Q2 LONG ANSWER 10M ★ High-yield

Define cardiac output. Describe its determinants and the factors regulating it. Add a note on the methods of its measurement.

Physiology · 1st Professional — Pre-Clinical

Cardiac output (CO) = volume of blood pumped by each ventricle per minute.CO = Heart rate × Stroke volume = 72 × 70 = 5 L/min

Term	Definition	Normal
Stroke volume	Blood ejected per beat	70–80 mL
Cardiac output	Per minute	5 L/min
Cardiac index	CO ÷ body surface area	3.2 L/min/m2
Cardiac reserve	Maximum − resting CO	4–5× resting (up to 25 L/min in athletes)

A. Preload — Frank–Starling mechanism

Preload = the degree of stretch of the ventricular fibres at the end of diastole — effectively the end-diastolic volume.

Starling's law — force of contraction is directly proportional to the initial length of the muscle fibre, within physiological limits
Basis — the length–tension relationship; stretch moves sarcomeres toward the optimal 2.2 µm
Also ↑ sensitivity of troponin C to Ca2+
Preload ↑ by — venous return, blood volume, venoconstriction, muscle pump, lying down, bradycardia

B. Afterload

The resistance against which the ventricle must eject — effectively aortic pressure and peripheral resistance
↑ Afterload → ↓ stroke volume
↑ in hypertension and aortic stenosis

C. Contractility (inotropic state)

Positive inotropic (↑ CO)	Negative inotropic (↓ CO)
Sympathetic stimulation, adrenaline	Parasympathetic (vagus) — mainly on atria
Digitalis, dopamine, dobutamine	β-blockers, Ca2+ channel blockers
↑ Extracellular Ca2+	Hypoxia, acidosis, hyperkalaemia
Thyroxine	Quinidine, barbiturates, myocardial infarction

↑ Cardiac output	↓ Cardiac output
Exercise (up to 25–30 L/min)	Standing from lying
Anxiety, excitement (50–100%)	Rapid arrhythmias
Eating (30%)	Heart failure, myocardial infarction
Pregnancy (30–40%)	Haemorrhage, shock
High environmental temperature	Valvular disease
Fever, hyperthyroidism, anaemia, beri-beri	Hypothyroidism

Intrinsic (heterometric) — Starling's law: matches output to venous return beat by beat, and equalises right and left ventricular output
Intrinsic (homometric) — Anrep effect: ↑ contractility on sudden ↑ afterload; Bowditch (Treppe) effect: ↑ contractility with ↑ heart rate
Extrinsic — nervous: sympathetic → ↑ rate and force; vagus → ↓ rate
Extrinsic — humoral: adrenaline, thyroxine, glucagon

A. Fick principle

CO = O2 consumed per minute ÷ Arteriovenous O2 difference= 250 mL/min ÷ 50 mL/L = 5 L/min

Requires mixed venous blood from the pulmonary artery — hence cardiac catheterisation

B. Other methods

Method	Principle
Dye dilution	Indocyanine green injected; concentration–time curve; Stewart–Hamilton equation
Thermodilution	Cold saline via Swan–Ganz catheter; commonest clinical method
Echocardiography	Non-invasive; SV from LV dimensions
Ballistocardiography	Body recoil with each ejection

External (stroke) work = Stroke volume × Mean arterial pressure
Volume work — the greater part is spent overcoming pressure, not moving volume
Kinetic energy component is only about 1% normally, but rises to 50% in aortic stenosis
Left ventricular work is about 6–7× that of the right
Cardiac efficiency = 20–25%; the rest appears as heat
Myocardial O2 consumption is determined chiefly by heart rate, wall tension and contractility — not by the work done

Heart failure → the heart operates on the descending limb of the Starling curve → further dilatation reduces output
Ejection fraction < 40% defines systolic heart failure
High-output failure — anaemia, thyrotoxicosis, beri-beri, arteriovenous fistula, Paget disease
Cardiac tamponade → restricted filling → ↓ preload → ↓ CO despite normal contractility

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