M-Mode Echocardiography

Historically, M-mode (motion mode) echocardiography was the first effective modality for the ultrasonic evaluation of the heart. M-mode echocardiography provides an ice pick, one-dimensional (1D; depth only) view of the heart. ^, The ultrasound echoes reflected from the various cardiac interfaces are represented as dots and their intensities by brightness (B-mode). With the sweep of the screen (or the recording paper), the location of each interface is represented by a line, which provides information about its temporal location ( Fig. 10.1 ). The two-dimensional (2D) appearance of the tracing is the result of presenting depth (expressed as the up–down dimension of the tracing) and width (left-to-right dimension, which expresses time).

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Left ventricle. A, Schematic shows the four principal M-mode views of the heart. AMV , Anterior mitral valve; ARV , anterior right ventricle; EKG , electrocardiograph; EN , endocardial edge; EP , epicardial surface; LA , left atrial; LS , left ventricular septum; LV , left ventricle; PLA , plasminogen activator; PLV , posterior left ventricle; PMV , posterior mitral valve; PPM , posterior papillary muscle; RS , right ventricular septum; RV, right ventricle. B, M-mode view of the mitral valve. C, M-mode transducer is directed through the RV, proximal aorta, and LA. Aortic valve leaflets and AC aortic closure line, which, in normal individuals, is in the midportion of the aorta. A , Anteriormost excursion of the anterior leaflet with atrial systole; C , closure of the mitral valve and end-diastole; D , posteriormost excursions of the mitral leaflets in early diastole; E , anteriormost excursions of the mitral leaflets in early diastole; F , closure point of the mitral valve in early diastasis; MV , mitral valve; RV , right ventricle.

Part A adapted with permission from Feigenbaum H: Echocardiography , ed 5. Philadelphia: Lea & Febiger; 1993.

An important attribute of M-mode echocardiography, compared with 2D imaging, is its superior temporal resolution. In the current 2D-directed M-mode echocardiography, data are acquired at a rate up to 1000 frames per second, compared with 15 to 100 frames per second for 2D echocardiography, depending on sector width and heart rate. ^, Thus M-mode echocardiography remains important in daily clinical use for its ability to time events during the cardiac cycle and assess fast-moving structures (e.g., valves) (see Fig. 10.1 ).

The limitations of M-mode echocardiography are related to its 1D nature. Although the interrogating ultrasound beam can be tilted to visualize different structures and their anatomic relations, it may miss other structures. Also, it may produce erroneous information about chamber and vessel dimensions by scanning them obliquely. Therefore, M-mode tracings should always be obtained with the guidance of the 2D images.

Some echocardiographers consider M-mode echocardiography to be an historical relic. Many of its initial uses during the earliest decade of echocardiography have been supplanted by the newer, more anatomically correct 2D, three-dimensional (3D), and Doppler modalities. We and others believe that, like history itself, older experience is still worth studying. Furthermore, in selected situations, M-mode echocardiography remains a fundamental part of the routine echocardiographic examination and provides an important supplement to the newer echocardiographic modalities. Accordingly, this chapter demonstrates classic M-mode images, with emphasis on its continued value in the era of 2D and 3D Doppler echocardiography.

Left Ventricle

The most common use for echocardiography, in any of its forms, is to assess size and function of the left ventricle (LV). At present, 2D-directed M-mode echocardiography is still used to establish cardiac chamber size and wall thickness ( Fig. 10.2 ). Given its superior temporal resolution, M-mode echocardiography is perfectly suited to diagnosing abnormal LV contraction patterns, such as those seen with left bundle branch block (see Fig. 10.2B ). ^,

It has long been appreciated that normal systolic function includes the descent of the mitral and tricuspid annulus toward the apex, which remains relatively stationary. The extent of the descent of the base, measured in millimeters, is reflective of global systolic function. Acronyms for this descent of the base are TAPSE (tricuspid annular plane systolic excursion) and MAPSE (mitral annular plane systolic excursion) (see Fig. 10.2E ).

Mitral Valve

Normal Motion

During the rapid ventricular filling in early diastole, the anterior mitral leaflet moves from its end-systolic closed position (D point) toward the opening position, that is, anteriorly toward the interventricular septum (E point); there is a reciprocal motion of the posterior leaflet (se Fig. 10.1A and C ). The nadir of this backward motion is called the F point . Atrial contraction reopens the leaflets (the A point). Near the onset of systole, the leaflets move to the closed position (at the C point), where they remain throughout systole.

Mitral Stenosis

The mitral leaflets open at early diastole. The leaflets are thickened with commissural fusion. The larger anterior leaflet moves anteriorly, which also pulls the smaller posterior leaflet anteriorly. Because of the pressure gradient across the valve throughout diastole and the lack of rapid ventricular filling phase, the leaflets do not return toward the closure position as in normal valves. ^, The E-F slope (see Fig. 10.1B ) is flatter than normal. The M-mode combination of leaflet thickening, anterior motion of the posterior mitral leaflet, and flat E-F slope is diagnostic of mitral stenosis ( Fig. 10.3A ).

Mitral Valve Prolapse

Unlike mitral stenosis, the valve diastolic motion is normal, and the leaflets remain closed in systole. In mitral valve prolapse, the closed mitral leaflets sag backward. This motion may occur in mid to late systole (see Fig. 10.3B ) and may be associated with midsystolic click or pansystolic (and be associated with a pansystolic mitral regurgitation murmur).

Systolic Anterior Motion of the Anterior Mitral Leaflet

An anterior mitral leaflet that demonstrates systolic anterior motion (SAM) is seen mainly in patients with hypertrophic cardiomyopathy; high velocity in the left ventricular outflow tract results in SAM of the anterior mitral leaflet ^, because of the Venturi effect. In these patients, the diastolic mitral valve motion is normal. During systole, however, the anterior leaflet moves toward the interventricular septum and may coapt with the ventricular surface. The duration and degree of coaptation are related to the pressure gradient (see Fig. 10.3C ). ^,

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