Effects of Arterial Carbon Dioxide Tension on the Newborn Lamb's Cardiovascular Responses to Rapid Hemorrhage

Abstract

SummaryNineteen newborn lambs, spontaneously breathing room air, were hemorrhaged of 50% of their measured blood volume over a 30-min period. They were then observed for the following 90 min. No fluid or blood was reinfused during the study. Eight of the 19 lambs survived beyond the study period, the other 11 died at the end of the hemorrhage or during the recovery period. All lambs became hypotensive and bradycardic during the hemorrhage. All became tachycardic after the hemorrhage. Blood pressure of the survivors returned towards baseline levels whereas it continued to fall in the non-survivors. All animals became anemic, acidotic, and hypocarbic but remained normoxemic.Myocardial blood flow fell to approximately 50% of baseline levels in both survivors and non-survivors during, and at the end of the hemorrhage. It returned to near baseline levels in the survivors by 90 min posthemorrhage. Cerebral blood flow remained at baseline levels during the hemorrhage in the survivors but cerebral oxygen delivery decreased. Flow rose 10% above baseline levels 90 min after hemorrhage but oxygen delivery remained low. In the non-survivors, cerebral blood flow fell to 60% of baseline and cerebral oxygen delivery was 50% of baseline by the end of the hemorrhage.A group of five lambs was hemorrhaged and studied in the same fashion as the 19, except that they breathed 4.5% CO 2 beginning 15 min before the hemorrhage, during the hemorrhage and 90 min recovery period. Four of these five survived the study period. They remained normocarbic and became more acidotic than the animals that breathed room air. Blood pressure and heart rate fell during hemorrhage but these decreases were less than in either group of room air breathing animals. During recovery they became tachycardic and blood pressure returned to baseline levels. Myocardial blood flow increased 20% above baseline by the end of hemorrhage and 40% above baseline by 90 min posthemorrhage. Cerebral blood flow increased 40% above baseline during the hemorrhage and remained at that level at the end of hemorrhage and 90 min afterwards. The increase in brain blood flow was sufficient to maintain oxygen delivery to the brain at baseline levels throughout the experiment. SpeculationWe speculate that the normal response for the normocarbic lambs is to increase cerebral and myocardial blood flow in response to anemia and hypotension. Spontaneous hyperventilation with hypocarbia tends to interfere with this response both in the brain and in the myocardium. The effect of hypocarbia is greater in the myocardium. The net result is a decrease in oxygen delivery to these tissues. If the same responses were to occur in the human infant it might be important to maintain normocarbia during resuscitation of infants suffering from hemorrhagic shock. 70

The Effect of Hematocrit Alterations on Cerebral Vascular CO2 Reactivity in Newborn Baboons

“…Therefore, a greater fraction of CO2 is dissolved in the plasma at any given Pacoz (32), which could also explain an increased Paco, reactivity in anemia (and a converse phenomenon in polycythemia). In contrast to the experiments of Melkumyants et al (25,28), Hudak et al (33) Sola et al (35) noted that 4.5% C 0 2 inhalation preserved cerebral and coronary flow in animals previously subjected to acute hemorrhage without replacement of shed blood; this occurred in part due to an increase in systemic blood pressure. However, the animals in our study were not hypovolemic and did not have hypotension or bradycardia during hypercarbia experiments.…”

Regional Cerebral Blood Flow: Studies in the Fetal Lamb during Hypoxia, Hypercapnia, Addosis, and Hypotension

“…Higher brainstem C02 responsitivity'could possibly be under neural control in order to maintain brain oxygenation, as suggested by recent studies in the newborn lamb (36). As compared to controls, survival, blood flow, and O2 delivery to the brain and myocardium were greater in newborn lambs given 4.5% C02 when stressed by 50% blood volume depletion (36).…”

“…As compared to controls, survival, blood flow, and O2 delivery to the brain and myocardium were greater in newborn lambs given 4.5% C02 when stressed by 50% blood volume depletion (36). Also, C02 responsitivity could possibly be modulated and enhanced by opiate receptors in the brainstem, as the C02 sensitivity of fetal breathing movements is affected by opiate receptor blockade with naloxone (27).…”

“…In the newborn puppy, autoregulation is maintained between 27 and 97 mm Hg, and in the newborn piglet between 40 to 150 mm Hg (12,21), In newborn lambs subjected to 50% blood volume reduction, CBF was maintained at 77.5 to 79.6 ml.min-'. 100 g-' as blood pressure decreased from 76 to 24 mm Hg (36). In fetal lambs, a 20% reduction in blood volume was accompanied with decreases in blood pressure of 21 %, in cardiac output of 40%, and in CBF of 18% when corrected for the 4-tom increase in Pco2 which occurred (39).…”

“…The results demonstrated that 1) fetal CBF increased linearly as oxygen tension or content decreased and a hierarchy of responsivity occurred (brainstem > subcortex and cortex), 2) fetal CBF increased as carbon dioxide tension increased with a different hierarchy of responsivity (brainstem > subcortex > cortex), and 3) autoregulation of fetal CBF over a wide range of blood pressure or cardiac output was maintained for both total CBF and the various brain regions examined. (Pediatr Res 18:1309-1316,1984 Abbreviations CBF, cerebral blood flow During the past several years, numerous investigators have used the radioactive microsphere technique to measure newborn and fetal cerebral blood flow in different species (1,19,24,25,36,40). In these studies, which have examined changes in flow as a function of changes in blood gas tensions, pH, and blood pressure, it has frequently been difficult to separate clearly the effects of these variables because of their physiological interactions.…”