Cardiac Cycle: Phases, Systole, and Diastole on ECG

1. Definitions

• Cardiac cycle — the sequence of mechanical and electrical events during a single heartbeat.
• At HR 75 bpm — duration is 0.8 s (systole ~0.33 s, diastole ~0.47 s).
• During tachycardia — diastole shortens predominantly (limiting ventricular filling).
• Systole — contraction, diastole — relaxation.

2. Phases of ventricular systole

• Isovolumetric contraction phase: AV valves closed (S1), semilunar valves still closed. Pressure rises sharply, volume remains constant.
• Rapid ejection phase: LV pressure exceeds aortic pressure (~80 mmHg), aortic valve opens, blood is ejected.
• Reduced ejection phase: ejection velocity drops, LV and aortic pressures nearly equalize.
• Completion: LV pressure falls below aortic pressure → closure of the aortic valve (onset of S2).

3. Phases of ventricular diastole

• Isovolumetric relaxation phase: semilunar valves closed (S2), AV valves not yet open. LV pressure drops rapidly.
• Rapid filling phase: when LV pressure falls below atrial pressure, AV valves open, blood flows from the atria.
• Reduced filling phase (diastasis): LV and atrial pressures equalize.
• Atrial systole (P wave on ECG): contributes an additional 15–25% of LV volume ("atrial kick").

4. Pressures in chambers and vessels

5. Stroke volume and cardiac output

• EDV (end-diastolic volume) of LV — 120 mL.
• ESV (end-systolic volume) — 50 mL.
• SV (stroke volume) = EDV − ESV = 70 mL.
• EF (ejection fraction) = SV / EDV × 100% = 58% (normal ≥ 52% in men, ≥ 54% in women).
• CO (cardiac output) = SV × HR = 70 × 75 ≈ 5.25 L/min at rest.
• Cardiac index = CO / body surface area; normal 2.5–4.0 L/min/m².

6. Heart sounds

• S1 — closure of mitral and tricuspid valves, onset of systole; best heard at the apex.
• S2 — closure of aortic and pulmonary valves, onset of diastole; best heard at the 2nd intercostal space. Physiological splitting during inspiration (aortic precedes pulmonary).
• S3 — early diastole (rapid filling); normal in youth under 40, pathological in adults (heart failure, volume overload).
• S4 — presystolic (atrial systole against a stiff LV); pathological (LVH, aortic stenosis, myocardial infarction).
• Additional sounds: opening snap (OS) in mitral stenosis; systolic click in mitral valve prolapse (MVP).

7. ECG correlation

• P wave → atrial systole (at the end of ventricular diastole).
• PR interval → AV nodal delay.
• QRS complex → onset of ventricular systole; immediately following QRS — S1.
• ST segment → reduced ejection phase + AP plateau.
• T wave → ventricular repolarization, end of systole; immediately following T — S2.
• QT interval → duration of ventricular systole (electrical).

8. Regulation of the cardiac cycle

• Frank-Starling law: ↑ EDV → ↑ contraction force (up to a certain limit).
• "All-or-none" law for the myocardium (unlike skeletal muscle).
• Preload — LV end-diastolic pressure; stretching of fibers prior to contraction.
• Afterload — the resistance against which the ventricle contracts (aortic blood pressure, SVR).
• Contractility — an intrinsic property of the myocardium, regulated by the sympathetic nervous system ($\beta_1$ $\to$ $\uparrow$) and inotropic drugs.
• Heart rate — regulated by the autonomic nervous system: sympathetic $\uparrow$, parasympathetic (vagus) $\downarrow$.

9. Clinical correlates

• Mitral stenosis — prolongation of atrial diastole, $\uparrow$ LA pressure $\to$ atrial fibrillation, pulmonary congestion.
• Aortic stenosis — $\uparrow$ afterload, LV hypertrophy, $S_4$, late systolic murmur.
• Mitral regurgitation — pansystolic murmur, regurgitation into the LA, volume overload, $S_3$.
• Heart failure with preserved ejection fraction (HFpEF) — diastolic dysfunction, impaired filling.
• Heart failure with reduced ejection fraction (HFrEF) — systolic dysfunction.
• Cardiac tamponade — limitation of diastolic filling in all chambers.

10. Wiggers diagram

• Graphical representation of the cardiac cycle: LV, aortic, and LA pressures + LV volume + ECG + heart sounds on a single time axis.
• Provides a clear visualization of: when valves open/close, active phases, and the correlation between mechanical and electrical events.
• A textbook classic; useful for understanding pathological conditions (AS, MR, MS, AI).

11. Myocardial blood supply during the cycle

• The LV receives blood almost exclusively during diastole (systolic compression of vessels impedes blood flow).
• In tachycardia ($\downarrow$ diastole) — risk of LV ischemia.
• RV — systolic-diastolic blood flow (lower pressure).
• Coronary reserve — the ability of blood flow to $\uparrow$ 4–5 fold during exertion.

12. FAQ

Why does diastole suffer predominantly during tachycardia?
Systole remains relatively constant (~0.3 s), while diastole shortens proportionally to the heart rate. At HR > 150, diastole is almost non-existent, impairing filling and coronary blood flow $\to$ ischemia, $\downarrow$ CO.

What is the "atrial kick" and when is it important?
Atrial systole contributes 15–25% of the ventricular EDV. In a stiff ventricle (LVH, AS, elderly), this share rises to up to 40%. Loss of kick (AF) $\to$ drop in CO, especially in AS and HFpEF.

Why does $S_2$ split during inspiration?
Inspiration $\uparrow$ venous return to the RV $\to$ prolongs its systole $\to$ pulmonic valve closure occurs after the aortic valve. Normal splitting is 20–40 ms. Paradoxical splitting (on expiration) occurs in LBBB.

What does a "gallop" mean?
A rhythm of three or four heart sounds ($S_1 + S_2 + S_3/S_4 \pm$ both). $S_3$ gallop ("protodiastolic") — in heart failure with systolic dysfunction. $S_4$ gallop ("presystolic") — in LVH, AS, hypertension, ischemia. Summation gallop — during tachycardia when both $S_3$ and $S_4$ are present.

How does the cardiac cycle change in atrial fibrillation?
Irregular rhythm $\to$ varying cycle lengths; absence of effective atrial systole $\to$ pulse deficit (difference between radial pulse and auscultated heart rate); $\uparrow$ risk of LA thromboembolism.

See also: Normal ECG · Heart Auscultation · Heart Failure · Valvular Heart Disease · Cardiology Directory