Cardiac output

In cardiac physiology, cardiac output (CO), also known as heart output and often denoted by the symbols ${\displaystyle Q}$, ${\displaystyle {\dot {Q}}}$, or ${\displaystyle {\dot {Q}}_{c}}$,^[2] is the volumetric flow rate of the heart's pumping output: that is, the volume of blood being pumped by a single ventricle of the heart, per unit time (usually measured per minute). Cardiac output (CO) is the product of the heart rate (HR), i.e. the number of heartbeats per minute (bpm), and the stroke volume (SV), which is the volume of blood pumped from the left ventricle per beat; thus giving the formula:

${\displaystyle CO=HR\times SV}$^[3]

Values for cardiac output are usually denoted as L/min. For a healthy individual weighing 70 kg, the cardiac output at rest averages about 5 L/min; assuming a heart rate of 70 beats/min, the stroke volume would be approximately 70 mL.

Because cardiac output is related to the quantity of blood delivered to various parts of the body, it is an important component of how efficiently the heart can meet the body's demands for the maintenance of adequate tissue perfusion. Body tissues require continuous oxygen delivery which requires the sustained transport of oxygen to the tissues by systemic circulation of oxygenated blood at an adequate pressure from the left ventricle of the heart via the aorta and arteries. Oxygen delivery (DO₂ mL/min) is the resultant of blood flow (cardiac output CO) times the blood oxygen content (CaO₂). Mathematically this is calculated as follows: oxygen delivery = cardiac output × arterial oxygen content, giving the formula:

${\displaystyle D_{O2}=CO\times C_{a}O2}$^[4]

With a resting cardiac output of 5 L/min, a 'normal' oxygen delivery is around 1 L/min. The amount/percentage of the circulated oxygen consumed (VO₂) per minute through metabolism varies depending on the activity level but at rest is circa 25% of the DO₂. Physical exercise requires a higher than resting-level of oxygen consumption to support increased muscle activity. Regular aerobic exercise can induce physiological adaptations such as improved stroke volume and myocardial efficiency that increase cardiac output.^[5] In the case of heart failure, actual CO may be insufficient to support even simple activities of daily living; nor can it increase sufficiently to meet the higher metabolic demands stemming from even moderate exercise.

Cardiac output is a global blood flow parameter of interest in hemodynamics, the study of the flow of blood. The factors affecting stroke volume and heart rate also affect cardiac output. The figure at the right margin illustrates this dependency and lists some of these factors. A detailed hierarchical illustration is provided in a subsequent figure.

There are many methods of measuring CO, both invasively and non-invasively; each has advantages and drawbacks as described below.