Phases of the Normal Cardiac Cycle in the Wiggers Diagram
The normal cardiac cycle consists of systole and diastole, with distinct electrical and mechanical phases that can be visualized in a Wiggers diagram showing the relationship between ECG, pressure changes, ventricular volume, heart sounds, and valve movements.
Electrical and Mechanical Phases
The cardiac cycle can be divided into key phases that correspond to specific electrical and mechanical events:
1. Atrial Systole (Late Diastole)
- Initiated by the P wave on ECG (atrial depolarization)
- Characterized by atrial contraction ("atrial kick")
- Contributes final 20-30% of ventricular filling
- Ventricular volume reaches end-diastolic volume
- Atrial pressure slightly exceeds ventricular pressure
2. Isovolumetric Contraction
- Begins with QRS complex (ventricular depolarization)
- Both AV and semilunar valves are closed
- Ventricular pressure rises rapidly while volume remains constant
- Ends when ventricular pressure exceeds arterial pressure
- First heart sound (S1) occurs as AV valves close
3. Rapid Ejection
- Begins when semilunar valves open
- Characterized by rapid ejection of blood into arteries
- Ventricular pressure continues to rise
- Ventricular volume decreases rapidly
- Corresponds to early part of ST segment on ECG
4. Reduced Ejection
- Later phase of systole
- Rate of ejection slows
- Ventricular pressure begins to fall
- Ventricular volume continues to decrease but at slower rate
- Corresponds to late ST segment and early T wave on ECG
5. Isovolumetric Relaxation
- Begins with closure of semilunar valves
- Second heart sound (S2) occurs
- Ventricular pressure falls rapidly while volume remains constant
- Both AV and semilunar valves are closed
- Corresponds with T wave (ventricular repolarization)
6. Rapid Ventricular Filling
- Begins when AV valves open
- Characterized by rapid passive filling of ventricles
- Ventricular pressure continues to fall then begins to rise slowly
- Ventricular volume increases rapidly
- Third heart sound (S3) may be heard (normal in children and young adults)
7. Diastasis
- Middle phase of diastole
- Slow filling of ventricles
- Minimal pressure and volume changes
- Period of relative cardiac "rest"
Key Relationships in the Wiggers Diagram
The Wiggers diagram illustrates several important relationships:
- Electrical-Mechanical Coupling: The ECG events precede and trigger the mechanical events
- Pressure Gradients: Blood flows from areas of high to low pressure
- Valve Function: Valves open when pressure is higher on one side and close when pressure gradient reverses
- Volume Changes: Ventricular volume decreases during systole and increases during diastole
Clinical Significance
Understanding the cardiac cycle phases helps in:
- Interpreting heart sounds and murmurs
- Diagnosing valvular disorders
- Understanding the effects of arrhythmias on cardiac output
- Interpreting pressure-volume relationships in heart failure
Common Pitfalls in Interpretation
- Confusing isovolumetric contraction with isovolumetric relaxation
- Misinterpreting the timing of valve opening and closure
- Failing to recognize that electrical events precede mechanical events
- Not appreciating that ventricular pressure must exceed arterial pressure for ejection to occur
The relationship between electrical and mechanical activity is critical, as noted by the American College of Cardiology, with the ECG waveforms driving the subsequent mechanical events of each phase 1.