Section Six:
Cardiac Output Determination
Cardiac output is the volume of blood ejected
from the heart per minute. It is expressed in
liters per minute. The normal cardiac output is
4 – 8 L/minute. Cardiac output is a function of
heart rate and stroke volume. Stroke volume is
the volume of blood (ml) ejected with each
ventricular contraction, and heart beat.
CO = HR x SV
Determinants of stroke volume, therefore
cardiac output, are determined by the volume of
blood in the ventricle at the end of diastole,
impedance to flow from the heart, and the
contractile ability of the myocardium. The left
ventricle must generate enough pressure in
systole to overcome aortic pressure and
systemic vascular resistance (SVR) and eject
sufficient blood volume to perfuse the organs of
the body.
CO = HR x SV
Preload Contractility Afterload
The measurement of cardiac output and
assessment of its determinants are important
adjuncts to the care of the critically ill patient.
Routine evaluation of cardiac output is essential
when technology such as a PA catheter is used.
Methods
Several methods exist to measure cardiac
output. Historically the “gold standard” has been
the Fick method, originally developed in the
1800s by Adolf Fick. The Fick method uses the
difference between arterial and venous
oxygenation, oxygen consumption, and CO2
production measured by spirometry to determine
the cardiac output.
BSA x 125cc x 10
(arterial O2 sat – mixed venous O2 sat) x 1.34 x
Hgb
The most common method to measure
cardiac output is by thermodilution. The
indicator is cold or room temperature solution
injected into the RA port of the PA catheter. The
thermistor near the end of the catheter
continuously measures the temperature of blood
flowing past it. The temperature curve is
generated by the rate of change in blood
temperature after indicator injection. Based on
this curve, a cardiac output is calculated by the
computer.
Procedure for Intermittent Thermodilution
Cardiac Output Measurement
Prior to beginning the procedure, explain it to
the patient, instructing the patient not to speak
or move during the injections. Ensure patient is
comfortable, in a supine position with the head
of the bed 20 or less. A computation
constant, based on the catheter size and
volume of injectate, is set on the computer. The
injectate solution used for the procedure is
sterile D5W (preferred) or normal saline. Five or
10 mL (more common) of solution is drawn into
the syringe, which is attached to the RA port by
a stopcock. Iced or room temperature solution
may be used. Iced solution may be necessary
in hypothermic patients and may improve
accuracy in very low cardiac output states.
When the injectate syringe is filled, the
computer is activated and signals the time for
injection. The injectate must be given quickly, in
less than 4 seconds, and injected smoothly.
When injected, the solution passes a
temperature probe in the system, through the
right atrium and right ventricle, and past the
thermistor at the tip of the PA catheter. The
change in temperature sensed by the thermistor
generates a cardiac output curve based on the
time taken for the bolus to travel through the
circulation and a CO calculation is make by the
computer.
The average of 3 cardiac outputs within 10%
of the median value is required to obtain a final
measurement. (AACN guideline) More than 3
measurements may need to be made before 3
within 10% of the median, each with a normal
curve are obtained. The values that are not
acceptable should be deleted. If unable to
obtain 3 CO values within 10% of the median,
51
Section Six – Cardiac Output Determination
with a normal waveform curve, then the Fick
method should be used. Review the summary
of AACN recommendations for cardiac output
determination on page 74.
Interpretation of Cardiac Output
Curves
Many bedside monitors and cardiac output
computers are equipped with a strip recorder or
modality to visualize the cardiac output curves.
A normal cardiac output curve has a smooth
upstroke and then a gradual decline (Fig.15).
High cardiac outputs generate a smaller curve
with a steeper upstroke and decline than curves
associated with a low cardiac output. Cardiac
output measurements associated with abnormal
curves are eliminated from the averaging
process.
Figure 15. Normal and abnormal CO curves.
Once the cardiac output values have been
edited and stored in the bedside monitor, most
systems then display the average cardiac output
obtained. In order for the computer to calculate
a complete hemodynamic profile, the following
values should be entered at the time the cardiac
output was done:

Blood pressure

Heart rate

CVP

Pulmonary artery pressure

PCWP

Height

Weight

SvO2 (if oxygenation parameters are
desired)
Section Six – Cardiac Output Determination
52
Determinants of Cardiac Output
Normal cardiac output at rest is considered to
be 4 to 8 L/min. The measurement may also be
calculated to reflect body size and is termed
cardiac index (CI). The CI is obtained by
dividing cardiac output by the patient’s body
surface area. Body surface area (BSA) can be
determined by the use of a special chart or
automatically calculated when the height and
weight are entered into the bedside
monitor/computer. The normal CI is 2.5 to 4
L/min/m2. It is a more meaningful value than
CO because size is considered.
Cardiac output or index is always evaluated
by analysis of its determinants, heart rate and
stroke volume. Tachycardia may initially
increase cardiac output, but further increases in
heart rate may reduce cardiac output as a result
of shortened diastole and decreased filling time
of the ventricles. Bradycardia is deemed
symptomatic when it causes cardiac output and
blood pressure to fall.
Preload, afterload, and contractility influence
stroke volume (SV), the volume of blood
ejected by the ventricles with each ventricular
contraction. The normal stroke volume is 50 –
100 mL/beat, the individual normal based on the
size of the person. The stroke index (SI) takes
the body surface area into account and is, as CI
is, a more meaningful number. The normal SI is
25 – 45 mL/m2.
Left ventricular preload is assessed by the
PCWP measurement.
Afterload
Pulmonary vascular resistance (PVR) or the
systemic vascular resistance (SVR) influences
afterload, which is impedance to ejection of
blood from the ventricles. Function of the
pulmonic and aortic valves may also effect PVR
or SVR. PVR is used to assess right ventricular
afterload. SVR is used to assess left ventricular
afterload. The MAP, CVP and CO are needed
in order to calculate the SVR which is calculated
automatically by most monitoring systems.
Contractility
Contractility is an inherent property of the
heart. It is not affected by end-diastolic volume
and cannot be directly measured. Sometimes
stroke work index for both the left and right
ventricles are used to assess contractility.
Myocardial contractility is influenced by the
balance of cardiac oxygen supply and demand,
and certain electrolytes and minerals such as
calcium.
See Table 3 for a summary of normal
measured and derived hemodynamic
parameters. Analysis of these parameters will
be discussed in more detail in section seven.
Review material from this section by
completing the Self-Test that follows, and then
compare your answers with those given.
Preload
Preload is the volume of blood in the
ventricles at the end of diastole. Within
physiological limits, increases in end-diastolic
volume cause stretch of the myofibrils and
increases the force of ventricular contraction.
Preload is primarily influenced by total blood
volume. Because the PA catheter measures
pressure, not volume, assumptions are made
that volume and pressure can be equated.
Although many factors alter the pressure-volume
relationship, pressures are used to evaluate the
adequacy of end-diastolic volume. Right
ventricular preload is assessed using the RAP.
Section Six – Cardiac Output Determination
53
Parameter
Normal
Formula
Heart rate (HR)
60 – 100 beats/minute
Direct measurement
Blood pressure (B/P)
90/60 – 140/90 mm Hg
Direct measurement
Mean arterial pressure (MAP)
70 – 105 mm Hg
Cardiac output (CO)
4 – 8 L/min
Cardiac index (CI)
2.5 – 4.0 L/min/m2
Stroke volume (SV)
50 – 100 ml/beat
CO  HR x 1000
Stroke index (SI)
25 – 45 ml/beat/m2
CI  HR x 1000
Right atrial pressure (RAP)
2 – 6 mm Hg
SBP – DBP  3 + DBP
Direct measurement
CO  BSA
Direct measurement
3 – 8 cm H2O
Pulmonary artery pressure (PAP)
20/8 – 30/15 mm Hg
Direct measurement
Mean: < 20 mm Hg
Pulmonary capillary wedge pressure
(PCWP)
8 – 12 mm Hg (Although varies
depending on the LV function)
Systemic vascular resistance (SVR)
900 – 1400 dynes/sec/cm
Pulmonary vascular resistance (PVR)
50 – 250 dynes/sec/cm
MPAP – PCWP  CO x 80
Left ventricular stroke work index
(LVSWI)
40 – 65 gm-m/m2/beat
(MAP – PCWP) x SI x
0.0136
Coronary artery perfusion pressure
(CAPP)
60 – 80 mm Hg
Mixed venous oxygen saturation (SvO2)
60% - 80%
Direct measurement
Arterial oxygen saturation (SaO2)
95% - 99% on room air
Direct measurement
Arterial oxygen content (CaO2)
12 – 16 ml/dl
Oxygen delivery (DO2)
900 – 1100 ml/min
CaO2 x CO x 10
Oxygen consumption (VO2)
225 – 275 ml/min
(SaO2 – SvO2) x hgb x
13.9 x CO
Direct measurement
MAP – CVP  CO x 80
DBP - PCWP
(Hgb x 1.39 x SaO2) +
(PaO2 x 0.003)
Table 3. Hemodynamic parameters and normal values.
Section Six – Cardiac Output Determination
54
Section Six:
Cardiac Output Determination
Self-Test
Number the following actions in the correct sequence in performing a cardiac output
procedure:
___ Enter all hemodynamic values needed for hemodynamic profile.
___ Edit values not within 10%.
___ Inject injectate smoothly within 4 seconds.
___ Pull injectate into syringe.
___ Assess the profile and intervene as needed.
___ Explain procedure to patient.
___ Average CO values and store in monitor/computer.
___ Connect cable to monitor and set the computation constant according to type of catheter in
use.
___ Attach syringe to the proximal port of the PA catheter.
___ Run a CO waveform strip.
You have just completed a cardiac output procedure and obtained the following values
(curves are normal). Place an M next to the median value and an X next to the one(s) that
you would delete before averaging acceptable values.
___ 2.3 L/min
___2.9 L/min
___ 2.5 L/min
___ 2.1 L/min
___ 2.2 L/min
Match the term with the correct definition:
___ Right ventricular preload
a) SVR
___ Right ventricular afterload
b) CO
___ Left ventricular preload
c) CVP
___ Left ventricular afterload
d) PCWP
___ Milliliters per beat the left ventricle ejects
e) PVR
___ Liters per minute pumped by the heart
f) SV
Section Six – Cardiac Output Determination
55
Fill in blank column below:
Parameter
Normal Value (fill in)
CVP
PCWP
CO
CI
SV
SI
SVR
SVO2
Section Six – Cardiac Output Determination
56
Self-Test – Answers Section Six:
Cardiac Output Determination
Number the following actions in the correct sequence in performing a cardiac output
procedure:
9
Enter all hemodynamic values needed for hemodynamic profile.
7
Edit values not within 10%.
5
Inject injectate smoothly within 4 seconds.
4
Pull injectate into syringe.
10
Assess the profile and intervene as needed.
1
Explain procedure to patient.
8
Average CO values and store in monitor/computer.
2or3 Connect cable to monitor and set the computation constant according to type of catheter in
use.
2or3 Attach syringe to the proximal port of the PA catheter.
6
Run a CO waveform strip.
You have just completed a cardiac output procedure and obtained the following values
(curves are normal). Place an M next to the median value and an X next to the one(s) that
you would delete before averaging acceptable values.
M 2.3 L/min
X 2.9 L/min
___ 2.5 L/min
___ 2.1 L/min
___ 2.2 L/min
Match the term with the correct definition:
C Right ventricular preload
a) SVR
E Right ventricular afterload
b) CO
D Left ventricular preload
c) CVP
A Left ventricular afterload
d) PCWP
F Milliliters per beat the left ventricle ejects
e) PVR
B Liters per minute pumped by the heart
f) SV
Section Six – Cardiac Output Determination
57
Fill in blank column below:
Parameter
Normal Value (fill in)
CVP
2 – 6 mm Hg
PCWP
8 – 12 mm Hg
CO
4 – 8 L/min
CI
2.5 – 4.0 L/m2/min
SV
60 – 80 ml/beat
SI
35 – 50 ml/beat/m2
SVR
900 – 1400 dynes/sec/cm-5
SVO2
60 – 80 %
Section Six – Cardiac Output Determination
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Section Six – Cardiac Output Determination
59