Beat-to-beat variability of left ventricular indexes measured by acoustic quantification: influence of heart rate and respiration--correlation with M-mode echocardiography.
Seliem MA, McWilliams ET, Palileo M.
J Am Soc Echocardiogr. 1996 May-Jun;9(3):221-30. doi: 10.1016/s0894-7317(96)90134-0.