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M-Mode Hemodynamics
Education>Expert TTE>Quantitative Assessment and Hemodynamics>M-Mode Hemodynamics>1

Quantitative M-Mode

M-Mode provides a quick assessment of multiple values of wall thickness, chamber diameters, ejection fraction, severity of aortic insufficiency, and more. M-Mode as a single line of sight so it does not account for changes in the heart in other areas besides it's single ultrasound beam. Despite these limitations, M-Mode is accurate in the majority of cases and is a standard measurement in TTE.

Left Ventricular Volume, Mass and Geometry

In response to valvular disorders, hypertension, heart failure the left ventricle undergoes remodeling. The mass, volume, and/or geometry of the left ventricle may change. Left ventricular measurements that characterize the remodeling are relative wall thickness (RWT), short axis (SAX), long axis (LAX), volume (V), mass (M), and the volume/mass (V/M) ratio. The measurements of the left ventricle can be predictive in certain disease states. For example, patients with idiopathic dilating cardiomyopathy have a poorer survival rate if their ventricle remodels to a more spherical model where the LAX and the SAX are similar.

Mass by Prolate Ellipsoid Method

A prolate ellipsis is an ellipsis with a LAX/SAX ratio of 2:1. The normal left ventricle is a prolate ellipsoid. To find the mass of the left ventricle, the volume of the left ventricular muscle must be calculated and then multiplied by its specific gravity, 1.04.  If the volume of the whole left ventricle can be calculated as the outer prolate ellipsis and the endocardial or blood volume of the left ventricle can be calculated, the difference is the left ventricular volume.  A correction factor, 0.8 is needed to calculate the final value for left ventricular volume.  The formula for left ventricular mass by the prolate ellipsoid method is:

 Prolate Ellipsis

LV mass = 0.8{1.04[(STd + LVIDd + PWTd)]3 - LVIDd3}} + 0.6g

 The measurements required for the calculation of LV mass are the: Septal Thickness in Diastole (STd) Left Ventricular Internal Diameter in Diastole (LVSIDd) Posterior Wall Thickness in Diastole (PWTd) Specific Gravity of the Myocardium (1.04). Since the calculation will overestimate the LV mass by 20%, a correction factor, 0.8, is used to arrive at the true LV mass. The correct beam angle for M Mode calculations can be obtained via the parasternal SAX view. The 2D view of the LVSAX is obtained and the M-Mode scan line should be just below the mitral valve leaflet tips. The beam must be perpendicular to the interventricular septum and the posterior wall. Since wall thickness is being measured, the interventricular septum and the posterior wall must not have regional wall motion abnormalities. 2D echocardiography is utilized to obtain and inspect the correct view before switching to M Mode to obtain the measurements.
 M Mode Left Ventricular Measurements

Volume

If the LVID dimension is measured, the volume of the left ventricle can be calculated from the volume formula of a sphere:
 VSphere = 4/3 x pi x r3 V = D3 or V = LVID3 Volume of a Sphere LV Volume (Spherical)

Overestimation of left ventricular volumes occurs in disease states such as a dilated cardiomyopathy or chronic valvular regurgitation. Teicholz et al published a formula that compensates for the remodeling of the left ventricle using only the LVSAX in a spherical heart. The formula can be used in systole or diastole to arrive at a stroke volume that is fairly accurate when compared to other methods. The formula is:

Vd = [7/{2.4+LVSAXd)] * LVSAXd3

Note: The Teicholz method is not used since more accurate methods (by 2D or 3D) are available.

Relative Wall Thickness

 Relative wall thickness increases in direct response to systolic pressure. In hypertensive disease the patients can be classified or subdivided based upon the response of the heart. Concentric hypertrophy is an increase in LV mass and relative wall thickness. Eccentric hypertrophy is an increase in LV mass but normal relative wall thickness. Concentric remodeling occurs when normal LV mass is present but increased relative wall thickness.
 Image Relative Wall Thickness

The last state is a normal LV mass and normal relative wall thickness.

Patients with concentric remodeling or concentric hypertrophy have higher incidence of cardiovascular events when compared to patients with normal relative wall thickness (normal or eccentric hypertrophy). A relative wall thickness > 0.8 indicates an inappropriately hypertrophied ventricle compared to the systolic pressure needed to generate a stroke volume.

 LV Remodeling State LV Mass RWT Normal Normal Normal Concentric Remodeling Normal Increased Concentric Hypertrophy Increased Increased Eccentric Hypertrophy Increased Normal

M-Mode Application

The left parasternal axis view shows the left ventricle, mitral valve, and aortic valve. An M-Mode scan can be done across the middle of the left ventricle, at the level of the mitral valve, or at the level of the aortic valve. While this is a single line of sight scans, a lot of information about the left ventricular function, mitral valve function and aortic valve function can be obtained via M-Mode.