What is Myocardial Stunning and Hibernation?
Coronary artery disease may cause issues with blood flow to the heart, both sudden (acute) and ongoing (chronic). Poor blood flow can lead to the heart muscle not working as well as it should, causing the lower left chamber of the heart (left ventricle) to function less effectively. Some people may be able to restore how well their heart works by getting a procedure that opens the blocked heart arteries (percutaneous coronary intervention) or going through heart bypass surgery. Scientists are actively researching how to determine if heart tissue can recover from these treatments.
The term “stunned” heart muscle is used to describe heart muscle that temporarily loses ability to contract properly due to sudden lack of blood supply. This condition usually occurs when the blood flow is almost fully restored and the heart muscle does not deteriorate. This term was originally used to describe a situation observed in lab experiments with dogs. In these experiments, the heart artery was completely blocked for 5-15 minutes and then blood flow was restored. Despite this, the left ventricle wall motion showed irregularities that lasted for several days – this is what ‘stunning’ refers to.
The phrase “hibernating” heart muscle describes a condition where the heart muscle doesn’t contract as well as it should because it’s not getting enough blood over a long period of time. This is usually because the blood vessel (coronary artery) supplying the heart is narrowed and cells within the heart muscle remain alive but their ability to contract is reduced. However, the function of this affected heart muscle may be partially or even fully restored by improving blood flow or reducing how much oxygen the heart muscle needs.
Patients with ongoing problems in the lower left chamber of the heart, which can include a range of issues from localized dysfunction to a disease where not enough blood gets to the heart tissue (ischemic cardiomyopathy), could have a hibernating heart muscle. Many of these patients might have naturally formed bypasses and newly formed blood vessels which keep the lower left chamber functioning. Therefore, the extent of the dysfunction in the lower left chamber doesn’t always directly relate to how severe the coronary artery disease is.
Since the hibernating heart muscle is still alive, it’s possible that the function of the lower left chamber could be partially or completely restored if the blockages in the heart arteries are properly treated in a timely manner. A variety of tests such as dobutamine echocardiography, a PET scan, testing that uses small amounts of radioactive materials (radionuclide myocardial perfusion imaging) and specialist imaging of the cardiovascular system (CMR imaging) can help identify if the heart muscle can contract if stimulated, or if there is any activity in the part of the heart muscle that isn’t functioning properly. Therefore, identifying the difference between tissue that can improve its ability to contract and tissue that is permanently damaged and won’t recover after treatment is very important.
Both “stunned” and “hibernating” heart muscles can regain their ability to contract with improved blood flow.
What Causes Myocardial Stunning and Hibernation?
Temporary blockage of the coronary blood supply can cause the heart muscle to temporarily lose function, a phenomenon known as myocardial stunning. This can happen for the same reasons that cause coronary artery disease, which is a condition that affects the blood vessels supplying the heart.
Signs and Symptoms of Myocardial Stunning and Hibernation
Acute myocardial infarction, considered a type of heart attack, is often followed by a phase called “Myocardial stunning.” During this phase, the heart’s performance decreases due to damage, but usually recovers nearly back to normal within two weeks. What’s interesting is that even sections of the heart not directly affected by the attack can also show this pattern. Eventually, these areas also recover, demonstrating improvements in the contraction of the heart muscle.
Stable angina, a type of chest pain caused by reduced blood flow to the heart, can also involve Myocardial stunning. Despite the presence of this condition, the function of the heart’s left ventricle tends to improve after restoring the blood flow. It is worth noting that the severity of angina doesn’t always match the extent of affected areas in the heart.
- “Unstable angina”, a condition where chest pain occurs unpredictably, can potentially lead to myocardial stunning and, in the long run, myocardial hibernation, a more severe condition where the heart muscle goes into a state of ‘dormancy’. This explains why hibernation typically occurs more often in people with unstable angina compared to those with stable angina.
- Certain cases of heart failure, especially those associated with severe left ventricular dysfunction or abnormal coronary arteries, can involve myocardial hibernation. In many of these cases, the left ventricle takes on a spherical shape and expands in volume, indicating noticeable changes or ‘remodeling’. However, this can reverse when blood flow is once again restored, meaning some areas of the heart can recover.
Testing for Myocardial Stunning and Hibernation
Tests to detect living heart muscle (myocardial) tissue are crucial as they can evaluate if the heart muscle is working properly and if it has a chance to fully or partially recover its function. This recovery can be achieved if blood supply (revascularization) can be performed on time, leading to improved long-term survival rates for patients.
These tests should be accurate, specifically for heart muscle tissue, widely accessible, and non-invasive if possible.
* Echocardiography: This test uses sound waves to produce pictures of the heart. If the heart wall thickness at the end of the muscle’s relaxation phase (end-diastolic wall thickness) is less than 0.6 cm, it suggests recovery may not be possible.[28]
* Dobutamine stress echocardiography (DSE): This procedure uses the drug dobutamine to stimulate your heart, simulating what it would undergo during exercise. This test checks the function of the heart muscle when it’s stressed. Dobutamine can be combined with other drugs to improve blood flow to the heart.[29]
* Tissue Doppler Echocardiography (TDE): This technique is similar to regular echocardiography but provides more detailed images of the heart. It can assist in determining the function of the left ventricle (LV – the heart’s main pumping chamber). When combined with DSE, the sensitivity greatly increases.[31]
The velocity at which blood flows through the heart’s mitral valve (the gateway between the left atrium and left ventricle) can also provide an insight into the condition of the heart muscle. A certain measure known as Deceleration Time (DT) is considered. If DT is greater than 150 milliseconds, it’s a positive sign for recovery following revascularization.
Other options include a special type of scan known as Stress Redistribution Single Photon Emission Computed Tomography (SPECT) or FDG-PET, as well as Cardiac MRI (with contrast), which might suggest the viability of heart muscle tissue.
Myocardial Contrast Echocardiography (MCE) is yet another tool that can be used. It can provide information about heart muscle function and identify heart muscle tissue that isn’t working properly but is still receiving blood.
Furthermore, endocardial electromechanical mapping is a new test that helps diagnose heart muscle dysfunction. It evaluates the strength of the electrical signals produced by the heart and correlates this data with heart motion studies to pinpoint areas of reversible heart tissue damage.
Treatment Options for Myocardial Stunning and Hibernation
Usually, myocardial stunning, a temporary loss of heart muscle function following a period of poor blood flow, doesn’t need any treatment. However, if the heart’s function is severely impacted, medication that helps to strengthen the heart’s contractions may be required temporarily.
Nisoldipine, a type of medication that prevents calcium from entering the heart and blood vessels, can help to improve the recovery of the heart muscle function if given before a blood and oxygen shortage occurs in the heart. ACE inhibitors, a common type of heart medication, can also improve how well the stunned heart muscle contracts. Other heart medications such as Hydralazine, Enalapril, and Captopril can reduce the dysfunction of heart muscles caused by the restoration of blood flow and enhance the heart muscle activity.
It has been observed that the reduction in the generation of harmful molecules and the improvement of the heart muscle function can start at the time when blood flow is restored, further supporting the idea that myocardial stunning is a result of injury from re-perfusion (restoring blood flow).
As for hibernating myocardium, a condition where parts of the heart muscle reduce their function due to limited blood supply, the treatment focuses on improving blood supply to these areas. This type of heart muscle abnormality improves after using nitrate medications, medications that enhance heart contractions, a technique known as post-extra-systolic potentiation used to assess heart function, along with procedures to open up or bypass the blocked arteries, such as percutaneous transluminal coronary angioplasty (PTCA) and coronary artery bypass grafting (CABG).
What else can Myocardial Stunning and Hibernation be?
The following list includes various medical conditions that share similar symptoms:
- Acute pericarditis (inflammation of the heart’s protective covering)
- Aortic dissection (a tear in the heart’s main artery)
- Aortic stenosis (narrowing of the heart’s aortic valve)
- Herpes Zoster (a viral infection also known as shingles)
- Idiopathic Pulmonary Arterial Hypertension (high blood pressure in the arteries supplying the lungs, with unknown cause)
- Infective Endocarditis (infection of the heart’s lining and valve)
- Mitral Valve Prolapse (improper closure of the heart’s mitral valve)
- Myocarditis (inflammation of the heart muscle)
- Myopericarditis (inflammation of both the heart muscle and it’s exterior layer)
- Pneumothorax (collapsed lung)
- Pulmonary Embolism (PE) (a blood clot in the arteries of the lungs)
- Respiratory pneumonia (infection that inflames the air sacs in one or both lungs)
- Unstable angina (chest pain due to reduced blood to the heart)
These conditions require careful consideration and appropriate testing by a healthcare professional to correctly diagnose.
