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, in assessing ventricular function, and in detecting abnormal flow patterns associated with cardiac disease. In many instances, the diagnostic accuracy of echocardiography is equivalent to cardiac catheterization and has 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 section to acquaint the reader with the basic concepts inherent to echocardiography, as well as the indications for each type of echocardiographic exam.
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 2-dimensional echocardiography.
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. 489-1). The frame rate in M-mode echocardiography is 1000 to 4000 frames per second, yielding excellent temporal 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, due to the narrow area of interrogation used in M-mode echocardiography, anatomic relationships are best left to other modalities.
This M-mode view from a normal subject is aligned properly for performing a fractional shortening, which is the percent 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, however, the dots were not scrolled over time and multiple parallel beams were emitted, it would provide the beginnings of a 2-dimensional echocardiogram. In today’s echocardiogram, emitted beams of sound are steered, usually over 30 times a second, over an arc of approximately 90 degrees, producing a fan-shaped or sector image. As the sector is made up of a series of lines of information, the information represented on each 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 ...