Cardiac output

Major factors influencing cardiac output – heart rate and stroke volume, both of which are variable.[1]

In cardiac physiology, cardiac output (CO), also known as heart output and often denoted by the symbols , , or ,[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:

[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:

[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. 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.

Trend of central venous pressure as a consequence of variations in cardiac output. The three functions indicate the trend in physiological conditions (in the centre), in those of decreased preload (e.g. in hemorrhage, bottom curve) and in those of increased preload (e.g. following transfusion, top curve).
  1. ^ Betts JG (2013). Anatomy & physiology. OpenStax College, Rice University. pp. 787–846. ISBN 978-1938168130. Archived from the original on 23 February 2022. Retrieved 11 August 2014.
  2. ^ Kenyon, Anna; Williams, David; Adamson, Dawn (10 June 2010). "Physiology". Basic Science in Obstetrics and Gynaecology. Elsevier. pp. 173–230. doi:10.1016/b978-0-443-10281-3.00014-2. ISBN 978-0-443-10281-3. OCLC 1023146175. Archived from the original on 30 June 2022. Retrieved 23 February 2022. edited by Catherine E. Williamson, Phillip Bennett
  3. ^ OpenStax (6 March 2013). "Cardiac Physiology". BC Open Textbooks – Open Textbooks Adapted and Created by BC Faculty. Archived from the original on 6 November 2021. Retrieved 7 April 2020.
  4. ^ Dunn, J.-Oc; Mythen, M. G.; Grocott, M. P. (1 October 2016). "Physiology of Oxygen Transport". BJA Education. 16 (10): 341–48. doi:10.1093/bjaed/mkw012. ISSN 2058-5349. Archived from the original on 23 February 2022.