What is Myocardial Viability?
Myocardium is the heart muscle, and it can become weak due to a lack of blood supply, a condition called ischemia. However, this muscle has the potential to recover if the blood flow is restored, a medical procedure known as revascularization. The heart muscle in this situation can be described as either ‘stunned’ or ‘hibernating.’
‘Stunned’ or ‘hibernating’ myocardium happens because the heart muscle adjusts to the lack of blood by slowing down its metabolism and its ability to contract. This is an important detail for doctors when they decide which patients with heart failure should undergo revascularization. Heart failure with reduced ejection fraction (HFrEF) happens when the heart doesn’t pump blood as well as it should. This type of heart failure is mainly caused by coronary artery disease worldwide, and when it reduces the heart’s pumping ability to less than 40%, it’s referred to as ischemic cardiomyopathy.
The heart muscle’s blood supply comes from the coronary arteries on the surface of the heart, called the epicardium. Coronary artery disease, which happens when the arteries become hardened and narrowed due to a build-up of plaque, is the most common reason for heart muscle ischemia. This can eventually lead to the death of heart muscle cells and result in ischemic cardiomyopathy over time.
Temporary disruption to the blood supply can cause the heart muscle to lose its ability to contract for a brief period. However, it can recover its function with time, usually from days to weeks. This condition is referred to as ‘myocardial stunning.’
There’s another phenomenon known as ‘chronic stunning’ where repetitive disruptions to the blood supply can lead to longer-lasting weakness in the heart muscle even after the blood supply is restored.
‘Myocardial hibernation’ is another adjustment the heart muscle makes when it experiences long periods of poor blood supply. Only a few heart cells die in this situation, and the sections of the heart muscle supplied by a narrowed coronary artery stop contracting. However, they maintain the potential to partially or completely recover their function if the blood supply is restored. This results in a long-term decrease in the left side of the heart’s ability to pump blood due to the heart muscle’s adjustment to the severely reduced blood supply. In essence, it’s a way for the heart muscle to balance the reduced blood supply and function.
What Causes Myocardial Viability?
Myocardial hibernation is a condition where the heart muscle experiences decreased blood flow due to blockages in the heart’s arteries, an event known as ischemic cardiomyopathy. These blockages decrease the heart’s dietary supply and results in lower activity by the affected part of the heart muscle. However, this part of the heart is still alive and can recover partially or totally if the blood flow is restored.
Myocardial stunning is a different situation brought on by an acute coronary syndrome, which occurs when plaque in the arteries ruptures. This event disrupts the blood flow downstream, leading to damage (called ischemic cascade) that impairs the heart’s pumping action. Despite efforts to restore blood flow quickly, the heart’s ability to contract normally might continue to suffer. This condition, known as stunning, can last for variable periods depending on the duration of the initial blood flow obstruction. It’s believed to follow an episode of acute inadequate blood supply, followed by damage when the blood flow returns (reperfusion injury).
Myocardial stunning can also occur post-heart surgery. Surgical procedures like coronary artery bypass graft (CABG) involve the use of heart-lung machines and a temporary halt in blood supply to the heart. These steps can cause inadequate blood flow to the heart muscle (ischemia) followed by damage when blood flow returns (reperfusion injury), leading to reversible dysfunction of the heart. In some cases, it may prolong hospital stays and require the use of medication or machines to help support blood circulation.
Another variant of myocardial stunning happens after treatments to correct irregular heart rhythms, such as atrial fibrillation or flutter. These treatments, known as cardioversion, can cause the left chamber of the heart and a small pouch coming off this chamber (left atrial appendage) to temporarily lose their ability to contract for about 4 to 6 weeks. This condition, known as left atrial stunning, can cause blood clots to form in these areas, which is why patients are usually placed on blood-thinning medications for at least four weeks.
Furthermore, myocardial stunning can occur following resuscitation after a cardiac arrest. When the heart’s regular rhythm is restored, the heart muscle may not contract properly leading to reduced blood circulation and low blood pressure. However, the heart muscle’s blood supply isn’t decreased during this period of dysfunction and the heart typically recovers in about 72 hours in survivors.
Risk Factors and Frequency for Myocardial Viability
In developed countries, around 70% of patients with Heart Failure with Reduced Ejection Fraction (HFrEF) is due to ischemic cardiomyopathy, a condition generally linked to coronary artery disease, which is the narrowing of the arteries that supply blood to the heart. Research indicates that half of these patients with this heart disease have a considerable amount of preserved heart muscle tissue.
However, the overall death rate for those patients with HFrEF due to ischemic cardiomyopathy is found to be around 39%. It’s interesting to note that patients with preserved heart muscle tissue have a slightly lesser death rate, 37%, compared to those with non-preserved heart muscle tissue, which stands at 51%.
Signs and Symptoms of Myocardial Viability
Patients with left ventricular dysfunction – a heart condition where the left ventricle (the main pumping chamber) doesn’t work as well as it should – may have no symptoms or could experience:
- Tiredness during activity (exertional fatigue)
- Heartbeat sensations that feel like fluttering or pounding (palpitations)
- Chest discomfort (angina)
- Suddenly waking up in the night as a result of trouble breathing (paroxysmal nocturnal dyspnea)
- Difficulty breathing while lying down (orthopnea)
- Sudden cardiac arrest or sudden cardiac death as the initial symptom
A physical exam can reveal different signs depending on the severity of the disease. These signs may include:
- Respiratory distress (difficulty breathing)
- Bluish discoloration of the skin (cyanosis)
- Increase in jugular venous pressure (JVP elevation), which could indicate heart failure
- Swelling in both lower legs (bilateral pitting pedal edema)
- Enlarged and congested liver due to fluid backup (congestive hepatomegaly)
- A shifted location where the heart’s beating can be felt (displaced apex beat)
- A murmur heard throughout the heartbeat cycle (pan systolic murmur of secondary mitral regurgitation)
Testing for Myocardial Viability
When a patient undergoes a heart stress test or a myocardial viability assessment, various imaging techniques are used at different stages of the process. The following imaging modalities are commonly used to evaluate the health of the heart muscle.
Dobutamine Stress Echocardiography: This technique assesses the heart muscle’s health in patients whose heart might not be pumping blood effectively. It’s based on a simple principle. When under stress, a healthy heart muscle will pump blood more forcefully, while a non-healthy one won’t. Dobutamine, a drug that stimulates the heart, is given to the patient to test the heart’s response. An initial improvement in heart muscle contraction at a low dose, followed by decreased motion at a higher dose, suggests the heart muscle is still viable and might recover after a procedure.
Radionuclide Techniques: These techniques involve the use of radiotracers, which are substances that emit radiation and can be traced within the body. They give indirect evidence of intact cellular function. When absorbed by heart muscle cells, these tracers emit radiation which is detected using a special camera system. Both the degree of tracer uptake and the extent of viability (health) can be calculated. Areas of the heart muscle that do not show tracer uptake could be damaged or dormant.
There are different protocols used to assess heart muscle health. One involves Thallium-201, which, if taken up by heart muscle cells, shows that the cells are viable. Another includes Technetium 99m Sestamibi, which if positively taken up, suggests that heart muscle cells are healthy.
18F-Fluorodeoxyglucose (FDG) PET Scan: This is another method of assessing heart cell health. It uses a type of glucose that has a radiotracer attached to it. It is trapped within cells once they’ve absorbed it and can be detected by a scanner, which can then provide an image of the heart.
Contrast-Enhanced Cardiac Magnetic Resonance (CMR): The CMR test is another way of assessing heart health and uses a contrast dye and magnetic fields to create detailed images of the heart. Healthy heart muscle tissue will appear different than damaged or scarred tissue, allowing doctors to determine the extent of heart muscle damage.
Each of these tests and techniques helps doctors evaluate the health of the heart muscle, especially in those patients who may have resting heart dysfunction. Understanding the results can guide treatment and interventions to improve the patient’s heart health.
Treatment Options for Myocardial Viability
It’s crucial to identify patients with heart-related conditions such as ischemic cardiomyopathy, who might benefit from a procedure that helps improve blood flow to the heart, along with certain medications. All patients with a type of heart failure called heart failure with reduced ejection fraction (HFrEF) should be on the best possible medical treatment, regardless of their heart’s ability to pump blood effectively.
Medications are important for managing heart conditions:
- Beta-Blockers: These are medications such as carvedilol, metoprolol, and bisoprolol, which can help lower blood pressure and slow down fast heart rhythms. When used in optimal doses, they have been shown to improve lifespan and reduce health complications in HFrEF patients.
- ACEIs/ARBs/Angiotensin-Neprilsyn inhibitors (ARNI): These medications, like Enalapril and Sacubitril-Valsartan, can help to decrease blood pressure and are known to improve survival rates. Usually, ACEI and ARNIs should not be used together. If changing from an ACEI to an ARNI, a waiting period of at least 36 hours is necessary.
- Mineralocorticoid Receptor Antagonists (MRA): Spironolactone and eplerenone are medications that can reduce the death rate and related health issues in patients with heart failure and reduced heart pumping efficiency.
- Hydralazine and isosorbide dinitrate: These medications work together to help reduce heart stress and improve blood flow and are mainly used in African American patients with HFrEF, despite them already taking beta-blockers and ACEI.
- Sodium-Glucose Cotransporter 2 (SGLT2i) Inhibitors: Medications like empagliflozin and dapagliflozin are used to reduce hospital admissions and decrease deaths due to cardiovascular issues in patients with symptomatic HFrEF, regardless of whether the patient also has type 2 diabetes.
Revascularization is a procedure that helps improve blood flow to the heart. Coronary artery bypass graft (CABG) surgery is often performed on patients with complex heart conditions such as coronary artery disease and reduced heart pumping efficiency. A less invasive procedure called percutaneous coronary intervention, which involves widening narrowed arteries and placing a small wire mesh tube to keep the artery open, is used for simpler cases of heart disease.
For heart-related rhythm disorders due to cardiomyopathy (heart muscle disease), device-based therapies are used:
- Implantable cardioverter defibrillator (ICD): ICD is used in patients with advanced cardiomyopathy as a preventive measure against sudden cardiac arrest caused by dangerous heart rhythms.
- Cardiac Resynchronization Therapy (CRT): CRT is a therapy which helps coordinate heart contractions, prevents heart failure progression, and increases the heart’s efficiency.
An ICD is recommended for patients with severe heart failure, who have been on optimal doses of beta blocker, ACEI/ARNI, and MRA for 90 days, and are expected to survive for more than a year. Patients with a specific type of heart block and symptoms on guideline-directed medications could benefit from upgrading to a CRT device.
What else can Myocardial Viability be?
Stress-related heart problems: Extreme stress, such as a severe illness or emotional and physical trauma, can cause a rush of adrenaline, resulting in tiny blood vessels in the heart to spasm and the heart muscle to not perform normally. When this is observed using an ultrasound of the heart, we notice an unusual pattern of the heart muscle – the tip doesn’t squeeze well, but the base does. This scenario often looks like a heart attack, and can only be diagnosed once a heart attack has been ruled out with a special heart test.
Heart muscle impairment due to inflammation: Viruses like Coxsackie and Parvovirus B19 can cause inflammation of the heart muscle, leading to a temporary and reversible impairment of the heart’s function.
Rapid-heart-rate-related heart muscle problems: Prolonged periods of elevated heart rates such as seen with conditions like atrial fibrillation, supraventricular tachycardia, and atrial flutter can cause reversible harms to the heart muscle’s ability to squeeze effectively. Once the heart rate returns to normal, the heart muscle function often improves.
Post-pregnancy heart problems: Peripartum cardiomyopathy is a condition where the heart’s function worsens, leading to symptoms of heart failure either in the last month of pregnancy or within the five weeks following childbirth. This condition is believed to occur due to the added workload on the heart during pregnancy. Most of the time, the heart’s function will get better with time.
What to expect with Myocardial Viability
It is crucial to identify a condition called hibernating myocardium in patients with a particular type of heart disease known as ischemic cardiomyopathy. This is because some patients might have an ongoing heart muscle weakness not because of permanent damage and scarring from past heart attacks (or myocardial infarctions, MIs), but rather due to reversible poor blood flow (ischemia) to the heart muscle. The term “hibernating myocardium” refers to such a situation.
In many patients with a history of known or suspected coronary artery disease, their chronic heart failure could predominantly be due to this hibernating myocardium. The degree of heart dysfunction these patients show can’t always be accounted for by previous MIs alone.
Patients with ischemic cardiomyopathy, scarred because of past MIs, face a quite bleak prognosis. The only treatment options for them might eventually be assistive devices to support the heart’s function or even a heart transplant. But the outlook could be better for patients with hibernating myocardium that appears as a chronic heart failure. This is because a procedure called revascularization could potentially improve their condition. The ACC/AHA (American College of Cardiology and the American Heart Association) thus recommend a test to assess the presence of viable, or living, heart muscle in certain patients with chronic ischemic cardiomyopathy, indicating it’s a reasonably beneficial procedure (Class IIa level of recommendation).
Possible Complications When Diagnosed with Myocardial Viability
If myocardial dysfunction, or a damaged heart muscle, is left untreated, it can result in:
- End-stage heart failure, where the heart is unable to pump blood efficiently.
- Multi-organ failure, where several organs in the body fail at once.
- Hospitalization due to heart failure
- Hospitalization due to abnormal heart rhythms (arrhythmia).
- Sudden death caused by heart disease.
- The need for device-based treatments or mechanical devices that help the heart pump better.
- The need for a heart transplant.
Preventing Myocardial Viability
It’s crucial for patients with heart failure due to decreased ability of the heart to pump blood (HFrEF) to adhere to their prescribed medication regiment. This is because these prescribed meds have been shown to prevent worsening of heart muscle disease (cardiomyopathy), improve how well the heart pumps blood (ejection fraction), and overall reduce risks associated with heart disease and its impact on everyday life. Regular check-ups are necessary to keep an eye on fluid levels, electrolyte balance, and kidney health. This is because the same medications can sometimes cause kidney damage and high potassium levels in the blood (hyperkalemia). Heart function can likely improve with a procedure that restores blood flow to heart muscle (Revascularization), particularly if the heart muscle is still ‘viable’, or capable of regaining function.
