be multiplied by 80 to obtain more conventional units of resistance
(dynes•sec
- 1
•cm
-5
/m
2
), but this conversion offers no advantage (18).
PulmonaryVascular Resistance Index
The pulmonary vascular resistance (PVR) has the same limitations as
mentioned for the systemic vascular resistance. The PVR is a global
measure of the relationship between pressure and flow in the lungs, and
is derived as the pressure drop from the pulmonary artery to the left atri-
um, divided by the cardiac output. Because the pulmonary artery wedge
pressure (PAWP) is equivalent to the left atrial pressure, the pressure gra-
dient across the lungs can be expressed as the difference between the
mean pulmonary artery pressure and the wedge pressure (PAP – PAWP).
PVRI
=
(PAP – PAWP)/CI
(8.9)
Like the SVRI, the PVRI is expressed in Wood units (mm Hg/L/min/m
2
),
which can be multiplied by 80 to obtain more conventional units of resist-
ance (dynes•sec
-1
•cm
- 5
/m
2
).
OxygenTransport Parameters
The oxygen transport parameters provide a global (whole body) measure
of oxygen supply and oxygen consumption. These parameters are de-
scribed in detail in Chapter 10 and are presented only briefly here.
Oxygen Delivery
The rate of oxygen transport in arterial blood is called the
oxygen delivery
(DO
2
), and is the product of the cardiac output (or CI) and the oxygen
concentration in arterial blood (CaO
2
).
DO
2
=
CI
×
CaO
2
(8.10)
The O
2
concentration in arterial blood (CaO
2
) is a function of the hemo-
globin concentration (Hb) and the percent saturation of hemoglobin with
oxygen (SaO
2
): CaO
2
=
1.3
×
Hb
×
SaO
2
. Therefore, the DO
2
equation can
be rewritten as:
DO
2
=
CI
×
(1.3
×
Hb
×
SaO
2
)
(8.11)
DO
2
is expressed as mL/min/m
2
(if the cardiac index is used instead of the
cardiac output), and the normal range is shown in table 8.1.
Oxygen Uptake
Oxygen uptake (VO
2
), also called oxygen consumption, is the rate at
which oxygen is taken up from the systemic capillaries into the tissues.
The VO
2
is calculated as the product of the cardiac output (or CI) and the
difference in oxygen concentration between arterial and venous blood
(CaO
2
– CvO
2
). The venous blood in this instance is “mixed” venous
blood in the pulmonary artery.
146
Hemodynamic Monitoring
