M-mode echocardiography is considered to be obsolete by many. The technique rarely is included in American Society of Echocardiography standards documents, except for M-mode measurements, which have limited value. The superior temporal resolution of M-mode echocardiography is frequently overlooked. Doppler recordings reflect blood velocity, whereas M-mode motion of cardiac structures reflect volumetric blood flow. The 2 examinations are hemodynamically complementary. In the current digital era, recording multiple cardiac cycles of two-dimensional echocardiographic images is no longer necessary. However, there are times when intermittent or respiratory changes occur. The M-mode technique is an effective and efficient way to record the necessary multiple cardiac cycles. In certain situations, M-mode recordings of the valves and interventricular septum can be particularly helpful in making a more accurate and complete echocardiographic cardiac assessment, thus helping to make the examination more cost-effective.
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The American Society of Echocardiography is accredited by the Accreditation Council for Continuing Medical Education to provide continuing medical education for physicians.
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Target Audience
This activity is designed for all cardiovascular physicians and cardiac sonographers with a primary interest and knowledge base in the field of echocardiography: in addition, residents, researchers, clinicians, intensivists, and other medical professionals with a specific interest in cardiac ultrasound will find this activity beneficial.
Target Audience
This activity is designed for all cardiovascular physicians and cardiac sonographers with a primary interest and knowledge base in the field of echocardiography: in addition, residents, researchers, clinicians, intensivists, and other medical professionals with a specific interest in cardiac ultrasound will find this activity beneficial.
Objectives
Upon completing the reading of this article, the participants will better be able to:
- 1.
Describe the evolution of echocardiographic techniques.
- 2.
Recognize clinical situations in which the sampling rate of M-mode provides specific and precise information unavailable with the 2D sampling rate.
- 3.
Predict certain hemodynamic parameters from specific patterns of valve motion as observed on M-mode.
- 4.
Correlate M-mode patterns with spectral Doppler findings.
- 5.
Identify specific M-mode patterns of the interventricular septum in various clinical scenarios, especially left ventricular dysfunction and volume overload, conduction abnormalities, and pericardial disease.
Background
Echocardiography is by definition the examination of the heart using reflected ultrasound. Over the years, the examination has developed into a multitude of different ways to examine the heart ultrasonically. These technologies are frequently described from a historical perspective. I and others began using real-time A-mode (amplitude-based) ultrasonography. The first long-lasting diagnostic cardiac ultrasound application, the detection of pericardial effusion, used A-mode ultrasonic signals. The B-mode (brightness-based) technique converted the ultrasonic A-mode “spike” to a “dot” and amplitude to brightness. This change left a dimension available for “time.” Thus, by sweeping the B-mode dot across the oscilloscope, we had M-mode (motion-based) ultrasonography. This M-mode approach was the principle technology available for approximately 10 years. Then a variety of two-dimensional (2D) approaches became available. Shortly thereafter, Doppler techniques for recording intracardiac blood flow were described. The next development was displaying the Doppler signal as a 2D image using color to denote the direction and character of the flow.
Because of these developments, there were 2 approaches to the ultrasonic examination of the heart. The first was a one-dimensional or “ice-pick view” displaying the location and motion of the heart, usually with a sampling rate of approximately 2000 frames per second. The second was a more anatomically correct 2D or cross-sectional display of the heart with an initial frame rate of approximately 30 frames per second. The 2D frame rate is now up to approximately 100 frames per second. A similar approach was done with the Doppler recordings of blood flow. Initially, there was a single-dimensional recording of the Doppler signal recorded against time on a strip chart recorder. This spectral display could be used with either pulsed- or continuous-wave Doppler. Then a 2D approach was developed using color Doppler that was displayed as a moving picture. In many ways, the single-dimensional or spectral Doppler recording of blood flow is essentially equivalent to the M-mode recording of the valves and walls. From a historical point of view, it is interesting that the 1-dimensional recording of spectral Doppler has persisted, but the 1-dimensional M-mode technique for recording tissue has been considered obsolete by many. Although there are still M-mode questions on the echocardiography board examinations, and they are sometimes a part of “quiz shows” at meetings, M-mode findings are rarely a part of official echocardiography standards documents except for M-mode dimensions, which now only have limited applications because M-mode measurements are heavily influenced by the location of the available acoustic windows. Thus, the M-mode left ventricular (LV) diameter is rarely the true minor dimension and varies depending on the location of the acoustic window. In addition, the M-mode beam is stationary while the heart is moving, that is, the diastolic measurement is at a slightly different location than the systolic measurement. Direct measurements made from 2D echocardiograms overcome these problems.
The exact reason for the clinical disappearance of M-mode recordings of valves and walls is difficult to explain. Admittedly, many of the applications that were done with the M-mode technique became obsolete when 2D echocardiography was developed. The single dimensions that one would record and measure with M-mode were no longer necessary once 2D echocardiography became available. The more correct anatomic shape and size of cardiac structures were now available with the 2D technique. Some M-mode diagnoses, such as fluttering of the mitral valve for the diagnosis of aortic regurgitation, were no longer relevant with the availability of Doppler techniques. However, the almost complete abandonment of M-mode echocardiography ignored the fact that we are still examining a moving object. Some of these structures, especially the valves and interventricular septum, move very quickly. There are circumstances in which temporal resolution or sampling rate can be critical in detecting very rapid or subtle motions. Even with the fastest 2D imaging device, it is unusual to record more than 100 frames per second. The original stand-alone M-mode instruments recorded at a sampling rate of 2000 per second. Even with the 2D-guided M-mode recording of today, the sampling rate is still at least 1000 samples per second. The introduction of 2D or color Doppler certainly did not make the spectral display of Doppler obsolete. The same should have been true with tissue imaging. The introduction of 2D tissue imaging does not make M-mode tissue imaging irrelevant.
This review of what M-mode echocardiography can provide in today’s echocardiographic examination will hopefully reintroduce some of the unique advantages of recording cardiac valves and walls with a temporal resolution of 1000 samples per second. This discussion is by no means intended to be an exhaustive review of all the possible uses of M-mode techniques in today’s echocardiography. I will only review what I consider to be some of the more relevant uses of the historically older technique.
M-Mode Recording of the Mitral Valve
The fastest moving structure within the heart is probably the mitral valve. Figure 1 describes an M-mode recording of a normal mitral valve. This is an old recording that was taken at a sampling rate of 2000 samples per second. The various labels that were placed on the valve are indicated. This particular normal valve was selected partially because it exhibits a finding that is distinctly different from a similar recording of mitral Doppler flow. In this normal subject, there is a mid-diastolic reopening of the mitral valve. This simulates a Doppler signal in mid-diastole called the “L-wave.” The L-wave, which reflects flow velocity, is usually considered to be an abnormal sign indicative of diastolic dysfunction or abnormal LV filling. However, the mid-diastolic reopening of the mitral valve in the M-mode recording in Figure 1 represents volumetric blood flow rather than velocity. Although this may also appear in patients with diastolic dysfunction, it is usually a normal finding. A detailed discussion as to the mechanism for the L-wave and mitral valve mid-diastolic reopening might help to explain the differences in these 2 echocardiographic findings; however, the main point is to illustrate that an M-mode recording may look similar to a Doppler recording and the 2 findings may occur in the same patient, but in reality they are not the same but rather complementary. One represents blood velocity, and the other reflects volumetric flow.
Figure 2 shows some of the obvious diagnostic value of an M-mode mitral valve recording. Figure 2 A again exhibits a normal M-mode mitral valve. The excursions of the anterior and posterior leaflets virtually fill the LV cavity. The mitral anterior leaflet reaches its peak at the E-wave that almost touches the interventricular septum. The posterior leaflet moves in an opposite direction as the 2 leaflets separate from each other. The M-mode recording in Figure 2 B, which has the same calibration as Figure 2 A, is grossly different. The LV cavity is much larger with the location of the posterior ventricular wall being much further from the transducer. The size and shape of the mitral valve in Figure 2 B are strikingly different from that in Figure 2 A. The distance between the anterior and posterior leaflets is substantially less than in Figure 2 A. This finding is indicative of decreased blood flow passing through the mitral orifice, or at least reduced flow relative to the size of the ventricle. The increased distance or separation between the peak of the mitral valve opening or E-point and the septum is obvious. This E-point septum separation (EPSS) has been used for years as an indicator of global LV function. Any EPSS greater than 1 cm is considered to be abnormal. There is a good theoretic reason why EPSS correlates with ejection fraction. Ejection fraction is stroke volume divided by diastolic volume. The flow going through the mitral valve is related to stroke volume. Therefore, the E-point is reduced because of the small amount of blood flowing through the mitral orifice. As the left ventricle dilates, the mitral valve apparatus, which is more closely aligned to the posterior ventricular wall, moves away from the interventricular septum. Thus, the E-point septal separation is related to the LV end-diastolic volume and the transmitral stroke volume, which are the 2 components of ejection fraction. The EPSS is not valid in some patients with primary valvular abnormalities. Mitral valve motion in patients with mitral stenosis and patients with aortic regurgitation is distorted by factors other than flow passing through the valve and cannot be used to measure EPSS.
Another observation in Figure 2 is that the A-wave of the mitral valve looks decidedly different between Figure 2 A and B. In Figure 2 B the amplitude of the A-wave is reduced and the slope of the mitral motion after the peak of the A-wave is much different than the slope in the normal recording. In Figure 2 B there is a more gradual closure of the mitral valve after the peak of the A-wave. There is also a slight interruption of mitral closure. Clearer examples of interrupted mitral valve closure are illustrated in Figure 3 . This finding is helpful in identifying patients with an elevated LV end-diastolic pressure.
Figure 4 shows how mitral valve closure is related to LV and left atrial pressures. Normally, mitral valve closure begins with atrial relaxation and then is completed with LV contraction. This process is usually smooth and uninterrupted. The corresponding pressure between the left atrium and the left ventricle is characterized by a gradual increase in left atrial pressure after atrial contraction, which in turn produces a gradual increase in LV pressure. When there is an elevated LV end-diastolic pressure as a result of the left atrium contracting against a stiff or already fully dilated left ventricle, there is a rapid increase in the LV pressure to a point that it exceeds the left atrial pressure earlier than is normal. This earlier reversal in the pressures causes the peak of the mitral valve A-wave to be earlier. Then there is a more prolonged closure of the mitral valve before ventricular contraction with a frequent interruption or plateau caused by equalization of the pressures. This interruption is called a “B-bump,” or “notch” or “shoulder,” between the A and C points of the mitral valve.
In current discussions of diastolic function and LV pressure, the M-mode mitral valve “B-bump” is almost never mentioned. It is not a strictly quantitative assessment; however, it almost never occurs unless the LV end-diastolic pressure is more than 20 mm Hg. Figure 3 A shows a distinct plateau between the A and C points of the mitral valve. This recording was made with the older technique using 2000 samples per second. Figure 3 B shows a more recent study with standard 2D-guided M-mode echocardiography again showing the interrupted closure of the mitral valve after the A-wave. This M-mode finding is still relevant today. It may be one of the easier ways to help identify mitral flow Doppler “pseudo-normalization” and an elevated LV end-diastolic pressure. A “B-bump” is not a normal finding and should not occur if the LV diastolic pressure is normal and the mitral flow is truly normal. This M-mode finding can also be useful in differentiating patients who have a mitral Doppler E/A ratio less than 1 because of abnormal LV relaxation from those in whom the abnormal ratio is caused by low LV filling pressures. Patients with diastolic dysfunction frequently may have elevated diastolic pressures and an M-mode B-bump, which will not be present with low LV filling pressures. This situation is another example of how M-mode and Doppler recordings can provide complementary hemodynamic information.
Figure 5 shows another mitral valve M-mode recording indicative of an elevated LV diastolic pressure. This older recording shows premature closure (C) of the mitral valve before electric depolarization. This finding is indicative of a patient with severe aortic regurgitation in whom the LV diastolic pressure increases dramatically to the point that it closes the mitral valve before ventricular contraction.
Several well-recognized findings on an M-mode recording of the mitral valve are also seen with 2D echocardiography, and the hemodynamic consequences are recorded with Doppler techniques. One of these is systolic anterior motion (SAM) of the mitral valve, which is indicative of a dynamic obstruction of the LV outflow tract. Historically, the M-mode technique was the first to describe this phenomenon. Figure 6 shows 3 different varieties of SAM. Figure 6 A shows a recording in a patient with known hypertrophic cardiomyopathy where the SAM gradually approaches the interventricular septum and then falls away before the onset of diastole. Figure 6 B shows another patient with SAM whereby the mitral valve apparatus only briefly touches the interventricular septum late in systole. Figure 6 C shows yet a more severe form of SAM that is almost undoubtedly caused by hypertrophic cardiomyopathy and fairly significant LV outflow tract obstruction. There is early apposition of the mitral valve to the interventricular septum, and the valve stays in contact with the septum almost throughout systole. The duration of contact between the mitral valve and the septum is one way of judging the severity of obstruction. However, now we rely on Doppler recordings for making this assessment.
SAM can be recorded with 2D echocardiography; however, the timing of the SAM in 2D echocardiography does not come close to appreciating the timing and duration of contact between the valve and the interventricular septum. The M-mode recording of SAM may not be considered critical in today’s management of patients with hypertrophic cardiomyopathy, but it does add to our understanding of the mechanism underlying any LV outflow obstruction and is confirmatory or complementary to the Doppler recording of dynamic LV outflow obstruction, especially if there is any question about the Doppler recording.
Figure 7 represents another M-mode recording that many would think has only historical value. These recordings are of patients with mitral valve prolapse. The M-mode criteria are rarely used today as a definitive way to make the diagnosis. The only real value is that the timing of the prolapse is better appreciated with the M-mode technique. For example, in Figure 7 A the posterior displacement of the posterior leaflet seems to begin fairly early in systole and peaks in the latter half of systole. This leaflet motion produces essentially holosystolic separation and regurgitation with late systolic accentuation. In contrast, Figure 7 B shows separation of the leaflets to occur only in the latter half of systole, resulting in a shorter duration of the separation and regurgitation, and becomes a factor in quantifying the degree of regurgitation.
M-Mode Recording of the Aortic Valve
Although the motion of the aortic valve is not as complex as the mitral valve, an M-mode recording of this structure also can provide useful clinical information even in today’s practice of echocardiography. Figure 8 A shows a normal recording of the aortic valve. The aortic valve produces a characteristic parallelogram whereby 2 of the 3 leaflets are seen moving parallel to each other throughout systole. Figure 8 B shows an M-mode recording of a thickened, somewhat stenotic aortic valve. Originally, we thought we could judge the severity of the aortic stenosis by seeing the separation of the 2 leaflets; however, this proved to be unreliable, especially with congenital aortic stenosis. The real value in looking at the valve in Figure 8 B is the observation that the leaflets are thickened and, more important, moving parallel to each other. In the setting of LV outflow obstruction, this finding clearly denotes that at least part if not all of the obstruction is at the aortic valve level because the leaflets are separated to the maximum degree throughout systole.
