 |
| Fig.1.2 |
Blood flow
through the heart and great vessels has certain characteristics
that can be measured using Doppler instruments designed for medical
use. For the purpose of understanding flow patterns in the heart,
it is important to recognize the difference between laminar flow
and turbulent (or disturbed) flow. Laminar flow is flow that occurs
along smooth parallel lines in a vessel so that all the red cells
in an area are moving at approximately the same speed and in the
same direction (Fig. 1.2). Due to friction, flow is always slightly
slower near the walls of a vessel. With the pulsations of the heart,
the red cells generally accelerate and decelerate at approximately
the same speed. Flow in most of the cardiovascular system, including
the heart and great vessels, is normally laminar and rarely exceeds
the maximum velocity of 1.5 m/sec.
In contrast, turbulent or disturbed flow is present when there is
some obstruction that results in a disruption of the normal laminar
pattern. This causes the orderly movement of red blood cells to
become disorganized and produces various whirls and eddies of differing
velocities and directions. Obstruction to flow usually also results
in some increase in velocity. Thus, turbulent flow is characterized
by disordered directions of flow in combination with many different
red cell velocities. If the obstruction is significant, some of
the red blood cells may be moving at higher velocities than normal
and may reach speeds of 7 m/sec. Turbulent flow is usually an abnormal
finding and is considered indicative of some underlying cardiovascular
pathology.
 |
| Fig.1.3 |
Abnormal flows are therefore generally characterized by turbulence
and any increase in velocity. As an example, consider blood flow
in the ascending aorta during systole. If the aorta and aortic valve
are normal, then this flow is laminar. However, the presence of
a valvular stenosis will induce a turbulent flow pattern. Figure
1.3 shows that a narrowed aortic valve orifice interrupts the parallel
lines of normal laminar flow and produces turbulent flow. The resulting
jet of blood creates a short segment within the proximal aorta with
complex flow and velocity characteristics.
2004