Concise Critical Appraisal: Cerebral Oxygen Metabolism and RBC Transfusion in Infants

The use of transfusion in critically ill children or those who have undergone major surgery has declined since the publication of the landmark articles by Hébert et al (N Engl J Med. 1999;340:409-417) and Lacroix  et al (N Engl J Med. 2007;356:1609-1619). It has now become the standard of care to limit the use of transfusion to most critically ill but hemodynamically stable children to maintain a transfusion trigger of 7 mg/dL. While most of the literature dealing with transfusions supports this trigger, the actual effect on oxygen delivery (DO₂) to specific organs and the issue of potential morbidity secondary to diminished DO₂ from anemia have not been fully elucidated. Neunhoeffer et al (Pediatr Crit Care Med. 2018;19:318-327) sought to use noninvasive means to determine the changes in cerebral oxygen metabolism in infants younger than 6 months who have undergone major surgery. By measuring cerebral oxygen saturation and cerebral microperfusion, the authors hypothesized that they could find a population of critically ill infants who had restricted DO₂ that would improve after transfusion.

For this prospective, observational cohort study, the authors used physiologic data obtained using tissue spectrometry and laser Doppler flowmetry from 58 infants who underwent surgery. These infants were divided into 3 groups based on the type of surgery—infants who underwent major noncardiac surgery (group 1), infants with congenital heart disease who underwent primary biventricular repair (group 2), and infants with functionally univentricular hearts who underwent palliative procedures (group 3). The authors then measured physiologic variables, including cerebral oxygen saturation (cSO₂) and relative cerebral blood flow (rcFlow), in order to calculate cerebral fractional tissue oxygen extraction (cFTOE), arteriocerebral difference in oxygen content (acDO₂) and cerebral metabolic rate of oxygen metabolism (CMRO₂), before and after transfusion.

Using these noninvasive modalities, the authors demonstrated that transfusion resulted in no significant change in arterial carbon dioxide, heart rate, lactate, or pH, although groups 1 and 2 had increases in mean arterial pressure (MAP). In addition to this change in MAP, the authors demonstrated that posttransfusion cSO₂ values were statistically higher in all groups and that cFTOE decreased, although rcFlow, acDO₂, and aCMRO₂ remained unchanged. These latter measures suggest that, even though oxygen extraction decreased, the fact that cerebral blood flow remained the same as well as aCMRO₂, there did not appear to be a significant pretransfusion oxygen deficit in these patients. Even though oxygen extractions decreased, these latter measures demonstrate that there was no pretransfusion oxygen deficit since cerebral blood flow and CMRO₂ remained the same The authors comment on a possible role of altered cerebral autoregulation as well. They also found that infants with a cFTOE > 0.4 had a steep increase in cSO₂ and that this degree of extraction was seen more commonly among the two groups of infants with congenital heart disease.

The authors comment that elevated cFTOE had been previously associated with poor neurologic outcomes in preterm infants and that cFTOE could serve as a surrogate measure of the adequacy of oxygenation in their patient population as well. They also suggest that infants who had more profound (> 0.4) extraction, and subsequently a more profound rise in cSO₂, may be better candidates for transfusion than infants with the same hemoglobin value but lower extraction.

This study supports the use of physiologic variables, obtained noninvasively, to make the decision of whether to transfuse a critically ill infant who has undergone surgery. This may be an important place to start in making this decision, but more outcome data must be obtained. While anemia may be bad for these infants, the risks of transfusion may be worse.

Author of this installment of Concise Critical Appraisal:

Daniel E. Sloniewsky, MD, is an associate professor in the Division of Pediatric Critical Care Medicine in the Department of Pediatrics at Stony Brook Long Island Children’s Hospital. Dr. Sloniewsky is an editor of Concise Critical Appraisal.