21. Cardiac Output Monitoring

Question 1

What is CO

Whats it determined by

Answer 1

Cardiac output (CO) is defined as the volume of blood ejected from the left ventricle per minute.

It is determined by a number of interplaying factors including heart rate, rhythm, preload, contractility and afterload.

Circulation (both in terms of pressure and flow) is essential to organ perfusion and O2 delivery.
However, it varies significantly under different physiological extremes and disease states.

Question 2

Monitor

Ideal method

Gold standard

Answer 2

Cardiac output monitoring measures various parameters associated with the central circulation and is currently the Holy Grail of haemodynamic assessment.

Over the last decade there has been a rapid expansion in the use of CO monitoring devices
within critical care and theatre settings to aid optimisation of haemodynamic variables
and O2 delivery and to facilitate
goal-directed therapy CO monitoring is also essential to many enhanced recovery programmes.

The ideal method of measuring CO would be non-invasive, accurate, continuous, safe, easy to use and operator independent. It would provide rapid data acquisition and be cost-effective. Unfortunately, none of the currently available CO monitoring devices possesses all these properties.

Conventional thermodilution techniques using a pulmonary artery floatation catheter (PAFC) remain the clinical gold standard for accuracy in CO monitoring.

However, newer, less invasive monitoring devices that provide continuous CO data are establishing a role in haemodynamic management.

Question 3

What methods are available to measure cardiac output?

Answer 3

1
> Fick’s principle

2
> Thermal/indicator dilution techniques

3
> Doppler ultrasound

4
> Electrical bioimpedance

5
>Arterial pulse pressure contour analysis

Question 4

What is the Fick’s principle?

Answer 4

Fick 
(a nineteenth-century German physiologist, 
credited for Fick’s law of
diffusion and the 
invention of the contact lens)

identified that the uptake or release

a substance (M)

by an organ is the

product of the blood flow (Q)

through that organ and the

arteriovenous concentration difference

(A-V) of the substance in question.

M = Q × (A-V)

In essence,

Fick’s principle allows the blood flow

to an organ (or the body) to be

calculated using a

suitable marker substance

(e.g. dye, temperature or O2).

Question 5

How can you calculate CO using

the Fick’s principle?

Answer 5

> The Fick method of calculating
CO uses the Fick’s principle to measure
the CO of the pulmonary circulation.

> The A-V oxygen content difference
across the lungs is measured via
arterial and venous blood gases

(i.e. a mixed venous sample from the pulmonary artery)

and the

rate of oxygen uptake is
measured via spirometry.
V .O2 = CO × (CaO2 − CvO2)

and therefore

CO = V .O2/(CaO2 − CvO2)

Where:

VO2 . = oxygen uptake
CaO2 = arterial oxygen content
CvO2 = mixed venous oxygen content
CO = cardiac output

> In the absence of intra-pulmonary or intra-cardiac shunts, the pulmonary
blood flow is equal to systemic blood flow and thus cardiac output.

Question 6

What do you understand
by the term ‘assumed Fick
determination’?

Answer 6

The Fick’s method is extremely accurate,
but in reality it is very time consuming
and cumbersome to obtain the required measurements.

Therefore, the assumed value for
O2 consumption (250 mL/min or 125 mL/
min/m2)

is sometimes used to calculate the CO.

This is called an assumed Fick determination.

Question 7

How is the dye or indicator dilution technique used to

measure CO?

Answer 7

> A known quantity of dye 
(e.g. indocyanine green) or 
indicator substance (e.g. lithium) 
is injected into a 
central vein and then 
measured distally from
a peripheral arterial blood sample.

> A graph of concentration over
time is then plotted.

> Due to recirculation of the substance,
 a second peak known as a
‘recirculation hump’ 
is seen on the concentration–time curves 
and this
limits the total number of 
measurements that can be taken.

> The graph is therefore plotted
semi-logarithmically in order to minimise
the effect of this recirculation.

> The CO is inversely related
to the area under the curve (AUC).

> Computer algorithms use the modified
Stewart–Hamilton equation to
calculate CO.

Question 8

What is a pulmonary artery flotation catheter (PAFC)?

Answer 8

> A PAFC is a device that can be used 
to measure cardiac filling pressures,
pulmonary artery occlusion pressure, 
central venous oxygen saturations
and core temperature.

> Cardiac output data may be acquired
through thermodilution methods using the PAFC.

> Global use of the PAFC is
falling as a result of newer,
relatively less invasive methods of CO monitoring becoming available.

Nevertheless, the PAFC is an
extremely accurate method of CO monitoring
and newer monitoring devices are routinely
validated against the PAFC thermodilution technique.

Question 9

Describe the key features of the PAFC.

Answer 9

> 110 cm long, balloon-tipped,
flow-directed catheter.

> Inserted via a 5 FG
introducer sheath

> Distal lumen should be positioned
in the pulmonary artery (PA)
to allow measurement of PA pressure
and allow sampling of mixed venous blood.

> Proximal lumen is 30 cm from
distal tip and should be
positioned in the
right atrium.

> Balloon at the tip is inflated with up to
1.5 mL of air
necessary to allow the catheter to advance
with blood flow and to enable measurement of
pulmonary capillary wedge pressure).

> Cardiac output is measured using
cold thermodilution or pulsed heating
bursts (latter available only in newer catheters).

> Pulmonary capillary wedge pressure 
(PCWP) provides an indication of 
left atrial filling pressure and 
thereby 
left ventricular end-diastolic pressure
(LVEDP), 
which is used as a surrogate for
 left ventricular end-diastolic
volume (LVEDV) that represents preload.

Question 10

How is cardiac output measured using a PAFC?

Answer 10

> A thermodilution technique is used
(which is an advance on the dye
dilution technique)

where heated
or cooled fluid is
now used to replace
the dye.

This eliminates the problems of recirculation,
allowing infinite measurements to be made.

> 10 mL ice-cold 0.9% saline
(or 5% dextrose) is injected via the proximal
port of the PAFC into the right atrium,
thereby reducing the blood temperature.

> The blood temperature is then measured by a 
distal thermistor on the PAFC and a 
thermodilution curve 
( temperature change against time) 
is generated.

Question 11

Stewart–Hamilton equation

Answer 11

> Cardiac output is inversely related to the AUC and computer programmes calculate it using the Stewart–Hamilton equation:

    V(TB – T1)K1K2 Q = \_\_\_\_\_\_\_\_\_\_\_\_
        TB(t)dt

where:

Q cardiac output
V volume of injectate
TB temperature of blood
T1 injectate temperature
K1K2 computer constants
TB(t)dt change in blood temperature over time
#

> Modern PAFCs are able to provide
continuous CO data.

They contain an electric heating coil
that is positioned in the right atrium
and which heats up the blood in a semi-random manner.

The pulsed heating bursts are detected by the thermistor, and via the same Stewart–Hamilton method
CO is calculated.

Question 12

Draw a pressure trace to
illustrate the PAFC passage to
the pulmonary artery.

Answer 12

Pressure
(mmHg)
RA
1-7

RV
(25/0)

PA
(25/10)

PCWP
(4–12)

Question 13

In what circumstances does

PCWP overestimate LVEDP?

Answer 13

Any condition creating an interfering pressure gradient that does not represent function of the left ventricle:

> Mitral stenosis

> Positive end expiratory pressure (PEEP)

> Pulmonary hypertension

Question 14

in what circumstances does

PCWP underestimate LVEDP?

Answer 14

Any condition causing increased pressure within the left ventricle, which the catheter tip cannot detect:

> Poorly compliant left ventricle

> LVEDP >25 mmHg

Question 15

How is a PCWP of 25 mmHg

interpreted?

Answer 15

> The most common interpretation of an
elevated PCWP in the

assumed setting of normal juxtacardiac pressure

and normal ventricular compliance would be

that of hypervolaemia with an increased

LVEDV causing an elevated PCWP.

> If juxtacardiac pressure is increased,

as in cardiac tamponade or
constrictive pericarditis, 
the same elevated PCWP 
may be associated
with normal or reduced LVEDV.

> Another scenario is possible if
ventricular compliance is reduced

(e.g. diastolic dysfunction arising
from myocardial ischaemia),

in which case again LVEDV
may be normal or reduced
despite elevated PCWP.

> These problems delineate the basis of
arguments that are now made
against the use of PCWP
as a marker of fluid responsiveness.

Question 16

What other methods of cardiac
output measurement do you
know of?

Answer 16

Examiners will expect an understanding
of the principles behind cardiac output
monitoring and may wish to
explore the advantages and disadvantages of
some of the commoner technologies available.

Non-invasive techniques:

1 Transcutaneous Doppler

2 Transthoracic electrical bioimpedance

3 Non-invasive cardiac output monitor (NICO)

Invasive techniques:

1 Transoesophageal Doppler

2 Dye and temperature dilution techniques

3 Arterial pulse contour analysis (e.g. PiCCO and LiDCO)

Question 17

> Transcutaneous Doppler

Answer 17

> Transcutaneous Doppler
(e.g. ultrasound cardiac output monitor – USCOM)

• Based on the Doppler effect.

• Transcutaneous probe is placed
on the supra-sternal notch of the patient.

• Measures blood flow in the pulmonary artery and across the semi-lunar valves.

• Device plots velocity of
trans-valvular and
trans-pulmonary blood flow
over time.

• Programmed algorithms within
the device then calculate the CO.

• User-dependent and requires training.

Question 18

Transthoracic electrical bioimpedance

Answer 18

Transthoracic electrical bioimpedance

• Based on the principle that

during ejection of blood
from the heart in systole

there is an associated change

in electrical impedance
of the thoracic cavity

due to the increased blood volume,

the rate of change of this impedance
is a reflection of CO.

• Four dual electrodes or sensors
are placed on the neck and thorax
and a low current is passed between them.

• Current seeks the path of least resistance,
which in this case is the blood-filled aorta.

• Blood volume and velocity within
the aorta changes from beat to beat
and this equates to changes in thoracic impedance.

• Device measures the corresponding changes in impedance and relates these to CO.
• Quick to set up and easy to use.
• Useful in estimating trends in CO but not for absolute measurements.

• Studies suggest the method is
accurate in healthy volunteers,
but its reliability decreases in critically ill patients.

• Technique has not gained wide clinical acceptance.

Question 19

> Non-invasive cardiac output monitor (NICO)

Answer 19

> Non-invasive cardiac output monitor (NICO)

• Applies the Fick’s principle to CO2.

• Changes in CO2 concentration
after intermittent periods of partial
rebreathing through a
special rebreathing loop are measured.

• Sensors are placed to measure CO2,
air flow and airway pressures.

• VCO2 . is calculated from 
minute ventilation including 
its CO2 content while PaCO2 
is estimated from 
end tidal CO2 measurements.

• System relies solely on airway gas measurement.

• Calculates effective lung perfusion 
(i.e. that part of the
pulmonary capillary blood flow 
that has passed through 
ventilated parts of the lung).

• Effects of unrecognised ventilation–perfusion

inequality in patients may explain

why the results with this method

show a lack of agreement
w thermodilution techniques.

Question 20

Invasive techniques:

> Transoesophageal Doppler

Answer 20

Invasive techniques:
> Transoesophageal Doppler

• Based on the Doppler effect.

• Velocity of blood flow in the
descending thoracic aorta is measured
using a flexible ultrasound probe
placed in the mid-oesophagus.

The frequency change (Doppler shift)
measured correlates directly with the
speed of blood travelling in the aorta.

• The integral of the blood velocity–time
waveform (area under the curve)
then represents stroke distance.

• This waveform is displayed on the monitor and,
in addition to the associated
sound of the waveform,
can help in positioning the probe.

• The product of the
cross-sectional area of the aorta
and the stroke distance gives the stroke volume

(nomograms are used to
calculate diameter of the descending aorta,
based on height, age and weight).

• Provides indicator of preload
(flow time corrected to 60 beats per min)

and contractility
(peak velocity and mean acceleration).

• Systemic vascular resistance is calculated.
• Rapid data acquisition.

• Only measures blood flow in the
descending aorta and hence flow to
head, neck and upper limbs is excluded.

• Not tolerated by awake patients.
• Suffers interference from surgical diathermy.

• Operator dependent and
requires skill to align probe and
obtain good signal.

• Contraindicated in those with oesophageal varices.
• Risk of oesophageal perforation.

Question 21

> Arterial pulse contour analysis (e.g. PiCCO and LiDCO)

Answer 21

> Arterial pulse contour analysis (e.g. PiCCO and LiDCO)

• The size of the pulsatile component
of the arterial waveform

(the pulse pressure – PP) 
and area under the systolic portion of the waveform
(AUC) are 
intimately linked to SV, 
vascular compliance 
and systemic vascular resistance. 

Under steady-state conditions,
when vascular compliance and resistance are relatively constant,

SV becomes
the main determinant of PP and AUC.

If CO is measured directly
under these static conditions
using indicator or temperature dilution
techniques then a 
mathematical relationship can be established
linking PP, AUC and SV. 

This indirect estimation of CO using
parameters such as PP and AUC
obtained from arterial waveforms
is known as ‘arterial pulse contour analysis’.

• Requires insertion of an arterial line
(special femoral arterial line
required for PiCCO).

• Physiological and therapeutic changes in vessel wall diameter are assumed to reflect changes in cardiac performance – this effect is minimised by intermittent calibration.

Question 22

PICCO AND LIDCO

Calibrartion

Advantages

disadvantages

Answer 22

• PiCCO device is calibrated via a transpulmonary thermodilution technique.
• LiDCO device is calibrated via a lithium dilution technique.

• PiCCO device identifies AUC by recognising the dicrotic notch on the arterial waveform and this is used to determine SV. It is converted to an absolute number by calibration.
In addition to CO and SV,
PiCCO device also provides
‘dynamic’ indicators of
volume responsiveness:
stroke volume variation (SVV),
pulse pressure variation (PPV),
systolic pressure variation (SPV)
as well as a number of volumetric markers of
preload:
global end-diastolic volume, intrathoracic blood volume and extravascular lung water.

• LiDCO utilises an existing arterial line and tracks the power of the arterial waveform rather than the contour in order to track changes in
SV. Theoretical advantage of LiDCO is the reduction of the effect of reflected waves because the device does not need to identify specific parts of the arterial waveform.
It also provides dynamic indicators of
preload (SVV, SPV and PPV).

LiDCO calibration is not possible in the
presence of atracurium.

• Relatively minimally invasive systems, providing continuous beat-tobeat monitoring.
• Can be used in awake patients.
• Both are not reliable with arrhythmias.