What is it?
Cardiac output is the volume of blood pumped by the heart per minute and is the product of the amount of blood per heart beat (called the stroke volume) times the number of heart beats in a minute (heart rate).
For an average size of adult (70 kg) at rest this flow of blood from the heart into the circulation would be about 5 litres per minute. During severe exercise it can increase to over 30 l/min, although not in the unfit! Miguel Indurain ("Big Mig"), who won the Tour de France in five successive years had a resting heart rate of 28 beats per minute and could increase his cardiac output to 50 litres per minute and his heart rate to 220 beats per minute.
Why Measure It?
It is frequently necessary to assess the state of a patient's circulation. A common clinical problem is that of hypotension (low blood pressure); this may occur because the cardiac output is low and/or because of low vascular resistance. This problem can occur in a wide range of patients, especially those in surgery, in intensive care or postoperative high dependency units. In these high risk patients more comprehensive monitoring will usually be required including arterial blood pressure and cardiac output.
The cardiac output is also one of the determinants of the amount of oxygen delivered from the lungs to the body’s tissues. Doubling the cardiac output doubles the oxygen delivered to the tissues. If cardiac output falls the oxygen delivered to organs like the brain may be inadequate and cause low levels of cellular oxygenation (hypoxia) which can cause tissue and organ failure
Continuous blood pressure, stroke volume and cardiac output measurements are crucial for:
Monitoring therapeutic interventions:
Establishing a patient's initial hemodynamic status and measuring the response to various therapeutic interventions such as: fluid transfusion and the use of inotropic and vasoactive drugs for supporting the heart and circulation.
The continuous monitoring of stroke volume, stroke volume change and stroke volume variation (SVV%) provides powerful insights into both the fluid status of the patient and the actual hemodynamic response to fluid administration in terms of blood pressure and / or cardiac output changes.
Improving Patient Outcomes:
Knowing the cardiac output and arterial oxygen content allows the calculation of continuous systemic oxygen delivery to the body. Avoiding low blood flow and low or excessive fluid administration reduces the chance of oxygen deficit building up during surgery. Avoiding surgical oxygen debt oxygen debt, or if incurred intra-operatively, ensuring the quick and complete payback of any accumulated oxygen debt has been shown to significantly reduce the incidence of complications and hospital length of stay.