E. Cardiac Output Monitoring
Cardiac output monitoring can be measured by two methodologies: the
Fick method (calculated from oxygen consumption--VO 2 --divided by
arterial/venous oxygen content difference), or the thermodilution method (utilizes
the injection of cold saline into the proximal port of a pulmonary arterial catheter
and measurements from a thermistor at the end of the catheter and calculates
the area under the curve as a function of decay over time). Optimal catheter
position requires that the injection or energy coil lie within the right ventricle. As
mentioned in the arterial catheter section, new dilution methods are being
employed using systemic arterial sampling as well. In a study of outcomes of
pediatric ICU patients stratified by severity of illness, patients with pulmonary
artery catheters had higher mortality. Since then, the use of these devices has
plummeted and most clinicians have turned to central venous catheters or non-
invasive measures.
There are numerous other methods to measure cardiac output either
intermittently or continuously, but none have been regularly employed in
American ICUs. Echocardiography may calculate CO based upon aortic
dimensions and Doppler. Impedance devices can calculate aortic flow via
oscillatory changes in electrical impedance across the thorax.
F. Peripheral Stroke Volume and Cardiac Output Monitoring