It’s February and if you are a bear, it’s the middle of hibernation season. During hibernation, the bear does not merely fall asleep; there are a complex series of changes that occur throughout its body and the heart is no exception. First, the heart rate slows dramatically. When active, a bear’s heart rate is around 70 to 80 beats per minute. During hibernation, the heart rate slows to 14 beats per minute. There are also a series of changes within the heart. The left ventricle stiffens, preventing stretching due to the low heart rate. In addition, there is a change in the biochemistry of the heart muscle itself. A protein called myosin controls heart muscle contractions. During hibernation, there is a switch from the usual myosin to a different variety which produces a weaker contraction. What can we learn from the study of animals to help us understand the human heart better? What happens when our heart beat is very low? Can human hearts hibernate?
The normal human heart beat ranges from 60 to 100 beats per minute. Very slow heart rates, 20 to 40 beats per minute, do occur and can be present in normal hearts as well as various disease states. The heart beat normally slows while sleeping. Athletes often have very slow rates as a result of training. In fact, athletes usually have a heart rate in the 40’s and are symptomatic. It is not considered abnormal until an athlete’s rate is below 30 beats per minute. The most common abnormality causing a slow heart beat is sick sinus syndrome. This occurs in older individuals and is a result of the electric system of the heart “wearing down” over the years. A heart attack can slow the heart rate, especially if it affects the blood flow to the electric system. Low heart rates are also seen in an under active thyroid (hypothyroidism), sleep apnea, and Lyme disease. There are many medications that can slow the heart beat. These include beta blockers (for example, metoprolol), calcium channel blockers (ex, diltiazem), digoxin, rhythm agents (ex, amiodarone, sotalol), eye drops (especially ones containing timolol, a beta blocker) and Alzheimer medications (donepezil and memantine). Patients with very slow heart rates may have no symptoms or they may feel lightheaded, dizzy, sweaty or pass out. If left untreated, a slow heart rate will lead to congestive heart failure or death. The diagnosis of a slow heart rate is made by capturing the event on an EKG strip. A variety of methods can be used. A Holter monitor is an EKG that is attached to patient and worn all day and night for one to three days. An event monitor is a patch that is applied to the chest. A constant signal is sent to monitoring center and if a significant rhythm change occurs, the doctor is notified. The patch is worn for two to four weeks. These monitors are good if the patient has frequent events. Sometimes, events occur weeks or months apart. In that case, an implantable loop recorder is placed. This is a small metal device that is placed under the skin and can monitor a patient for months or years at a time. In the case of an event, the EKG can be downloaded on a computer. For patient centric devices, there is the Apple watch and Kardia mobile. Kardia mobile provides a single lead EKG when the patient places two fingers on the device and an EKG is stored on a cell phone. It is commercially available on Amazon and can be bought without a prescription. The treatment for a slow heart is to correct the underlying cause or stop the offending medication. If that does not improve the heart rate or symptoms, then a pacemaker is placed.
The human heart can hibernate, but for a different reason compared to the hibernating bear’s heart. In humans, the whole heart doesn’t hibernate, like the bear, but only a portion of the heart muscle. If there is a region of the heart muscle that is supplied by a blocked artery for many months or years, the region will hibernate. Normally if there is lack of blood flow to the heart muscle, there is a heart attack and the affected muscle dies (it stops working, contracting). In an area of hibernation, there is enough blood flow to keep the muscle alive, but it doesn’t contract normally. In effect, the area of heart muscle adapts by downregulating- reducing or ceasing contraction and changing metabolism to try to keep the area alive. Similar to the bear’s hibernating heart, there is a change in the biochemistry. Biopsies of hibernating heart muscle in humans has shown a reduction in and disorganization of the contracting proteins, including myosin. If the blocked artery supplying the area is opened (with a stent or bypass surgery) and blood flow restored, the affected hibernating heart muscle returns to normal function.
In the animal kingdom, the adaptations made by the heart are beneficial, helping the animal to survive cold winters and to ultimately thrive. In humans, the hibernating heart can keep us alive, but in the long run the changes are detrimental. Hibernating areas of heart muscle weaken the overall contraction of the heart resulting in congestive heart failure and ultimately death. Similarly, a slow heart rate seems to confer a survival advantage in the animal kingdom while not a factor in man. Animals with fast heart rates such as the shrew with a heart rate of 220 beats per minute tend to have short lifespans, a few years at most. On the other hand, the Galapagos giant tortoise, whose average heart rate is six, can live for more than 100 years. If the human heart were to slow down to that level blood would pool in the four chambers, the heart would start to enlarge, the muscle would weaken leading to congestive heart failure and death. Aside from interesting physiology, the study of animal adaptations may lead to advances in human heart disease. The research has already borne fruit by producing new medications that target myosin. Omecamtiv activates myosin, improving the heart’s contractility and helping with heart failure due to a weakened heart muscle. Mavacamten inhibits myosin, decreasing the force of contraction in patients with a thickened heart muscle who need less vigorous heart pumping. Let sleeping bears lie, we can learn a lot from them.
