Fig. 1.

Ultrasound radio frequency data acquired from the pulsed wave Doppler gate. The pulsed wave gate data acquired during minimum diastolic (A) and peak systolic right ventricular (RV) pressure (B) phases are shown. The Fourier domain representations of the pulses in A and B are shown in C and D, respectively. The bandwidth from which the subharmonic signal was extracted from each pulse is shaded in gray in C and D. The subharmonic signal amplitude decreases from about 56 dB (C) to about 51 dB (D) as the pressure increases during systole. During the systolic pressure rise, there is no change in fundamental signal amplitude confirming that subharmonic signal alone can be calibrated to indicate absolute pressure values, consistent with an inverse relationship between subharmonic signals and ambient pressure values (1, 13, 15, 17, 18, 32).