Blood Flow and Oxygen Delivery to the Organs of the Neonatal Lamb as a Function of Hematocrit

Abstract

ABSTRACT. We chronically catheterized 15 newborn lambs (9.5 f 2.8 days) and measured the distribution of cardiac output by the radionuclide-microsphere technique at hematocrits ranging from 10 volumes % to 55 volumes %. Seven animals were made progressively anemic and eight polycythemic by means of exchange transfusions. Cardiac output and heart rate increased with decreasing hematocrit while whole body oxygen consumption showed a small decrease during severe anemia. Both cerebral and cardiac blood flow markedly increased during anemia which assured a relatively stable oxygen delivery to both organs. The changes seen for blood flow to the carcass (skin, bones, and muscle) were predictable from the effects of blood viscosity: small decreases in flow at the highest hematocrits and small increases in flow at the lowest hematocrits. Consequently, oxygen delivery was as low as 1 ml of oxygen/min/100 g at a hematocrit of 10 volumes %. Renal blood flow remained unchanged while oxygen delivery fell when hematocrit was decreased. Hepatic oxygenation was measured using a modification of the Fick principle. Hepatic blood flow showed only a small decrease as hematocrit increased and changed minimally during anemia resulting in a falling delivery of oxygen with anemia. A stable hepatic oxygen consumption was assured by a marked increase in oxygen extraction during anemia. Two differing organ responses to changes in hematocrit can be seen in the newborn: the brain and heart vary blood flow to assure an adequate delivery of oxygen while a number of other organs show less blood flow regulation and, most likely, vary oxygen removal from blood. Over a wide range of hematocrits, compensatory responses occur in the newborn which effectively prevent the development of tissue hypoxia. (Pediatr Res 20: 1274-1279, 1986 The physiologic responses that occur with isovolemic anemia and polycythemia reflect the continual interplay between the requirements for tissue oxygenation and the rheologic effects of changes in blood viscosity. While information is available concerning the changes in the distribution of cardiac output for both the fetus (1) and adults (2-8) with anemia and polycythemia, there are little data for the newborn. Two patterns of organ blood flow response can be seen in conditions of decreased oxygen availability: the brain, heart and adrenal glands meet their oxygen requirements by increasing organ blood flows; the majority of other organs regulate the extraction of oxygen by varying capil- lary density and the oxygen diffusion gradient. When both oxygen content and blood viscosity are changing, complex physiologic adjustments are clinically important for the maintenance of a stable oxygen delivery to the organs of the newborn. Both anemia and polycythemia can occur from a variety of causes related to chronic and acute events during pregnancy and delivery. Understanding the limits within which adequate tissue oxygenation occurs offers both prognostic and therapeutic possibilities in the care of infants, especially as we tr...

Capillary Blood Cell Velocity in Full-Term Infants as Determined in Skin by Videophotometric Microscopy

“…Thus, the peripheral blood flow does not change in response to decreased (19) and increased (20) oxygen delivery, i.e., the hematocrit, in newborn lambs and infants, respectively. Hence, our results suggest that the hematocrit plays a major role as a flow resistance factor in neonatal skin capillaries.…”

Effect of Exchange Transfusion on Brain Perfusion and Electrocortical Brain Activity in Newborn Lambs

“…Changes in hematocrit may also modify Q car [21,22]. Determination of hematocrit values at the end of each withdrawal and infusion period was considered to be of no value, because hematocrit values do not change immediately after the withdrawal or infusion of blood.…”

Left Ventricular Oxygen and Substrate Uptake in Chronically Hypoxemic Lambs

“…Conversely, myocardial blood flow decreases during acute polycythemia (I. 3,5). These adjustments in myocardial blood flow are the result of alterations in vascular tone.…”