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 The purpose of vasoactive drugs in the ICU is to improve the mean arterial pressure (MAP) and cardiac output (CO) by affecting left ventricular contractility, volume status, and systemic vascular resistance (SVR). Vasopressors are generally indicated in the setting of circulatory shock. MAP is related to CO and SVR by the equation:

 Cardiac output is the volume of blood the heart is able to pump through the circulatory system per minute. The resistance to blood flow due to the entire systemic vasculature is known as the systemic vascular resistance, and is primarily a function of vascular smooth muscle tone. CO is directly related to heart rate (HR) and stroke volume (SV) as seen by the equation:

 Stroke volume is a function of left ventricular end‐diastolic filling pressure (preload), the resistance against which the ventricle has to eject blood during systole (afterload), and the intrinsic ability of the cardiac muscle to contract (contractility). Each of these hemodynamic factors needs to be interrogated to find the reason for circulatory shock, and will be critical in understanding how to tailor therapy to the patient’s physiology.