Quantitative M-mode and Two-dimensional Echocardiography

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Quantitative M-mode and Two-dimensional Echocardiography

Varun Dixit, John C. Sciarra and Christopher J. Gallagher

M-mode provides a one-dimensional, “ice pick” view through the heart and updates the B-mode images at a very high rate.¹ Doppler recordings reflect blood velocity, whereas M-mode motion of cardiac structures reflects volumetric blood flow. The two examinations are hemodynamically complementary. In certain situations, M-mode recordings of the valves and inter-ventricular septum can be particularly helpful.² M-mode is a superior method of examining the timing of the cardiac events especially when displayed with the electrocardiogram.

Edge Recognition

Seeing the edges is easiest when the signal hits the objects at 90 degrees. (Don’t get fooled here, Doppler does best at 0 degrees or 180 degrees, but the best 2-D images are picked up when the beam is 90 degrees to the object.) The clearest edges are right down the middle of the image. Picking out edges along the side is trickier, less clear.

Knowing where the edge ends is important when you are looking at ventricular function. Good function shows good wall thickening and good chamber emptying. Without seeing the edges clearly, it’s hard to read either wall thickening or chamber emptying.

M-mode gives you a good edge, but to make sense of those squiggles I think you need the $250 Italian leather designer shoes and snappy silk ties of the cardiologist.

Edge Components

When measuring “where is the real live edge” in the ventricle, the back of the mid-papillary muscle view is the most appropriate. Then, the whole deal centers on where does the blood stop and where does the ventricle start. For this, you are best off using contrast, as noted above. Other than that anemic explanation, I don’t know beans about how they’d ask you a test question on “Edge Components”.

Temporal Resolution

Anything that takes more scan lines will slow down your temporal resolution. In other words, anything that makes the transducer process more data will slow it down. That makes sense, if you think about it. Even your PC at home, when processing a lot of stuff at once, slows down.

Deeper field?—That would take more information in, so temporal resolution would be worse.

Shallower field?—That would take less information in, so temporal resolution would be better.

Wider field?—More information coming in, therefore worse temporal resolution.

Narrower field?—You’re getting it. Less info, therefore better temporal resolution.

Aortic Valve in M-Mode

The aortic valve can be evaluated in different views but one of the best for M-mode is the mid-esophageal long-axis view. On the right-hand corner is the view through the ME AV LAX view and the cursor is through the aortic valve. If you press the M-mode button on your ECHO machine, you will get this image. Let’s try to make sense of this picture.

First you will see the chest wall. The next hypoechoic shadow is the right ventricular outflow tract. The next ‘ice pick’ line you get is the aortic anterior wall.

The ‘box car’ represents the cusp separation of the aortic valve.

The second ice pick line is the posterior wall of the aorta. Behind the posterior wall is the left atrium, depending your view.

The picture explains the same landmarks in greater details. Try to correlate the diagram shown below with the real life ECHO picture above.

The diagram shown on the left is of an aortic valve in case of aortic stenosis. Try to appreciate the small cusp separation, and the thickened cusps of the aortic valve (small box cars). Cusp separation below 8 mm represents severe aortic stenosis. Always correlate the M-mode with the EKG.

This diagram shows the aortic regurgitation in M-mode. Note continuous flow during the two consecutive ‘box cars’ representing the leak in the aortic valve.

Mitral Valve in M-Mode

Again, the mitral valve can reviewed in various views, in fact it should be evaluated in different views. One of the easiest is to get the mid-esophageal four-chamber view and then get the curser through the mitral valve and hit the M-mode button.

In the image shown above, the first hypoechoic image will be of the left atrium. Then there will be the mitral valve leaflets.

Note, the two leaflets of the mitral valve move in M-shaped mirror image pattern in diastole. At the onset of systole the two leaflets come together and generate the first heart sound.

Ventricular Wall Assessment with M-Mode

If M-mode is performed in trans-gastric view, the left ventricular wall can be evaluated during the systole and diastole. Try to get the inter-ventricular septum and the left ventricular wall. The first hypoechoic shadow is of the right ventricle.

The next ‘ice picked’ line is the inter-ventricular septum (IVS). The IVS will move inwards during the systole as the left ventricle contracts, and at the same time the posterior wall of the left ventricle moves anteriorly as it contracts. From this view the end-systolic and end-diastolic measurments can be calculated. It is also possible to make measurements of the left ventricular wall thickness. The left ventricular end-diastolic dimension is measured at the onset of the QRS complex.

The end-systolic dimension is measured at the maximum movement of the ventricular septum and postero basal left ventricular free wall. The end-diastolic dimension is measured at the minimum movement of the ventricular septum and wall.

In a normal heart the thickness of the inter-ventricular septum and the posterior left ventricular wall is between 0.8 and 1 cm. Also note behind the posterior wall pericardium and epicardium are closely placed.

In case of collection of pericardial effusion, the space between posterior wall pericardium and the epicardium is increased and the collection of fluid can be appreciated.

Note the paradoxical motion of the inter-ventricular septum (instead of moving in, the septal wall is moving out). This wall motion abnormality can be seen in post-cardiac surgery and right ventricular hypertrophy.

Global Function: Measurements and Calculations

At last, something familiar!

Even someone with little echo experience can see pretty quickly that the heart is functioning well but is empty (give volume!) or the heart is functioning poorly (give inotrope, put in a balloon, do something!). Without getting wild about specific measurements, the naked eye can tell pretty well when the ventricle is pumping all its volume out and contracting well, and when the ventricle is just sitting there, doing nothing. Obviously, experience will improve your assessment over time but it’s hard to miss the elephant in the room.

This is worth dwelling on for a moment, not so much for the test, but for real life. Plus, you will hear this lesson over and over and over again in this book.

Real Life Note

You’re in the OR, or the unit, and a patient has unexplained hypotension. Is it hypovolemic shock or cardiogenic shock? Or is this cardiac tamponage? Or has the patient low SVR due to sepsis? Maybe you have a Swan, but then the numbers might be ambiguous. Does a PA of 30/15 mean empty or full? What were the baseline numbers? Is the Swan working OK? Maybe you don’t have a Swan. Now, what do you delay while you try to get in a Swan? What if the Line Gods are not with you and you have a hard time sticking the neck, or the subclavian, or the femorals? Now what?

BINGO! Put in a TEE. Just like that you’ll SEE what’s going on and won’t have to INFER anything! Tamponade? You’ll see it, you won’t have to figure whether the numbers support it. LV failure? You’ll see it. Empty (say your patient had an allergic reaction and the SVR is zippo)? You’ll see it.

When you are in trouble, transesophageal echo may keep you from crashing.

Now, on to more mundane stuff, the types of calculato-equationo-testo stuff that might appear on the test.

With a true long-axis or short-axis view, you can get the fractional area of the ventricle with the following equation:

You can trace an outline of the ventricle at diastole and systole and run the numbers, but most often you just eyeball it and make your own assessment. Errors in LVEF estimates can be diminished by increased experience of the echocardiographer, the use of cine loop technology, and frequent, continuous training for quality improvement.³

Geometric, Spectral, and Other Measurements

You can go a little more gaga on this measurement stuff. Measure, for example, the area of the LVOT, measure a VTI there, then you’ll get the following:

Once you have the stroke volume, multiply that by the heart rate and you have the cardiac output. There you have it, a geometric way to calculate the LV function.

Spectral? Things get a little hairier here. The fancy gadgets we have nowadays can filter out the high-velocity, low-amplitude signals of blood flow, and reveal the high-amplitude, low-velocity signals of cardiac tissue itself. So instead of looking at how the heart moves the blood, you look at how the heart itself moves.

From this examination of the heart tissue itself, you can judge strain (a dimensionless quantity that shows the percentage change from a resting state to one achieved following the application of a force [that force is called stress]).

Strain rate measures the rate of deformation of a tissue segment and is measured in s⁻¹. Infarcted myocardial tissue does not demonstrate shortening or lengthening activity and shows no or minimal systolic strain rate or strain.⁴

The echocardiogram is that visual assessment of wall motion which relies mainly on evaluation of inward motion of the myocardium (the transverse component of the contraction). Strain measurement allows evaluation of the longitudinal component as well, thus enabling identification of abnormalities that are not readily seen by the naked eye.⁵

Questions

1. Which is the best view to assess the left ventricular function:

B Deep trans-gastric view

C TG mid short-axis view

D ME long-axis view

2. The best angle to get a 2-D image is:

3. Which combination will give you best temporal resolution:

A Deeper and wider field

B Deeper and narrower field

C Shallow and wider field

D Shallow and narrower field

4. Calculate the fractional area of shortening with the following data (without using a calculator or your 4-year-old): end-diastolic area 4.8 cm², end-systolic area 3.6 cm², right atrial diameter 3.4 cm, left ventricular wall thickness 1.6 cm

5. A patient admitted for CABG has central crushing chest pain. You place a TEE probe and, guess what, you got M-mode. What ECHO findings will you see during systole?

A Reduced wall motion

B No systolic thickening

C Normal ventricular thickness

D All of the above

6. What do you see in this picture?

A Aortic insufficiency

B Aortic stenosis

C Mitral stenosis

D Aortic regurgitation

Answers

1. C. The left ventricular function can be evaluated in number of views but the trans-gastric short-axis view will give an impression of the ventricular walls, which are supplied by all the three main coronary arteries.

2. B. A quick revision: Doppler is best at 0 or 180 degrees, but 2-D echo images are best viewed at 90 degrees.

3. D. Remember temporal resolution is like CA 1 resident, if you give him too much information he will slow down. So tell him one thing a day and give time to process. So a shallow and narrow field will give best results.

4. C. Too bad!!! You already forgot the formula

5. D. All these are the findings of ischemia in ventricular walls. Note that the ventricular wall should thicken during the systole; if there is normal ventricular thickness it signifies ischemia. I even made a diagram for you.

6. B. There is severe stenosis of the two cusps of the aortic valve, with one cusp hardly moving at all.

References

Perrino AC, Reeves ST. A practical approach to transesophageal echocardiography, 2nd ed. Lippincott: Williams & Wilkins, 2003.

Feigenbaum H. Role of M-mode technique in today’s echocardiography. J Am Soc Echocradiogr. 2010;23(3):240–257.

Mathew JP, Fontes ML, Garwood S, et al. Transesophageal echocardiography interpretation: a comparative analysis between cardiac anesthesiologists and primary echocardiographers. Anesth Analg. 2002;94:302–309.

Pellerin D, Sharma R, Elliott P, Veyrat C. Tissue Doppler, strain, and strain rate echocardiography for the assessment of left and right systolic ventricular function. Heart. 2003;89(Suppl. 3):iii9–iii17.

Gila Perk Paul A, Tunick Itzhak Kronzon. Non-Doppler two-dimensional strain imaging by echocardiography—from technical considerations to clinical applications. J Am Soc Echocardiogr. 2007;20(3):234–243.

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