Dynamics of breathing in the hypoxic awake lamb

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Abstract
Newborn mammals respond to hypoxia with an immediate hyperventilation that is rapidly dampened. Changes in mechanical properties of the respiratory system during hypoxia have been considered an important reason for this fall in minute ventilation (.ovrhdot.VE). We have studied the dynamic mechanical behavior of the respiratory system in eight unanesthetized intact newborn lambs (mean age 2 days) during normoxia and hypoxia (FIO2 = 0.08). Mouth pressure (P), airflow (.ovrhdot.V), and volume (V) were recorded while lambs were breathing through a leak-proof face mask and a pneumotachograph. Active compliance (C'') and resistance (R'') of the respiratory system were computed from P developed during an inspiratory effort against airway closure at end expiration and .ovrhdot.V and V of the preceding breaths. Tidal expiratory .ovrhdot.V-V curves were analyzed to estimate the elevation in functional residual capacity (FRC) over resting volume (Vr). After hypoxia, there was an immediate increase in .ovrhdot.VE in the first 2 min, from 0.49 to 1.13 l .cntdot. kg-1 .cntdot. min-1, followed by a rapid decrease to 0.80. After 8 min of hypoxia, C'' was unchanged. The inspiratory R'' decreased during hypoxia, probably reflecting a drop in inspiratory laryngeal resistance. The expiratory .ovrhdot.V=V curves during hypoxia showed considerable braking, often with a double peak in expiratory .ovrhdot.V. This pattern was only occasionally seen during normoxia. In animals with a linear segment of the expiratory .ovrhdot.V-V curves the FRC-Vr difference could be calculated and averaged 1.93 ml/kg during normoxia and 3.7 during hypoxia. The recoil P of the respiratory system at end expiration was 0.75 cmH2O during normoxia vs. 1.63 cmH2O during hypoxia (P < 0.01). Thus, during hypoxia in the awake newborn lamb 1) inspiratory R'' decreases; 2) expiratory .ovrhdot.V is interrupted or reduced, yielding an increase in FRC; and 3) changes in dynamic mechanical properties should not contribute to the lack of sustained hypoxic hyperventilation.