Corrected Flow Time in the Carotid Artery as a Predictor of Fluid Responsiveness in Mechanically Ventilated Patients With Low Tidal Volume in the Intensive Care Unit

Inclusion Criteria: 1. Adult patients with age more than 19 2. Undergoing postoperative mechanical ventilation with low tidal volume 3. Clinically diagnosed with hypovolemic status 4. If one of follow criteria is satisfied 1) Systolic blood pressure \<90mmHg 2) Mean arterial pressure(MBP)\<70mmHg or fall more than 20% from baseline 3) Heart rate \>100bpm 4) Urine output \<0.5mg/kg/hr more than 1 hour 5) Arterial lactate \>2mmol/L 6) Capillary refilling time \>3secs Exclusion Criteria: 1. Body mass index \<40 or \>15 kg/m2 2. Medical history of carotid artery stenosis \>50% or diagnosed while examination 3. Cardiac arrhythmia other than normal sinus rhythm 4. Moderate or severe valvular heart disease, intracardiac shunt, heart failure, pulmonary hypertension 5. Moderate or severe chronic obstructive pulmonary disease 6. Increased intracranial pressure 7. Using vasopressor 8. Pregnancy