Cardiac Output
What Is It?
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Cardiac output is the volume of blood pumped by the heart per minute.
For an average size of adult (70 kg) at rest this would
be about 5 litres/min. During severe exercise it can
increase to over 30 l/min, although not in the unfit!
Miguel Indurain ("Big Mig", who won the Tour de France
in five successive years) had a resting heart rate of
28 beats per minute and could increase his cardiac output
to 50 litres per minute and his heart rate to 220 beats
per minute. Cardiac output is often divided by body
surface area to take into account the size of the subject.
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Real-time, continuous monitoring of cardiac output and oxygen delivery. Jonas M, Hett D, Morgan J. (2002)
(View paper)
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Why Measure It?
It is frequently necessary to assess
the state of a patient's circulation. The simplest measurements,
such as heart rate and blood pressure, may be adequate
for many patients, but if there is a cardiovascular
abnormality then more detailed measurements are needed.
A common clinical problem is that of hypotension (low
blood pressure); this may occur because the cardiac
output is low and/or because of low systemic vascular
resistance (SVR). This problem can occur in a wide range
of patients, especially those in intensive care or postoperative
high dependency units. In these high risk patients more
detailed monitoring will usually be established and
will often include measuring central venous pressure
via a central venous catheter and continuous display
of arterial blood pressure via a peripheral arterial
catheter. In addition, measurement of cardiac output
can be carried out and this, together with arterial
pressure measurements, allows SVR to be calculated.
These measurements are useful both in establishing a
patient's initial cardiovascular state and in measuring
the response to various therapeutic interventions such
as transfusion, infusion of inotropic drugs, infusion
of vasoactive drugs (to increase or reduce SVR) or altering
heart rate either pharmacologically or by adjusting
pacing rate.
Methods of Measuring Cardiac Output.
Existing methods of measuring cardiac output are unsatisfactory for various reasons.
The Fick Method
Cardiac Output = oxygen consumption / arteriovenous oxygen content difference
The original
method described by Fick in 1870 is difficult to carry
out. Oxygen consumption is derived by measuring the
expired gas volume over a known time and the difference
in oxygen concentration between this expired gas and
inspired gas. Accurate collection of the gas is difficult
unless the patient has an endotracheal tube because
of leaks around a facemask or mouthpiece. Analysis of
the gas is straightforward if the inspired gas is air
but if it is oxygen enriched air there are two problems,
(a) the addition of oxygen may fluctuate and produce
an error due to the non-constancy of the inspired oxygen
concentration, and (b) it is difficult to measure small
changes in oxygen concentration at the top end of the
scale. The denominator of the equation, the arteriovenous
oxygen content difference, presents a further problem
in that the mixed venous (i.e. pulmonary arterial) oxygen
content has to be measured and therefore a pulmonary
artery catheter is needed to obtain the sample. Complications
may arise from these catheters. If carefully carried
out, the Fick method is accurate, but it is not practicable
in routine clinical practice. Several variants of the
basic method have been devised, but usually their accuracy
is less good. There are many other methods of measuring
cardiac output nowadays, but the most accurate are those
which use some form of indicator dilution.
Other Methods
Bioimpedance
- this method was described by Kubicek et al in 1966
and has recently been reviewed by Critchley (1998).
It has the advantages of providing continuous cardiac
output measurement at no risk to the patient. A small
high frequency current is passed through the thorax
from a pair of spot electrodes stuck to the skin. Sensing
electrodes are used to measure the changes in impedance
within the thorax; the normal value for an adult is
20-48 ohms at a frequency of 50-100 Hz. Contraction
of the heart produces a cyclical change in transthoracic
impedance of about 0.5%, unfortunately giving a rather
low signal to noise ratio. Although the method has been
reported to give accurate results in normal subjects,
several studies have some inaccuracy in critically ill
patients eg. Genoni et al (1998), Marik et al (1997)
and Imhoff et al (2000).
Echocardiography - transoesophageal
echocardiography (TOE) provides diagnosis and monitoring
of a variety of structural and functional abnormalities
of the heart (for review see Poelaert et al. (1998).
It can be used to derive cardiac output from measurement
of blood flow velocity by recording the Doppler shift
of ultrasound reflected form the red blood cells. The
time velocity integral, which is the integral of instantaneous
blood flow velocities during one cardiac cycle, is obtained
for the blood flow in the left ventricular outflow tract
(other sites can be used). This is multiplied by the
cross-sectional area and the heart rate to give cardiac
output. In a study of patients having coronary artery
revascularisation (Krishnamurthy et al 1997) the authors
concluded that 'undue reliance placed on the absolute
values may be unwise'. Others have compared TOE with
thermodilution and reported agreements ranging from
good (Perrino et al 1998) to poor (Estagnasie et al
1997). The main disadvantages of the method are that
a skilled operator is needed (Lefrant et al 1998), the
probe is large and therefore heavy sedation or anaesthesia
is needed, the equipment is very expensive and the probe
cannot be fixed so as to give continous cardiac output
readings without an expert user being present.
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