactory for measuring the increased tidal volume in salicylate overdose. As after other respiratory stimuli in man, increased ventilation occurred predominantly through changes in tidal volume and expiratory time while inspiratory time changed very little.