Cardiac output (CO) can be determined by indicator dilution (typically
thermodilution), where CO is a function of the quantity of the indicator
divided by the area under the dilution curve as measured at a downstream
location. In practice, a bolus of cold fluid is injected into the
circulation in the vena cava through the pulmonary artery catheter
(PAC) and the area is measured under the temperature-change curve
in the pulmonary circulation at the PAC balloon tip. This chapter will
describe the bolus method to characterize the general technique
of thermodilution, although continuous CO measurement is increasingly
- ▪ Thermodilution: Measurement of blood flow in the circulation
based on an induced change in the heat content of blood flowing
downstream from the heat change.
- ▪ Thermistor: A temperature sensing resistor integrated into
- ▪ Bolus CO: In which a bolus of indicator (typically cold
saline) is injected into the circulation as the indicator.
- ▪ Continuous CO: In which the blood is heat in pulses upstream
of the thermistor and CO is determined by a mathematical transformation
using the heat current changes (continuous cardiac output calculation
is performed automatically by a computer attached to the pulmonary
- ▪ Fick method: An approach to calculating CO relying on assumptions
about patient systemic oxygen consumption, using the following formula:
- —Cardiac out put = ((125
mL/min oxygen × body surface
area (m2)/(arteriovenous oxygen difference)) × 100
- ▪ Manual thermodilution bolus CO:
- —A predetermined bolus (5-10 cc) of saline
of known temperature (as based on the measurement of the injectate
temperature) is injected into the central venous pressure (CVP)
port of PAC (Figures 39-1 and 39-2).
- —A CO-analysis computer characterizes the heat change
of the blood at the PAC tip as a curve (Figure 39-3), and the area
under the curve is used to calculate CO by dividing that area into
the amount of indicator, consequently:
- • Lesser area under the curve is consistent
with faster blood flow and higher CO (Figure 39-4).
- • Greater area under the curve is consistent with
slower blood flow and lower CO (Figure 39-5).
- —A series of determinations are typically averaged
together to determine CO after discarding unreliable measurements
Figure 39-1. Figure 39-2. Figure 39-3.