Ganong's Review of Medical Physiology, 23rd Edition

514
SECTION VI
Cardiovascular Physiology

A popular indicator dilution technique is
thermodilution,
in which the indicator used is cold saline. The saline is
injected into the right atrium through one channel of a dou-
ble-lumen catheter, and the temperature change in the blood
is recorded in the pulmonary artery, using a thermistor in the
other, longer side of the catheter. The temperature change is
inversely proportionate to the amount of blood flowing
through the pulmonary artery; that is, to the extent that the
cold saline is diluted by blood. This technique has two impor-
tant advantages: (1) the saline is completely innocuous; and
(2) the cold is dissipated in the tissues so recirculation is not a
problem, and it is easy to make repeated determinations.

CARDIAC OUTPUT IN

VARIOUS CONDITIONS

The amount of blood pumped out of the heart per beat, the

stroke volume,

is about 70 mL from each ventricle in a resting

man of average size in the supine position. The output of the

heart per unit of time is the

cardiac output.

In a resting, supine

man, it averages about 5.0 L/min (70 mL

×

72 beats/min). There

is a correlation between resting cardiac output and body surface

area. The output per minute per square meter of body surface

(the

cardiac index

) averages 3.2 L. The effects of various condi-

tions on cardiac output are summarized in Table 31–3.

FACTORS CONTROLLING

CARDIAC OUTPUT

Predictably, changes in cardiac output that are called for by

physiologic conditions can be produced by changes in cardiac

rate or stroke volume or both (Figure 31–5). The cardiac rate is

controlled primarily by the autonomic nerves, with sympathetic

stimulation increasing the rate and parasympathetic stimula-

tion decreasing it (see Chapter 30). Stroke volume is also deter-

mined in part by neural input, with sympathetic stimuli making

the myocardial muscle fibers contract with greater strength at

any given length and parasympathetic stimuli having the oppo-

site effect. When the strength of contraction increases without

an increase in fiber length, more of the blood that normally re-

mains in the ventricles is expelled; that is, the ejection fraction

increases. The cardiac accelerator action of the catecholamines

liberated by sympathetic stimulation is referred to as their

chro-

notropic action,

whereas their effect on the strength of cardiac

contraction is called their

inotropic action.

FIGURE 31–4
Determination of cardiac output by indicator
(dye) dilution.

5.0

4.0

3.0

2.0

1.0

0.8

0.6

0.4

0.3

0.2

0.1

048 12 16 20

Time (s)

24 28 32 36

Rest

Exercise

mg/L

F=

F= flow

E= amount of indicator injected

C= instantaneous concentration of indicator in

arterial blood

In the rest example above,

Cdt

α

∫ο

E

Flow in 39 s

(time of first passage) =

5 mg injection

1.6 mg/L

(avg concentration)

Flow= 3.1 L in 39 s

Flow (cardiac output)/min= 3.1 ×=^6039 4.7 L

For the exercise example,

Flow in 9 s== 3.3 L

5 mg

1.51 mg/L

Flow/min= 3.3×=^609 22.0 L

TABLE 31–3

Effect of various conditions on

cardiac output.

Condition or Factor

a

No change Sleep

Moderate changes in environmental temperature

Increase Anxiety and excitement (50–100%)

Eating (30%)

Exercise (up to 700%)

High environmental temperature

Pregnancy

Epinephrine

Decrease Sitting or standing from lying position (20–30%)

Rapid arrhythmias

Heart disease

a

Approximate percent changes are shown in parentheses.