Cardiac Output & Its Regulation

Cardiac output (CO) is the central integrating variable of the cardiovascular system, linking the heart's pump function to tissue perfusion. This topic ties together the Frank-Starling mechanism, the four determinants of stroke volume, neuro-humoral heart-rate control, and the Fick principle — all repeatedly examined and feeding directly into shock classification and haemodynamic monitoring.

Definition & basic relationships

Cardiac output (CO) = volume of blood ejected by each ventricle per minute.

CO = Stroke Volume (SV) × Heart Rate (HR)

• Resting adult CO ≈ 5 L/min (SV ≈ 70 mL × HR ≈ 70/min).
• Stroke volume (SV) = End-Diastolic Volume (EDV) − End-Systolic Volume (ESV) ≈ 120 − 50 = 70 mL.
• Ejection fraction (EF) = SV / EDV × 100 ≈ 55–70% (normal). EF < 40% defines heart failure with reduced EF (HFrEF).
• Cardiac Index (CI) = CO / Body Surface Area = 2.5–4.0 L/min/m² (normalises CO for body size; the value actually used clinically and in shock tables).

High-yield: Cardiac index, not absolute CO, is the parameter used to define and grade shock. CI < 2.2 L/min/m² with raised filling pressures = cardiogenic shock.

The four determinants of CO are: preload, afterload, contractility (these govern SV), and heart rate. Examiners love to test how each shifts SV and the ventricular function curve.

Stroke volume determinant 1 — Preload (Frank-Starling Law)

Preload = the degree of myocardial fibre stretch at end-diastole, i.e. the load on the muscle before contraction. The best clinical surrogates are EDV and, indirectly, filling pressures (LVEDP, pulmonary capillary wedge pressure / PCWP for the left heart; CVP/right atrial pressure for the right heart).

Frank-Starling Law of the Heart

"The energy of contraction (and hence stroke volume) is proportional to the initial length of the cardiac muscle fibre." — within physiological limits, increased EDV → increased force of contraction → increased SV.

Mechanism (modern view): stretching sarcomeres toward the optimal length (~2.2 µm) increases myofilament calcium sensitivity and improves actin–myosin overlap, so more cross-bridges form for a given intracellular Ca²⁺. (The older "optimal overlap" explanation alone is incomplete — length-dependent Ca²⁺ sensitivity is the dominant factor.)

Physiological role: the Frank-Starling mechanism beat-to-beat matches output of the two ventricles and lets the heart adapt to changes in venous return without external nervous input (intrinsic regulation).

High-yield: The principal physiological role of Frank-Starling is to equalise the output of the right and left ventricles so blood does not pool in the pulmonary or systemic circulation.

Factors that raise preload: increased venous return, increased blood volume, gravity/recumbency, increased atrial contraction ("atrial kick" — lost in atrial fibrillation), bradycardia (longer filling time), negative intrathoracic pressure.

Stroke volume determinant 2 — Afterload

Afterload = the load/resistance the ventricle must overcome to eject blood; functionally, ventricular wall tension during ejection. Surrogates: aortic pressure / systemic vascular resistance (SVR) for LV, pulmonary artery pressure / PVR for RV.

By Laplace's law, wall tension T = (P × r) / 2h (P = pressure, r = radius, h = wall thickness). Dilated ventricles (large r) and hypertensive states (high P) raise wall tension → increase afterload and myocardial O₂ demand; hypertrophy (high h) is a compensatory tension-reducing response.

Effect on SV: ↑ afterload → ↑ ESV → ↓ SV (for any given preload and contractility). This is why vasodilators (afterload reduction) improve SV in heart failure.

Parameter	Preload	Afterload
Definition	Stretch before contraction (end-diastolic)	Resistance to ejection during systole
Surrogate (LV)	LVEDV, LVEDP, PCWP	Aortic pressure, SVR
↑ effect on SV	↑ SV (Starling)	↓ SV
Raised by	Venous return, volume, atrial kick	Hypertension, aortic stenosis, vasoconstriction
Therapeutic lowering	Diuretics, venodilators (nitrates)	Arterial dilators (hydralazine, ACEi/ARB)

Stroke volume determinant 3 — Contractility (inotropy)

Contractility = the intrinsic strength of contraction independent of preload and afterload. It reflects the rate and amount of cross-bridge cycling at a fixed fibre length, governed largely by intracellular Ca²⁺ availability.

• Positive inotropes: sympathetic stimulation / catecholamines (β₁ → ↑cAMP → ↑Ca²⁺ entry), digoxin (Na⁺/K⁺-ATPase inhibition → ↑intracellular Na⁺ → ↓Na⁺/Ca²⁺ exchange → ↑Ca²⁺), dobutamine, milrinone (PDE-3 inhibitor), increased HR (Bowditch / Treppe / staircase effect — faster rate raises intracellular Ca²⁺).
• Negative inotropes: β-blockers, non-dihydropyridine Ca²⁺ blockers (verapamil, diltiazem), acidosis, hypoxia, hypercapnia, myocardial ischaemia/infarction, barbiturates, heart failure.

High-yield: The Anrep effect = an abrupt rise in afterload causes an autoregulatory increase in contractility over a few minutes (intrinsic). The Bowditch/Treppe effect = increased contractility caused by increased heart rate. Examiners frequently swap these two.

Anrep vs Bowditch one-liner: Anrep — pressure (afterload) raises contractility; Bowditch — beats (rate) raise contractility.

The ventricular function (Frank-Starling) curve

Plot SV or stroke work (Y-axis) against EDV / LVEDP / preload (X-axis). A family of curves exists:

↑ contractility / sympathetic stimulation → curve shifts up and left (more SV for same preload). ↓ contractility / heart failure / ↑ afterload → curve shifts down and right (less SV for same preload; needs higher filling pressure).

Stepwise interpretation:

1. Start on the normal curve at a given LVEDP.
2. Give a positive inotrope → move to a higher curve → SV rises at the same LVEDP → upward-left shift.
3. Acute MI / myocardial depressant → drop to a lower curve → SV falls → compensatory fluid retention raises LVEDP, pushing rightwards along the flatter curve → pulmonary congestion appears once LVEDP > ~18–20 mmHg.
4. Diuretic in congested failure → move leftwards on the same depressed curve → ↓LVEDP relieves congestion with little SV loss (because the failing curve is flat).

High-yield: In heart failure, the curve is depressed and flattened, so the heart operates on the plateau — venous return rises markedly with little SV gain, producing congestion. This is why preload reduction (diuretics/venodilators) relieves symptoms with minimal CO penalty.

In high-output states (anaemia, thyrotoxicosis, beri-beri/thiamine deficiency, AV fistula, pregnancy, Paget's disease, sepsis), CO is raised (often >8 L/min) chiefly because SVR is low and venous return high — the curve is normal or shifted up; SvO₂ tends to be high in distributive/high-output states.

Cardiac output determinant 4 — Heart rate & its regulation

HR is set by the SA node and modulated by:

• Parasympathetic (vagus, ACh on M₂ receptors): dominant at rest → "vagal tone." Slows SA node (negative chronotropy), slows AV conduction (negative dromotropy). Right vagus → mainly SA node; left vagus → mainly AV node.
• Sympathetic (noradrenaline/adrenaline on β₁): ↑HR, ↑conduction, ↑contractility.
• Intrinsic HR (after full autonomic blockade with atropine + propranolol) ≈ 100–110/min, showing resting vagal tone normally dominates.

Reflexes affecting HR/CO:

1. Baroreceptor reflex (carotid sinus, aortic arch → glossopharyngeal/vagus → NTS): ↑BP → ↑firing → ↑vagal, ↓sympathetic → ↓HR, ↓CO. Buffers acute BP changes.
2. Bainbridge reflex (atrial stretch reflex): ↑venous return / atrial stretch → ↑HR (prevents back-up of blood). Explains tachycardia with volume loading.
3. Bezold-Jarisch reflex: stimulation of ventricular C-fibres (e.g. inferior MI, certain drugs) → bradycardia, hypotension, apnoea.
4. Chemoreceptor & CNS ischaemic responses, respiratory sinus arrhythmia (HR ↑ in inspiration, ↓ in expiration — vagal).

High-yield: Increasing HR raises CO only up to a point. Tachycardia beyond ~150–180/min reduces CO because diastolic filling time shortens → ↓EDV → ↓SV. The diastolic interval (filling) shortens proportionately more than systole as HR rises.

Venous return and the guyton analysis

CO cannot exceed venous return (VR) in the steady state. Venous return depends on the pressure gradient between mean systemic filling pressure (MSFP, ~7 mmHg) and right atrial pressure (RAP), divided by resistance to venous return:

VR = (MSFP − RAP) / Resistance to venous return

• ↑MSFP (volume, venoconstriction) → ↑VR.
• ↑RAP → ↓VR (the venous return curve falls; at RAP ≈ MSFP, VR = 0).
• The operating point of the circulation is where the cardiac function curve and the venous return curve intersect (Guyton's graphical analysis) — a favourite for showing how blood volume, contractility, and resistance interact.

Measurement of cardiac output

Fick principle (gold standard / reference method)

CO = O₂ consumption (VO₂) / (Arterial O₂ content − Mixed venous O₂ content)

CO = VO₂ / (CaO₂ − CvO₂)

The amount of O₂ taken up by the lungs equals the amount carried away by blood (rate of uptake = blood flow × arteriovenous concentration difference). Mixed venous blood is sampled from the pulmonary artery.

Worked example: VO₂ 250 mL/min; CaO₂ 200 mL/L; CvO₂ 150 mL/L → AV difference = 50 mL/L → CO = 250 / 50 = 5 L/min.

Method	Principle	Notes
Fick (direct)	O₂ uptake / AV-O₂ difference	Reference standard; needs PA blood
Indicator/dye dilution	Indocyanine green; area under conc–time curve	Stewart-Hamilton equation
Thermodilution	Cold saline injected via Swan-Ganz, temperature change in PA	Commonest bedside method; PA catheter
Doppler echo	LVOT velocity-time integral × CSA × HR	Non-invasive, operator-dependent

High-yield: Fick principle is the reference (gold) standard; thermodilution via Swan-Ganz (pulmonary artery) catheter is the classic clinical method and also measures PCWP (≈ LVEDP/preload).

Haemodynamic profiles & shock classification

The Swan-Ganz–derived trio — CI, PCWP (preload), SVR (afterload) — classifies shock and is heavily tested:

Shock type	CO/CI	PCWP (preload)	SVR	SvO₂
Cardiogenic	↓	↑	↑	↓
Hypovolaemic	↓	↓	↑	↓
Distributive (septic/anaphylactic/neurogenic)	↑ or normal	↓/normal	↓	↑
Obstructive (PE, tamponade, tension PTX)	↓	variable*	↑	↓

*In tamponade, equalisation of diastolic pressures and raised CVP/PCWP; in massive PE, RA/RV pressures rise. Neurogenic shock classically has hypotension with bradycardia (loss of sympathetic tone).

High-yield: Warm shock with low SVR and high CO = early distributive (septic) shock. Cold, clamped-down periphery with low CO and high SVR = cardiogenic/hypovolaemic.

Factors that increase vs decrease cardiac output

Increase CO	Decrease CO
Exercise (up to 5×, mainly ↑HR & ↑SV)	Heart failure, MI
Pregnancy, fever, anaemia	Haemorrhage/hypovolaemia
Thyrotoxicosis, beri-beri	Standing (↓venous return), Valsalva strain phase
AV fistula, Paget's disease	Tachy-/bradyarrhythmias, β-blockade
Sympathetic stimulation	Positive-pressure ventilation, PEEP (↓preload)
Increased temperature	Hypothyroidism, deep anaesthesia

Complications / clinical correlates of deranged CO

• Low-output failure: fatigue, cold extremities, oliguria, narrow pulse pressure, raised JVP, pulmonary congestion (left) or peripheral oedema (right).
• High-output failure: the heart cannot meet metabolic demand despite raised CO; bounding pulses, warm peripheries (thyrotoxicosis, severe anaemia, beri-beri, AV fistula).
• Forward vs backward failure: forward = ↓perfusion (fatigue, hypotension); backward = congestion behind the failing chamber.
• Loss of atrial kick (AF) drops CO ~15–20%, important in stiff/hypertrophied ventricles (HFpEF, HOCM, mitral stenosis).

Key differentials & concept clarifications

• Preload vs afterload: preload = volume/stretch before contraction; afterload = pressure/resistance during ejection. Don't confuse PCWP (preload marker) with SVR (afterload marker).
• Anrep (afterload→contractility) vs Bowditch/Treppe (rate→contractility).
• Bainbridge (↑VR→↑HR) vs Baroreceptor (↑BP→↓HR).
• HFrEF vs HFpEF: reduced EF with dilated ventricle and low contractility vs preserved EF with diastolic stiffness (preload-dependent, sensitive to AF and tachycardia).
• High-output vs distributive shock overlap: both show high CO and low SVR; sepsis is the classic bridge.

Recently asked / exam angle

• Frank-Starling curve shifts: identify upward-left (inotrope/sympathetic) vs downward-right (failure/↑afterload) — image-based MCQs are common.
• Fick principle calculation: given VO₂ and AV-O₂ difference, compute CO (and vice versa to find VO₂).
• Anrep vs Bowditch effect — direct one-line discriminator questions.
• Haemodynamic table of shock (CI/PCWP/SVR/SvO₂) — match the profile to the shock type; warm septic shock especially.
• Intrinsic heart rate (~100–110/min) and resting vagal dominance after autonomic blockade.
• Determinant of CO most reduced by tachycardia → diastolic filling time → ↓SV.
• Laplace law linking ventricular dilatation to increased wall stress and O₂ demand.
• Role of Frank-Starling = balancing the two ventricular outputs.
• PCWP as the best bedside surrogate of LV preload (LVEDP).

Rapid revision

1. CO = SV × HR; normal ≈ 5 L/min; Cardiac Index 2.5–4.0 L/min/m² is the clinically used value.
2. Four determinants: preload, afterload, contractility (→SV) and heart rate.
3. Frank-Starling: ↑EDV → ↑force → ↑SV via length-dependent Ca²⁺ sensitivity; its job is to balance RV and LV output.
4. ↑Afterload → ↓SV (↑ESV); reduce it with arterial vasodilators (ACEi, hydralazine) in failure.
5. Contractility is independent of preload/afterload; raised by catecholamines, digoxin, dobutamine, milrinone.
6. Anrep = afterload raises contractility; Bowditch/Treppe = rate raises contractility.
7. Failure curve is depressed and flat → diuretics relieve congestion with little SV loss.
8. Intrinsic HR ≈ 100–110/min; at rest vagal tone dominates.
9. Baroreceptor ↑BP→↓HR; Bainbridge ↑VR→↑HR; Bezold-Jarisch → bradycardia + hypotension (inferior MI).
10. Tachycardia >150–180/min lowers CO by shortening diastolic filling.
11. Fick: CO = VO₂ / (CaO₂ − CvO₂); reference standard; thermodilution (Swan-Ganz) is the bedside method.
12. Shock table: cardiogenic ↓CI/↑PCWP/↑SVR; hypovolaemic ↓CI/↓PCWP/↑SVR; septic ↑CO/↓SVR/↑SvO₂.