This chapter discusses the use of M-mode (movement/motion mode) and its role in neonatal functional echocardiography, as well as common measurements obtained with two-dimensional (2D) imaging.

The original description of M-mode echocardiography in 1953 by Dr. Inge Edler and physicist Hellmuth Hertz marked the beginning of a new noninvasive diagnostic technique. This technique was initially used primarily for the preoperative study of mitral valve stenosis. M-mode is also frequently referred to as an “ice pick view of the heart” for the following reason: Dr. Edler, while pioneering this “new” technology, had realized that a characteristic pattern of echo signals was obtained in patients who displayed mitral stenosis. Determined to find the origin of these signals, he initially performed experiments on calf hearts and later, to verify his observations, he performed ultrasound examinations on dying patients. Upon completion of an examination, he marked the direction of the ultrasonic beam on the patient’s chest. After the patient died, he reportedly passed an ice pick through the chest wall in the direction taken by the ultrasonic beam. On postmortem examination, Edler found that the needle had pierced the anterior wall of the right ventricle, had passed through the right ventricular outflow tract, the interventricular septum, and the upper part of the left ventricle, and had entered the left atrium via the mitral valve. Dr. Edler realized that the anterior cusp of the mitral valve was responsible for the echo signal in question.¹

Another key element in the development of medical ultrasonography was making ultrasound waves visible on the screen. In other words, the returning echoes needed to be displayed in a way that would enable visualization of the cardiac structures. The first display mode was called the amplitude modulated or A-mode display. In A-mode, the presence of a “structure” on the ultrasound beam’s path is represented on the screen by a positive deflection (spike) and the strength of the returning signal is represented by its height (amplitude). In intensity modulated display—also known as B-mode (B = brightness)—the signals are converted from spikes to dots. The taller the spikes (the greater the amplitude of the signal), the brighter the dots. This forms the basis of M-mode as well as 2D imaging (Figure 5-1).

Since the cardiac structures are constantly moving, the echo dots from these moving structures will move back and forth along an imaginary vertical line toward and away from the transducer. By electronically sweeping this tracing from left to the right, the “dots” will inscribe the motion pattern of the moving structures (like running an electrocardiogram [ECG] or seismograph paper). In contrast, a fixed (nonmoving) structure will be represented by a straight, horizontal line. Hence, the “M” in M-mode stands for “motion,” and the concept is like the ECG. M-mode scans produce a running display of data obtained from the interrogation of a single straight line through a cardiac structure. Imagine a single ultrasound beam piercing the heart to disclose its contents. The ultrasound beam hits all structures in its path, and the ultrasound waves are then reflected. The reflected waves are displayed in B-mode, assigning a brightness value based on their strength and a distance based on time traveled. While 2D echocardiography is essentially a “picture” of the heart, an M-mode echocardiogram is a “diagram” that shows how the positions of its structures change during the cardiac cycle. M-mode recordings allow in-vivo noninvasive measurement of cardiac dimensions and motion patterns of its structures.

M-mode has a higher frequency of sampling than a 2D scan and can therefore give a more detailed analysis of the change in cardiac structures’ positions over time. A graph is essentially created on the screen that plots time on the x-axis versus distance from the transducer on the y-axis. This displays motion of a cardiac structure along a single axis in real time. M-mode provides excellent time resolution, since its pulse repetition frequency (PRF, also known as sampling rate; see Chapter 1), which is given in the number of samples obtained in 1 second (usually >1000 cycles per second), is much greater than the heart rate. Therefore, even at a heart rate of 200 beats per minute, 300 pulses of information are acquired for each heartbeat. Assuming a heart rate of 200 beats per minute, there will be 3.33 beats every second, and in a sampling at 1000 cycles per second, there will be 300 looks per heartbeat (1000/3.3=300). Thus, M-mode recordings allow excellent noninvasive measurement of cardiac dimensions and motion patterns. The sweep rate of the M-mode display is analogous to the speed of an ECG paper. This speed varies between 25 mm/s and 100 mm/s and is usually set at 50 mm/s. A series of M-mode recordings may be obtained from the parasternal long-axis view, as illustrated in Figure 5-2.

Currently, 2D imaging, coupled with spectral/color flow Doppler capability, have largely replaced M-mode echocardiography. Noninvasive and bedside diagnostic evaluation of congenital heart defects is accomplished almost entirely with 2D and Doppler echocardiography. M-mode echocardiography, however, does retain a role in functional echocardiography and, due to its superior temporal and spatial resolution, is most helpful when used for the timing of rapid cardiac motion and precise linear measurements of cardiac dimensions.

In our institutions’ neonatal intensive care unit (NICU), we routinely perform M-mode scanning for the following measurements:

1. 
   

   Wall thickness

   
   
2. 
   

   Left ventricular (LV) systolic function

   
   
3. 
   

   LV mass

   
   
4. 
   

   Chamber sizes

Since M-mode is a diagram of the heart made over time, it is important for the examiner to have a simultaneous ECG tracing while obtaining echo images. The ECG tracing will help the examiner time the cardiac events that will be recorded by M-mode. This is accomplished easily by connecting the ECHO machine to the patient’s bedside cardiac monitor. In certain patients (micropremies) with very delicate skin, ECG lead placement might not be feasible.

Currently, M-mode images are generated from all standard 2D echocardiographic imaging systems. The 2D imaging plane is used to guide the placement of the M-mode ultrasound beam. Therefore, the first step is to obtain a good-quality cross-sectional (2D) image of the heart prior to placing the M-mode beam. It is important to remember that the best resolution is obtained if interrogated structures are perpendicular to the ultrasound beam (see Chapter 1). Most machines are capable of simultaneously displaying a miniaturized version of the 2D image along with the M-mode. In this way, the operator gets to see precisely where the M-mode cursor lies, preventing misplacement of the cursor and consequent misleading images.

The best views for evaluating the left side of the heart are the parasternal views. Both the parasternal long- (LAX) and short-axis (SAX) views can be used to generate M-mode images of the left ventricle (Figure 5-3) and aortic root and left atrium (Figure 5-4). With the SAX view, it is much easier to be certain that the ultrasound beam is perpendicular to the septum and left ventricular posterior wall (LVPW) and, therefore, through the center of the LV cavity. We believe this allows for more accurate and reproducible measurements than the LAX, where the structures can be sectioned tangentially and distort the measurements. Therefore, in our daily practice we prefer to use the parasternal SAX view when performing M-mode evaluation of the LV cavity (Figure 5-5). The two routinely used parasternal SAX views are (1) at the level of the mitral valve tips and (2) at the level of the aortic root. Measurements are made from “leading edge” to “leading edge.” The “leading edge” is, by definition, the surface of the structure first encountered by the ultrasound beam.²

Right ventricular (RV) anterior wall, interventricular septum (IVS), and LV posterior wall (LVPW) thicknesses in systole and diastole can be measured using M-mode (Figure 5-5). In the evaluation for hypertrophy, the diastolic measurements must be examined carefully. In the normal newborn, the IVS to LVPW ratio during diastole should be < 1.3:1; ratios greater than 1.3 are indicative of septal hypertrophy. In the neonate, the presence of myocardial hypertrophy (symmetric and or asymmetric) can be found in several important conditions (Table 5-1).

Evaluation of LV systolic function is an essential part of all echocardiographic examinations. Determination of global systolic function is based on changes in ventricular size and volume. LV dimensions are usually measured from 2D guided M-mode echocardiograms at the mitral tips level via the parasternal SAX or LAX views.

First, a parasternal view should be obtained and the M-mode cursor placed through the septal and LV posterior walls just beyond the tip of the mitral leaflets (Figure 5-5). In the resultant M-mode image, measurements should be obtained of the RV internal dimension (RVID), IVS thickness, LV internal dimension (LVID), and LVPW at end-diastole and at end-systole (see Table 5-2 for determination of end-diastole and end-systole). With this information, most machines can generate two numbers that give an objective assessment of LV function: the fractional shortening (FS) and ejection fraction (EF).

Table 5-3 is an example of normal values for M-mode measurements in preterm and normal term neonates in the first week after birth.³