The human heart, a tireless engine, rhythmically pumps blood throughout our bodies, sustaining life itself. This intricate process, known as the cardiac cycle, involves a precise sequence of events within the heart chambers during each beat. Understanding the cardiac cycle is crucial for comprehending cardiovascular health and various heart conditions. This comprehensive guide delves into the seven phases of the cardiac cycle, exploring the physiological mechanisms, electrical activity, and associated heart sounds. We’ll also explore clinical correlates, providing insights into various cardiac arrhythmias and their implications.
Phases Of The Cardiac Cycle
The cardiac cycle is a continuous loop, seamlessly transitioning between phases. For clarity, we divide it into seven distinct phases:
Atrial Systole (The End of Diastole)
Prior to atrial systole, blood passively flows from the atria into the ventricles through open atrioventricular (AV) valves. This passive filling represents the majority of ventricular filling. During atrial systole, the atria contract, adding a final, small volume of blood to the ventricles. This ‘topping off’ ensures the ventricles are optimally filled before ventricular contraction begins. Atrial contraction is complete before ventricular contraction commences.
The ‘a’ wave of atrial pressure, reflecting atrial contraction, is observable on a pressure tracing. As the atrium contracts, increasing atrial pressure, blood arriving at the heart cannot enter the atrium; instead, it flows back up the jugular vein, creating the first discernible wave in the jugular venous pulse. Atrial pressure subsequently drops when atrial contraction ceases.
From an electrocardiographic (ECG) perspective, an impulse originating from the sinoatrial (SA) node triggers atrial depolarization and contraction. The right atrium contracts slightly before the left atrium. This atrial depolarization is represented by the P wave on the ECG. The PR segment, an electrically quiet interval, reflects the conduction delay as the depolarization wave traverses the atrioventricular (AV) node. This brief pause allows complete ventricular filling.
Heart Sounds: An abnormal fourth heart sound (S4) can be heard at the end of atrial emptying after atrial contraction. This sound is associated with conditions like hypertrophic congestive heart failure, massive pulmonary embolism, tricuspid incompetence, or cor pulmonale. Understanding these abnormal heart sounds requires further investigation into the underlying pathology.
Isovolumetric Contraction (The Beginning of Systole)
The beginning of this phase is marked by the closure of the AV valves. Electrically, ventricular systole is defined by the QRS complex (ventricular depolarization) and the end of the T wave (ventricular repolarization) on the ECG – the QT interval. Mechanically, it’s defined as the interval between AV valve closure and semilunar valve (aortic and pulmonary) opening.
AV valve closure occurs when ventricular pressure surpasses atrial pressure. As the ventricles contract isovolumetrically (volume remains constant), intraventricular pressure rises, approaching aortic and pulmonary artery pressures.
The electrical impulse travels from the AV node through the His bundle and Purkinje fibers, causing coordinated ventricular contraction from the apex towards the base. The QRS complex on the ECG signifies ventricular depolarization and the start of ventricular systole. Its amplitude masks the atrial repolarization signal.
Heart Sounds: The first heart sound (S1, ‘lub’), a crucial diagnostic marker, results from the closure of the AV valves and associated blood turbulence. The intensity and timing of S1 can provide valuable clinical information.
Rapid Ejection
This phase begins with the opening of the semilunar valves. As ventricular contraction continues, ventricular pressure exceeds aortic and pulmonary artery pressures, causing semilunar valve opening. Blood rapidly exits the ventricles, leading to a decrease in ventricular volume. Increased arterial blood flow causes a pressure buildup until peak flow is reached.
The ‘c’ wave of atrial pressure is partially due to slight backflow of blood into the atria at the onset of ventricular contraction, but primarily due to the backward bulging of the AV valves into the atria from increasing ventricular pressure. This produces a small, usually indiscernible wave in the jugular venous pulse, typically coinciding with the carotid pulse.
ECG: No characteristic deflections are typically observed during this phase on the ECG.
Reduced Ejection (The End of Systole)
This phase concludes with the closure of the semilunar valves. After peak ventricular and arterial pressures, ventricular blood outflow decreases, and ventricular volume reduction slows. When ventricular pressure drops below arterial pressure, arterial blood backflows towards the ventricles, causing semilunar valve closure. This mechanically marks the end of ventricular systole.
ECG: The T wave, representing ventricular repolarization, appears on the ECG. The end of the T wave electrically signifies the end of ventricular systole.
Heart Sounds: No distinct heart sounds are typically associated with this phase.
Isovolumetric Relaxation (The Beginning of Diastole)
This phase begins with closed AV valves. During this phase and the preceding two, the atria, in diastole, fill with blood accumulating on top of the closed AV valves, gradually increasing atrial pressure.
The ‘v’ wave of atrial pressure is caused by the slow venous blood flow into the atria while the AV valves are closed (and subsequent backflow upon encountering the closed valves) during ventricular contraction. This is the second discernible wave in the jugular venous pulse. Ventricular pressure continues to drop without significant volume change; ventricular volume is at its minimum, ready for refilling.
ECG: No characteristic deflections are typically seen during this phase on the ECG.
Heart Sounds: The second heart sound (S2, ‘dup’) occurs with the closure of the semilunar valves. S2 is usually split, as the aortic valve closes slightly before the pulmonary valve. The timing and splitting of S2 can offer important diagnostic clues.
Rapid Ventricular Filling
Once the AV valves open, blood rapidly flows from the atria into the ventricles. Ventricular volume increases rapidly during this phase.
ECG: No specific ECG deflections mark this phase.
Heart Sounds: A third heart sound (S3), usually abnormal, indicates rapid passive ventricular filling. It’s associated with conditions such as dilated congestive heart failure, severe hypertension, myocardial infarction, or mitral incompetence. The presence of S3 warrants further investigation.
Reduced Ventricular Filling (Diastasis)
The remaining atrial blood slowly flows into the ventricles. Ventricular volume increases more slowly, nearing complete filling.
ECG: No specific ECG deflections are observed.
Heart Sounds: No distinct heart sounds are typically associated with this phase.
Clinical Correlates: Cardiac Arrhythmias
Cardiac arrhythmias represent abnormalities in the heart’s rhythm or rate. These include:
- Tachycardia: A faster-than-normal heart rate.
- Bradycardia: A slower-than-normal heart rate.
- Sinus tachycardia: Increased rate, regular rhythm, originating from the SA node. Learn more about heart rate regulation
- Sinus bradycardia: Slow, regular rhythm.
- Supraventricular tachycardia: Fast, regular rhythm due to an abnormal electrical pathway causing continuous impulse circulation, overriding the SA node.
- Ventricular tachycardia: Rapid, irregular beats originating in the ventricles.
- Atrial flutter: Regularly and rapidly beating atria, but not all impulses reach the ventricles, leading to slower ventricular rates.
- Atrial fibrillation: Uncoordinated, rapid atrial beating, causing totally irregular ventricular beats. Explore more about heart conditions
- Ventricular fibrillation: A life-threatening cardiac arrest where the ventricles twitch rapidly and disorganized. Immediate intervention is crucial.
- Heart block: Partial or complete blockage of electrical impulse conduction through the heart muscle, resulting in slow, irregular heartbeats. Consult reliable medical resources for accurate information
Understanding the cardiac cycle is fundamental to comprehending normal cardiovascular function and the pathophysiology of various heart diseases. Further study and consultation with healthcare professionals are essential for a comprehensive understanding and appropriate management of cardiac conditions. For additional learning resources, you can consult sites like Medical Note, MedNotes, and Med Student Notes.