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The heart beat is regulated by autonomic nervous system(ANS) more than the central nervous system (CNS) (Brain). So in the process of transplantation these autonomic nervous system get disrupted and it will not have that influence on heart rate. Usually in heart ANS have a more parasympathetic system, which is reducing the rate. And once it is disrupted that resting parasympathetic tone is gone and the heart beat little faster than the previous rate, not as the donor or as the recipient . For eg the resting heart rate in donor was 72 , recipient before was having 84, after successful transplantation it will be on higher side like 100.