Fluid administration is a commonly performed in the ICU for critically ill patients. However, it can lead to complications such as fluid overload, pulmonary edema, and increased mortality in some patients. Therefore, identifying patients who are likely to respond to fluid therapy is crucial for optimizing their management. Several methods have been used to assess fluid responsiveness, such as passive leg raising, stroke volume variation, and cardiac output monitoring. However, these methods have limitations and may not be feasible in all patients. In this study, the investigators aim to evaluate the use of velocity time integral (VTI) and Trendelenburg positioning in predicting fluid responsiveness in ICU patients.

The investigators will perform an echocardiogram both in the supine and Trendelenburg positions for research purposes prior to the ICU team assessing fluid responsiveness (see paragraph below).

Critically ill patients in which fluid responsiveness is unclear typically admitted to the ICU routinely undergo a bedside echocardiogram in the supine position and then another echocardiogram still in the supine position after receiving IV fluids to assess the patient's fluid responsiveness (i.e., whether giving a small IV fluid bolus increases cardiac output). VTI is an echocardiographic surrogate for cardiac output and is routinely used in the ICU setting in addition to providing additional information on whether a patient is fluid responsive. This method is already standard practice in the medical and surgical ICU in our PCCM department.

Unfortunately, administering IV fluids can potentially cause adverse events such as pulmonary edema, heart failure, interstitial edema, respiratory failure, and death. The use of a method, such as the passive leg raise maneuver (i.e., raising patient's legs by 30 degrees for 1 minute and then evaluating hemodynamic measurements), help predict whether a patient would benefit from IV fluids prior to giving them fluids in order to prevent these aforementioned adverse events since it is a reversible process. Literature and current practice support the use of the passive leg raise (PLR) maneuver in predicting fluid responsiveness, where an increase of at least 15% signifies a positive fluid response. Similar to IV fluids, the PLR maneuver can cause adverse events, is cumbersome, and not feasible in certain circumstances. There is recent research which suggests that Trendelenburg positioning (TP) can be used as an alternative approach that is potentially safer and less cumbersome. This study aims to evaluate a cutoff increase in VTI that would be accurate in predicting fluid responsiveness in patients undergoing TP. As stated above, in our study we will see if placing the patient in TP (i.e., tilt head of the bed 15 degrees downward) will predict if the patient is fluid responsive. The investigators want use ROC analysis to determine the cutoff VTI increase and the accuracy of using TP in predicting fluid responsiveness. The gold standard is to check VTI prior to and after administering an IV fluid bolus and the current literature demonstrates that a VTI increase by 15% is indicative of an appropriate fluid response.

The patients in the study in the ICU will receive IV fluids regardless of our study based on the clinician's discretion (i.e., our study will not affect the decision of the clinician in any way). The bedside echocardiogram takes less than 2 minutes to perform and will not delay care.

Inclusion Criteria:

• Adult patients (≥ 18 years old) admitted to the medical or surgical ICU.
• Patients who require fluid administration for suspicion of hypovolemia or indicated for volume expansion due to any one of the following: hypotension (systolic blood pressure < 90 mm Hg or mean arterial pressure < 65 mm Hg), tachycardia (heart rate > 100 beats per min), blood lactate > 2.0 mmol/L, skin mottling, oliguria (urine output < 30 ml/hr), or requiring vasopressor/inotrope support.
• Patients who are able to tolerate the Trendelenburg position.

Exclusion Criteria:

• Pregnancy.
• Prisoners and institutionalized patients.
• Patients who are not able to tolerate the Trendelenburg position. This includes patients with increased intra-cranial hypertension, intra-abdominal hypertension and gastric retention which places a risk for stomach fluid aspiration.
• Unsatisfactory cardiac echogenicity (an inability to correctly align the Doppler beam to generate reliable VTI measurements at the left ventricular outflow tract [LVOT]).