Changes in the precordial Doppler sound test (S-test)
The two raters (YO, MT) listened to the recorded Doppler sounds in a quiet location outside the operating suites; this was performed independently on different days to avoid bias. After listening to the audio files, the raters documented whether they identified a change in Doppler sounds. Research investigators (AO, TK) played the audio files, and each rater listened to the sound. The raters were blinded from the computer display that showed graphical waveforms of Doppler sound to avoid providing clues of Doppler sound changes. Changes in Doppler sounds were considered to have occurred when both raters reported a noticeable increase in pitch and volume in the S-test.