Measurement of Cardiac Output
Several direct and indirect techniques for measurement of cardiac output are available. The thermodilution technique uses a special thermistor-tipped catheter (Swan-Ganz catheter) that is inserted from a peripheral vein into the pulmonary artery. A cold saline solution of known temperature and volume is injected into the right atrium from a proximal catheter port. The injectate mixes with the blood as it passes through the ventricle and into the pulmonary artery, thus cooling the blood. The blood temperature is measured by a thermistor at the catheter tip, which lies within the pulmonary artery, and a computer is used to acquire the thermodilution profile and a computer calculates flow (cardiac output from the right ventricle) using the blood temperature information, and the temperature and injected volume. Injections are repeated a few times and the cardiac output averaged. Because cardiac output changes with respiration, it is important to inject the saline at a consistent time point during the respiratory cycle. In normal practice, this is done at the end of expiration.
Echocardiographic techniques and radionuclide imaging techniques can estimate real-time changes in ventricular dimensions, thus computing stroke volume, which, when multiplied by heart rate, gives cardiac output.
An old technique based on the Fick Principle can compute cardiac output (CO) indirectly from whole body oxygen consumption (VO2), and the mixed venous (CvO2) and arterial oxygen concentrations (CaO2). Although this technique is seldom used to determine CO, it can be calculated by the following equation:
CO = VO2/(CaO2 − CvO2)
To calculate CO, the oxygen contents of arterial and venous blood samples are measured, and whole body oxygen consumption is measured by analyzing expired air. The blood concentration of oxygen is expressed as mL O2/ml blood, and the VO2 is expressed in units of mL O2/min. If CaO2 and CvO2 are 0.2 mL and 0.15 mL O2/ml blood, respectively, and VO2 is 250 mL O2/minute, then CO = 5000 mL/min, or 5 L/min. Ventricular stroke volume would be the cardiac output divided by the heart rate.