Elevated diaphragm electromyogram during neonatal hypoxic ventilatory depression

Diaphragmatic electromyogram (EMG) was obtained in eight 48-h-old unanesthetized monkeys while breathing air and then either of two different hypoxic gas mixtures (12 or 8% O2 in N2) for 5 min. Minute ventilation (VI) rose significantly above control levels by 1 min of hypoxemia while animals were breathing either of the hypoxic gas mixtures as tidal volume (VT) and slope and rate moving average EMG increased. The relative gains in VI were associated with comparable increases in diaphragmatic neural activity per minute (EMG/min = peak EMG X frequency) during this early phase of hypoxemia. VI subsequently fell to control levels (inspired O2 fraction = 12%, arterial PO2 = 23 +/- 3 Torr) or significantly below (inspired O2 fraction = 8%, arterial PO2 = 18 +/- 0.4 Torr) by 5 min of hypoxemia, secondary to changes in VT. Despite the decline in VI, slope and rate moving average EMG, and EMG/min remained statistically above control values by 5 min of hypoxemia, although there was a trend for EMG/min to decrease slightly from the 1-min peak response. These findings demonstrate that hypoxic-induced depression of neural input to the diaphragm is not independently responsible for the biphasic nature of the newborn ventilatory response, although it cannot be ruled out as a contributor. The fall in inspiratory volumes despite constant elevated EMG activity suggests the presence of a change in respiratory mechanics and/or an impairment in diaphragmatic contractile function without offsetting neural compensatory activity.