Cardiac Loading using Passive Left Atrial Pressurization and Passive Afterload for Graft Assessment

Abstract

Ex vivo machine perfusion or normothermic machine perfusion is a preservation method that has gained great importance in the transplantation field. Despite the immense opportunity for assessment due to the beating state of the heart, current clinical practice depends on limited metabolic trends for graft evaluation. Hemodynamic measurements obtained from left ventricular loading have garnered significant attention within the field due to their potential as objective assessment parameters. In effect, this protocol provides an easy and effective manner of incorporating loading capabilities to established Langendorff perfusion systems through the simple addition of an extra reservoir. Furthermore, it demonstrates the feasibility of employing passive left atrial pressurization for loading, an approach that, to our knowledge, has not been previously demonstrated. This approach is complemented by a passive Windkessel base afterload, which acts as a compliance chamber to maximize myocardial perfusion during diastole. Lastly, it highlights the capability of capturing functional metrics during cardiac loading, including left ventricular pulse pressure, contractility, and relaxation, to uncover deficiencies in cardiac graft function after extended periods of preservation times (˃6 h).

Figure 1:. Perfusion System Setup.

(A) Perfusion setup. Black lines indicate the flow of perfusate during both perfusion modalities. The blue dashed line indicates the flow of perfusate during Langendorff perfusion only, and the orange lines indicate the flow of perfusate during load perfusion. (B) Location of the aortic (solid line) and atrial (dashed line) pressure sensors. Since the atrial pressure sensor is located spatially below the atrium, the height difference was taken into consideration for proper LA pressurization.

Figure 2:. Conventional assessment metrics.

Metrics are denoted as conventional if their acquisition is possible during Langendorff perfusion. (A) Heart rate was calculated from the pressure waves obtained using the left intraventricular pressure sensor. However, as this metric can easily be calculated from EKG traces, also available during Langendorff, it is considered a conventional metric. The solid line is the median, and the shaded region is the interquartile range (IQR). (B) Area under the curve (AUC) of heart rate data for every 30 min of perfusion time. (C) Vascular resistance (CVR) of the grafts. (D) Oxygen uptake rate (OUR). (E) Lactate accumulation. (F) Glucose consumption. (G) Potassium accumulation. (H) Percent weight gain as a proxy for edema. All data is expressed as median ± interquartile range (IQR, n = 4). Repeated measures two-way ANOVA and Tukey-Kramer HSD post-hoc analysis on standard least squares means. * = p <0.01.

Figure 3:. Non-conventional assessment metrics.

Metrics are denoted as non-conventional when their acquisition is only possible during loaded perfusion. (A) Left Ventricular (LV) pulse pressure plotted over time. The solid line is the median, and the shaded region is the IQR. (B) The area under the curve (AUC) of the LV pulse pressure for every 30 min of perfusion time. (C) AUC of cardiac contractility was obtained from the maximum derivative of the LV pulse pressure for every 30 min of perfusion. (D) AUC of cardiac relaxation was obtained from the minimum derivative of the LV pulse pressure for 30 min of perfusion time. All data is expressed as median ± interquartile range (IQR, n = 4). Repeated measures two-way ANOVA and Tukey-Kramer HSD post-hoc analysis on standard least squares means. * = p <0.01, ^** = p <0.05, ^*** = p < 0.001, ^**** = p <0.0001.