Reproducibility of left ventricular internal dimensions with M mode echocardiography: effects of heart size, body position and transducer angulation.

Analysis was made of the variables of heart size, body position and transducer angle affecting the reproducibility of left ventricular internal dimensions as measured with M mode echocardiography. Echocardiograms were recorded in 24 subjects as the thorax was incrementally rotated and tilted. Transducer angle was noted from a three plane level attached to the probe. Constants were the technician, transducer placement and the interpreter. Heart rates varied insignificantly; respirations were held. Groups A and B were defined by their initial left ventricular internal dimensions at end-diastole (LVIDd): 49 +/- 5.9 and 73 +/- 8.6 mm (group mean +/- standard deviation). With body position constant the measurement error between duplicate recordings of LVIDd was +/- 1.2 mm (coefficient of variation = 1.8 percent) in Group A and +/- 4.5 mm (coefficient of variation = 4.6 percent) in Group B (p less than 0.001). Transducer angle varied 12 degrees between duplicate recordings in both groups. As the position of the thorax changed, the transducer followed, maintaining approximately the same incline with the chest wall. In both groups errors for combined LVIDd recorded with rotation and tilt, respectively, were unchanged from the duplication error. Thus, when the spatial orientation between the transducer and heart is held constant, it is the size of the heart that determines the reproducibility of the measurement of left ventricular internal dimensions.