Search

Respiratory alkalosis (hypocapnia) is used to treat persistent pulmonary hypertension in newborns. With the exception of the brain, there are no data on the effects of hypocapnia on organ blood flow in the neonate. Therefore, we examined the effects of 2 h of hypocapnia on blood pressure, oxygen consumption, cardiac output and organ blood flows in puppies.

The purpose of the study was to evaluate the near infrared spectroscopy technique for determination of the cerebral venous oxygen saturation. By tilting the patient's head down 15°, changes in OD may be caused by changes in cerebral blood volume. On the assumption that increases in cerebral blood volume consist of venous blood only, cerebral venous oxygen saturation can be calculated as the measured change in oxygenated Hb divided by the change in total Hb. Two groups of mechanically ventilated, newborn infants were investigated: 10 asphyxiated, term infants and another 22 preterm infants with respiratory distress syndrome. All were monitored by near infrared spectroscopy during tilting, and cerebral blood flow was estimated by the 133Xe clearance technique immediately before tilting. Cerebral venous oxygen saturation could not be calculated in 13 preterm infants, as the blood volume remained constant during tilting. In the remaining 19, cerebral venous oxygen saturation averaged 0.67 (SD = 0.09) in asphyxiated infants and 0.53 (SD = 0.15) in preterm infants (p= 0.03). The corresponding values of cerebral blood flow were closely and inversely related to oxygen extraction calculated from cerebral venous saturation as estimated by near infrared spectroscopy.

We examined autoregulation of cerebral blood flow (CBF) over the range of oxygen tension commonly observed in the chronic fetal lamb preparation. Seventeen animals were surgically prepared under general anesthesia for chronic in uterostudies. Based on measured resting arterial Po2and calculated saturation 24–48 h after surgery, two groups were defined: a normoxic group of eight with saturation of 57 or higher (our laboratory normal for physiologically stable preparations) and an hypoxic group of nine with saturation less than 57. Regional CBF was measured with radiolabeled 15-μm microspheres. Autoregulation of CBF was assessed by measuring the change in CBF when fetal mean arterial blood pressure (MABP) was acutely decreased and increased by withdrawal and reinfusion of fetal blood. In normoxic animals CBF was constant over an MABP range of 42–61 torr in all four areas of the brain examined: cerebral hemispheres, basal ganglia, cerebellum, and brain stem. In hypoxic animals CBF was pressure dependent in all areas over an MABP range of 46–73 torr, i.e., autoregulation was abolished. These studies demonstrate that the mechanism of autoregulation is functionally developed in the mature fetal lamb, but operationally dependent upon arterial oxygen concentration. Below a saturation of approximately 50–60 CBF varies directly with perfusion pressure.