89. Cardiac Cycle – Quiz

📌 Professor’s Notes: Quick Revision

High-yield revision notes for rapid review before university examinations.

• The cardiac cycle is the sequence of electrical and mechanical events occurring during one heartbeat. It includes coordinated changes in chamber pressure, blood flow, blood volume, valve movements, heart sounds, and the electrocardiogram. At a heart rate of 75 beats/min, one cardiac cycle lasts 0.8 second (0.3 second systole and 0.5 second diastole). Changes in heart rate mainly alter the duration of diastole.
• The cardiac cycle has three phases: atrial systole, ventricular systole, and ventricular diastole.
  • Atrial systole completes ventricular filling and normally contributes 10–20% of ventricular filling. Its contribution increases during tachycardia and exercise. It produces the a wave in the jugular venous pulse and may generate the fourth heart sound in a stiff ventricle.
  • Ventricular systole consists of isovolumetric contraction and ventricular ejection (rapid and reduced ejection). During isovolumetric contraction, all valves remain closed, ventricular pressure rises rapidly, and the first heart sound (S1) is produced by closure of the atrioventricular valves. Rapid ejection expels about two-thirds of the stroke volume, while reduced ejection ends with closure of the semilunar valves.
  • Ventricular diastole includes isovolumetric relaxation, rapid filling, and diastasis. During isovolumetric relaxation, all valves remain closed, ventricular pressure falls rapidly, the second heart sound (S2) is produced, and the dicrotic notch appears in the aortic pressure curve. Rapid filling may produce the third heart sound (S3), while diastasis is the slow filling phase before atrial systole.
• The pressure–volume loop illustrates ventricular filling, isovolumetric contraction, ejection, and relaxation. In systolic dysfunction, the loop shifts to the right with increased end-systolic volume and reduced stroke volume. In diastolic dysfunction, the loop shifts to the left because impaired ventricular relaxation reduces end-diastolic volume.
• The jugular venous pulse (JVP) reflects right atrial pressure changes. It has a, c, and v waves, and x and y descents.
  • a wave: atrial contraction.
  • c wave: bulging of the tricuspid valve during ventricular systole.
  • v wave: atrial filling during ventricular systole.
  • Raised JVP occurs in right heart failure, constrictive pericarditis, tricuspid regurgitation, and superior vena cava obstruction. A cannon a wave occurs in complete atrioventricular block, a wave is absent in atrial fibrillation, and a prominent v wave suggests tricuspid regurgitation.
• Heart sounds:
  • S1: Closure of mitral and tricuspid valves; marks the onset of ventricular systole.
  • S2: Closure of aortic and pulmonary valves; marks the beginning of ventricular diastole. Physiological splitting occurs during inspiration.
  • S3: Produced during rapid ventricular filling. It is normal in children and young adults but suggests ventricular volume overload or heart failure in older adults.
  • S4: Produced by atrial contraction into a stiff ventricle; indicates decreased ventricular compliance and is absent in atrial fibrillation.
• A cardiac murmur is produced by turbulent blood flow due to valvular stenosis, regurgitation, increased blood flow, or septal defects. Murmurs are assessed by timing, site, character, radiation, and relation to respiration.