CHAPTER 31
The Heart as a Pump 513The major—but certainly not the only—cause of cardiac mur-
murs is disease of the heart valves. When the orifice of a valve is
narrowed
(stenosis),
blood flow through it is accelerated and
turbulent. When a valve is incompetent, blood flows through it
backward (
regurgitation
or
insufficiency
), again through a
narrow orifice that accelerates flow. The timing (systolic or dia-
stolic) of a murmur due to any particular valve (Table 31–2) can
be predicted from a knowledge of the mechanical events of the
cardiac cycle. Murmurs due to disease of a particular valve can
generally be heard best when the stethoscope is directly over the
valve. There are also other aspects of the duration, character,
accentuation, and transmission of the sound that help to locate
its origin in one valve or another. One of the loudest murmurs is
that produced when blood flows backward in diastole through a
hole in a cusp of the aortic valve. Most murmurs can be heard
only with the aid of the stethoscope, but this high-pitched musi-
cal diastolic murmur is sometimes audible to the unaided ear
several feet from the patient.
In patients with congenital interventricular septal defects,
flow from the left to the right ventricle causes a systolic mur-
mur. Soft murmurs may also be heard in patients with intera-
trial septal defects, although they are not a constant finding.
Soft systolic murmurs are also common in individuals, espe-
cially children, who have no cardiac disease. Systolic murmurs
are also heard in anemic patients as a result of the low viscosity
of the blood and associated rapid flow (see Chapter 32).
ECHOCARDIOGRAPHY
Wall movement and other aspects of cardiac function can be
evaluated by the noninvasive technique of
echocardiography.
Pulses of ultrasonic waves are emitted from a transducer that also
functions as a receiver to detect waves reflected back from various
parts of the heart. Reflections occur wherever acoustic impedance
changes, and a recording of the echoes displayed against time on
an oscilloscope provides a record of the movements of the ven-
tricular wall, septum, and valves during the cardiac cycle. When
combined with Doppler techniques, echocardiography can be
used to measure velocity and volume of flow through valves. It
has considerable clinical usefulness, particularly in evaluating and
planning therapy in patients with valvular lesions.
CARDIAC OUTPUT
METHODS OF MEASUREMENT
In experimental animals, cardiac output can be measured with
an electromagnetic flow meter placed on the ascending aorta.
Two methods of measuring output that are applicable to hu-
mans, in addition to Doppler combined with echocardiography,
are the
direct Fick method
and the
indicator dilution method.
The
Fick principle
states that the amount of a substance
taken up by an organ (or by the whole body) per unit of time
is equal to the arterial level of the substance minus the venous
level
(A-V difference)
times the blood flow. This principle
can be applied, of course, only in situations in which the arte-
rial blood is the sole source of the substance taken up. The
principle can be used to determine cardiac output by measur-
ing the amount of O
2
consumed by the body in a given period
and dividing this value by the A-V difference across the lungs.
Because systemic arterial blood has the same O
2
content in all
parts of the body, the arterial O
2
content can be measured in a
sample obtained from any convenient artery. A sample of
venous blood in the pulmonary artery is obtained by means of
a cardiac catheter. It has now become commonplace to insert
a long catheter through a forearm vein and to guide its tip into
the heart with the aid of a fluoroscope. The procedure is gen-
erally benign. Catheters can be inserted through the right
atrium and ventricle into the small branches of the pulmonary
artery. An example of the calculation of cardiac output using a
typical set of values is as follows:Output of left ventricle =
O
2
consumption (mL/min)
[A
0
2
] – [V
0
2
]= 250 mL/min
190 mL/L arterial blood –
140 mL/L venous blood in
pulmonary artery=
250 mL/min
50 mL/L
= 5 L/min
In the indicator dilution technique, a known amount of a
substance such as a dye or, more commonly, a radioactive iso-
tope is injected into an arm vein and the concentration of the
indicator in serial samples of arterial blood is determined.
The output of the heart is equal to the amount of indicator
injected divided by its average concentration in arterial blood
after a single circulation through the heart (Figure 31–4). The
indicator must, of course, be a substance that stays in the
bloodstream during the test and has no harmful or hemody-
namic effects. In practice, the log of the indicator concentra-
tion in the serial arterial samples is plotted against time as the
concentration rises, falls, and then rises again as the indicator
recirculates. The initial decline in concentration, linear on a
semilog plot, is extrapolated to the abscissa, giving the time
for first passage of the indicator through the circulation. The
cardiac output for that period is calculated (Figure 31–4) and
then converted to output per minute.TABLE 31–2
Heart murmurs.Valve Abnormality Timing of Murmur
Aortic or pulmonary Stenosis Systolic
Insufficiency Diastolic
Mitral or tricuspid Stenosis Diastolic
Insufficiency Systolic