Nurse 1 will perform an USCOM scan and record values of Vpk and SV on the clinical data sheet.

Nurse 2 will perform the USCOM second scan within five minutes of Nurse 1's scan and record the values of Vpk and SV on the clinical data sheet.

Nurse 1 and 2 will be blinded to each other's scans by using separate clinical data forms

Nurse 1 will perform an USCOM scan and record values of Vpk and SV on the clinical data sheet.

Nurse 2 will also be notified of the hypotensive event, will then perform the USCOM second scan within five minutes of PI's scan and record the values of Vpk and SV on the clinical data sheet.

The two USCOM scans will be completed within 10 minutes of the hypotensive episode.

Nurse 1 and Nurse 2 will be blinded to each other's scans by using separate clinical data forms.

At the time that an enrolled patient meets one of the following criteria; SBP drops below 95 mmHg or MAP drops below 65 mmHg the PI will be notified to PI to perform the USCOM scan and record the Vpk, SV, systolic blood pressure and mean arterial pressure on the clinical data sheet within 10 minutes of the hypotensive episode, prior to the patient receiving a fluid bolus (fluid bolus is part of standard of care).

The USCOM scan will then be repeated within 5 minutes after fluid bolus delivery.

Before the hemodynamically unstable patient receives subsequent fluid boluses (multiple boluses are common standard of care), the PI will perform an USCOM scan. Then within 5 minutes after the delivered fluid bolus, the PI will perform another USCOM scan. This will continue, until no further boluses are prescribed.

Pairs of USCOM measurements from multiple raters for SV and Vpk, both of which are interval-level measures. Inter-rater reliability will be assessed using the Intraclass Correlation Coefficient (ICC); our design fits into case 1 using the Shrout and Fleiss (1979) classification system.

pairs of USCOM measurements from multiple raters for SV and Vpk. We will use the ICC to evaluate rater agreement here, as well, with the same sample size and the same confidence interval. For Aim 2, though, if the second rating cannot be completed within 5 minutes of the first, that episode will be excluded from the study.

After two raters have measured SV and Vpk, bolus will be administered. Determining the success of bolus administration, we will simply record whether the clinician judged the bolus to be successful. In addition, one rater will measure SV and Vpk again using USCOM within 5 minutes of bolus administration. We will then calculate the percentage of patients whose stroke volume and peak velocity increased at least 15% after bolus administration using the post-bolus USCOM reading and the same rater's pre-bolus USCOM reading. We chose an increase of 15% based on a review of the research as the threshold that would best indicate a therapeutic response to bolus administration.