60 Cardiac output
Cardiac output (CO) is the volume of blood pumped by each ventricle per minute. It equals stroke volume × heart rate. At rest, an adult heart pumps about 4–8 L/min; during intense exercise, cardiac output can rise four- to sixfold in trained people. It is determined by heart rate, contractility, preload (filling of the heart) and afterload (resistance to ejection). Cardiac output, together with haemoglobin and oxygen saturation, determines oxygen delivery to the body.
How cardiac output is measured
The reference methods are invasive. In thermodilution, cold fluid is injected through a pulmonary artery (Swan–Ganz) catheter and the temperature change downstream is used to calculate flow. The Fick principle calculates cardiac output from oxygen consumption divided by the difference in oxygen content between arterial and mixed venous blood. Less invasive methods include transpulmonary thermodilution, pulse contour analysis, echocardiography and Doppler. Cardiac index (cardiac output divided by body surface area) allows comparison between people of different size.
Low cardiac output
Causes include heart failure and cardiomyopathy, myocardial infarction, myocarditis, valve disease, arrhythmias (very fast or very slow), low blood volume (bleeding, dehydration), pulmonary embolism, cardiac tamponade and medicines that depress the heart. Symptoms are fatigue, poor exercise tolerance, breathlessness, cold extremities, dizziness, low urine output and confusion. A cardiac index below about 2.2 L/min/m² with signs of hypoperfusion is typical of cardiogenic shock.
High cardiac output
Occurs physiologically during exercise, pregnancy and fever, and pathologically in sepsis (early phase), hyperthyroidism, severe anaemia, arteriovenous fistulas, liver cirrhosis and beriberi ("high-output" states).
for breathlessness at rest, chest pain, fainting, confusion, cold clammy skin or a very fast or very slow pulse.
References
- Vincent JL. Understanding cardiac output. Crit Care. 2008;12(4):174. doi:10.1186/cc6975
- Vander A, Sherman J, Luciano D. Human Physiology: The Mechanisms of Body Function. 8th ed. Boston: McGraw-Hill; 2001. Fig. 14-9 (after Chapman CB, Mitchell JH. Sci Am. 1965). Data: BioNumbers BNID 109465
This article is for general information and does not replace a consultation with a doctor. Measurements must be interpreted by a physician together with symptoms and other tests.
Published on 29 April 2024