The signs and symptoms associated with myocarditis are varied, and relate either to the actual inflammation of the myocardium or to the weakness of the heart muscle that is secondary to the inflammation. Signs and symptoms of myocarditis include the following:

- Chest pain (often described as "stabbing" in character)

- Congestive heart failure (leading to swelling, shortness of breath and liver congestion)

- Palpitations (due to abnormal heart rhythms)

- Sudden death (in young adults, myocarditis causes up to 20% of all cases of sudden death)

- Fever (especially when infectious, e.g. in rheumatic fever)

- Symptoms in young children tend to be more nonspecific, with generalized malaise, poor appetite, abdominal pain, and chronic cough. Later stages of the illness will present with respiratory symptoms with increased work of breathing, and is often mistaken for asthma.

Since myocarditis is often due to a viral illness, many patients give a history of symptoms consistent with a recent viral infection, including fever, rash, diarrhea, joint pains, and easily becoming tired.

Myocarditis is often associated with pericarditis, and many people with myocarditis present with signs and symptoms that suggest myocarditis and pericarditis at the same time.

Myocardial infarction (MI) refers to tissue death (infarction) of the heart muscle (myocardium). It is a type of acute coronary syndrome, which describes a sudden or short-term change in symptoms related to blood flow to the heart. Unlike other causes of acute coronary syndromes, such as unstable angina, a myocardial infarction occurs when there is cell death, as measured by a blood test for biomarkers (the cardiac protein troponin or the cardiac enzyme CK-MB). When there is evidence of an MI, it may be classified as an ST elevation myocardial infarction (STEMI) or Non-ST elevation myocardial infarction (NSTEMI) based on the results of an ECG.

The phrase "heart attack" is often used non-specifically to refer to a myocardial infarction and to sudden cardiac death. An MI is different from—but can cause—cardiac arrest, where the heart is not contracting at all or so poorly that all vital organs cease to function, thus causing death. It is also distinct from heart failure, in which the pumping action of the heart is impaired. However, an MI may lead to heart failure.

Chest pain is the most common symptom of acute myocardial infarction and is often described as a sensation of tightness, pressure, or squeezing. Pain radiates most often to the left arm, but may also radiate to the lower jaw, neck, right arm, back, and upper abdomen. The pain most suggestive of an acute MI, with the highest likelihood ratio, is pain radiating to the right arm and shoulder. Similarly, chest pain similar to a previous heart attack is also suggestive. The pain associated with MI is usually diffuse, does not change with position, and lasts for more than 20 minutes. Levine's sign, in which a person localizes the chest pain by clenching one or both fists over their sternum, has classically been thought to be predictive of cardiac chest pain, although a prospective observational study showed it had a poor positive predictive value. Pain that responds to nitroglycerin does not indicate the presence or absence of a myocardial infarction.

Signs and symptoms of ischemic cardiomyopathy include sudden fatigue, shortness of breath, dizziness and palpitations.

Subjects' symptoms from non-compaction cardiomyopathy range widely. It is possible to be diagnosed with the condition, yet not to have any of the symptoms associated with heart disease. Likewise it possible to have severe heart failure, which even though the condition is present from birth, may only manifest itself later in life. Differences in symptoms between adults and children are also prevalent with adults more likely to have heart failure and children from depression of systolic function.

Common symptoms associated with a reduced pumping performance of the heart include:

- Breathlessness

- Fatigue

- Swelling of the ankles

- Limited physical capacity and exercise intolerance

Two conditions though that are more prevalent in noncompaction cardiomyopathy are: tachyarrhythmia which can lead to sudden cardiac death and clotting of the blood in the heart.

Myocarditis, also known as inflammatory cardiomyopathy, is inflammation of the heart muscle. Symptoms can include shortness of breath, chest pain, decreased ability to exercise, and an irregular heartbeat. The duration of problems can vary from hours to months. Complications may include heart failure due to dilated cardiomyopathy or cardiac arrest.

Myocarditis is most often due to a viral infection. Other causes include bacterial infections, certain medications, toxins, and autoimmune disorders. A diagnosis may be supported by an electrocardiogram (ECG), increased troponin, heart MRI, and occasionally a heart biopsy. An ultrasound of the heart is important to rule out other potential causes such as heart valve problems.

Treatment depends on both the severity and the cause. Medications such as ACE inhibitors, beta blockers, and diuretics are often used. A period of no exercise is typically recommended during recovery. Corticosteroids or intravenous immunoglobulin (IVIG) may be useful in certain cases. In severe cases an implantable cardiac defibrillator or heart transplant may be recommended.

In 2013, about 1.5 million cases of acute myocarditis occurred. While people of all ages are affected, the young are most often affected. It is slightly more common in males than females. Most cases are mild. In 2015 cardiomyopathy, including myocarditis, resulted in 354,000 deaths up from 294,000 in 1990. The initial descriptions of the condition are from the mid-1800s.

Ischemic cardiomyopathy is a type of cardiomyopathy caused by a narrowing of the coronary arteries which supply blood to the heart. Typically, patients with ischemic cardiomyopathy have a history of acute myocardial infarction, however, it may occur in patients with coronary artery disease, but without a past history of acute myocardial infarction. This cardiomyopathy is one of the leading causes of sudden cardiac death.

Non-compaction cardiomyopathy (NCC), also called spongiform cardiomyopathy, is a rare congenital cardiomyopathy that affects both children and adults. It results from the failure of myocardial development during embryogenesis.

During development, the majority of the heart muscle is a sponge-like meshwork of interwoven myocardial fibers. As normal development progresses, these trabeculated structures undergo significant compaction that transforms them from spongy to solid. This process is particularly apparent in the ventricles, and particularly so in the left ventricle. Noncompaction cardiomyopathy results when there is failure of this process of compaction. Because the consequence of non-compaction is particularly evident in the left ventricle, the condition is also called left ventricular noncompaction. Other hypotheses and models have been proposed, none of which is as widely accepted as the noncompaction model.

Symptoms range greatly in severity. Most are a result of a poor pumping performance by the heart. The disease can be associated with other problems with the heart and the body.

The risk of PVF during acute myocardial infarction is related to the amount of ST elevation, the presence of hypokalemia, the absence of pre-infarction angina, the size of the infarction, and the presence of a blocked left coronary artery. Other risk factors could include younger age, male gender, and history of sudden cardiac death in first degree relatives.

Myocardial scarring is fibrous tissue that replaces normal tissue destroyed by injury or disease pertaining to the muscular tissue of the heart.

Myocardial scarring occurs in myocardial infarction, and also results from the surgical repair of congenital heart disease. Such scarring can disrupt the electrical conduction system of the heart, and may also irritate surrounding viable ventricular myocytes making them more likely to depolarize spontaneously.

Boxer cardiomyopathy is an adult-onset disease with three distinct clinical presentations:

The concealed form is characterized by an asymptomatic dog with premature ventricular contractions (PVCs).

The overt form is characterized by ventricular tachyarrhythmias and syncope. Dogs with overt disease may also have episodic weakness and exercise intolerance, but syncope is the predominant manifestation.

The third form, which is recognized much less frequently, is characterized by myocardial systolic dysfunction. This may result in left-sided, right-sided, or bi-ventricular congestive heart failure. It is not known if this form represents a separate clinical entity, or whether it is part of the continuum of disease.

Approximately 10% of all myocardial infarctions lead to PVF. The incidence peaks between 20 and 50 minutes after the start of the MI. 2/3 of events occur before medical attendance, and of these medically unattended events, 2/3 occur after more than 30 minutes of warning symptoms.

Unstable angina (UA) is a type of angina pectoris that is irregular. It is also classified as a type of acute coronary syndrome (ACS).

It can be difficult to distinguish unstable angina from non-ST elevation (non-Q wave) myocardial infarction (NSTEMI). They differ primarily in whether the ischemia is severe enough to cause sufficient damage to the heart's muscular cells to release detectable quantities of a marker of injury (typically troponin T or troponin I). Unstable angina is considered to be present in patients with ischemic symptoms suggestive of an ACS and no elevation in troponin, with or without ECG changes indicative of ischemia (e.g., ST segment depression or transient elevation or new T wave inversion). Since an elevation in troponin may not be detectable for up to 12 hours after presentation, UA and NSTEMI are frequently indistinguishable at initial evaluation.

Boxer cardiomyopathy (also known as "Boxer arrhythmogenic right ventricular cardiomyopathy") is a disease of the myocardium primarily affecting Boxer dogs. It is characterized by the development of ventricular tachyarrhythmias, resulting in syncope and sudden cardiac death. Myocardial failure and congestive heart failure are uncommon manifestations of the disease.

Unstable angina (UA) (also ""crescendo angina""; this is a form of acute coronary syndrome) is defined as angina pectoris that changes or worsens.

It has at least one of these three features:

1. it occurs at rest (or with minimal exertion), usually lasting more than 10 minutes

2. it is severe and of new onset (i.e., within the prior 4–6 weeks)

3. it occurs with a crescendo pattern (i.e., distinctly more severe, prolonged, or frequent than before).

UA may occur unpredictably at rest, which may be a serious indicator of an impending heart attack. What differentiates stable angina from unstable angina (other than symptoms) is the pathophysiology of the atherosclerosis. The pathophysiology of unstable angina is the reduction of coronary flow due to transient platelet aggregation on apparently normal endothelium, coronary artery spasms, or coronary thrombosis. The process starts with atherosclerosis, progresses through inflammation to yield an active unstable plaque, which undergoes thrombosis and results in acute myocardial ischemia, which, if not reversed, results in cell necrosis (infarction). Studies show that 64% of all unstable anginas occur between 22:00 and 08:00 when patients are at rest.

In stable angina, the developing atheroma is protected with a fibrous cap. This cap may rupture in unstable angina, allowing blood clots to precipitate and further decrease the area of the coronary vessel's lumen. This explains why, in many cases, unstable angina develops independently of activity.

Also known as 'effort angina', this refers to the classic type of angina related to myocardial ischemia. A typical presentation of stable angina is that of chest discomfort and associated symptoms precipitated by some activity (running, walking, etc.) with minimal or non-existent symptoms at rest or after administration of sublingual nitroglycerin. Symptoms typically abate several minutes after activity and recur when activity resumes. In this way, stable angina may be thought of as being similar to intermittent claudication symptoms. Other recognized precipitants of stable angina include cold weather, heavy meals, and emotional stress.

Heart problems are very important in people with Human Immunodeficiency Virus (HIV) as Acquired ImmunoDeficiency Syndrome (AIDS) patients with left ventricular dysfunction have a median survival of 101 days as compared to 472 days in AIDS patients with healthy hearts. HIV is a major cause of cardiomyopathy (problems with the heart muscle that reduce the efficiency with which the heart pumps blood). The most common type of HIV induced cardiomyopathy is dilated cardiomyopathy also known as eccentric ventricular hypertrophy which leads to impaired contraction of the ventricles due to volume overload. The annual incidence of HIV associated dilated cardiomyopathy was 15.9/1000 before the introduction of highly active antiretroviral therapy (HAART). However, in 2014, a study found that 17.6% of HIV patients have dilated cardiomyopathy (176/1000) meaning the incidence has greatly increased.

Among the causes of LBBB are:

- Aortic stenosis

- Dilated cardiomyopathy

- Acute myocardial infarction

- Extensive coronary artery disease

- Primary disease of the cardiac electrical conduction system

- Long standing hypertension leading to aortic root dilatation and subsequent aortic regurgitation

- Lyme disease

- Side effect of some cardiac surgeries (e.g., aortic root reconstruction)

A myocardial infarction may compromise the function of the heart as a pump for the circulation, a state called heart failure. There are different types of heart failure; left- or right-sided (or bilateral) heart failure may occur depending on the affected part of the heart, and it is a low-output type of failure. If one of the heart valves is affected, this may cause dysfunction, such as mitral regurgitation in the case of left-sided coronary occlusion that disrupts the blood supply of the papillary muscles. The incidence of heart failure is particularly high in patients with diabetes and requires special management strategies.

Left bundle branch block (LBBB) is a cardiac conduction abnormality seen on the electrocardiogram (ECG). In this condition, activation of the left ventricle of the heart is delayed, which causes the left ventricle to contract later than the right ventricle.

The pathophysiology of unstable angina is controversial. Until recently, unstable angina was assumed to be angina pectoris caused by disruption of an atherosclerotic plaque with partial thrombosis and possibly embolization or vasospasm leading to myocardial ischemia. However, sensitive troponin assays reveal rise of cardiac troponin in the bloodstream with episodes of even mild myocardial ischemia. Since unstable angina is assumed to occur in the setting of acute myocardial ischemia without troponin release, the concept of unstable angina is being questioned with some calling for retiring the term altogether.

Dilated cardiomyopathy can be due to pericardial effusion or infective endocarditis, especially in intravenous drug users which are common in the HIV population. However, the most researched cause of cardiomyopathy is myocarditis (myocardial inflammation and infection) caused by HIV-1, which the main subtype of HIV (the other being HIV-2), with greater likelihood of transmission and shorter period between infection and illness. HIV-1 virions infect cardiomyocytes in patches but there is no direct correlation between viral infection and dysfunction of cardiomyocytes.

HIV-related cardiomyopathy is often not associated with any specific opportunistic infection, and approximately 40% of patients have not experienced any opportunistic infection before the onset of cardiac symptoms.

Myocardial infarction complications may occur immediately following a heart attack (in the acute phase), or may need time to develop (a chronic problem). After an infarction, an obvious complication is a second infarction, which may occur in the domain of another atherosclerotic coronary artery, or in the same zone if there are any live cells left in the infarct.

This condition can cause complications such as vasospasm, angina pectoris, arrhythmia, ventricular tachycardia. Additionally many patients express discomfort in specific positions, (i.e. lying on the left side for a prolonged period of time).

Substernal or left precordial pleuritic chest pain with radiation to the trapezius ridge (the bottom portion of scapula on the back), which is relieved by sitting up and bending forward and worsened by lying down (recumbent or supine position) or inspiration (taking a breath in), is the characteristic pain of pericarditis. The pain may resemble the pain of angina pectoris or heart attack, but differs in that pain changes with body position, as opposed to heart attack pain that is pressure-like, and constant with radiation to the left arm and/or the jaw. Other symptoms of pericarditis may include dry cough, fever, fatigue, and anxiety. Due to similarity to myocardial infarction (heart attack) pain, pericarditis can be misdiagnosed as an acute myocardial infarction (a heart attack) solely based on the clinical data and so extreme suspicion on the part of the diagnostician is required. Acute myocardial infarction (heart attack) can also cause pericarditis, but the presenting symptoms often differ enough to warrant diagnosis. The following table organizes the clinical presentation of pericarditis: