Anaes · Applied cardiovascular & respiratory physiology

Cardiac output & its determinants

Also known as Cardiac output · Cardiac index · Fick principle · Venous return · Mean systemic filling pressure · Guyton curves

Cardiac output — about 5 litres per minute at rest — is the product of heart rate and stroke volume, but in the steady state it is set equally by the venous return the circulation delivers to the heart. The framework rests on five exam-critical ideas: cardiac output equals heart rate times stroke volume, and the cardiac index normalises it to body surface area; the Fick principle measures cardiac output from oxygen consumption and the arteriovenous oxygen difference; heart rate is the autonomic lever on cardiac output, but tachycardia beyond an optimum cuts diastolic filling and lowers stroke volume; stroke volume is governed by preload, afterload and contractility (see the cardiac-cycle topic); and, in Guyton's framework, cardiac output equals venous return in the steady state, and the operating point is the intersection of the cardiac function curve and the venous return curve, set by the mean systemic filling pressure. Built on the venous-return and mean-systemic-filling-pressure review (Persichini 2022), the determinants-of-systemic-blood-flow review (Joyce 2020), the venous-return physics review (Brengelmann 2019), and three cardiac-output-measurement papers comparing thermodilution and the Fick method (Flick 2026 BJA, Rivera-Robles 2025, Abualsaud 2024), and the levosimendan-versus-dobutamine survival trial (SURVIVE 2007).