Cardiac Cycle and Heart Sounds, Basics of ECG

The cardiac cycle refers to the sequence of events that occur during one complete heartbeat. It involves the contraction and relaxation of the heart chambers, which results in blood being pumped to the lungs and the rest of the body. Alongside the cardiac cycle, the sounds produced by the heart (heart sounds) and the electrical activity of the heart, recorded through an electrocardiogram (ECG), are crucial in understanding the heart’s function and diagnosing various cardiac conditions.

1. Cardiac Cycle

The cardiac cycle is the continuous cycle of the contraction (systole) and relaxation (diastole) of the heart chambers, which allows the heart to pump blood efficiently throughout the body. The cardiac cycle can be divided into two main phases: systole and diastole.

1. Phases of the Cardiac Cycle

1. Atrial Systole (Contraction)
   • The cycle begins with the atrial contraction. During this phase, the atria contract and push blood through the atrial-ventricular valves (tricuspid and mitral valves) into the ventricles.
   • This phase accounts for about 20% of the ventricular filling, with the remainder occurring passively during the early part of ventricular diastole.
2. Ventricular Systole (Contraction)
   • After the atria contract, the ventricles contract, which increases the pressure within the ventricles.
   • This pressure causes the atrioventricular (AV) valves (tricuspid and mitral) to close, preventing backflow of blood into the atria.
   • Once the ventricular pressure exceeds the pressure in the pulmonary artery and aorta, the semilunar valves (pulmonary and aortic valves) open, and blood is ejected into the lungs and systemic circulation.
   • During this phase, the ventricles eject approximately 70% of the blood in the ventricles (the stroke volume).
3. Ventricular Diastole (Relaxation)
   • After the ventricles contract, they relax, and the semilunar valves close to prevent the backflow of blood into the ventricles.
   • The ventricles begin to fill with blood from the atria, which are still in the relaxed state.
   • The AV valves (tricuspid and mitral) open when ventricular pressure is lower than atrial pressure, allowing blood to flow into the ventricles.

1. Phases in Detail

1. Isovolumetric Contraction: This occurs at the beginning of ventricular systole when the ventricles contract, but the pressure is not yet high enough to open the semilunar valves.
2. Ejection Phase: Blood is pushed out of the heart into the pulmonary artery (from the right ventricle) and the aorta (from the left ventricle). This is the phase where the ventricles actually eject blood into the systemic and pulmonary circulations.
3. Isovolumetric Relaxation: After the semilunar valves close, the ventricles begin to relax, but the AV valves have not yet opened. The volume of blood remains constant in the ventricles during this phase.
4. Ventricular Filling: Blood flows passively from the atria to the ventricles during ventricular diastole. This phase is divided into:
   • Early filling: Blood flows into the ventricles quickly.
   • Late filling (Atrial contraction): The atria contract, finishing the ventricular filling process.

1. Duration of the Cardiac Cycle

• The duration of one complete cardiac cycle is typically around 0.8 seconds at a heart rate of 75 beats per minute.
  • Systole lasts about 0.3 seconds.
  • Diastole lasts about 0.5 seconds.

2. Heart Sounds

The heart sounds are caused by the closing of the heart valves during the cardiac cycle. There are typically four heart sounds, but the first two are the most commonly heard and are clinically significant.

1. First Heart Sound (S1):
   • Sound: “Lubb”
   • Cause: The closure of the atrioventricular (AV) valves (mitral and tricuspid) at the beginning of ventricular systole.
   • Significance: This sound is heard when the ventricles contract and the AV valves close to prevent blood from flowing back into the atria.
2. Second Heart Sound (S2):
   • Sound: “Dupp”
   • Cause: The closure of the semilunar valves (aortic and pulmonary) at the end of ventricular systole.
   • Significance: This sound is heard when the ventricles stop contracting and the semilunar valves close to prevent blood from flowing back into the ventricles.
3. Third Heart Sound (S3):
   • Sound: This sound is softer and is often referred to as a “gallop” rhythm.
   • Cause: It is associated with rapid ventricular filling during early diastole.
   • Significance: It is normal in children and young adults but can indicate heart failure or volume overload in older adults.
4. Fourth Heart Sound (S4):
   • Sound: It occurs just before S1, and it is often described as a “gallop.”
   • Cause: It is caused by the atrial contraction during late diastole, typically when the ventricles are stiffened (e.g., hypertension or ventricular hypertrophy).
   • Significance: It is generally abnormal and can indicate pathological conditions like left ventricular hypertrophy or stiffness of the heart muscle.

3. Basics of Electrocardiogram (ECG)

An electrocardiogram (ECG) is a recording of the electrical activity of the heart. It provides important information about the heart’s rhythm, electrical activity, and possible abnormalities in the heart’s structure or function. The ECG is composed of waves that correspond to different phases of the cardiac cycle.

1. Components of an ECG

1. P Wave:
   • Represents atrial depolarization (the electrical activity leading to atrial contraction).
   • It is a small, upward deflection on the ECG tracing.
2. QRS Complex:
   • Represents ventricular depolarization (the electrical activity leading to ventricular contraction).
   • The Q wave is the first downward deflection, the R wave is the tall upward deflection, and the S wave is the downward deflection following the R wave.
   • The QRS complex is typically narrow and sharp and lasts around 0.08–0.12 seconds.
3. T Wave:
   • Represents ventricular repolarization (the recovery phase of the ventricles).
   • It is a small, upward deflection that follows the QRS complex.
4. U Wave (occasionally seen):
   • It is a small wave that follows the T wave and is thought to represent repolarization of the Purkinje fibers.
5. PR Interval:
   • The time interval from the beginning of the P wave to the beginning of the QRS complex. It represents the time it takes for the electrical impulse to travel from the atria to the ventricles through the AV node.
   • Normal range: 0.12–0.20 seconds.
6. QT Interval:
   • The time interval from the beginning of the QRS complex to the end of the T wave. It represents the time required for the ventricles to contract and relax.
   • Normal range: 0.36–0.44 seconds (varies with heart rate).

1. Purpose of ECG

• Diagnosis of Arrhythmias: ECG helps detect irregular heart rhythms like atrial fibrillation or ventricular tachycardia.
• Ischemia and Infarction: ECG can identify myocardial ischemia or heart attack (myocardial infarction) through changes in the ST segment or the presence of abnormal waves.
• Conduction Disorders: ECG helps diagnose conditions like AV block or bundle branch block, where the electrical conduction through the heart is impaired.
• Electrolyte Imbalances: Changes in potassium, calcium, or sodium levels can be detected on the ECG, often altering the T wave or QT interval.

Conclusion

The cardiac cycle and heart sounds reflect the mechanical and functional events that occur in the heart during each heartbeat. The ECG provides valuable insight into the electrical activity of the heart, allowing for the diagnosis of various cardiovascular conditions. A well-coordinated cardiac cycle ensures that the heart effectively pumps blood, while proper heart sounds and an ECG tracing can help clinicians evaluate heart health and detect any abnormalities.