The Pulmonary Artery Catheter
145
The CVP is used as a measure of the right ventricular filling pressure. The
normal range for the CVP is 0 – 5 mm Hg, and it can be a negative pres-
sure in the sitting position. The CVP is a popular measurement in critical
care, and is described in more detail in the next chapter.
Pulmonary ArteryWedge Pressure
The pulmonary artery wedge pressure (PAWP) is described earlier in the
chapter. The PAWP is a measure of left-atrial pressure (LAP), which is
equivalent to the left-ventricular end-diastolic pressure (LVEDP) when
mitral valve function is normal.
PAWP
=
LAP
=
LVEDP
(8.5)
The wedge pressure is a measure of the left ventricular filling pressure. It
is slightly higher than the CVP (to keep the foramen ovale closed), and
the normal range is 6 – 12 mm Hg.
Cardiac Index
The thermodilution cardiac output (CO) is the average stroke output of
the heart in one-minute periods. It is typically adjusted to body surface
area (BSA), and is called the
cardiac index
(CI).
CI
=
CO/BSA
(8.6)
In the average-sized adult, the cardiac index is about 60% of the cardiac
output, and the normal range is 2.4 – 4 L/min/m
2
.
Stroke Index
The heart is a stroke pump, and the stroke volume is the volume of blood
ejected in one pumping cycle. The stroke volume is equivalent to the
average stroke output of the heart per minute (the measured cardiac out-
put) divided by the heart rate (HR). When cardiac index (CI) is used, the
stroke volume is called the
stroke index
(SI).
SI
=
CI/HR
(8.7)
The stroke index is a measure of the systolic performance of the heart
during one cardiac cycle. The normal range in adults is 20 – 40 mL/m
2
.
SystemicVascular Resistance Index
The hydraulic resistance in the systemic circulation is not a measurable
quantity for a variety of reasons (e.g., resistance is flow-dependent and
varies in different regions). Instead, the systemic vascular resistance (SVR)
is a global measure of the relationship between systemic pressure and flow.
The SVR is directly related to the pressure drop from the aorta to the right
atrium (MAP – CVP), and inversely related to the cardiac output (CI).
SVRI
=
(MAP – CVP) /CI
(8.8)
The SVRI is expressed in Wood units (mm Hg/L/min/m
2
), which can
