Tissue Oxygenation
The hemodynamic patterns just described can identify a hemodynamic
problem, but they provide no information about the impact of the prob-
lem on tissue oxygenation. The addition of the oxygen uptake (VO
2
) will
correct this shortcoming, and can help identify a state of clinical shock.
Clinical shock can be defined as a condition where tissue oxygenation is
inadequate for the needs of aerobic metabolism. Since a VO
2
that is below
normal can be used as indirect evidence of oxygen-limited aerobic
metabolism, a subnormal VO
2
can be used as indirect evidence of clinical
shock. The following example shows how the VO
2
can add to the evalu-
ation of a patient with cardiac pump failure.
Without the VO
2
measurement in Table 8.3, it is impossible to differenti-
ate compensated heart failure from cardiogenic shock. This illustrates
how oxygen transport monitoring can be used to determine the conse-
quences of hemodynamic abnormalities on systemic oxygenation.
Oxygen transport monitoring is described in more detail in Chapter 10.
A FINALWORD
Despite the wealth of physiologically relevant information provided by
the PA catheter, the catheter has been vilified and almost abandoned in
recent years because clinical studies have shown added risk with little or
no survival benefit, associated with use of the catheter (2–4). The follow-
ing points are made in support of the PA catheter.
1. First and foremost, t
he PA catheter is a monitoring device, not a therapy
.
If a PA catheter is placed to evaluate a problem and it uncovers a dis-
order that is untreatable (e.g., cardiogenic shock), the problem is not
the catheter, but a lack of effective therapy. Clinical outcomes should
be used to evaluate therapies, not measurements.
2. In addition, surveys indicate that
physicians often don’t understand the
measurements provided by PA catheters
(19,20). Any tool can be a
weapon in the wrong hands.
3. Finally, the incessant use of mortality rates to evaluate critical care
interventions is problematic because
the presumption that every inter-
vention has to save lives to be of value is flawed.
Interventions should (and
e
Heart Failure
High CVP
Low CI
High SVRI
Normal VO
2
Table 8.3
Compensated Heart Failure vs. Cardiogenic Shock
Cardiogenic Shock
High CVP
Low CI
High SVRI
Low VO
2
148
Hemodynamic Monitoring
