6.2 Cardiac cycle and heart sounds

The cardiac cycle is the heartbeat's rhythm, featuring systole and diastole phases in both atria and ventricles. This dance of contraction and relaxation pumps blood through the heart, creating the familiar "lub-dub" sounds we associate with a healthy ticker.

Understanding the cardiac cycle is crucial for grasping how the heart functions within the cardiovascular system. It's like learning the steps of a dance – once you know the rhythm, you can spot when something's off-beat and potentially problematic.

Cardiac Cycle Phases and Heart Sounds

Phases of the Cardiac Cycle

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• The cardiac cycle consists of one complete heartbeat which includes systole (contraction) and diastole (relaxation) of both atria and ventricles
• The cardiac cycle is divided into four phases:
  • Atrial systole: Atria contract and pump blood into the ventricles while the ventricles are relaxed and filling with blood
  • Ventricular systole: Ventricles contract and pump blood out of the heart to the lungs and body while the atria are relaxed and filling with blood
  • Atrial diastole: Atria relax and fill with blood from the venous system
  • Ventricular diastole: Ventricles relax and fill with blood from the atria

Heart Sounds and their Timing

• The first heart sound (S1) occurs at the beginning of ventricular systole caused by the closure of the mitral and tricuspid valves
• The second heart sound (S2) occurs at the beginning of ventricular diastole caused by the closure of the aortic and pulmonic valves
• The duration of the cardiac cycle is represented by the time between two consecutive R waves on an ECG known as the R-R interval (one complete heartbeat)
• The R-R interval can be used to calculate heart rate by dividing 60 by the R-R interval in seconds (e.g. R-R interval of 1 second = 60 bpm)

ECG Components and Significance

Waves and Intervals

• The P wave represents atrial depolarization which initiates atrial systole
• The QRS complex represents ventricular depolarization which initiates ventricular systole
  • The Q wave is the initial downward deflection
  • The R wave is the first upward deflection
  • The S wave is the downward deflection following the R wave
• The T wave represents ventricular repolarization which allows the ventricles to relax and enter diastole
• The PR interval represents the time from the beginning of atrial depolarization to the beginning of ventricular depolarization normally 120-200 ms
  • A prolonged PR interval (>200 ms) may indicate AV block or other conduction abnormalities
• The QT interval represents the time from the beginning of ventricular depolarization to the end of ventricular repolarization normally less than 440 ms
  • A prolonged QT interval may indicate electrolyte abnormalities, certain medications, or genetic disorders and increases the risk of ventricular arrhythmias

ST Segment

• The ST segment represents the time between the end of ventricular depolarization and the beginning of ventricular repolarization normally isoelectric (flat)
• Elevation of the ST segment may indicate myocardial infarction, pericarditis, or other conditions
• Depression of the ST segment may indicate myocardial ischemia, strain, or other conditions

Physiological Basis of Heart Sounds

First Heart Sound (S1)

• S1 is caused by the closure of the mitral and tricuspid valves at the beginning of ventricular systole
  • S1 is typically described as a "lub" sound and is longer and lower-pitched than S2
• The intensity of S1 is determined by the force of ventricular contraction and the position of the AV valves at the onset of systole
  • A loud S1 may indicate a hyperdynamic state (e.g. fever, anemia, hyperthyroidism) or mitral stenosis
  • A soft S1 may indicate a hypodynamic state (e.g. heart failure, cardiomyopathy) or mitral regurgitation

Second Heart Sound (S2)

• S2 is caused by the closure of the aortic and pulmonic valves at the beginning of ventricular diastole
  • S2 is typically described as a "dub" sound and is shorter and higher-pitched than S1
• The intensity of S2 is determined by the pressure gradient between the ventricles and the great arteries at the end of systole
  • A loud S2 may indicate pulmonary hypertension or a dilated aorta
  • A soft S2 may indicate aortic stenosis or a low cardiac output state
• The splitting of S1 and S2 can occur due to the slight difference in timing of the closure of the respective valves
  • Physiologic splitting of S2 is normal and occurs during inspiration as the pulmonic valve closes slightly later than the aortic valve
  • Pathologic splitting of S2 may indicate conditions such as bundle branch block or atrial septal defect

Abnormalities in Heart Sounds

Extra Heart Sounds

• A third heart sound (S3) may be heard in early diastole caused by rapid ventricular filling in conditions such as heart failure or volume overload
  • S3 is a low-pitched sound best heard with the bell of the stethoscope at the apex and is normal in children and young adults but abnormal in older individuals
• A fourth heart sound (S4) may be heard in late diastole caused by forceful atrial contraction in conditions such as hypertension or hypertrophic cardiomyopathy
  • S4 is a low-pitched sound best heard with the bell of the stethoscope at the apex and is always abnormal

Murmurs and Other Abnormal Sounds

• Murmurs are abnormal heart sounds caused by turbulent blood flow often due to valvular abnormalities or septal defects
  • Systolic murmurs occur during ventricular systole and may indicate conditions such as aortic stenosis or mitral regurgitation
  • Diastolic murmurs occur during ventricular diastole and may indicate conditions such as aortic regurgitation or mitral stenosis
• Pericardial friction rubs are abnormal heart sounds caused by inflammation of the pericardium often due to pericarditis
  • Pericardial friction rubs are scratchy, grating sounds best heard with the diaphragm of the stethoscope along the left sternal border
• Abnormalities in heart sounds can provide important diagnostic clues and may indicate the need for further evaluation with imaging studies (echocardiography, cardiac MRI) or invasive procedures (cardiac catheterization)