CLINICAL MEASUREMENT

Cardiac output measurementMahesh Prabhu

AbstractMeasurement of cardiac output provides an estimate of whole body

perfusion, oxygen delivery and ventricular function. This article aims to

provide an overview of cardiac output measurement techniques, with

an emphasis on their principles of operation and limitations. Cardiac

output can be measured in terms of volume displacement or velocity of

blood flow. Although thermodilution with a pulmonary artery catheter

has been used extensively, less invasive technologies such as Doppler

echocardiography, pulse contour analysis, impedance plethysmography

and Fick partial rebreathing are increasingly used. The advantages and

limitations of each technique are different and each technique may find

a specific niche in the armamentarium of the clinician.

Keywords Doppler; Fick principle; indicator dilution; pulse contour

analysis; thermodilution

Definition and significance

Cardiac output is the volume of blood pumped by the heart per minute, and is the product of the heart rate and stroke volume. The stroke volume of the ventricle is determined by the inter­actions between its preload, contractility and afterload.

Clinical indicators such as capillary refill, urine output, serum lactate, core–peripheral temperature gradient and level of con­sciousness are inaccurate and non­specific, especially with large variations in cardiac output. The measurement of cardiac output provides an estimate of whole­body perfusion, oxygen delivery and ventricular function. Systemic vascular resistance cannot be measured directly; however, monitoring cardiac output allows a better understanding of the causes of blood pressure variation.

Methods of measurement

The output of the ventricle is pulsatile and intermittent. By esti­mating the velocity or volume of blood through the aorta and integrating this volume with time, flow (i.e. cardiac output) can be calculated. Thus, cardiac output can be measured in terms of volume displacement or velocity. ‘Gold standard’ techniques for

Mahesh Prabhu, FRCA, is Consultant Cardiothoracic Anaesthetist at

the Freeman Hospital, Newcastle upon Tyne. He qualified from the

University of Bombay, India, and trained at Cambridge, Peterborough

and Newcastle upon Tyne. His interests include basic science,

cardiothoracic anaesthesia and intensive care.

ANAESTHESIA AND INTENSIVE CARE MEDICINE 8:2 6

cardiac output measurement, such as aortic electromagnetic or ultrasound transit­time flowmetry, are highly invasive (Table 1).

Measurement of volume displacementThe Fick principle states that the total uptake or release of a substance by an organ is the product of the blood flow through that organ and the arteriovenous concentration difference of the substance. Thus, oxygen uptake in the lungs is the product of cardiac output and the difference in oxygen concentration in the arterial and venous blood in the lungs.

The Fick principle method is limited by invasive monitoring and the need for steady­state respiratory and haemodynamic conditions. However, portable metabolic monitors have been adapted for paediatric use.

Indicator dilution techniques: an indicator, when mixed com­pletely into a unit volume of constantly flowing blood, can be used to identify the volume of blood displaced in a given time provided that the indicator remains in the system between the two points of injection and measurement.

Dye dilution – an inert dye such as indocyanin green (ICG) is injected into the central or peripheral vein. Arterial concentra­tion is measured using a calibrated cuvette densitometer. Plot­ting the concentration of dye against time on a semi­logarithmic scale and extrapolating the primary curve with exponential decay will create an indicator dilution curve (Figure 1). Recircula­tion of the dye elevates the baseline and limits the number of measurements. Pulse dye densitometry technique uses a pulse dye densitogram analyser with a finger or nose probe. To elimi­nate re­circulation, the device automatically calibrates to zero ICG plasma concentration when a new measurement starts. A dye densitogram is displayed on a computer screen, and cardiac output can be derived from the area under the curve.

Thermodilution – is an intermittent technique, which is widely accepted as the clinical reference technique. An injection of cold injectate into the right atrium causes a decrease in blood temperature, which is measured by a thermistor in the pulmon­ary artery. The decrease in temperature is inversely proportional to the dilution of the injectate (i.e. cardiac output). The cardiac output can be derived from the modified Stewart­Hamilton con­servation of heat equation. The pulmonary artery catheter (PAC) is attached to a cardiac output computer that displays a curve and calculates the output and derived indices automatically. The accuracy of this technique can be improved by injecting boluses of 10 ml ice­cold saline at a constant, consistent rate using a closed­loop injectate system throughout the ventilatory cycle for 2 minutes and rejecting any uneven curves. Since there is no recirculation peak or elevation of baseline, the measurements can be repeated.

Factors that may cause this technique to be inaccurate include intracardiac shunts, tricuspid regurgitation, cardiac arrhythmias, abnormal respiratory pattern and low cardiac output. Other drawbacks include invasiveness of the PAC, the concomitant morbidity and mortality and possible lack of cost­effectiveness.

Continuous cardiac output (CCO) catheters work on the basis of the thermodilution principle, with a randomly pulsed heat­ing coil placed in the right ventricle. The temperature change is detected in the pulmonary artery by a rapid­response thermistor and a computer produces a continuously updated, time­averaged

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CLINICAL MEASUREMENT

Comparison of methods of cardiac output measurement

Method Invasiveness Continuous

measurement

Validated by

trials in all

groups of

patients

Accuracy

(compared with

thermodilution

intermittent bolus)

Skills

required

Limitations

Thermodilution

intermittent bolus

Pulmonary artery,

central venous

canulation

No Yes Considered ‘gold

standard’

Cognitive,

technical skills

Intracardiac shunts,

arrhythmias, abnormal

respiratory pattern

Thermodilution

continuous

Pulmonary artery,

central venous

cannulation

Yes Yes Overestimates

by 4.3%

Cognitive,

technical

skills

As above

Pulse dye

densitometry

Central venous

canulation

No No Underestimates

(bias 0.42 litre/min,

LOA ± 1.91 litre/min)

Iodine allergy,

presence of an aortic

counterpulsation

device, severe renal

disease or liver

dysfunction

Lithium dilution Arterial, central

venous/peripheral

cannulation

No Yes Underestimates

by 5%

Cognitive Intracardiac shunts,

cannot be used in

patients on lithium

therapy

Oesophageal

Doppler

Oesophagus Yes Yes Underestimates by

10–15% (bias 0.24

litre/min, LOA +2 to

–1.5 litre/min)

Positioning

of probe

User dependent,

needs sedation,

physiological

assumptions

Transoesophageal

echo

Oesophagus No Yes Underestimates

by 7% (bias 0.5 ±

1 litre/min)

Specialist

expertise

User dependent,

needs sedation

PulseCO Arterial Yes No Cognitive Needs calibration,

dependent on

waveform

Pulse waveform

cardiac output

Arterial, central

venous cannulation

Yes Yes Overestimates

by 6% (bias 0.38

litre/min)

Hemodynamic

instability, severe

arrhythmia and

major changes in

SVR may affect

accuracy

Inert gas

throughflow

Double-lumen

endobronchial

intubation

Yes Yes Underestimates

by 5.2% (bias 0.68

litre/min, LOA +1.18

to –1.48 litre/min)

Double-lumen

intubation

needed

Lung separation is

vital

Impedance

plethysmography

Non-invasive Yes No Variable Electrocautery,

mechanical

ventilation and

surgical manipulation,

perioperative fluid

shifts

Non-invasive

cardiac output

measurement

Endotracheal

intubation, arterial

cannulation

Yes No Overestimates

by 10% (bias ± 1.8

litre/min)

Controlled

mechanical

ventilation

Stable CO2

elimination essential,

not real-time, poor

correlation in lung

disease

LOA, limits of agreement; SVR, systemic vascular resistance

Table 1

ANAESTHESIA AND INTENSIVE CARE MEDICINE 8:2 64 © 2007 Elsevier Ltd. All rights reserved.

CLINICAL MEASUREMENT

(every 3–6 minutes) value for cardiac output. This reduces work­load for healthcare staff and also decreases procedural complica­tions. This system is not truly continuous as it takes a variable time to update.

Lithium dilution – is an invasive technique, and is based on the principle of indicator dilution. Lithium is safe, non­toxic, rapidly redistributed and does not have any first­pass effect. Lithium chloride (0.002–0.004 mmol/kg, maximum cumulative dose 3 mmol) is injected into a central or peripheral vein. Arte­rial plasma concentrations are measured by withdrawing blood across a lithium­selective electrode at a rate of 4 ml/min. Cardiac output can be calculated from lithium dose and the area under the primary concentration–time curve before re­circulation. This technique cannot be used in patients on lithium therapy or atracurium.

Inert gas throughflow: this technique measures continuous cardiac output using differential Fick method. A double­lumen endobronchial tube is used to achieve functional separation of the two lungs. Nitrous oxide is administered exclusively to one lung. The continuous transfer of inert gas from one lung to the other through the circulation is termed ‘through flow’. Simulta­neous measurement of fresh gas and mixed expiratory concentra­tion in each lung allows pulmonary blood flow to be calculated. Smooth, synchronous ventilation of both lungs as well as correct placement of the double­lumen tube is essential for accuracy.

Non-invasive cardiac output measurement (NICO): this tech­nique, also known as differential carbon dioxide Fick partial rebreathing method, derives pulmonary capillary blood flow from the ratio of change in carbon dioxide elimination to change in end­tidal carbon dioxide (EtCO2), in response to a brief period of partial rebreathing. The production of carbon dioxide (VCO2) is calculated from minute ventilation and the instantaneous

Indicator dilution curve

Source: Hinds C J, Watson D. Intensive care. Philadelphia:

W B Saunders, 1997.

Time

1

1

2

3

4

5

2

4

3

5

Transit timeInjection

Log

co

nce

ntr

ati

on

of

dye

Primary curve

Recirculation peak

Extrapolation of primary decay curve

Elevation of baseline secondary to circulation of dye

Appearance time

Area under

the curve

Figure 1

ANAESTHESIA AND INTENSIVE CARE MEDICINE 8:2 6

carbon dioxide content, and the arterial carbon dioxide content is estimated from EtCO2. Cardiac output is proportional to the change in carbon dioxide elimination divided by the change in EtCO2 resulting from rebreathing. This technique assumes that the mixed venous CO2 content and dead­space are constant. An intrapulmonary shunt correction factor derived from fractional inspired oxygen concentration and pulse oximetry is added to the final equation. The NICO monitor should be used only whilst the patient is under fully controlled mechanical ventilation. Arterial blood samples are required for shunt estimation.

Measurement of blood velocity and flowDoppler: with the Doppler technique blood flow measurements are usually taken at the aorta, from either the transthoracic or transoesophageal approach. Blood velocity is calculated from the frequency shift of reflected ultrasound waves using the Doppler principle. The ultrasonic beam (continuous wave or pulse wave Doppler) is positioned parallel to the direction of blood flow to display the maximum blood velocity signal. The cross­sectional area (CSA) of the aorta is measured or computed. The integral of velocity versus time (i.e. velocity time integral (VTI)) during a cardiac cycle, the CSA and a correcting factor based on anatomi­cal and mathematical assumptions is used to calculate stroke volume (stroke distance) and cardiac output. Limitations of this technique include incorrect probe positioning and the possibility of having to anaesthetize the patient.

Echocardiography allows the measurement of cardiac output by two­dimensional imaging (measurement of ventricular volumes) or Doppler. The velocity of blood flow across the aortic/mitral valve or the left ventricular outflow tract is measured with a pulse­wave Doppler. With two­dimensional echocardiography, the CSA of the particular structure is determined. Stroke volume (SV) can be determined using the CSA and the stroke distance travelled by the column of blood at a given anatomicial site.

SV = VTI × CSA

Echocardiography can also be used to define indices of dia­stolic dysfunction. Two­dimensional echocardiography usually underestimates volumes of the left ventricle (LV). Transoesopha­geal echocardiography is repeatable but is user dependent and requires the patient to be anaesthetized.

PulseCO haemodynamic monitor: the PulseCO system (LiDCO Ltd) provides beat­to­beat calculation of cardiac output by analysis of peripheral arterial pulse waveform using an auto­correlation algorithm to convert the analogue pressure waveform into nomi­nal stroke volume. Changes in blood volume in the arterial tree are measured over the range of arterial distensibility. This provides an estimate of the volume of blood flowing out of the arterial circuit during a period nearly equal to diastole. This volume is equivalent to stroke volume. This technique is comparable to thermodilution and provides an early warning of haemodynamic changes. However, it needs calibration by the lithium dilution technique and may be limited by damping of the waveform or fluctuations in temperature and vascular resistance.

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CLINICAL MEASUREMENT

Pulse contour cardiac output: cardiac output is computed by analysis of the area under the systolic portion of the arterial pressure waveform, from the end­diastole to the end of the ejection phase; this corresponds to stroke volume. A continuous pulse waveform contour analysis is obtained using a long arterial catheter (with thermistor) placed in the femoral, axillary or brachial artery and connected to a pulse contour device (PiCCO, PULSION Medical Systems). A beat­to­beat analysis of cardiac output, averaged for 30 seconds, is displayed. Central venous can­nulation is required for calibration using transcardiopulmonary thermodilution technique. Changes in aortic impedance caused by haemodynamic instability, severe arrhythmia, and major changes in systemic vascular resistance may interfere with accuracy.

Impedance plethysmography: measurement of changes in trans­thoracic electrical impedance due to ejection of blood into the ascending aorta during each cardiac cycle allows the assessment of stroke volume. An alternating current of low amplitude and high frequency is introduced and simultaneously sensed by two sets of electrodes placed around the neck and thorax. By using mathematical models, alterations in thoracic impedance can be used to derive stroke volume and cardiac output.

ANAESTHESIA AND INTENSIVE CARE MEDICINE 8:2

Electrocautery, mechanical ventilation, surgical manipula­tion, perioperative fluid shifts and pulmonary oedema can cause distortion of impedance signal. Cardiac output measurement is unpredictable in a perioperative and critical care environment. ◆

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66 © 2007 Elsevier Ltd. All rights reserved.