Association Is Not Causation
Viviane Nasr MD, Susan Nicolson MD, LIndsey Loveland Baptist MD, James DiNardo MD
Today’s Pediatric Anesthesia Article of the Day is brought to us by the PAAD’s cardiac anesthesia reviewers. After the ASA/SPA meeting and as promised, we’ve developed a more formal relationship with SPA, created an executive council, increased the number of reviewers, and you will start to notice a wider variety of journals we review. The editors of Anesthesia & Analgesia thought today’s original article was important enough to have an accompanying editorial AND infographic. Myron Yaster MD
ORIGINAL ARTICLE
Justin B Long, Branden M Engorn, Kevin D Hill, Liqi Feng, Karen Chiswell, Marshall L Jacobs, Jeffrey P Jacobs, Dheeraj Goswami. Postoperative Hematocrit and Adverse Outcomes in Pediatric Cardiac Surgery Patients: A Cross-Sectional Study From the Society of Thoracic Surgeons and Congenital Cardiac Anesthesia Society Database Collaboration. Anesth Analg. 2021 Nov 1;133(5):1077-1088. PMID: 33721876
EDITORIAL
David Faraoni, James A DiNardo. Red Blood Cell Transfusion and Adverse Outcomes in Pediatric Cardiac Surgery Patients: Where Does the Blame Lie? Anesth Analg. 2021 Nov 1;133(5):1074-1076. PMID: 34673720
Pediatric patients with congenital heart disease (CHD) undergoing cardiac surgery are frequently exposed to packed red blood cells (RBCs). RBCs are used to prime the cardiopulmonary bypass (CPB) circuit to minimize hemodilution and again after weaning from CPB to replace blood loss and to optimize oxygen delivery to vital organs. The threshold for transfusion during surgery and the target hematocrit during CPB in infants have been studied. Despite recommendations that liberal transfusion is not superior to restrictive transfusion in children with CHD, wide variation in RBC transfusion practices exist with many favoring liberal transfusion.
The literature on thresholds for transfusion and hematocrit levels during CPB is abundant, but there is limited discussion on the upper limit of “safe hematocrit” threshold. The study by Long et al. in this month’s A&A aims to evaluate the association between the hematocrit levels on arrival to the ICU and postoperative outcomes. The authors reviewed 27,462 patients from the Society of Thoracic Surgeons Congenital Heart Surgery Database divided into 4909 (17.9%) cyanotic (arterial oxygen saturation of < 92%) patients and 22,553 (82.1%) acyanotic patients. The hematocrit levels were split into three groups, low, medium, and high. In the cyanotic patients the levels were <36%, 36-42%, and >42% and in the acyanotic, <32%, 32-40%, and >40%. For acyanotic patients, increases in hematocrit >38% and for cyanotic patients, increases in hematocrit >42% were associated with increased odds of major complication and/or operative mortality.
We’d like to remind the PAAD readers that Association is a statistical relationship between two variables, in this case RBC transfusion and adverse outcomes. However, two variables may be associated without a causal relationship.
The goal of RBC transfusion is to ensure oxygen carrying capacity while providing volume and preventing dilution and anemia. RBC transfusion helps restore or maintain hematocrit level that is necessary to maintain oxygen delivery (DO2) matched to oxygen consumption (VO2). This matching will provide adequate organ perfusion and avoid utilization of anaerobic metabolism by the tissues. (1) Under general anesthesia, VO2 decreases, and the critical hematocrit level might be lower than when the patient is awake and VO2 increases. In addition, patients who are transfused could be patients who are bleeding acutely, patients with low cardiac output or patients that require higher DO2. As the clinical conditions change, the interaction of VO2, DO2 and target hematocrit change. It is a dynamic process. Hence without understanding the patient’s condition that triggered the decision to transfuse, one cannot conclude whether the indication for transfusion, the higher hematocrit or both led to worse outcomes. As stated in the editorial by Faraoni and DiNardo that accompanies Long’s article: “ A single transfusion threshold for all patients with CHD will never be a viable approach, clinical tools to rapidly and reliably determine DO2 and VO2 so that RBC transfusion can be evidence based are urgently needed.”
The importance of patient blood management in pediatric cardiac and noncardiac patients is well recognized. The formation of the SPA Patient Blood Management Committee and the CCAS Hemostasis Interest Group is a testimony of the need for additional investigations in this area.
References:
1. Nicolescu LC, Nicolescu CM, Mihu AG, Balta C. The effect of red blood cell transfusion on peripheral tissue oxygen delivery and consumption in septic patients. Transfus Clin Biol. 2021 Feb;28(1):5-10. doi: 10.1016/j.tracli.2020.12.002. Epub 2020 Dec 8. PMID: 33307215.
