232
Dynamics
of
the
Vascular
System
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
is
the right atrium and the sampling site
is
the
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
of
the balloon, the catheter
is
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.
The
faster the flow, the greater is the temperature increase.
Ganz et al.
(1
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
digital computer.
For the thermodilution technique (Li, 2000), the standard cardiac
output (CO) determination in-vivo
is
normally calculated from the
following formula:
(8.1.20)
where
Vi
=
volume of the injectate in ml
Tb, Ti
=
temperature of the blood and injectate, respectively
sb,
Si
=
specific gravity of the blood and injectate, respectively
cb,
C,
=
specific heat of blood and injectate, respectively
The ratio
of
(sici)/(sbcb) is
1.08
when 5% dextrose in water is used
as an indicator. This ratio is
1.
I0
when normal saline is used.
The indicator heat
loss
along the catheter between the site of injection
and the delivery site is accounted for by a correction factor, F,:
