Expansion
Fick principle, indicator dilution, thermodilution, Doppler and pulse contour analysis
Expansion
- Fick principle - oxygen consumption divided by arteriovenous oxygen difference. The reference standard, but cumbersome
- Indicator dilution - inject a known dose of dye and measure its concentration downstream; output is inversely proportional to the area under the curve
- Thermodilution - the same principle with cold saline as the indicator and a thermistor measuring the temperature change in the pulmonary artery. Requires a pulmonary artery catheter
- Oesophageal Doppler - measures descending aortic blood velocity and multiplies by cross-sectional area; minimally invasive and good at tracking trends
- Pulse contour analysis - derives stroke volume from the shape of the arterial waveform, calibrated or uncalibrated
- Echocardiography - stroke volume from the left ventricular outflow tract diameter and velocity-time integral
The falling apparent output when tricuspid regurgitation is present, and the errors introduced by shunts, are the classic limitations of thermodilution.
In current practice, the trend and the response to an intervention matter more than the absolute number.