What is the duration of the entire cardiac cycle, including all its phases, at a normal heart rate?

Duration of the Cardiac Cycle and Its Phases

At a normal heart rate of 60-100 beats per minute, the entire cardiac cycle lasts approximately 0.8 seconds (800 milliseconds), with systole occupying about one-third (0.3 seconds) and diastole about two-thirds (0.5 seconds) of the cycle.

Cardiac Cycle Components and Duration

Electrical Components

• P wave: 80-100 ms - Represents atrial depolarization
• PR interval: 120-200 ms - From onset of P wave to onset of QRS complex
• QRS complex: ≤110 ms in adults - Represents ventricular depolarization 1
  • 100 ms in children 4-16 years
  • 90 ms in children <4 years
• QT interval: 350-440 ms (varies with heart rate) - From onset of QRS to end of T wave
• T wave: 160 ms - Represents ventricular repolarization

Mechanical Components

• Systole: ~0.3 seconds at normal heart rate
  • Isovolumic contraction: 0.05 seconds
  • Ventricular ejection: 0.25 seconds
• Diastole: ~0.5 seconds at normal heart rate
  • Isovolumic relaxation: 0.08 seconds
  • Early rapid filling (E wave): ~0.1 seconds 2
  • Diastasis: Variable (decreases with increasing heart rate)
  • Atrial contraction (A wave): ~0.1 seconds 2

Heart Rate Effects on Cardiac Cycle Duration

The duration of the cardiac cycle is inversely proportional to heart rate:

• At 60 bpm: Cycle length = 1.0 second
• At 75 bpm: Cycle length = 0.8 seconds
• At 100 bpm: Cycle length = 0.6 seconds

As heart rate increases, the cardiac cycle shortens primarily through reduction of diastolic duration, particularly the diastasis phase 2. Research shows that:

• E-wave and A-wave durations remain relatively constant regardless of heart rate
• A 100% increase in heart rate results in only an 18% decrease in E-wave duration and 16% decrease in A-wave duration 2
• Diastasis is the first phase to be eliminated as heart rate increases

Variations in Total Heart Volume

During the cardiac cycle, the total heart volume (atria plus ventricles) varies by approximately 8-9% 3. This variation:

• Is most pronounced at the midcardiac level at the widest diameter of the heart
• Shows left-sided predominance
• Occurs primarily in the region of atrioventricular plane movement

Clinical Implications

Understanding the duration of cardiac cycle phases is critical for:

1. ECG interpretation: Abnormal durations may indicate pathology

   • QRS duration ≥120 ms suggests intraventricular conduction delays like RBBB or LBBB 1
   • Prolonged QT interval may indicate risk for torsades de pointes 4

2. Hemodynamic assessment:

   • Mean arterial pressure calculations must account for the changing systole/diastole ratio at different heart rates 5
   • At higher heart rates, the systolic fraction of the cardiac cycle increases

3. Therapeutic interventions:

   • QRS duration ≥120 ms may indicate need for cardiac resynchronization therapy in heart failure patients 1

Common Pitfalls in Measurement

• Failure to consider age-appropriate norms for ECG intervals
• Assuming fixed systole/diastole ratios across different heart rates
• Using single-lead measurements instead of global intervals (earliest onset to latest offset in all leads)
• Not accounting for respiratory variation in heart rate

Remember that the cardiac cycle duration and its phases are dynamic and respond to physiological demands, with diastole being the most variable component that shortens significantly as heart rate increases.