Cardiac output, expressed in liters/minute, is the amount of blood the heart pumps in 1 minute. Cardiac output is logically equal to the product of the stroke volume and the number of beats per minute (heart rate). Easy enough, one may think, but the term cardiac in cardiac output is potentially misleading – with clinician’s sometimes assuming that to interpret cardiac output they must focus on the heart. The heart is just one part of the much larger cardiovascular system, however, and the amount of blood it pumps is dependent on both cardiac and extracardiac factors.

Although most clinicians should/will be able to recite the four determinants of cardiac output – heart rate, contractility, preload, and afterload – understanding of the applicability and practical relevance of each of these four components is all too often less well ingrained.

Heart rate

The heart rate is perhaps the simplest determinant of cardiac output to visualize: the faster the heart beats, the more blood can be pumped over a particular period of time. if the heart rate is too slow, usually easily identified as part of a severe bradyarrhythmia, or is too fast, then cardiac output can be impaired. Acute supraventricular or ventricular tachycardia may also be a cause of low cardiac output, and even of cardiogenic shock.

Contractility

Impaired contractility, will reduce cardiac output; however, too much effort will result in fatigue, sometimes leading to a complete collapse, with the need to slow down substantially or even to stop.

Preload

Preload is the degree of myocardial distension prior to shortening. An intrinsic property of myocardial cells is that the force of their contraction depends on the length to which they are stretched: the greater the stretch (within certain limits), the greater the force of contraction. An increase in the distension of the ventricle will therefore result in an increase in the force of contraction, which will increase cardiac output.

Preload largely depends on the amount of ventricular filling. It should not, however, be confused with the venous return. The amount of blood returning to the heart in any period of time must be equal to the amount of blood pumped by the heart in the same period, as there is no place for storage of blood in the heart. Venous return therefore equals cardiac output, whereas preload is only one component of cardiac output.

Afterload

Afterload is the force against which the ventricles must act in order to eject blood, and is largely dependent on the arterial blood pressure and vascular tone. Reducing afterload can increase cardiac output, especially in conditions where contractility is impaired.