Ultrasonography is a valuable and widely available technique
for evaluating the patient with suspected heart disease. Standard 2-dimensional
and Doppler echocardiography play a major role in defining cardiac anatomy,
assessing ventricular function, and detecting abnormal flow patterns
associated with cardiac disease. In many instances, the diagnostic
accuracy of echocardiography is equivalent to that of cardiac catheterization,1 and
has dramatically decreased the need for diagnostic catheterization
preoperatively. In addition to transthoracic echocardiography, other
modalities such as fetal, transesophageal, and stress echocardiography
are widely used. It is the goal of this chapter to acquaint the
reader with the basic concepts, as well as the indications for each
type of echocardiographic examination.
Ultrasound was first used during World War II, when the technique of
sending sound waves through water and observing the returning echoes
to identify submarines was widely used. After the war, medical investigators
transferred this technology for use in medical diagnosis. The major
application of diagnostic ultrasound has been imaging of tissue
structures. Imaging modalities of ultrasound include M-mode echocardiography and
M-mode echocardiography provides an “ice-pick” view
of the heart by emitting a narrow ultrasound beam. Structures encountered
by the beam are reflected back and displayed as a dot, and as the
image scrolls through to display time, the motion of this site over
the cardiac cycle is displayed (Fig. 495-1). The frame rate in M-mode echocardiography
is 1000 to 4000 frames per second, yielding excellent time resolution.
Thus, M-mode provides the most precise display of events that occur
rapidly within the cardiac cycle, such as the opening and closing
of valves, the motion of the ventricular and atrial walls, and the
changes in size of the cavities during contraction and relaxation.
However, owing to the narrow area of interrogation used in M-mode echocardiography,
anatomic relationships are best left to other modalities.
This M-mode tracing from a normal subject is aligned
properly for performing a fractional shortening, which is the percentage
of change between the left ventricular end-diastolic dimension (LVEDD)
and the left ventricular end-systolic dimension (LVESD). In addition,
this view is useful for measuring septal and posterior wall thickness
and for confirming abnormal septal wall motion.
If the dots are scrolled over time and multiple parallel beams
are emitted, the beginnings of a 2-dimensional echocardiogram are noted.
In today’s echocardiogram, emitted beams of sound are steered,
usually over 30 times a second, over an arc of approximately 90,
producing a fan-shaped or sector image. As the sector is made up
of a series of lines of information, the ultrasound information
represented on each one of the lines within this sector fan produces
an image. The replay of this ultrasound image from the sector fan in
the time that is almost instantaneous with its generation has led
to the technique being called real-time imaging.