Ventricular ejection physiology

Ventricular Ejection - The Big Squeeze

• Begins when ventricular pressure surpasses aortic and pulmonary artery pressures, pushing the semilunar valves open.
• Blood is forcefully expelled, marking the primary work phase of the heart.
• Phases:
  • Rapid Ejection: Initial, powerful surge of blood.
  • Reduced Ejection: Slower flow as ventricular and arterial pressures start to equalize.
• Key Formulas:
  • Stroke Volume (SV): $SV = EDV - ESV$
  • Ejection Fraction (EF): $EF = (SV / EDV) * 100%$. A critical index of contractility; normal is >55%.

⭐ Afterload is the primary determinant of end-systolic volume (ESV). High afterload (e.g., aortic stenosis, hypertension) increases the pressure the ventricle must overcome, reducing stroke volume and leaving more blood behind (↑ ESV).

Ejection Fraction - The Heart's Report Card

• Definition: The percentage of blood ejected from the ventricle with each beat; a key index of systolic function.
• Formula: $EF = (SV / EDV) * 100%$
  • SV: Stroke Volume
  • EDV: End-Diastolic Volume
• Normal Range: 55-70%.
• Clinical Ranges:
  • Heart Failure with reduced EF (HFrEF): $≤ \textbf{40}%$
  • Heart Failure with preserved EF (HFpEF): $≥ \textbf{50}%$
• Primary Indicator Of: Ventricular contractility. Changes in preload, afterload, or contractility will alter EF.

⭐ High-Yield Fact: EF can be normal in diastolic heart failure (HFpEF). Patients present with dyspnea and exercise intolerance, but the ventricle's pumping percentage is preserved; the issue is with filling (diastolic dysfunction).

Stroke Volume Determinants - The Three Bosses

Stroke Volume ($SV$) is governed by three factors: Preload, Afterload, and Contractility. $SV = EDV - ESV$.

• Preload: Ventricular stretch at end-diastole (EDV).
  • ↑ Venous return → ↑ Preload → ↑ SV (Frank-Starling Law).
• Afterload: Resistance the ventricle ejects against (approximated by SVR).
  • ↑ Afterload → ↓ SV.
• Contractility (Inotropy): Intrinsic pump strength.
  • ↑ Sympathetic tone (Ca²⁺) → ↑ Contractility → ↑ SV.

⭐ Afterload is clinically critical; reducing it with vasodilators is a key strategy in treating systolic heart failure to improve forward flow.

📌 Mnemonic: SV depends on CAP: Contractility, Afterload, Preload.

Clinical Correlations - When Ejection Fails

• Heart Failure with Reduced Ejection Fraction (HFrEF): The ventricle fails to eject an adequate stroke volume (SV) due to impaired contractility.
• Key Metric: Ejection Fraction (EF) = $(SV / EDV) \times 100$.
  • Normal EF: 55-70%.
  • HFrEF is diagnosed when EF < 40%.
• Consequences:
  • ↓ Cardiac Output → fatigue, weakness (hypoperfusion).
  • ↑ End-Diastolic Volume (EDV) → pulmonary & systemic congestion (e.g., dyspnea, edema).
• Common Causes: Ischemic heart disease, chronic hypertension, dilated cardiomyopathy, valvular disease.

⭐ An S3 heart sound is a hallmark of HFrEF, representing tensing of the chordae tendineae during rapid ventricular filling into a distended ventricle.

High‑Yield Points - ⚡ Biggest Takeaways

• Ventricular pressure must exceed aortic and pulmonary artery pressures to open the semilunar valves.
• The majority of stroke volume is ejected during the initial rapid ejection phase.
• Aortic pressure rises and peaks during this phase due to the rapid influx of blood.
• This phase corresponds to the ST segment and T wave on the ECG.
• Ejection ends when ventricular pressure falls below aortic/pulmonic pressure, causing semilunar valve closure.