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Chapter 15
Assessment of Cardiac Output
Mosby items and derived items © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.
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Learning Objectives
After reading this chapter you will be able to:
 Define cardiac output, cardiac index,
stroke volume, and venous return
 Describe the following regarding cardiac
output:



Method of calculation
Range of normal values
Effect of sympathetic nervous stimulation
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Learning Objectives (cont’d)

Describe regarding the distribution of blood
flow:





Effect of metabolism and reduced oxygen
availability on the regulation of blood flow
through organs
Percentage of total blood volume in venous
system
Effect of blood loss (hypovolemia) on
circulatory function
Basal distribution of blood flow to organs
versus distribution during cardiac failure
Effect of mechanical ventilation
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Learning Objectives (cont’d)

Explain the significance of the following
indicators of cardiac output:

Cardiac index, ejection fraction, stroke volume,
end-diastolic volume, cardiac work, ventricular
stroke work
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Learning Objectives (cont’d)

Describe the following regarding preload:


Definition
Values used to measure preload of the left and
right ventricles
 Factors affecting
 Clinical value of ventricular function curves
 Effect of mechanical ventilation
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Learning Objectives (cont’d)

Describe the following for afterload:
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

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Definition
Factors affecting
Measurement
Effect of vasodilators
Calculation of systemic and pulmonary
vascular resistance
Effect of mechanical ventilation
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Learning Objectives (cont’d)

Describe the following regarding
contractility:



Definition
Factors affecting
Assessment
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Learning Objectives (cont’d)

Describe the technique for obtaining
cardiac output via the following invasive
methods:




Thermodilution
Fick
Pulse contour
Doppler ultrasonic transducers
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Learning Objectives (cont’d)

Describe the noninvasive methods for
evaluating cardiac performance:



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Transthoracic electrical bioimpedance
Echocardiography
Radionuclide cardiac imaging
Partial CO2 rebreathing
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Cardiac Output (CO)

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The amount of blood pumped out of the
left ventricle in 1 minute is the CO
A product of stroke volume and heart rate
Stroke volume: amount of blood ejected
from the left ventricle with each contraction
Normal stroke volume: from 60 to 130 ml
Normal CO: from 4 to 8 L/min at rest
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Venous Return


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Amount of blood returning to the right atria
each minute is the venous return
Normally venous return is the same as CO
Venous return increases with peripheral
vasodilation and decreases with
vasoconstriction
In a healthy heart, as venous return
increases so does CO
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Measures of Cardiac Output and
Pump Function

Cardiac index (CI)

Determined by dividing the CO by body surface
area
 Normal CI is 2.5 to 4.0 L/min/m2
 CI measurement allows a standardized
interpretation of the cardiac function
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Measures of Cardiac Output and
Pump Function (cont’d)

Cardiac work

A measurement of the energy spent ejecting
blood from the ventricles against aortic and
pulmonary artery pressures
 It correlates well with the amount of oxygen
needed by the heart
 Normally cardiac work is much higher for the
left ventricle
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Measures of Cardiac Output and
Pump Function (cont’d)

Ventricular stroke work

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A measure of myocardial work per contraction
It is the product of stroke volume times the
pressure across the vascular bed
Ventricular volume

Estimated by measuring end-diastolic pressure
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Measures of Cardiac Output and
Pump Function (cont’d)

Ejection fraction

The fraction of end-diastolic volume ejected
with each systole; normally 65% to 70%; drops
with cardiac failure
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Determinants of Pump Function
1. Heart rate (HR)
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Normally not a major factor in control of CO
Extreme abnormalities can alter CO
A low HR is normally compensated for by an
increase in stroke volume (SV)
A significantly elevated HR often causes SV to
drop in people with heart disease when it
reduces filling time
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Determinants of Pump Function
(cont’d)
2. Preload


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Created by end-diastolic volume
The greater the stretch on the myocardium
prior to contraction the greater the subsequent
contraction will be
When preload is too low, SV and CO will drop
This occurs with hypovolemia
Too much stretch on the heart can also reduce
SV
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Determinants of Pump Function
(cont’d)

Ventricular function curves
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A measure of heart output is placed on the
vertical axis and end-diastolic volumes are
placed on the horizontal axis
Allows clinicians to see the changes in CO
associated with various levels of preload
In general, increases in preload will increase
SV and CO until a physiologic end limit is
reached
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Determinants of Pump Function
(cont’d)
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Determinants of Pump Function
(cont’d)

Ventricular compliance


The stiffer the left ventricle the higher the
preload needs to be to obtain an adequate SV
Reduced ventricular compliance is caused by:
• Myocardial infarction
• Shock
• Pericardial effusions
• PEEP
• Positive inotropic drugs
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Determinants of Pump Function
(cont’d)

Factors that affect venous return, preload,
and CO



Circulating blood volume
Distribution of the blood volume
Atrial contraction (adds 30% to subsequent
ventricular SV)
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Determinants of Pump Function
(cont’d)

Effect of mechanical ventilation
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Spontaneous inspiration lowers intrapleural
pressures; improves venous return and CO
Positive pressure breaths increase intrapleural
pressures and reduce venous return and CO
Degree of alteration in venous return with
positive pressure breaths depends on the lung
and chest wall compliance
Venous return maximally reduced when lung
compliance high, chest wall compliance is low
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Determinants of Pump Function
(cont’d)
3. Afterload




Two components: peripheral vascular
resistance and tension in the ventricular wall
Increases with ventricular wall distention and
peripheral vasoconstriction
As afterload increases, so does the oxygen
demand of the heart
Decreasing afterload with vasodilators may
help improve SV but can cause BP to drop if
the blood volume is low
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Determinants of Pump Function
(cont’d)

Calculating SVR and PVR

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SVR is a measure of resistance to blood flow
through the systemic circulation
SVR increases with peripheral vasoconstriction
and occurs with hypertension and use of
vasoconstrictors
PVR is a measure of pulmonary vascular
resistance and increases with pulmonary
vasoconstriction as seen in hypoxemia and
acidosis
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Determinants of Pump Function
(cont’d)
4. Contractility



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The final factor that determines pump function
A measure of myocardial contraction strength
Determined by:
• Amount of stretch on ventricle prior to contraction
• Inotropic state of the heart
Reduced with hypoxia, acidosis, electrolyte
abnormalities, and myocardial ischemia
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Methods of Measuring CO
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Invasive Techniques

Thermodilution
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Most common technique
Requires placement of a pulmonary artery
catheter and use of a computer
A cold bolus of saline injected into a proximal
port; temperature change over time measured
by a thermometer at the tip of the PAC
The degree of temperature change between
the proximal port and the distal tip is a function
of CO
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Mosby items and derived items © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.
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Invasive Techniques (cont’d)

Fick method


Based on the fact that CO can be calculated if
the oxygen consumption, the arterial oxygen
content, and the mixed venous oxygen content
are simultaneously measured
Because this technique is very difficult to use
regularly, it is not popular in the clinical setting
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Continuous Cardiac Output
Monitoring (CCO)

Useful when patient is hemodynamically
unstable and frequent measurements
needed


Indicator dilution techniques
Arterial pulse contour analysis
• Can be done with invasive or noninvasive techniques
• After calibration CO is determined by the shape of
the arterial pressure waveform
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Transtracheal, Transesophageal,
and Intravascular Ultrasound


Doppler ultrasound can be placed on tubes
or catheters to measure blood flow and CO
The transtracheal ultrasound is attached to
the end of an endotracheal tube and aimed
at the descending aorta
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Transtracheal, Transesophageal, and
Intravascular Ultrasound (cont’d)



Transesophageal ultrasound placed in the
esophagus and aimed at descending aorta
The intravascular ultrasound is placed on
the distal end of a pulmonary artery
catheter
All three techniques require a bedside
ultrasound technician and are expensive
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Periodic Noninvasive
Measurement of CO

Echocardiography

Provides periodic measurements of cardiac
performance (ejection fraction, SV, CO)
 Can also diagnose shunts and other pathology
 Air trapping in the lung can make it difficult to
image the heart
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Radionuclide Cardiac Imaging


Thallium-201 can be injected into the
coronary arteries to look for areas of poor
perfusion
At the same time, the imaging can
measure ventricular wall movement and
ejection fraction
Mosby items and derived items © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.
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Blood Pressure

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Not a good indicator of blood flow
A blood pressure cuff that examines the
arterial pulse contour to estimate CO is
available
Pulse pressure is a reflection of stroke
volume in many cases; narrow pulse
pressures = low stroke volume
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Summary

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Cardiac output monitoring a valuable part
of hemodynamic assessment in critically ill
patients
Variety of invasive and noninvasive
techniques are available
The most popular invasive technique is the
thermodilution
RTs must be knowledgeable about the
techniques used and the results obtained
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