M-mode echocardiogram in left ventricular dysfunction
M-mode echocardiogram is commonly used to measure left ventricular dimensions and ejection fraction. Ejection fraction is indicative of the left ventricular systolic function. In this case left ventricular systolic function is grossly depressed, with a left ventricular ejection fraction (EF) of only 31.1%. Usually the left ventricular cavity is also significantly dilated when there is severe left ventricular dysfunction. But here it is within the normal range. IVSd: Interventricular septal septum, diastolic; LVIDd: Left ventricular internal diameter, diastolic; LVPWd: Left ventricular posterior wall, diastolic; IVSs: Interventricular septum, systolic; LVIDs: Left ventricular internal diameter, systolic; EDV: End diastolic volume; IVS/LVPW: Septal to posterior wall ratio (elevated in hypertrophic cardiomyopathy with asymmetric septal hypertrophy); ESV: End systolic volume; FS: Fractional shortening. Here the sweep speed is 75 mm/s.
FS = LVIDd – LVIDs/LVIDd × 100. This will give fractional shortening as a percentage.
Limitations of ejection fraction measurement by M-mode
It measures ejection fraction in only one plane and does not represent global left ventricular function. The contractions of anterior and posterior walls alone are represented in this measurement. It can give erroneous values if there is regional wall motion abnormality of these two walls. Ejection fraction may be overestimated if the basal segments are contracting well while other segments are not contracting so well. Teichholz formula which is used by the echocardiography machine to calculate the ejection fraction is currently not the best one recommended for clinical use . Two dimensional and three dimensional methods are better for assessment of overall left ventricular systolic function. But very often M-mode is the only mode which is used to calculate left ventricular ejection fraction in routine echocardiography because other methods are more cumbersome.
Spherical formula for calculation of ejection fraction vs Teichholz formula
Spherical formula assumes that the left ventricular shape is spherical. Diastolic volume is taken as the cube of LVIDd and systolic volume as the cube of LVIDs. The difference will correspond to the stroke volume. When that is divided by the diastolic volume, we get the ejection fraction. Teichholz formula on the other hand uses a correction factor for the difference in shape of the left ventricle from a true sphere:
LV Volume = [7/(2.4 + LVID)] * LVID3
By this formula, diastolic and systolic volumes can be calculated using LVIDd and LVIDs respectively. Then the ejection fraction is calculated just like the case of the spherical formula.
It may be noted that a failing heart tends to be more globular and the spherical formula is probably more representative.
Chengode S. Left ventricular global systolic function assessment by echocardiography. Ann Card Anaesth. 2016 Oct; 19(Suppl 1): S26–S34.