M-Mode Imaging

Traditionally, m-mode measurements have been used to quantify aortic root size, aortic valve cusp separation, and left atrial dimensions. These measurements are obtained from the parasternal long-axis imaging plane. By convention, m-mode measurements are made leading edge to leading edge, which differs from two-dimensional measurements, which are made inner edge to inner edge. The m-mode cursor is placed perpendicular to the structure being measured. Fig. 2.4 illustrates the proper m-mode technique for measuring the aortic root, aortic valve cusps, and left atrium. The aortic root is measured in end-diastole, just before the onset of the QRS complex. The aortic valve cusp separation is measured in midsystole. The normal appearance of the aortic cusps during systole is that of an “open box,” which reflects holosystolic opening of the valve leaflets. By convention, the left atrium is measured during ventricular systole or atrial diastole, when the left atrium is maximally filled with blood.

The m-mode examination of the left ventricle is also obtained from the parasternal long-axis imaging plane. By convention, left ventricular dimensions are made at end-diastole and end-systole, whereas measurements of left ventricular wall thicknesses, including the interventricular septum and posterior wall of the left ventricle, are usually measured only at end diastole. By convention, the m-mode cursor is placed perpendicular to the long axis of the left ventricle at the level of the mitral valve chordae. Fig. 2.5 illustrates the proper m-mode technique for measuring left ventricular internal dimensions at end systole and end diastole, as well as septal and posterior wall thicknesses in end diastole. In the absence of left ventricular regional wall motion abnormalities, m-mode recordings of the left ventricle have been shown to be an accurate method for calculating left ventricular ejection fraction via the method of Teicholtz. According to this method, the left ventricular dimension in diastole squared minus the left ventricular dimension in systole squared is divided by the left ventricular dimension in diastole squared.

The normal m-mode recording of the mitral valve—like that of the aortic root, left atrium, and left ventricle—is also obtained from the parasternal long-axis imaging plane. The m-mode cursor is placed perpendicular to the long axis of the left ventricle at the level of the tips of the mitral leaflets. The anterior leaflet and posterior leaflet are noted to open fully in diastole and close completely during systole (Fig. 2.6).

The normal m-mode recording of the pulmonic valve is usually obtained from the parasternal short-axis view, but it can also be obtained from the right ventricular outflow tract view and main pulmonary artery and bifurcation view (Fig. 2.7). Like the normal aortic valve, the normal pulmonic valve opens throughout systole and has the appearance of “an open box.” Normal m-mode letter designations for the pulmonic valve are as follows: a = atrial contraction, b = onset of ventricular systole, c = ventricular ejection, d = during ventricular ejection, and e = end of ventricular ejection.

The normal m-mode recording of the tricuspid valve is obtained from the right ventricular inflow view (Fig. 2.8). Usually, only the anterior leaflet of the tricuspid valve is transected by the m-mode cursor. M-mode letter designations for the tricuspid valve are as follows: D = onset of diastole, E = maximal opening of the leaflet, F = most posterior position of the leaflet, E–F slope = closing motion of the leaflet, A = leaflet reopening with atrial contraction, and C = leaflet closure following ventricular systole.

M-mode echocardiography can often be useful in helping establish the diagnosis of a bicuspid aortic valve (Fig. 2.9). The classic appearance of a bicuspid aortic valve on m-mode echocardiography is eccentric closing of the valve leaflets. If present, this is strongly suggestive of a bicuspid aortic valve, although in some cases, bicuspid aortic valves may open symmetrically.

M-mode echocardiography can also be helpful in confirming the diagnosis of a fixed subaortic membrane (Fig. 2.10). In this case, an m-mode cursor placed through the aortic valve leaflets will demonstrate early closure of the leaflets in systole. In this case, the subaortic membrane decreases the pressure differential between the systemic circulation and left ventricle, causing the aortic valve to close early.

M-mode echocardiography has also been used to diagnose mitral valve prolapse (Fig. 2.11). An m-mode cursor placed at the tip of the mitral leaflets in the parasternal long-axis view can demonstrate late systolic prolapse of the mitral leaflets into the left atrium. Because of the dependence of the ultrasound beam, however, mitral valve prolapse can be missed or overdiagnosed with m-mode echocardiography alone. For this reason, the diagnosis of mitral valve prolapse be confirmed by two-dimensional methods, which should demonstrate systolic prolapse of greater than 2 mm beyond the plane of the mitral annulus and into the left atrium.

M-mode echocardiography is especially useful for establishing the presence of systolic anterior motion (SAM) of the mitral valve, causing dynamic left ventricular outflow tract obstruction (Fig. 2.12). This is often seen in the setting of hypertrophic obstructive cardiomyopathy; however, it can also occur in the absence of hypertrophic cardiomyopathy. In the parasternal long-axis view, as the left ventricular chamber decreases in systole, the anterior cusp of the mitral valve comes into forceful contact with the protruding interventricular septum. The m-mode recording is especially useful in providing information pertaining to the timing of the SAM of the mitral valve (i.e., early systolic, holosystolic, or late systolic).

M-mode echocardiography can also provide clues with respect to quantifying the severity of aortic insufficiency. In cases of severe aortic insufficiency, the aortic regurgitant jet can impinge on the anterior leaflet of the mitral valve, causing diastolic fluttering as well as early closure of the anterior leaflet of the mitral valve (Figs. 2.13–2.15), or the so-called Austin Flint murmur heard on clinical exam, which can be mistaken for the murmur of mitral stenosis. In cases of severe acute aortic insufficiency, a sudden increase in diastolic volume overload causes increased resistance by the ventricle to diastolic filling, resulting in early diastolic closure of the mitral valve.

Because of its higher frame rates (i.e., number of times per second the image is updated on ultrasound), m-mode echocardiography can sometimes identify vegetations that may be missed with two-dimensional echocardiography. M-mode echocardiography may demonstrate the presence of a mobile mass on one of the valves with independent motion (Fig. 2.16), which is highly suspicious for vegetation in patients with a strong clinical suspicion of endocarditis.

M-mode imaging can also be helpful in establishing the diagnosis of rheumatic mitral stenosis (Fig. 2.17). In this case there is reduced opening of the mitral leaflets during diastole due to fusion of the commissures. When used in combination with spectral Doppler of the mitral valve to measure gradients, pressure half-time–derived mitral valve areas, and direct planimetry of the mitral valve area, the added information provided by m-mode can often assist in making a more accurate judgment as to the severity of mitral stenosis, particularly when there are discrepant data.

The m-mode examination of the mitral valve can also provide insight into hemodynamics in patients with cardiomyopathy. A classic m-mode finding in patients with dilated cardiomyopathy is the “b-notch” on the anterior mitral valve leaflet (Fig. 2.18). Although not always present, the b-notch, when identified, is indicative of a markedly elevated left ventricular end-diastolic pressure.

M-mode interrogation of the mitral valve has also been commonly used to estimate left ventricular ejection fraction in patients with global left ventricular systolic dysfunction. The m-mode examination in this case will demonstrate an enlarged E point septal separation (EPSS) (Fig. 2.19). This is due to a reduced stroke volume with poor left ventricular systolic function. In general, a normal EPSS should be less than 1 cm. The greater the EPSS distance on m-mode, the worse the overall left ventricular systolic function.

M-mode interrogation of the left ventricle has also been used to establish the presence of left ventricular dyssynchrony in patients with heart failure being considered for cardiac resynchronization therapy (CRT). Fig. 2.20 illustrates an m-mode recording of the left ventricle in a patient with left bundle branch block (LBBB). In this patient, there is marked paradoxical septal motion or delayed contraction of the interventricular septum in systole. Measurement of the septal-to-posterior wall motion delay (SPWMD), which is defined as the distance between the timing of septal and posterior wall contraction, has been used to predict a positive response to CRT therapy, with greater than 130 ms being used as the cutoff to predict a positive response to CRT.

M-mode interrogation of the left ventricle can also be helpful in detecting an exaggerated respiratory variation of the position of the interventricular septum (“septal bounce”) (Fig. 2.21). Although this is a nonspecific finding in suspected constrictive pericarditis, its identification, along with a strong clinical suspicion of cardiac constriction, should warrant additional directed comprehensive two-dimensional and Doppler evaluation for other markers of cardiac constriction.

M-mode examination of the left ventricle can also be helpful in identifying echocardiographic evidence of right heart failure or cor pulmonale complicated by evidence of both pressure and volume overload of the right ventricle (Fig. 2.22). M-mode interrogation can readily identify both systolic and diastolic flattening of the interventricular septum (the “D-shaped septum”). Systolic flattening of the septum represents pressure overload of the right ventricle from pulmonary hypertension, whereas diastolic flattening of the septum represents volume overload of the right ventricle, which is often secondary to severe wide-open tricuspid insufficiency.

M-mode examination of the pulmonic valve from the parasternal short-axis or main pulmonary artery view can also assist in identifying the presence of severe pulmonary hypertension (Fig. 2.23). This can be readily identified by demonstrating early systolic closure of the pulmonic valve (“flying w”) associated with severe pulmonary hypertension and due to the elevated filling pressure of the right ventricle. Often there may also be an absent a wave (atrial wave) of the m-mode tracing as well.

Ebstein anomaly is a congenital deformity characterized by downward displacement of part or all of the tricuspid valve into the right ventricular cavity. M-mode examination of the tricuspid valve from the right ventricular inflow view can be helpful in establishing the diagnosis of Ebstein anomaly (Fig. 2.24). Typically there is decreased diastolic opening of the anterior leaflet due to deformation (compare with Fig. 2.8, a normal m-mode view of the tricuspid valve).

An elevation in right atrial pressure—seen in different cardiac pathologies including cardiac tamponade, cardiac constriction, and both left- and right-sided heart failure—can be confirmed by an m-mode tracing of the inferior vena cava near its communication with the right atrium. Fig. 2.25 illustrates an m-mode examination of the inferior vena cava obtained in the subcostal view in the presence of markedly elevated right atrial pressure. Note the markedly dilated (greater than 2 cm) and plethoric (no inspiratory collapse) inferior vena cava (IVC) (arrows). The m-mode cursor is placed at the junction of the inferior vena cava and right atrium. The estimated right atrial pressure in this scenario is at least 20 mm Hg. In situations of low filling pressures, m-mode recordings of the IVC can confirm greater than 50% inspiratory collapse of the IVC.

M-mode recordings can also be useful in differentiating various types of prosthetic valves, particularly when patients are unaware of the type of valve they may have. M-mode can differentiate single-disk, bileaflet, and ball-in-cage mechanical valves and bioprosthetic valves. It can also confirm normally functioning prosthetic valves from those with evidence of valve dysfunction. Fig. 2.26 illustrates an m-mode examination of a normally functioning St. Jude bileaflet mechanical aortic valve prosthesis; it was obtained with transesophageal echocardiography from a midesophageal longitudinal view demonstrating opening of both prosthetic valve leaflets in systole (arrows). Fig. 2.27 illustrates a transthoracic m-mode examination of a normally functioning St. Jude bileaflet mechanical mitral valve prosthesis; it was obtained from the parasternal long-axis view, demonstrating opening of both prosthetic valve leaflets in diastole (arrows).

Color m-mode echocardiography is an ideal modality for confirming the timing of events with respect to the cardiac cycle, which can be especially useful in valvular regurgitant lesions. Color m-mode has also been used in the assessment of left ventricular diastolic function and is thought to be less pre- and afterload-dependent than spectral Doppler. In this situation, the initial propagation velocity (Vp) of blood flow in the left ventricle is measured. Fig. 2.28 illustrates a color m-mode examination of the aortic root and left atrium from the parasternal long-axis view in a patient with mild diastolic mitral regurgitation (arrow) attributable to heart block. Fig. 2.29 illustrates a color m-mode examination of the left ventricle, from the apical four-chamber view, demonstrating a normal Vp of blood flow in the left ventricle of 81.6 cm/s during diastole, which indicates a normal diastolic filling pattern. Normal propagation velocities are measured by the initial slope of the E wave on color m-mode and are always greater than 45 cm/s. To make these color m-mode recordings, the Nyquist color scale is moved up to 39.4 cm/s (arrow), allowing color Doppler flow to be visualized all the way from the base of the mitral annulus to the left ventricular apex. Fig. 2.30 illustrates a color m-mode examination of the left ventricle from the apical four-chamber view, demonstrating a diastolic filling pattern consistent with impaired left ventricular relaxation. In this case, the Vp measures 40.2 cm/s and is mildly reduced. Fig. 2.31 demonstrates a color m-mode examination of the left ventricle from the apical four-chamber view, showing a diastolic filling pattern consistent with markedly delayed propagation or restrictive physiology. In this example, the Vp measured significantly less than 45 cm/s (blue arrow). Note that the accuracy of the measurement of the Vp of early diastolic filling on color m-mode is improved by increasing the sweep speed to 100 mm/s (yellow arrow).

Strain imaging represents a load-independent technology, now well validated, for measuring regional myocardial deformation of the ventricular myocardium; this is measured in terms of percent to reflect the relative shortening of the myocyte during systole. Fig. 2.32 illustrates a parametric curved m-mode examination of the left ventricle from the apical four-chamber view demonstrating a normal strain pattern of the left ventricle. In this example, a curved m-mode line is traced along the interventricular septum from apex to base. The wide orange band in systole represents shortening, whereas the wide yellow band in diastole represents lengthening. The four red curves below the parametric image represent four separate strain measurements from apex to base, with the apex (top curve) showing the least amount of strain or deformation during systole (arrow). Different strain patterns have been shown to differentiate various cardiomyopathies, including cardiac amyloidosis as well as hypertrophic and hypertensive cardiomyopathies. This is an exciting and active area of investigation in the field of echocardiography that is likely to significantly improve the diagnostic capabilities of cardiac ultrasound in the future.