DIAPHRAGMATIC ACTIVITY AND VENTILATION IN PRETERM INFANTS .1. THE EFFECTS OF SLEEP STATE

To determine the effects of sleep on diaphragmatic activity and ventil ation we studied 10 preterm infants (birth weight 1,840 +/- 50 g, gest ational age 33 +/- 0.6 weeks, and postnatal age 9.4 +/- 1.4 days). We measured surface and esophageal diaphragmatic electromyograms (EMG(di) ). Ventilation was measured using a nasal flowmeter and a flow-through system. Diaphragmatic activity was analyzed for total phasic activity , expiratory phasic activity, the expiratory to total phasic activity ratio, and the presence of 'tonic' activity. The latter was defined by the presence of electrical activity and the end of expiration. There was a decrease in the average total phasic activity (1.25 vs. 0.71 s, p = 0.001), expiratory phasic activity (0.67 vs. 0.21 s, p = 0.002), t he expiratory to total phasic activity ratio (0.51:0.27 s, p = 0.001) and tonic activity (51 to 5%, p = 0.01) from quiet to REM sleep in the surface EMG(di). Similar changes were found in the esophageal EMG(di) , except that tonic activity was rarely observed. In parallel with the se changes in electrical activity of the diaphragm, minute ventilation and alveolar ventilation increased from quiet to REM sleep. This incr ease was primarily related to an increase in frequency with a negligib le change in tidal volume. The increase in frequency was primarily due to shortening of inspiratory time. The findings that tonic activity r ecorded via surface electrodes decreased substantially from quiet to R EM sleep and was not observed in the esophageal EMG(di) suggests that this tonic activity may represent electrical activity of the postural muscles of the chest wall rather than the diaphragm.