Biomedical Engineering Reference
In-Depth Information
in which a cuff is used to apply sufficient pressure to an artery, usually in the arm, to
prevent the flow of blood through the artery, and a stethoscope is used to listen to the
change in sounds as the cuff is slowly deflated. The first Korotkoff sounds occur when
the systolic pressure, the highest pressure reached when the ventricles contract and eject
blood, first exceeds the pressure in the cuff so blood once again flows through the artery
beneath the stethoscope. The Korotkoff sounds become muffled and disappear when the
pressure in the cuff drops below the diastolic pressure, the minimum pressure that occurs
at the end of ventricular relaxation. Another indirect measurement is the oscillometric
method, which uses a microprocessor to periodically inflate and slowly deflate a cuff. When
blood breaks through the occlusion caused by the cuff, the walls of the artery begin to
vibrate slightly due to the turbulent nature of the blood flow. The onset of these oscillations
in pressure correlates with the systolic pressure. The oscillations decrease in amplitude
over time with the diastolic pressure event corresponding to the point at which the rate
of amplitude decrease suddenly changes slope. A third indirect measurement, the ultra-
sonic method, depends on the Doppler shift of sound waves that hit red blood cells that
are flowing with the blood.
Blood in the systemic circulation leaves the heart through the aorta with an average
internal pressure of about 100 mmHg (maximum systolic pressure of about 120 mm Hg,
with a diastolic pressure of about 80 mm Hg in a normal adult) and moves to medium-sized
arteries (see Figure 3.17a) and arterioles. Arterioles lead to capillaries (average internal pres-
sure of about 30 mm Hg), which are followed by venules. Venules lead to medium-sized
veins, then to large veins, and finally to the venae cavae (average internal pressure of about
10 mm Hg), which return blood to the heart at the right atrium. Blood in the pulmonary
circulation (see Figure 3.19) leaves the pulmonary artery and moves to arterioles and then
the capillary beds within the lungs. It returns to the heart through the left atrium. Blood flow
is highest in the large arteries and veins (30-40 cm/s in the aorta; 5 cm/s in the vena cavae)
and slowest in the capillary beds (1 mm/s), where the exchange of nutrients, metabolic
wastes, gases, and hormones takes place. Pressures in the pulmonary circulation are lower
(25 mm Hg/10 mm Hg) than in the systemic circulation due to the decreased pumping
power of the smaller right ventricle as compared to the left and to the lower resistance of
blood vessels in the lungs.
EXAMPLE PROBLEM 3.8
What would be the pulse pressure and the mean arterial pressure for a person with a blood
pressure reading of 118 mmHg/79 mmHg?
Solution
The pulse pressure is defined as the difference between the systolic (118 mmHg) and diastolic
(79 mmHg) pressures, which would be 39 mmHg in this case.
Mean arterial pressure is the average blood pressure in the arteries and is estimated as the
diastolic pressure plus one-third of the pulse pressure, which would be 92 mmHg in this example.
