Cardiac Cycle & Haemodynamics

❤️ Cardiac Cycle & Haemodynamics

Cardiac output, Frank-Starling law, preload vs afterload, and the Wiggers diagram — the core cardiovascular physiology for MRCS.

Cardiac Output — The Fundamental Equation

Cardiac output (CO) is the volume of blood ejected by the heart per minute. It is the product of heart rate and stroke volume — and every clinical intervention aimed at improving cardiac function targets one or both of these components.

🧠 The Core Equation

CO = HR × SV
Cardiac Output (L/min) = Heart Rate (beats/min) × Stroke Volume (mL/beat)

Normal values: CO = 70 bpm × 70 mL = ~5 L/min
Cardiac Index (CI) = CO / Body Surface Area (BSA) — normalises for body size. Normal: 2.5–4.0 L/min/m²

CI <2.2 L/min/m² = cardiogenic shock threshold

Normal CO

~5 L/min

At rest. Rises to 20–25 L/min during maximal exercise.

Normal HR

60–100 bpm

SAN automaticity. Modified by ANS.

Normal SV

~70 mL/beat

EDV (~120 mL) − ESV (~50 mL)

Ejection Fraction

SV / EDV × 100

Normal: 55–65%. <40% = reduced EF heart failure.

Cardiac Index

2.5–4.0 L/min/m²

Shock if <2.2

SVR (normal)

800–1200 dynes·s/cm⁵

Afterload. ↑ in shock (vasoconstriction).

Determinants of Cardiac Output

Determinant	Effect on CO	Clinical Modifiers
Heart Rate	↑HR → ↑CO (up to a point — at very high rates, diastole too short to fill adequately → ↓SV)	Sympathetic (↑), vagal (↓), beta-blockers (↓), atropine (↑), arrhythmias, fever (↑), hypothyroidism (↓)
Preload (EDV)	↑ preload → ↑ SV (Frank-Starling mechanism — up to optimal point)	Venous return, fluid status, posture, haemorrhage, venodilators (nitrates reduce preload)
Afterload (SVR)	↑ afterload → ↓ SV (heart works against higher resistance → less ejected)	Hypertension, aortic stenosis ↑ afterload. ACE inhibitors, vasodilators reduce afterload.
Contractility (inotropy)	↑ contractility → ↑ SV at any given preload. Shifts Frank-Starling curve upward.	Sympathetic stimulation (↑), catecholamines (↑), digoxin (↑), beta-blockers (↓), heart failure (↓), acidosis (↓)

Fick Principle — Measuring CO

Fick Principle

CO = O₂ consumption / (arteriovenous O₂ difference)
CO = VO₂ / (CaO₂ − CvO₂)
Gold standard for CO measurement (thermodilution via pulmonary artery catheter also widely used clinically). The Fick principle states that the amount of O₂ consumed by the body per minute equals the CO multiplied by the difference in O₂ content between arterial and venous blood. Mixed venous saturation (SvO₂) normally ~75% — falls in low CO states as tissues extract more O₂.

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