Data collection
Data collection (i.e., recording the sounds and blood flow velocity captured using the precordial Doppler machine) was performed by the investigators (AO, TK) in the operating suites after the patient’s airway was secured with an endotracheal tube or a supraglottic airway device.All data were collected using a precordial Doppler ultrasound machine (ES-100V3®, Hadeco®, Kanagawa, Japan). Doppler sounds were recorded using a microphone (Sanwa Supply USB microphone®, Sanwa Supply®, Okayama, Japan) and recording software (Audacity 2.0®, The Audacity Team®, California, USA). The blood flow velocity was captured using a precordial Doppler machine and recorded using software (Wavetest®, Hadeco®, Kanagawa, Japan).A precordial Doppler probe (BF8M1558A®, Hadeco®, Kanagawa, Japan) with the following properties was used: maximum intensity of < 310 W/cmData collection was performed using the following procedure: (1) a syringe filled with an allocated amount of NS was connected to the three-way stopcock, which was positioned at the most proximal location to the insertion site of the PIV catheter; (2) the baseline precordial Doppler sound was confirmed; (3) the baseline Doppler sound and blood flow velocity were recorded for 10 s, followed by injection of the allocated amount of NS through the three-way stopcock at the highest speed possible; and (4) the recordings were continued until 10 s after the initiation of the NS bolus. The research investigator (TK) labelled each data recording with the participant number (based on the inclusion order) provided by the two data managers (YS, TY). The data managers (YS, TY) created a correspondence table that matched the participant number and random allocation results. This correspondence table was concealed from the raters (YO, MT), research investigators (AO, TK), and data analyst (AO) until the completion of the data analysis. The data managers (YS, TY) were not involved in data collection or analysis during the study.