What is the cardiac cycle and how is it managed in clinical practice?

The Cardiac Cycle: Phases, Physiology, and Clinical Implications

The cardiac cycle consists of a coordinated sequence of electrical and mechanical events that enable the heart to efficiently pump blood through the circulatory system, characterized by alternating periods of ventricular contraction (systole) and relaxation (diastole). 1

Phases of the Cardiac Cycle

Electrical Events

• The cardiac cycle begins with electrical activation via the sinoatrial node, which initiates atrial depolarization represented by the P wave on ECG 2
• Ventricular depolarization follows, represented by the QRS complex, triggering ventricular contraction 2
• Ventricular repolarization occurs during the T wave, allowing for ventricular relaxation 2

Mechanical Events - Systole

• Isovolumetric contraction: Ventricular pressure rises rapidly while all valves remain closed 1
• Ventricular ejection: When ventricular pressure exceeds arterial pressure, semilunar valves open and blood is ejected 1
• During systole, total heart volume decreases by approximately 8-11% as blood is ejected from the ventricles 3

Mechanical Events - Diastole

• Isovolumetric relaxation: Ventricular pressure falls rapidly while all valves remain closed 1
• Early diastolic filling (E wave): When ventricular pressure falls below atrial pressure, AV valves open allowing rapid filling 2
• Diastasis: A period of slow filling as pressures equilibrate 2
• Atrial contraction (A wave): Final filling phase caused by atrial systole 2

Hemodynamic Parameters and Assessment

Ventricular Function Assessment

• Left ventricular ejection fraction (LVEF) is a key parameter for assessing systolic function 2
• Diastolic function is evaluated through mitral inflow patterns (E/A ratio), tissue Doppler imaging (e′ velocity), and E/e′ ratio 2
• Delayed e′ velocity relative to mitral E velocity (TE2e′ > 49 ms) indicates impaired relaxation 2

Pressure-Volume Relationship

• The pressure-volume loop provides comprehensive assessment of both systolic and diastolic function 4
• Preload, afterload, and contractility can be evaluated using pressure-volume relationships 4

Clinical Implications and Management

Heart Failure Assessment

• Heart failure can occur with reduced ejection fraction (HFrEF) or preserved ejection fraction (HFpEF) 2
• HFpEF is characterized by abnormal ventricular filling, often with normal LVEF, and is more common in elderly women with hypertension 2
• Diastolic dysfunction is present in both HFrEF and HFpEF patients 2

Management of Cardiac Cycle Abnormalities

• Control of systolic and diastolic hypertension is essential for patients with diastolic dysfunction 2
• Ventricular rate control is crucial in patients with atrial fibrillation to prevent tachycardia-induced cardiomyopathy 2
• Diuretics are recommended to control pulmonary congestion and peripheral edema in patients with heart failure 2
• Coronary revascularization may be reasonable in patients with coronary artery disease affecting cardiac function 2

Arrhythmia Management

• Ventricular arrhythmias disrupt the normal cardiac cycle and can be classified based on morphology, duration, and hemodynamic impact 2
• Sustained ventricular tachycardia (VT) is defined as VT lasting >30 seconds or requiring termination due to hemodynamic compromise 2
• Premature ventricular complexes (PVCs) are common and increase with age, with very frequent PVCs (>10,000-20,000/day) potentially associated with depressed LV function 2

Heart Murmur Evaluation

• Heart murmurs represent abnormal blood flow through cardiac structures and can provide insights into cardiac cycle abnormalities 5
• Diastolic murmurs always require echocardiographic evaluation regardless of intensity, as they virtually always represent pathological conditions 5
• Holosystolic or late systolic murmurs at the apex or left sternal edge require echocardiography 5
• Midsystolic murmurs of grade 3 or greater intensity require echocardiographic evaluation 5

Special Considerations

Atrial Fibrillation

• Atrial fibrillation disrupts the cardiac cycle by eliminating atrial contraction and causing irregular ventricular response 2
• Loss of atrial contraction can significantly decrease cardiac output, especially in patients with impaired diastolic ventricular filling 2
• The irregularity of RR intervals during atrial fibrillation can further impair hemodynamics 2
• Cycle-to-cycle changes in myocardial contractility occur during atrial fibrillation due to force-interval relationships 2

Cardiogenic Shock

• Severe disruption of the cardiac cycle can lead to cardiogenic shock, characterized by inadequate cardiac output 2
• Management requires multidisciplinary approach with prompt assessment of severity and etiology 2
• Transthoracic echocardiography is essential for diagnosis, assessment of severity, and monitoring 2
• Mechanical circulatory support may be necessary in severe cases to maintain adequate circulation 2