VO
2
=
CI
×
(CaO
2
– CvO
2
)
(8.12)
If the CaO
2
and CvO
2
are each broken down into their component parts,
the VO
2
equation can be rewritten as:
VO
2
=
CI
×
1.3
×
Hb
×
(SaO
2
– SvO
2
)
(8.13)
(where SaO
2
and SvO
2
are the oxyhemoglobin saturations in arterial and
mixed venous blood, respectively). VO
2
is expressed as mL/min/m
2
(when the cardiac index is used instead of the cardiac output), and the nor-
mal range is shown in Table 8.1. An abnormally low VO
2
(
<
100
mL/min/m
2
) is evidence of impaired aerobic metabolism.
Oxygen Extraction Ratio
The oxygen extraction ratio (O
2
ER) is the fractional uptake of oxygen from
the systemic microcirculation, and is equivalent to the ratio of O
2
uptake
to O
2
delivery. Multiplying the ratio by 100 expresses it as a percent.
O
2
ER
=
VO
2
/ DO
2
(
×
100)
(8.14)
The O
2
ER is a measure of the balance between O
2
delivery and O
2
uptake.
It is normally about 25%, which means that 25% of the oxygen delivered
to the systemic capillaries is taken up into the tissues.
APPLICATIONS
Hemodynamic Patterns
Most hemodynamic problems can be identified by noting the pattern of
changes in three hemodynamic parameters: cardiac filling pressure (CVP
or PAWP), cardiac output, and systemic or pulmonary vascular resist-
ance. This is demonstrated in Table 8.2 using the three classic forms of
shock: hypovolemic, cardiogenic, and vasogenic. Each of these condi-
tions produces a distinct pattern of changes in the three parameters. Since
there are 3 parameters and 3 possible conditions (low, normal, or high),
there are 3
3
or 27 possible hemodynamic patterns, each representing a
distinct hemodynamic condition.
Parameter
Hypovolemic Cardiogenic Vasogenic
Shock
Shock
Shock
CVP or PAWP
Low
High
Low
Cardiac Output
Low
Low
High
Systemic Vascular
High
High
Low
Resistance
Table 8.2
Hemodynamic Patterns in Different Types of Shock
The Pulmonary Artery Catheter
147
