A. A. Rosenberg

Distribution of Cardiac Output in Fetal and Neonatal Lambs with Acute Respiratory Acidosis

Summary: The effects of changes in Paco2 on the circulation are complex, involving local vasodilation, vasodilation mediated by the pulmonary inflation reflex, and vasoconstriction due to effects on central vasomotor centers and peripheral chemoreceptors. One might anticipate that some or all of these might differ between the fetus in utero and the newborn. Distribution of cardiac output was measured in unanesthetized fetal (n = 6) and newborn (n = 7) sheep, using the radioactive microsphere technique. Paco2 rose from 44 to 70 (fetus) and 38 to 60 torr (newborn) with the addition of CO2 to room air. In the fetus, there were significant increases in central nervous system (CNS), diaphragm, and lung blood flows. No organ showed a significant decrease in flow. Local vasodilation by CO2 was the likely cause of the increased flow to CNS. The large increase in pulmonary blood flow was most likely due to the associated rise in fetal Pao2 (23 to 28 torr) that accompanied respiratory acidosis and the presence of fetal breathing movements. The rise in diaphragmatic blood flow was likely the result of fetal breathing. In the newborn, CNS and diaphragm flows rose, but unlike the fetus, spleen and stomach flows decreased. These decreased flows in the hypercapnic newborn may have been due to stimulation of either central vasomotor centers or peripheral chemoreceptors.

EFFECT OF HEMATOCRIT ON CEREBRAL BLOOD FLOW |[lpar]|CBF|[rpar]| IN LAMBS

In anemia and polycythemia the relative contributions of changes in arterial O2 content (CaO2) and blood viscosity are unclear. We studied this question in 9 unanesthetized lambs (<7 days of age). After initial measurements of arterial and sagittal sinus PO2, PCO2, CaO2, CBF (microsphere technique), cerebral O2 consumption (CMRO2), cerebral fractional oxygen extraction (E= CMRO2/CBFxCaO2) and the position of the oxyhemoglobin dissociation curve (P50) were made (State 1), we performed an exchange transfusion with methemoglobin-containing adult sheep red cells. This changed hematocrit without changing CaO2. After exchange, the hematocrit increased from 20 to 40% while CaO2 changed from 8.8±0.2 (X±SEM) to 10.1±0.2 ml/100 ml. Repeat measurements were made in 1 hour (State 2). PCO2 and P50 did not change (paired t test). CBF fell (138.8±14.1 vs 102.4±5.4 ml/100g/min, p<0.02), and E rose in each experiment (0.51±0.01 vs 0.60±0.01, p<.001). CMRO2 was constant (6.2±0.5 vs 6.2±0.4 ml/100g/min). Viscosity rose from 1.7 to 3.7 centipoise (measured at a shear rate of 230/sec.). Four of these animals were given methylene blue to reduce methemoglobin and CaO2 rose to the value appropriate for the hematocrit. Measurements were repeated (State 3). The ratio (CBF1-CBF2)/(CBF1-CBF3) represents the proportion of the change in flow resulting from a change in viscosity. This ratio is 0.45. Thus changes in viscosity account for about half of the changes in CBF.(NIH Grant HD 13830)

Oxygen Delivery to the Brain Before and After Birth

We studied the relationship between cerebral oxygen consumption and cerebral oxygen delivery (cerebral blood flow x arterial oxygen content) in fetal, newborn, and adult sheep, Relative to the amount of oxygen consumed, cerebral oxygen delivery in the fetus exceeds that in the lamb and adult by 70 percent. This may represent a protective advantage for the fetus or simply a necessary adaptation to the low arterial oxygen pressure in the intrauterine environment.