Cardiac Output

What Is It?

Cardiac output is the volume of blood pumped by the heart per minute. For an average size of adult (70 kg) at rest this would be about 5 litres/min. 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. Cardiac output is often divided by body surface area to take into account the size of the subject.

Why Measure It?

It is frequently necessary to assess the state of a patient's circulation. The simplest measurements, such as heart rate and blood pressure, may be adequate for many patients, but if there is a cardiovascular abnormality then more detailed measurements are needed. 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 systemic vascular resistance (SVR). This problem can occur in a wide range of patients, especially those in intensive care or postoperative high dependency units. In these high risk patients more detailed monitoring will usually be established and will often include measuring central venous pressure via a central venous catheter and continuous display of arterial blood pressure via a peripheral arterial catheter. In addition, measurement of cardiac output can be carried out and this, together with arterial pressure measurements, allows SVR to be calculated. These measurements are useful both in establishing a patient's initial cardiovascular state and in measuring the response to various therapeutic interventions such as transfusion, infusion of inotropic drugs, infusion of vasoactive drugs (to increase or reduce SVR) or altering heart rate either pharmacologically or by adjusting pacing rate.

Methods of Measuring Cardiac Output.

Existing methods of measuring cardiac output are unsatisfactory for various reasons.

The Fick Method

Cardiac Output = oxygen consumption / arteriovenous oxygen content difference

The original method described by Fick in 1870 is difficult to carry out. Oxygen consumption is derived by measuring the expired gas volume over a known time and the difference in oxygen concentration between this expired gas and inspired gas. Accurate collection of the gas is difficult unless the patient has an endotracheal tube because of leaks around a facemask or mouthpiece. Analysis of the gas is straightforward if the inspired gas is air but if it is oxygen enriched air there are two problems, (a) the addition of oxygen may fluctuate and produce an error due to the non-constancy of the inspired oxygen concentration, and (b) it is difficult to measure small changes in oxygen concentration at the top end of the scale. The denominator of the equation, the arteriovenous oxygen content difference, presents a further problem in that the mixed venous (i.e. pulmonary arterial) oxygen content has to be measured and therefore a pulmonary artery catheter is needed to obtain the sample. Complications may arise from these catheters. If carefully carried out, the Fick method is accurate, but it is not practicable in routine clinical practice. Several variants of the basic method have been devised, but usually their accuracy is less good. There are many other methods of measuring cardiac output nowadays, but the most accurate are those which use some form of indicator dilution.

Other Methods

Bioimpedance this method was described by Kubicek et al in 1966 and has recently been reviewed by Critchley (1998). It has the advantages of providing continuous cardiac output measurement at no risk to the patient. A small high frequency current is passed through the thorax from a pair of spot electrodes stuck to the skin. Sensing electrodes are used to measure the changes in impedance within the thorax; the normal value for an adult is 20-48 ohms at a frequency of 50-100 Hz. Contraction of the heart produces a cyclical change in transthoracic impedance of about 0.5%, unfortunately giving a rather low signal to noise ratio. Although the method has been reported to give accurate results in normal subjects, several studies have some inaccuracy in critically ill patients eg. Genoni et al (1998), Marik et al (1997) and Imhoff et al (2000).

Echocardiography - transoesophageal echocardiography (TOE) provides diagnosis and monitoring of a variety of structural and functional abnormalities of the heart (for review see Poelaert et al. (1998). It can be used to derive cardiac output from measurement of blood flow velocity by recording the Doppler shift of ultrasound reflected form the red blood cells. The time velocity integral, which is the integral of instantaneous blood flow velocities during one cardiac cycle, is obtained for the blood flow in the left ventricular outflow tract (other sites can be used). This is multiplied by the cross-sectional area and the heart rate to give cardiac output. In a study of patients having coronary artery revascularisation (Krishnamurthy et al 1997) the authors concluded that 'undue reliance placed on the absolute values may be unwise'. Others have compared TOE with thermodilution and reported agreements ranging from good (Perrino et al 1998) to poor (Estagnasie et al 1997). The main disadvantages of the method are that a skilled operator is needed (Lefrant et al 1998), the probe is large and therefore heavy sedation or anaesthesia is needed, the equipment is very expensive and the probe cannot be fixed so as to give continous cardiac output readings without an expert user being present.