Background
The Transfusion Requirement After Cardiac Surgery (TRACS) study showed that restrictive perioperative red blood cell transfusion strategy was as safe as a liberal strategy in patients undergoing elective cardiac surgery1). However, it is unknown whether restrictive perioperative red blood transfusion is safe in a subgroup of patients who required prolonged mechanical ventilation (PMV) after cardiovascular surgery, which is one of the risk factors of poor outcome2).

Methods
We retrospectively collected patient’s data at Yokohama City University Medical Center 2008 to 2012. We enrolled patients who required mechanical ventilation for more than 72 hours after cardiovascular surgery. Patients were categorized into two groups according to the hematocrit (Ht) on ICU admission; equal or more than 30% (≥30%) (Group H) or Ht less than 30% (Group L). The Mortality of two groups was compared using Kaplan-Meier technique and Log-rank test. To determine the predictor of mortality, we performed the Cox regression  analysis.

Results
Out of 252 patients, data from 187 patients were collected. Ninety-two and 95 patients were classified as Group L and H, and their mean (±standard deviation) Hematocrits were 33.79% (±3.56) and 26.70% (±2.44), retrospectively. Mortality between the two groups was significantly different with Kaplan-Meier technique (HR=0.53, 95%CI=0.32-0.90, P=0.014). Multivariate analysis revealed that seven factors were correlated with mortality.　

Discussion
Ht <30% on ICU admission and lower preoperative LVEF may be associated with poor outcome, even after other factors that may influence Ht such as gender and blood loss were considered. Several studies suggested that restrictive red blood cell transfusion strategy was not as safe as liberal one for patients undergoing cardiovascular surgery3) or with ischemic heart disease4). Their oxygen demand increases due to ventricular hypertrophy, and their oxygen supply decreases due to low cardiac output or coronary artery stenosis. The oxygen supply-demand balance would easily perturbed by restrictive transfusion strategy. 

Conclusion
Our retrospective analysis suggest that in patients who required PMV after cardiovascular surgery, mortality was low in the group with Ht ≥30% on ICU admission. Further studies are required to confirm our results.

References
1) Hajjar LA, Vincent JL, Galas FR, et al. Transfusion requirements after cardiac surgery: the TRACS randomized controlled trial. JAMA. 2010;304:1559-67.
2) Van Caenegem O, Jacquet LM, Goenen M. Outcome of cardiac surgery patients with complicated intensive care unit stay. Curr Opin Crit Care. 2002;8:404-10.
3) Murphy GJ, Pike K, Rogers CA, et al. Liberal or restrictive transfusion after cardiac surgery. N Engl J Med. 2014;372:997-1008.
4) Docherty AB, O'Donnell R, Brunskill S, et al. Effect of restrictive versus liberal transfusion strategies on outcomes in patients with cardiovascular disease in a non-cardiac surgery setting: systematic review and meta-analysis. BMJ. 2015;352:i1351.

Low hematocrit on ICU admission is a risk factor of long term outcome in patients who required prolonged mechanical ventilation after cardiovascular surgery. A single center retrospective cohort study