A Change of Heart: Heart Transplants and the Nervous System
Written by Lillian Brinkmann
Edited by Kelly Chau
Jan 23rd 2022
Edited by Kelly Chau
Jan 23rd 2022
Ever had a change of heart? For some people, a change of heart can be a pretty big deal, involving hefty medical intervention and life-long consequences. You don’t have to be a cardiac surgeon to know that heart transplants are no joke. But what goes into replacing this vital organ? As outlined in this article from the European Heart Journal, the balance between our cardiac and nervous systems is thrown off when a new heart comes into play.
Our hearts don’t operate alone; rather, they constantly receive input from our autonomic nervous system that allows them to work optimally in different situations. When we’re in danger, exercising, or stressing out, the sympathetic nervous system (SNS) sends signals for the heart to beat more quickly, optimizing blood flow to the body in strenuous times. During periods of rest, the parasympathetic nervous system (PNS) keeps the heart relaxed. The SNS and PNS communicate with the heart via nerves, but a transplant requires those nerves to be cut. In the time following the operation, the connections between the heart and these regulatory systems continue to degenerate until the heart is fully denervated. In other words...the new heart is cut off from SNS and PNS input.
Denervation has some noticeable consequences. Without PNS input, the SA node, which helps set the heart’s pace, doesn’t get the memo that it’s time to relax. Therefore, heart transplant patients have higher resting heart rates. According to a 2009 study, this change can be detrimental, for researchers found that post-transplant patients experiencing higher-than-average heart rates over a ten year period were 4.7 times more likely to pass away than subjects with average heart rates. Similarly, the lack of SNS input means that these patients don’t experience the typical, immediate rise in heart rate when exercising, which leads to decreased performance capacity.
Does this mean that a transplanted heart will forever operate in the dark? As a matter of fact, the new heart actually undergoes something known as reinnervation in the years following the operation. During this process, the gradual regrowth of nerve connections facilitates the reestablishment of communications between the heart and nervous system. Researchers have observed this phenomenon using methods ranging from PET imaging to the injection of tyramine, a chemical that allows observers to determine whether the nerves are functioning.
Reinnervation is a slow, incomplete process, progressing up to fifteen years post-op and never quite reaching pre-op efficiency. While some areas of the heart, like the SA node, do experience reinnervation, others like the right coronary artery undergo little or none. Regardless, the effects of reinnervation are measurable and beneficial. One study suggests that, in comparison to their still-denervated counterparts, patients experiencing reinnervation have greater exercise capacity, exhibiting higher heart rates and greater stamina. Another study found that reinnervated subjects had higher ejection fractions during exercise, i.e., more blood being pumped to the rest of the body.
Although a change of heart can be scary, especially when the nervous system is involved, the body’s ability to adapt is truly amazing. Not only does a donor heart save the life of its recipient, but it can also mesh with its new environment and connect with pre-existing systems. In these cases, it seems that a big change can be a great thing.