Cardiac output determinants

9 min read Last updated April 8, 2026

Core framework

  • Cardiac output (CO) = heart rate (HR) × stroke volume (SV)
    • SV is determined by preload, afterload, contractility, and ventricular interaction (including pericardial constraint) plus rhythm/AV synchrony
  • At steady state: CO equals venous return (VR)
    • VR is determined by mean systemic filling pressure (MSFP), right atrial pressure (RAP), and resistance to venous return (RVR): VR = (MSFP − RAP) / RVR
  • Determinants can be grouped into: cardiac pump function (Frank–Starling + inotropy + chronotropy) and the circulation (stressed volume/venous tone + intrathoracic pressure + vascular resistance)

Bedside mapping (what changes CO?)

  • Low CO differential: low preload (hypovolaemia/venodilation), high afterload (vasoconstriction), low contractility (ischaemia/drugs), brady/tachyarrhythmia, RV failure/PE, tamponade/tension pneumothorax
  • Interventions map to determinants: fluids/venoconstrictors (preload/MSFP), vasodilators (afterload), inotropes (contractility), pacing/antiarrhythmics (HR/AV synchrony), treat RV afterload (PE/hypoxia/acidosis), relieve pericardial/pleural constraint

1) Heart rate (chronotropy) and rhythm

  • CO rises with HR up to a point; excessive tachycardia reduces diastolic filling time and coronary perfusion, lowering SV and potentially CO
    • Loss of atrial contribution (AF) reduces LV filling especially with impaired relaxation (elderly, LVH, diastolic dysfunction)
  • Bradycardia reduces CO unless SV increases sufficiently; limited by preload reserve and ventricular compliance
  • AV synchrony: atrial systole contributes ~10–20% of LV end-diastolic volume at rest; more important at high HR and in stiff ventricles
  • Regularity matters: frequent ectopy reduces effective SV; AF causes beat-to-beat SV variability due to variable filling

2) Stroke volume determinants

  • SV = EDV − ESV; influenced by preload (EDV), afterload (ESV), and contractility (ESV) plus ventricular compliance and interaction
  • Preload (ventricular end-diastolic wall stress) approximated clinically by EDV/EDP, not by CVP alone
    • Frank–Starling: increasing preload increases SV via increased sarcomere length → increased force of contraction (within physiological range)
    • Preload responsiveness depends on position on the Starling curve; flat portion (e.g. dilated cardiomyopathy) gives little SV increase with fluids
  • Afterload: the force opposing ejection; relates to arterial pressure and ventricular wall stress (Laplace: wall stress ∝ P × r / (2h))
    • Increased afterload increases ESV and reduces SV (more marked in failing ventricle)
    • SVR is not identical to afterload; afterload also depends on arterial elastance/impedance, compliance, wave reflection, and aortic stenosis
  • Contractility (inotropy): intrinsic myocardial performance at a given preload and afterload; increased contractility decreases ESV and increases SV
    • Cellular basis: Ca2+ availability/sensitivity; affected by sympathetic tone, ischaemia, acidosis, hypoxia, electrolytes, and drugs (volatile agents reduce contractility)
  • Compliance/diastolic function: impaired relaxation or reduced compliance increases filling pressures for a given EDV → limits preload reserve and makes CO sensitive to tachycardia and AF
  • Ventricular interaction and pericardial constraint: RV dilation (e.g. PE) shifts septum left → reduces LV filling and SV; tamponade limits diastolic filling of both ventricles

3) Venous return approach (Guyton) — circulation determinants of CO

  • At equilibrium, CO = VR; the heart can only pump what it receives (except transiently)
  • Mean systemic filling pressure (MSFP): pressure in systemic circulation if the heart stopped and pressures equilibrated; reflects stressed volume and venous tone
    • Stressed vs unstressed volume: only stressed volume generates pressure driving VR; venoconstriction converts unstressed → stressed volume, raising MSFP
    • Fluids increase total volume and can increase stressed volume (depending on venous compliance), raising MSFP
  • Right atrial pressure (RAP) / CVP: back-pressure opposing VR; increasing RAP reduces VR (for a given MSFP)
    • High RAP may reflect RV failure, tamponade, high intrathoracic pressure, or volume overload; it does not necessarily mean high preload reserve
  • Resistance to venous return (RVR): mainly determined by venous/venular resistance and extravascular pressure; increased RVR reduces VR and CO
  • Intrathoracic pressure: positive pressure ventilation increases RAP (and can reduce VR); spontaneous inspiration lowers RAP and augments VR
    • PEEP: may reduce VR (↑RAP) but can improve RV afterload if it recruits lung and reduces hypoxic pulmonary vasoconstriction; net effect depends on volume status and lung mechanics
  • Splanchnic reservoir: sympathetic activation mobilises venous blood (venoconstriction) increasing MSFP and VR

4) Right ventricle and pulmonary circulation (often the limiting step)

  • RV output must equal LV output in steady state; acute RV failure reduces LV preload and CO
  • Determinants of RV output: preload, RV contractility, and RV afterload (pulmonary vascular resistance, PVR)
    • PVR increases with hypoxia, hypercapnia, acidosis, high lung volumes (alveolar vessel compression), thromboembolism; decreases with oxygenation, mild hypocapnia, pulmonary vasodilators

5) Measurement and interpretation (exam-relevant)

  • CO is not synonymous with blood pressure: BP = CO × SVR; a normal BP can coexist with low CO if SVR is high (and vice versa)
  • CVP is a poor predictor of fluid responsiveness; dynamic indices (SVV/PPV) and functional tests (passive leg raise, end-expiratory occlusion) are better when applicable
    • Dynamic indices require controlled ventilation, regular rhythm, adequate tidal volume, and no major RV failure or high intra-abdominal pressure
  • Fick principle: CO = VO2 / (CaO2 − CvO2); links determinants of oxygen delivery (DO2 = CO × CaO2)
Define cardiac output and list its main determinants.

Aim: state equation and expand SV determinants plus venous return concept.

  • CO = HR × SV
  • HR determinants: autonomic tone, intrinsic pacemaker rate, drugs, temperature, reflexes; rhythm/AV synchrony affects effective SV
  • SV determinants: preload, afterload, contractility, compliance/diastolic function, ventricular interaction/pericardial constraint
  • At steady state CO = venous return; VR depends on MSFP, RAP, and RVR
Explain the Frank–Starling mechanism and how it affects cardiac output.

Common viva: describe curve, mechanism, and clinical implications.

  • Increasing preload (EDV/wall stress) increases SV due to increased sarcomere length → increased actin–myosin overlap and Ca2+ sensitivity
  • Curve is steeper in normal hearts; flatter in systolic failure—less SV gain for a given preload increase
  • Clinical: fluid bolus increases CO only if patient is preload responsive; otherwise mainly increases filling pressures and congestion
Define preload and explain why CVP is an unreliable measure of preload and fluid responsiveness.

Examiners want definition (wall stress) and separation of pressure vs volume vs responsiveness.

  • Preload = end-diastolic myocardial fibre stretch; best conceptualised as end-diastolic wall stress (not simply a pressure)
  • CVP reflects RAP and is influenced by intrathoracic pressure, venous tone, RV compliance/function, tricuspid disease, and PEEP—so it does not map reliably to LV EDV
  • Fluid responsiveness depends on position on Starling curve; a single static pressure cannot predict slope
  • Better: dynamic indices (PPV/SVV) or functional tests (PLR) when conditions are met
Define afterload. Why is SVR not the same as afterload?

Key distinction: ventricular wall stress/arterial impedance vs lumped resistance.

  • Afterload = load opposing ventricular ejection; relates to LV wall stress during systole (Laplace: ∝ P × r / (2h))
  • SVR is a steady-state resistance term; afterload also includes arterial compliance, characteristic impedance, wave reflections, and outflow obstruction (e.g. aortic stenosis)
  • Increased afterload increases ESV and reduces SV, especially in impaired contractility
What is contractility? How can it be assessed clinically?

Definition plus practical surrogates.

  • Contractility = intrinsic myocardial ability to generate force at a given preload and afterload (inotropy)
  • Invasive/echo surrogates: dP/dt, ejection fraction (load dependent), end-systolic elastance (more load independent), tissue Doppler/strain
  • Clinical clues: low pulse pressure, cool peripheries with high SVR, rising lactate/low ScvO2, echo showing poor LV function
Using Guyton’s model, explain how venous return determines cardiac output.

Often asked as a conceptual diagram explanation.

  • At equilibrium CO = VR; intersection of cardiac function curve and venous return curve sets operating point
  • VR = (MSFP − RAP) / RVR
  • Increase MSFP (fluids, venoconstriction) shifts VR curve right → higher CO (if heart can respond)
  • Increase RAP (tamponade, RV failure, PEEP) reduces gradient → lower VR and CO
  • Increase RVR (venoconstriction in some beds, raised intra-abdominal pressure) reduces slope of VR curve → lower CO
Define mean systemic filling pressure (MSFP). What determines it and how can anaesthesia change it?

High-yield physiology + anaesthesia effects.

  • MSFP = equilibrated systemic pressure if the heart stopped; reflects stressed volume and venous tone (capacitance)
  • Determinants: blood volume, venous compliance, sympathetic tone (especially splanchnic venous reservoir)
  • Anaesthesia: induction agents/volatile agents reduce sympathetic tone → venodilation → reduced stressed volume and MSFP → reduced VR/CO (particularly if hypovolaemic)
  • Vasopressors (e.g. noradrenaline) increase venous tone and can increase MSFP, augmenting VR as well as arterial pressure
Explain the effect of positive pressure ventilation and PEEP on cardiac output.

Expect both VR and RV afterload components; net effect depends on context.

  • Increased intrathoracic pressure increases RAP → reduces (MSFP − RAP) gradient → reduced VR and CO, especially if preload dependent
  • High lung volumes can increase PVR (alveolar vessel compression) → increased RV afterload → reduced RV output and hence LV preload
  • Potential benefit: recruitment improves oxygenation and may reduce hypoxic pulmonary vasoconstriction → lower PVR; also reduces LV afterload (transmural pressure) in LV failure
  • Overall: PEEP often reduces CO in hypovolaemia; may improve CO in cardiogenic pulmonary oedema by reducing LV afterload and work of breathing
A patient is hypotensive after induction. Describe how changes in determinants of CO and SVR explain this, and outline immediate management.

Classic FRCA scenario: integrate BP = CO×SVR and venodilation effects.

  • Induction commonly causes venodilation (↓MSFP) and reduced sympathetic tone → ↓VR → ↓CO; also arterial dilation → ↓SVR
  • Some agents reduce contractility; bradycardia may contribute (e.g. opioids, vagal stimulus)
  • Immediate management: check rhythm/HR, treat bradycardia (atropine/glycopyrrolate), vasopressor bolus (metaraminol/phenylephrine/ephedrine depending on HR), consider noradrenaline infusion, give fluid if preload responsive, reduce anaesthetic depth, exclude anaphylaxis/bleeding
How does right ventricular failure reduce cardiac output? Give common perioperative causes.

Emphasise ventricular interdependence and PVR.

  • RV failure reduces forward flow to pulmonary circulation → reduced LV preload → reduced LV SV and CO
  • RV dilatation shifts interventricular septum left and increases pericardial constraint → further limits LV filling
  • Causes: PE, hypoxia/hypercapnia/acidosis (↑PVR), high PEEP/high airway pressures, RV infarction, pulmonary hypertension, protamine reaction, air/fat embolism
Describe the relationship between cardiac output and arterial blood pressure. Give examples where BP misleads you about CO.

Frequently examined: separating flow from pressure.

  • Mean arterial pressure (MAP) ≈ CO × SVR (plus CVP, usually small); BP is a pressure, CO is flow
  • High SVR states (cold shock, high-dose vasopressors): BP may be normal/high despite low CO and poor perfusion
  • Low SVR states (sepsis, neuraxial block): BP may be low despite normal/high CO
  • Therefore assess perfusion markers (lactate, urine output, capillary refill), echo, and CO monitoring when indicated
Explain how aortic stenosis affects determinants of cardiac output and why anaesthetic management differs.

Afterload obstruction + diastolic dependence.

  • Fixed outflow obstruction increases LV afterload and limits ability to increase SV; CO becomes more dependent on HR and preload
  • LVH reduces compliance → relies on atrial contraction; AF or tachycardia can markedly reduce filling and CO
  • Maintain sinus rhythm, avoid hypotension (coronary perfusion), avoid tachycardia/bradycardia extremes, maintain preload; vasopressors often preferred to vasodilators

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