The trial groups will be:

1. Routine fluid management. Clinicians will be blinded to RI monitoring and use clinical judgement to determine how much fluids should be given, and when. Clinical judgement will be according to their standard practice and may include interpretation of blood pressure, and heart rate.
2. RI-guided fluid administration. RI fluid guidance will be initiated before anesthesia induction and will maintain until end of anesthesia. Clinicians will titrate fluids with the goal of keeping RI above 90- always using good clinical judgement for individual patients which may include avoiding fluid when RI is <90 or giving additional fluid when RI >90. For RI scores below 90, 1 cc/kg actual body weight fluid bolus of any crystalloid solution (normal saline or Ringer's lactate) will be given; colloids and blood products can also be given if clinically indicated. The target will be maintained until end of anesthesia.

Anesthesia will be maintained for the entire surgical procedure per clinical routine. At the end of the surgical procedure, patients will be extubated and transferred to the post anesthesia care unit (PACU).

Patients will be premedicated with 0-2 mg midazolam per preference of the attending anesthesiologist. General anesthesia will be induced as preferred by the attending anesthesiologist, usually with a combination of lidocaine 1 mg/kg, propofol 1-4 mg/kg, fentanyl 1-2 µg/kg, and rocuronium 0.6-1.2 mg/kg or succinylcholine 1.5 mg/kg. The trachea will be intubated, and the lungs mechanically ventilated per clinical routine. General anesthesia will be maintained with an initial sevoflurane target concentration of 0.8% which will be adjusted based on apparent clinical need, supplemented with fentanyl. Vasopressors, antihypertensives, and drugs to control heart rate may be given as clinically indicated. BIS will be recorded to estimate hypnotic depth.

Patients will be randomized 1:1, stratified for chronic use of antihypertensive medication, in random-sized blocks to RI-guided fluid management or routine care. The randomization table will be prepared by trial statisticians, and allocation will be concealed until shortly before anesthetic induction with a web-based randomization system. The Zynex RI system will be applied and calibrated before anesthetic induction in all patients. A continuous fluid infusion of 1cc/ kg actual body weight/hour of crystalloid solution will be started in all patients before anesthesia induction and maintained until end of anesthesia. The trial groups will be:

1. Routine fluid management. Clinicians will be blinded to RI monitoring and use clinical judgement to determine how much fluids should be given, and when. Clinical judgement will be according to their standard practice and may include interpretation of blood pressure, and heart rate.
2. RI-guided fluid administration. RI fluid guidance will be initiated before anesthesia induction and will maintain until end of anesthesia. Clinicians will titrate fluids with the goal of keeping RI above 90- always using good clinical judgement for individual patients which may include avoiding fluid when RI is <90 or giving additional fluid when RI >90. For RI scores below 90, 1 cc/kg actual body weight fluid bolus of any crystalloid solution (normal saline or Ringer's lactate) will be given; colloids and blood products can also be given if clinically indicated. The target will be maintained until end of anesthesia.

Anesthesia will be maintained for the entire surgical procedure per clinical routine. At the end of the surgical procedure, patients will be extubated and transferred to the post anesthesia care unit (PACU).

Inclusion Criteria:

1. Adults having major non-cardiac surgery expected to last ≥2 hours
2. American Society of Anesthesiologists physical status 1-3
3. Age 21-85 years old
4. Planned endotracheal intubation and general anesthesia with or without any regional blocks.

Exclusion Criteria:

1. Non-sinus heart rhythm;
2. amputation of any extremity;
3. eGFR < 30 including end-stage kidney disease;
4. cardiac ejection fraction < 50;
5. temporary or permanent pacemaker;
6. BMI > 40 kg/m2. -

Adult patients having major non-cardiac surgery expected to last ≥2 hours. The ASA physical status 1-3, and their age 21-85 years old. The patient should have a planned endotracheal intubation and general anesthesia with or without any regional blocks.