excellent choices as indicators. This was demonstrated by Fronek and
Ganz (1960) in the measurement of flow in single vessels including
cardiac output by local thermodilution. The advent of thermodilution has
made cold saline and dextrose popular indicators.
In thermodilution, normal saline or isotonic dextrose (5%) in water
is used as the injectate, either at 0°C or at room temperature. The most
popular site of injection
the right atrium and the sampling site
pulmonary artery. By this choice of the sites, the effect of recirculation is
minimized. In this approach, a flow-directed balloon-tipped catheter can
be introduced into a vein and upon inflation
the balloon, the catheter
guided with the flow into the right atrium, the right ventricle or the
pulmonary artery. The thermodilution catheter typically has a thermistor
near the tip of the catheter to monitor sampling site temperature.
faster the flow, the greater is the temperature increase.
Ganz et al.
971) demonstrated this method (Fig. 8.1.12) by injecting
a 10 ml of cold (0.5-5°C) isotonic dextrose solution into the superior
vena cava of a patient with normal circulation.
The injectate was
delivered in 1-2 seconds. The area under the thermal curve was found by
planimetry which is now substituted with an analog integrator or with a
For the thermodilution technique (Li, 2000), the standard cardiac
output (CO) determination in-vivo
normally calculated from the
volume of the injectate in ml
temperature of the blood and injectate, respectively
specific gravity of the blood and injectate, respectively
specific heat of blood and injectate, respectively
when 5% dextrose in water is used
as an indicator. This ratio is
when normal saline is used.
The indicator heat
along the catheter between the site of injection
and the delivery site is accounted for by a correction factor, F,: