Stroke Volume vs. Cardiac Output: Definitions, Formulas & Clinical Use

These two hemodynamic parameters describe pump function at different scales.

Definitions

• Stroke Volume (SV): Volume ejected by LV per beat. Normal 60–100 mL. SV = EDV − ESV.
• Cardiac Output (CO): Volume pumped per minute. CO = SV × HR. Normal 4–8 L/min.
• Cardiac Index (CI): CO/BSA. Normal 2.5–4.0 L/min/m².

Determinants of SV (Frank-Starling +)

• Preload: LV end-diastolic volume (venous return). ↑ preload → ↑ SV (up to plateau).
• Afterload: Systemic vascular resistance (SVR) + aortic impedance. ↑ afterload → ↓ SV.
• Contractility: Inotropic state (catecholamines, Ca²⁺). ↑ contractility → ↑ SV at same preload/afterload.
• Heart rate: ↑ HR → ↓ filling time → ↓ EDV → ↓ SV (if excessive).

Clinical Applications

• Shock states: Cardiogenic (↓ SV, ↓ CO), distributive (↑ CO early, ↓ SVR), hypovolemic (↓ preload → ↓ SV).
• Heart failure: HFrEF (↓ SV), HFpEF (normal SV, ↑ filling pressures).
• Exercise: CO ↑ 4–6× (SV ↑ 30–50%, HR ↑ 2–3×).
• Monitoring: Echo (SV), thermodilution/PiCCO (CO), echocardiography, NICOM.

Key Insight

SV reflects beat efficiency; CO reflects total systemic delivery. Both needed for complete picture.