Recording-2025-03-09T22:22:54.430Z

Overview of the Cardiac Cycle

• The cardiac cycle consists of alternating contractions and relaxations of the heart's atria and ventricles.

• Blood pressure increases within the contracting chamber, with significant pressure differences between the left and right sides of the heart.

Cardiac Cycle Duration

• Duration of a full cardiac cycle is approximately 0.8 seconds (normal resting heart rate).

• The cycle involves ten distinct steps to understand the heart's function.

Steps of the Cardiac Cycle

Atrial Phase

1. Atrial Depolarization (P Wave)

   • Triggered by the SA node, precedes atrial contraction.

   • Important to note: P wave does not represent atrial contraction but depolarization.

2. Atrial Systole

   • Atrial depolarization leads to contraction, exerting pressure on blood to flow through open AV valves into ventricles.

   • This contributes 25 mL of blood to each ventricle, leading to an end diastolic volume (EDV) of 130 mL.

Ventricular Phase

3. Ventricular Depolarization (QRS Complex)

   • Follows atrial systole and signals the beginning of ventricular contraction.

4. Isovolumetric Contraction

   • Ventricular pressure rises, causing AV valves to close (5 seconds duration).

   • Cardiac fibers contract but volume remains constant, termed isovolumetric.

5. Ventricular Ejection

   • Pressure in ventricles rises above aortic pressure (80 mmHg), opening semilunar valves.

   • Blood is ejected into the aorta; this phase lasts approximately 0.3 seconds, ejecting about 70 mL of blood per heartbeat.

   • Stroke Volume (SV) calculation: SV = EDV - ESV = 130 mL - 60 mL = 70 mL.

6. Ventricular Repolarization (T Wave)

   • Marks the onset of ventricular diastole. The ventricles relax and pressure inside decreases.

Isovolumetric Relaxation

7. Aortic Valve Closure

   • As pressure falls below atrial pressure, aortic valve closure occurs with a brief rebound from blood producing a dicrotic wave.

   • No change in volume during this phase, termed isovolumetric relaxation.

Final Stages of the Cardiac Cycle

8. Opening of AV Valves

   • When ventricular pressure drops below atrial pressure, AV valves open, allowing blood from the atria to fill the ventricles.

   • At the end of this relaxation, the ventricles are about 75% full.

9. Heart Sounds

   • S1: Closure of AV valves (first heart sound).

   • S2: Closure of semilunar valves (second heart sound).

   • Additional sounds (S3 and S4) may occur but are often inaudible without special equipment.

Disorders and Cardiac Output

• Atrial fibrillation: Less dangerous but can lead to complications. Characterized by irregular atrial contractions due to rapid stimulation of the AV node.

• Ventricular fibrillation: More critical and life-threatening where ventricles fail to pump.

• Normal cardiac output can be calculated as HR x SV.

• Example calculation:

  • HR at 70 bpm and SV at 70 mL results in a cardiac output of 4.9 L.

Factors Regulating Stroke Volume

1. Preload: Degree of stretch influencing ventricular muscle tension.

2. Contractility: Forcefulness of contraction of individual contractile fibers.

3. Afterload: Pressure required to open the semilunar valves for ejection.

Remembering these factors and processes can lead to better understanding and management of cardiac health.