Ethical approval not obtained

Fluid management is a fundamental aspect of anesthesia. Several monitorization methods have been introduced to optimize fuid management.

To date neither of them were ideal. Corrected flow time measurement (FTc) in the carotid artery was recently introduced to detect fluid responsiveness.

Spinal anesthesia causes hypotension and fluid preloading is suggested to overcome this problem.

The aim of this study is to evaluate the effect of FTc guided fluid loading on spinal anesthesia induced hypotension.

Patients will be randomly allocated into 3 groups. Group C (Control) will receive spinal anesthesia after initiation of intravenous (iv) access without fluid preloading and will receive 2 ml/kg Ringer's lactate during anesthesia..

Group P (preloading) will receive 10 ml/kg Ringer's lactate before spinal block.

In Group FTc, ultrasonographic measurement of corrected flow time in carotid artery will be measured using 10-5 megahertz linear probe. On the two-dimensional image, the optimal image of the long-axis view will be obtained at the left common carotid artery. The sample volume will be placed on the center of the lumen, 2 cm proximal to the bulb, and a pulsed wave Doppler examination will be performed. Cardiac cycle time and carotid flow time will be measured. Carotid flow time will be measured between the upstroke of the flow tracing and the dicrotic notch, and it will be corrected for pulse rate by dividing flow time by the square root of the cardiac cycle time to calculate corrected carotid artery flow time (flow time/√cycle time).

The blood pressure will be measured 5 minutes before spinal anesthesia is commenced (baseline) and every minute afterwards until end of surgery.

Group FTc will receive a 500 mL Ringer's lactate bolus if the FTc is <349 ms (fluid responder); the FTc measurement will be repeated 15 minutes later and additional 500 mL Ringer's lactate bolus will be applied if the patient is still considered fluid responder; this sequence will be repeated until the patient's FTc is > 349 ms (non responder).

Ephedrine bolus will be used in allh groups if the mean arterial pressure drops below 30% of the baseline value.

Inclusion Criteria:

• Adult patients (19-80 years of age) who were scheduled to undergo arthroscopic knee surgery under spinal anesthesia

Exclusion Criteria:

• Mean blood pressure < 70 mmHg before induction of general anesthesia
• Patients who have currently taken angiotensin-converting enzyme inhibitor
• Patients who have currently taken angiotensin receptor blocker
• the presence of carotid artery stenosis > 50%
• cardiac rhythm other than sinus
• unstable angina
• a left ventricular ejection fraction of < 40%
• severe vascular disease
• implanted pacemaker/cardioverter
• autonomic nervous system disorders
• pregnancy