All adult DCD heart transplant recipients at a single institution from October 2021 to December 2024 were reviewed. Only DCD hearts recovered using TA-NRP and transported using static cold storage were included in this study. Allografts transported using a machine perfusion device, multiorgan transplant recipients, and recipients with congenital etiology of heart failure were excluded. Transplants without any cardiopulmonary bypass (CPB) parameters recorded or with missing donor warm ischemic times were also excluded.

Our institutional NRP protocol includes a standard median sternotomy followed by the insertion of a right atrial cannula and direct aortic cannula. Our modified extracorporeal circuit was then used to initiate NRP. The NRP protocols used in this study have been previously described in the literature (1, 2). Our clinical practice is to use NRP for recovery of DCD cardiac allografts, except in instances where NRP is not permitted.

While various definitions of functional warm ischemic time (FWIT) using systolic blood pressure and oxygen saturations exist, our recently published institutional results determined FWIT defined as oxygen saturation < 80% as the best predictor for outcomes (4). Therefore, in this study FWIT was defined as the duration between donor oxygen saturation < 80% and the initiation of NRP during the DCD heart recovery process. DO2i was captured and extracted from our institutional perfusion system (Quantum Perfusion System, Spectrum Medical). DO2i is dependent on several variables and defined as:DO2i=CardiacOutput×[(1.34×HbxSaO2)+(0.003×PaO2)]/BSAHb was hemoglobin, SaO2 was arterial oxygen saturations, PaO2 was partial pressure of oxygen in arterial blood, and BSA was the patient's body surface area.

The critical area—defined as total area under a DO2i threshold and above the continuous perfusion curve—was calculated to represent both magnitude and duration below a specific threshold (Figure 1). This was calculated using the trapezoidal method for integration on a minute by minute basis, where the boundaries were determined by the points where the curve goes above the threshold. In other words, this calculated the area between a continuous curve and a target threshold, but only when the curve is below the threshold. Therefore, a greater critical area for DO2i indicates greater deviation below the perfusion target and/or longer time below the target, which represents lower overall DO2i. Thresholds chosen for DO2i were 300, 280, and 260 mL/min/m2. These thresholds were chosen since the most commonly cited DO2i threshold for GDP in general adult cardiac surgery is 280 mL/min/m2, with some protocols targeting thresholds ranging from 270 to 300 mL/min/m2. (11, 12) Since GDP has not previously been explored in heart transplantation, we selected a higher and lower threshold in addition to the known 280 mL/min/m2 threshold in this exploratory analysis.

Outcomes of interest include primary graft dysfunction (PGD), as defined by the International Society of Heart and Lung Transplantation (ISHLT) consensus statement in 2014 (13), post cardiopulmonary bypass left ventricular ejection fraction (LVEF) and right ventricular function, postoperative cardiac indices, postoperative vasoactive inotrope scores (VIS), acute kidney injury (AKI), LVEF at post operative day 7, duration of mechanical ventilation, ICU and hospital length of stay, and 30-day and 1-year survival. AKI was staged using the KDIGO criteria (14). VIS was calculated as dopamine (µg/kg/min) + dobutamine (µg/kg/min) + 100 * epinephrine (µg/kg/min) + 100 * norepinephrine (µg/kg/min) + 15 * milrinone (µg/kg/min). A common closing follow-up of January 31, 2025, was applied to all patients to ensure complete severe PGD and 30-day mortality data in all patients.

First, univariate logistic regression models were used to compare the strengths of association between DO2i critical areas defined with the three different thresholds and severe PGD. Model performance was compared using the Akaike Information Criterion (AIC), with lower AIC values indicating better model fit relative to model complexity. Once the DO2i critical area definition most predictive of severe PGD was identified, receiver operating characteristic (ROC) curve analysis was used to dichotomize the parameter. Subsequently, patients were stratified into four groups based on the dichotomized DO2i critical area, and 23 min of FWIT. A break point of 23 min for FWIT was selected based on prior literature (4). Demographics and outcomes were compared between the four groups. A multivariate logistic regression model was created to assess the effect of FWIT and DO2i critical area on rates of severe PGD while controlling for known risk factors for worsened outcomes after transplant. The covariates we adjusted for included donor age, total ischemic time, static cold storage modality, recipient ischemic etiology of heart failure, pre-transplant intra-aortic balloon pump support, pre-transplant Impella support, and pre-transplant durable left ventricular assist device support.

A significance level of p < 0.05 was used throughout this study. All analysis was conducted using R 4.3.3 (R Core Team). Categorical variables are reported as number (percentage) and compared using Fisher exact test. Continuous variables are reported as mean (standard deviation) and compared using ANOVA. post-hoc pairwise comparisons were performed for variables significantly different across four groups, using the Holm-Bonferroni method to adjust for multiple comparisons in categorical variables and Tukey's Honestly Significant Difference test for continuous variables. The Institutional Review Board (IRB) or equivalent ethics committee of the Vanderbilt University Medical Center approved the study protocol and publication of data, under protocol #241909. Patient written consent for the publication of the study data was waived by the IRB due to the retrospective, chart review nature of the study.

A total of 102 patients met inclusion criteria. The median total warm ischemic time from donor withdrawal to allograft reperfusion on NRP was 26 min (IQR 21–34). The median functional warm ischemic time, defined as oxygen saturation < 80% to reperfusion, was 22 min (IQR 18–29). All other summary statistics of different warm ischemic intervals were shown in Table 1. The average DO2i was 296 mL/min/m2 (IQR 270–319) in the entire cohort. Summary statistics for duration below and critical areas below the pre-specified DO2i thresholds of 260, 280, and 300 mL/min/m2 were also shown in Table 1. On univariate logistic regression, the area under 260 (p = 0.018), 280 (p = 0.024), and 300 mL/min/m2 (p = 0.030) were all significantly associated with severe PGD. The AIC for the model with area under 260 mL/min/m2 was 49.45, compared to an AIC of 50.18 and 50.61 for areas under 280 and 300 mL/min/m2, respectively. This indicates that of the three pre-specified DO2i thresholds, the area under 260 mL/min/m2 was the most predictive of severe PGD. Going forwards, critical area was defined as the DO2i area under 260 mL/min/m2. ROC analysis then found a DO2i area under 260 mL/min/m2 threshold of 1,424 mL/m2 with an area under the curve of 0.82 (95% CI 0.67–0.97), with a sensitivity of 0.88 and specificity of 0.66.

The patients were stratified into four groups based on low/high FWIT and critical area below/above 1,424 mL/m2. There were 39 (38.2%) patients with low FWIT/ high DO2i, 18 (17.6%) with low FWIT/ low DO2i, 24 (23.5%) with high FWIT/high DO2i, and 21 (20.6%) with high FWIT/low DO2i.

Donor age, sex distribution, body mass index (BMI), total ischemic time, and static cold storage technique (ice or 10 °C cooler) were similar between the four groups (Table 2). Recipient age, sex, and BMI were also similar. The proportions of recipients with ischemic cardiomyopathy, re-do sternotomy, and pre-transplant mechanical circulatory support were also comparable. There was also no difference in the distribution of recipient waitlist status at transplant between the groups.

Total bypass length was greater, and minimum hemoglobin and mixed venous saturations were lower, in the two groups with lower DO2i compared to the two groups with high DO2i (Table 3). Pairwise comparisons confirmed that the most drastic differences were between the high FWIT/low DO2i group and both the low FWIT/high DO2i and high FWIT/high DO2i groups. Intraoperative transfusion requirements were also different between the groups, with the high FWIT/low DO2i (average 4,344 mL) requiring significantly higher total transfusions compared to both the low FWIT/high DO2i group (average 2,219 mL) and high FWIT/high DO2i group (average 1,248 mL). When broken down into transfusion requirements of specific blood components, we found that transfusion requirements for packed red blood cells and fresh frozen plasma followed the same pattern as total intraoperative transfusion. Intraoperative LVEF was lower in the high FWIT/low DO2i group compared to the high FWIT/high DO2i group (mean LVEF 49% vs. 55%, p = 0.028). Rates of intraoperative severe RV dysfunction were greater in the high FWIT/low DO2i group compared to both the low FWIT/high DO2i (23.8% vs. 5.1%, p = 0.038) and high FWIT/high DO2i groups (23.8% vs. 0%, p = 0.014). Rates of severe PGD were greater in the high FWIT/low DO2i group compared to the low FWIT/high DO2i group (23.8% vs. 0%, p = 0.004) (Figure 2). Rates of 30-day mortality were higher in the high FWIT/low DO2i group compared to the low FWIT/high DO2i group (14.3% vs. 0%, p = 0.039). Multivariate regression adjusting for potential confounders demonstrated that increasing FWIT (per minute increase: OR 1.04, 95% CI 1.01–1.09, p = 0.013) and lower DO2i captured by increased critical area (per 100 mL/m2 increase: OR 1.05, 95% CI 1.01–1.10, p = 0.019) were both independently associated with severe PGD. All p-values of pairwise comparisons were shown in Supplementary Table 1.