Overview
This lecture explains the phases of the cardiac cycle, details Wiggers diagram, and describes how pressure, volume, and valve events correspond to heart sounds and cardiac output.
Cardiac Cycle Phases
- The cardiac cycle includes both diastole (relaxation/filling) and systole (contraction/ejection).
- Ventricular filling starts passively (80% of blood, driven by gravity), then actively by atrial contraction ("atrial kick," 20% of blood).
- Ventricular systole begins with isovolumetric contraction (ventricles contract, all valves closed, no ejection).
- When ventricular pressure exceeds aortic pressure, the aortic valve opens, and ventricular ejection occurs.
- Ventricular relaxation (diastole) starts with isovolumetric relaxation (all valves closed), followed by reopening of AV valves once ventricular pressure falls below atrial pressure.
- The cycle repeats continuously, with systole taking about one-third and diastole two-thirds of the cycle time.
Wiggers Diagram & Key Events
- Wiggers diagram shows changes in pressure, volume, valve status, ECG, and heart sounds over time.
- Rapid ventricular filling, diastasis, and atrial systole are phases of diastole; rapid and slow filling account for 80%, atrial systole for 20% of filling.
- Mechanical events (pressure/volume changes) follow electrical events (as seen on ECG).
- S1 ("lub") is due to AV (mitral/tricuspid) valve closure; S2 ("dub") is due to semilunar (aortic/pulmonic) valve closure.
- The time between S1 and S2 is systole; between S2 and the next S1 is diastole.
- S3 heart sound may occur with increased ventricular compliance (e.g., in heart failure).
Pressure & Volume Relationships
- Ventricular ejection only occurs when ventricular pressure exceeds aortic pressure.
- Isovolumetric contraction/relaxation are periods where all valves are closed and no blood moves.
- Atrial pressure curve consists of a wave (atrial contraction), c wave (mitral valve bulging during ventricular contraction), and v wave (atria filling).
ECG Correlates
- P wave: atrial depolarization/contraction.
- QRS complex: ventricular depolarization/contraction.
- T wave: ventricular repolarization.
Aortic Pressure Curve
- Systolic blood pressure is the highest aortic pressure during ejection.
- Diastolic blood pressure is the lowest aortic pressure before the next cycle.
- Dicrotic notch is the brief rise in aortic pressure due to aortic valve closure.
Cardiac Output Regulation
- Cardiac output = stroke volume × heart rate.
- Increased right atrial pressure increases cardiac output up to a plateau.
- Sympathetic stimulation shifts the cardiac output curve up and left; parasympathetic shifts it down and right.
- Stroke volume plateaus, so increases in cardiac output at high workload are mainly through increased heart rate.
- High arterial pressure (afterload) makes the heart work harder; chronic hypertension can reduce cardiac output and cause heart failure.
- Normally, the heart maintains stable output (~5 L/min) across a range of blood pressures, unless pressures become pathologically high.
Key Terms & Definitions
- Cardiac Cycle — one complete heartbeat, including systole and diastole.
- Isovolumetric Contraction/Relaxation — periods with all heart valves closed and no blood flow.
- Atrial Kick — atrial contraction contributing the final 20% of ventricular filling.
- Wiggers Diagram — graphical representation of cardiac cycle events.
- Afterload — the pressure the heart works against during ejection (mainly arterial pressure).
- Systolic/Diastolic Blood Pressure — maximum/minimum aortic pressures during the cardiac cycle.
Action Items / Next Steps
- Review Wiggers diagram figures to reinforce pressure, volume, and valve event timing.
- Study the phases of the cardiac cycle and relate them to heart sounds and ECG findings.