Relationship between mean airway pressure, cardiac output, and organ blood flow with normal and decreased respiratory compliance

We investigated the relation between blood flow and mean airway pressure in two groups of anesthetized newborn piglets. The first group had normal respiratory compliance; the second group had pulmonary surfactant depleted by repeated saline lavage, which decreased static respiratory compliance by 42%. In the normal group, cardiac output decreased linearly from 292±43 mL/min/kg at 5 cm H₂O airway pressure to 134±37 ml/min/kg at 20 cm H₂O airway pressure, a drop of 43% (r²=0.79). Blood flow to the heart, kidney, and intestines had a similar decline, but brain, hepatic artery, and adrenal flow were constant. Mean arterial blood pressure did not decrease significantly until the highest airway pressure was reached, whereas sagittal sinus pressure increased as mean airway pressure increased. In contrast, the surfactant-depleted group maintained cardiac output up to a mean airway pressure of 15 cm H₂O. At 20 cm H₂O, cardiac output fell to 40% of the original value. Blood flow to the heart and kidneys fell at a mean airway pressure of 20 cm H₂O; intestinal blood flow decreased beginning at 10 cm H₂O. As in the normal piglets, brain, hepatic arterial, and adrenal blood flow were not affected by increasing ventilation pressure. Our data show that positive pressure ventilation in the neonate has important cardiovascular effects that are blunted when respiratory compliance is decreased. More important, because cardiac output decreased prior to a significant decline in arterial blood pressure, these data suggest that in a clinical setting considerable cardiovascular alterations can occur before a decline in arterial blood pressure is detected.