Unreliability of M-mode left ventricular dimensions for calculating stroke volume and cardiac output in patients without heart disease.

Single-dimension left ventricular echocardiographic measurements are currently being used in investigational studies as the basis for evaluating cardiac output parameters in normal subjects, even though validity of the method for normal subjects has not been established. We prospectively compared stroke volume derived from M mode left ventricular dimensions (LVID) to Fick stroke volume in 20 patients with no objective evidence of cardiac disease. Based on simultaneous studies, stroke volume by Fick ranged from 39 to 121 ml and cardiac output ranged from 3.9 to 10.4 L/min. Comparing the LVID cubed method with Fick, the correlation coefficient was r = .47 for stroke volume and r = .36 for cardiac output. LVID absolute error in cardiac output ranged -2.11 to +3.21 L/min. Use of other published formulas for calculating stroke volume from LVID did not improve accuracy. These data indicate that stroke volume and cardiac output cannot be accurately measured or reliably estimated from M mode left ventricular internal dimensions.