Prolonged Hypercarbia in the Awake Newborn Piglet: Effect on Brain Blood Flow and Cardiac Output

Abstract

ABSTRACT. In adults, exposure to prolonged hypercarbia results in a normalization of the extravascular brain pH associated with a reduction in brain blood flow (BBF). Following prolonged hypercarbia, sudden normalization of the arterial PCOz also produces a change in the extravascular brain pH to an alkaline state, resulting in a marked decrease in BBF. We examined these physiologic phenomena in newborn subjects by exposing seven awake, spontaneously breathing newborn piglets to 4 h of sustained hypercarbia (PaC02: 60-70 mm Hg) followed by a 45-min normocarbic period. Total and regional BBF, cardiac output (radionuclide-labeled microsphere method), arterial blood pressure, plasma catecholamine and lactate concentrations, blood gases, oxygen contents, and hematocrits were measured during a baseline period, at %, 2, and 4 h of sustained hypercarbia and ' /4 and 3h h following an abrupt onset of normocarbia. The initial 2.5-fold increase in total BBF during hypercarbia persisted for 2 h and at 4 h decreased significantly below the level of the 30-min hypercarbic measurement, although the values still remained 2-fold above the baseline values. Brain tissue pH was reduced at the onset of hypercarbia, remaining unchanged throughout the hypercarbic period. Both total BBF and brain tissue pH returned to baseline values following the return to normocarbia. Changes in regional BBF were similar to that of total BBF with the exception of the boundary zone (periventricular area in the frontoparietal region of the cerebrum, adjacent to the caudate nucleus) and the parietal area (site of the brain tissue pH electrode), where a significant decrease from the peak hyperemia was not observed. Choroid plexus blood flow demonstrated a small initial increase over normocarbic baseline values. Plasma bicarbonate concentrations did not change during the studies. Following the onset of hypercarbia, heart rate and cardiac output increased significantly whereas plasma lactate concentrations decreased significantly. There was also an increase in the percentage of cardiac output distributed to the brain. The increase in cardiac output persisted throughout the hypercarbic period and returned to baseline pH, 2) prolongation of hypercarbia for 4 h was associated with a slight but significant reduction in the cerebral hyperperfusion independent of changes in brain tissue pH, and 3) in contrast to that observed in adult subjects, the abrupt normalization of arterial PCOz was not associated with a fall of BBF below the prehypercarbic values. (Pediatr Res 21: 29-33, 1987) Abbreviations BBF, brain blood flow IVH, intraventricular hemorrhage MABP, mean arterial blood pressure Hypercarbia results in cerebral vasodilation and increased BBF in fetal and newborn animals (1-3) and may be an important factor in the development of IVH in premature infants (4). In infants with respiratory disorders, PaC02 may exceed the physiologic range for considerable amounts of time despite appropriate ventilatory therapy.In adult animals and humans, acid-base adjustm...

The Effect of Arterial Pco2-Variations on Ocular and Cerebral Blood Flow in the Newborn Piglet

“…In the present study, there apparently was a fall in CBF from level I to level I1 hypercarbia. This tendency to a slight decrease in CBF during prolonged exposure to hypercarbia has also been reported by others (30). This may partly be due to reduced vasodilative response of pial arterioles as suggested by Levasseur (3 1).…”

“…(19) variations in PaC02 levels did not alter CO significantly, whereas Brubakk et al (30) found an increase in CO during hypercarbia. Both in our study and in that of Hansen, the animals were paralyzed with pancuronium and mechanically ventilated.…”

Retinal, Choroidal and Total Ocular Blood Flow Response to Hypercarbia during Spontaneous Breathing and Mechanical Ventilation

“…Similar results have also been reported by other investiga tors. Thus, in the study by Brubakk et al [ 17] on awake and spontaneously breathing pig lets, a significant increase in CO was found during hypercarbia. This also agrees with our previous study [6], where a significant in crease in CO was found during sustained hypercarbia.…”

Cerebral Vascular Responses to Changes in Carbon Dioxide Tension in Term and Preterm Infants with Apnea

“…This response may be less predictable during the neonatal period due to immaturity of the autonomic nervous system and autoregulatory limitations [4]. Results of studies of the CBF in newborn animals have been contradictory, with some finding a markedly reduced response to changes in pCO 2 and others suggesting that the response is similar to that of adults [5,6]. Well full-term human infants are reported to have normal cerebral vasoregulation, while studies in preterm infants have reported both normal and diminished sensitivity of CBF to changes in pCO 2 [7][8][9].…”