Measurement of Cardiac Output
Carmen C. Cosio
Thomas A. Vargo
Cardiac output is an important hemodynamic variable in the assessment of cardiovascular function. Cardiac output (liters per minute) equals the product of the heart rate (beats per minute) and average effective stroke volume (liters per beat). In pediatrics, it usually is indexed to body surface area in square meters (liters per minute per square meter). If cardiovascular anatomy is normal, and the systemic and pulmonary circulations are in series, the systemic and pulmonary blood flows are equal to each other, and each equals the cardiac output. However, for some patients in the intensive care unit (ICU), the two circulations are not in series because of left-to-right or right-to-left shunting. In these cases, separate determinations of the systemic blood flow ([Q with dot above]s) and pulmonary blood flow ([Q with dot above]p) may be necessary to evaluate hemodynamic intricacies. The thorough evaluation of [Q with dot above]s and [Q with dot above]p is best performed in the cardiac catheterization laboratory.
Serial determinations of cardiac output made in the ICU can be useful in guiding fluid management, vasoactive drug therapy, and ventilator management in patients with compromised myocardial function, abnormal pulmonary or systemic vascular resistance, or elevated mean airway pressures. Several methods are available to estimate cardiac output, but only a few are applicable to the critically ill child. In an ICU patient, the methods used most frequently to estimate cardiac output are indicator-dilution methods, the Fick method, and echocardiographic volumetric flow analysis.
INDICATOR DILUTION METHODS
Thermodilution and dye-dilution techniques for determining cardiac output are indicator-dilution methods. The dye-dilution method to determine cardiac output rarely is used, even in the cardiac catheterization laboratory. The thermodilution method is the method used most commonly in an ICU to determine cardiac output. This method requires the placement of a pulmonary artery catheter with a proximal injection port and a distal thermistor for continuous temperature measurement. Thermodilution measurements are fairly accurate, reproducible, and easy to perform. The technique requires minimal subjective interpretation of collected data, and serial estimates can be obtained rapidly. The variance of cardiac output estimates by the thermodilution method range from 10% to 15%, so that the average of three to five successive measurements usually is taken to provide the best estimate of cardiac output.