Background This study investigated the effects of position on heart rate variability (HRV) in patients some years after orthotopic heart transplantation (OHT) surgery. Methods Spectral HRV analysis was performed on 15 patients after OHT and 16 patients with coronary artery disease (CAD). HRV measures were compared between OHT and CAD patients in four randomly ordered positions [supine, right lateral decubitus (RLD), left lateral decubitus (LLD), and upright]. Multivariable linear regression analysis was used to identify the factors associated with cardiac function and HRV of OHT patients in supine position, and the factors associated with the outcome (OHT or CAD) of the patients. Results The powers in all frequency ranges were significantly decreased in all four positions in OHT patients about 9 years after OHT surgery, as compared with those of CAD patients. Both RLD and LLD positions can lead to a significantly higher normalized high-frequency power in OHT patients than the supine position, as compared with the CAD patients. The LLD position seemed to be better than the other recumbent positions in terms of vagal enhancement in the OHT patients. Multivariable linear regression analysis showed that the left ventricular ejection fraction of OHT patients can be predicted from a linear combination of the OHT to HRV interval, and normalized very low-frequency power in the supine position. Furthermore, better cardiac function and the presence of cardiomyopathy would increase the necessity of OHT surgery, while the use of nitrates would decrease the necessity of OHT surgery. Conclusion Both the RLD and LLD positions, especially the LLD position, can lead to a higher vagal modulation in OHT patients about 9 years after OHT surgery, provided that the HRV measures can still be regarded as indicators of autonomic nervous modulation in such patients. Moreover, left ventricular ejection fraction, cardiomyopathy, and the use of nitrates were all associated with the necessity of OHT surgery.
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