Major abdominal surgery predisposes elderly patients to increased risk of morbidity and mortality. They experience variety of complications that affect every system of the body like transient ischemic attack, stroke, cognitive dysfunction, postoperative Myocardial infarction, pulmonary embolism, acute kidney injury, renal failure requiring dialysis, intraabdominal sepsis, delayed return of bowel function, and prolonged mechanical ventilation,Intraoperative hemodynamic fluctuations, blood loss, fluid shift, electrolyte imbalances and increased duration of surgery are known to affect postoperative outcomes. Systemic oxygen delivery to vital organs may be compromised during intraoperative hypotension, anemia, fluid overload induced oedema. Additionally, elderly has multiple comorbid illnesses and these illnesses put them at the risk of impaired oxygen delivery due to macro and micro vessel atherosclerosis and dysfunction. Intraoperative hypotension may further compromise systemic oxygen delivery. Ensuring a higher systemic oxygen delivery by avoiding hypotension and higher oxygenation might reduce the incidence the postoperative morbidity and mortality.Hypotension and hemodynamic fluctuations during intraoperative period may compromise systemic oxygen delivery. In addition, elderly patients have multiple comorbid illness and suboptimal physiologic reserve atherosclerotic burden that can reduce oxygen delivery to major organs. Therefore, improvement in tissue oxygen delivery by augmenting cardiac output, systemic blood pressure and increased oxygenation would ensure better intraoperative metabolic mileu in vital organs. Central venous oxygen saturation (ScvO₂) is known as a surrogate of systemic oxygen delivery and systemic oxygen consumption.  ScvO₂ monitoring is a feasible option during intra operative period to ensure systemic oxygen delivery. Direct systemic oxygen delivery calculation appears challenging during intraoperative period because the metabolic oxygen consumption using Lafarge equation may not be valid during anesthesia. Furthermore, monitoring of mixed venous oxygen saturation requires pulmonary artery catheterization.Central venous oxygen saturation monitoring during intraoperative period in elderly patients undergoing major abdominal surgery may provide a comprehensive indicator of intraoperative hypotension, systemic oxygen delivery, and oxygen consumption and it would help in ensuring protection of vital organs. Maintaining central venous oxygen saturation at a higher cut-off by eliminating hemodynamic fluctuations and ensuring proper oxygenation by both inotropic agent and supplemental oxygenation is expected to improve postoperative outcomes.

After obtaining approval from Department postgraduate research monitoring committee, Institute ethics committee and completing CTRI registration, patients satisfying eligibility criteria will be included in this study. Written informed consent explaining about the procedure, risks and any adverse events will be obtained from the patients before being enrolled into the study. Enrollment and consent will be preferably obtained a day to prior to surgery. The patient will be enrolled in 1:1 ratio. Relevant demographic data and investigation details will be obtained from the case sheet.

Anesthetic management: On arrival standard monitors, i.e. pulse oximetry (SpO₂), non-invasive blood pressure, 5 lead electrocardiography (ECG) will be connected. A wide bore iv cannula will be inserted and fluid (Ringer lactate @ 5ml/kg) will be infused before induction of anesthesia. Regional anesthesia technique, choice and dosage of local anesthetic agent will be decided by the attending anesthesia team. Bispectral index (BIS) will be attached before induction of anesthesia.Anesthesia induction will begin with preoxygenation for a minimum of 3 minutes and will be achieved by inj. Fentanyl (2 mcg/kg), thiopentone (4-5 mg/kg) and rocuronium (1 mg/kg). BIS will be maintained between 40-60. Hypotension and hypertension during induction will be managed by boluses of noradrenaline (1mcg/kg) and esmolol (0.5 mg/kg), respectively. Maintenance of anesthesia will be preferably done using Isoflurane (variable dial settings) in FiO2 (40-80% with air) and supplemental fentanyl boluses (0.5 mcg/kg) to maintain BIS between 40-60. Ventilator settings will be adjusted to maintain End tidal carbon dioxide (ETCO₂) between 35-45 mm Hg. Initially FiO₂ will be kept at 0.4.  After induction of anesthesia, central venous catheter will be placed in the right internal jugular vein under aseptic precaution. The arterial cannula will be secured in left radial artery and will be connected to a pressure transducer.

INTERVENTION ARM     Study will commence once patient positioning is achieved post intubation and all   monitoring devices are in place. Baseline hemodynamic data will be recorded and   repeated at 30 minutes interval. Fluids will be infused at the rate of 5 ml/kg/hr in open abdominal surgery and 3ml/kg/hour in laparoscopic surgery. Hemoglobin will be maintained > 8 gm/dl and PRBC will be transfused accordingly. Blood loss will be replaced with crystalloid, colloid or PRBC in 1:3 and 1:1 ratio. Baseline central venous oxygen saturation (ScVo₂) will be obtained after patient positioning. If the baseline ScVo₂ < 75% or if it falls to < 75% during surgery, we will follow a specific treatment protocol mentioned in annexure I. We will measure ScvO₂ again 30 minutes after intervention. Thereafter, we will measure ScvO₂ every 2 hourly.

STANDARD CARE ARM Baseline central venous oxygen saturation will be measured, and the value is monitored every two hours till the end of surgery. Remaining intervention including fluid management, regional anesthesia, anesthetic agents will be left according to the attending anesthesiologist team. However, BIS will be maintained between 40-60.

Intraoperative blood sugar will be maintained between 100-200 mg/dl. Intraoperative temperature will be kept between 36-37⁰C with the help of prewarmed fluid and external body warming devices.

On the day of surgery, data concerning the performed procedure will be collected, including duration of surgery, total fluid administration, central venous oxygen saturation, norepinephrine dosage, FiO₂, urine output, hemodynamic parameters, blood loss, type of surgery and type of surgical access (open vs. laparoscopic). Furthermore, data regarding number and types of abdominal drains, nasogastric tubes, urinary catheters, and postoperative analgesic will be collected

1^st postoperative day

All postoperative outcomes will be assessed according to the defined criteria. Additionally, we will collect all investigation data from the case sheet of the patient. Hemodynamic parameter, urine output, fluid balance, transfusion requirement will also be collected.

Postoperative visit 1 day before discharge

Fluid balance, postoperative outcomes as defined will be collected from the case sheet, investigation, Chest Xray, EQ-5DL filled questionnaire will be collected. Postoperative outcomes assessors will be blinded to study randomization arm. All uncertainty related to outcomes will be resolved with the help of neurologist, cardiologist, radiologist and surgeons. 

After 3 months EQ-5DL filled questionnaire will be collected from patient either in person or over telephone.

Outcome Definitions

 CVS COMPLICATIONS Perioperative myocardial infarction reflected by changes in electrocardiography like ST segment changes, new onset pathological q wave, any new onset bundle branch block, atypical chest pain, persistent hypotension less than 90 mm Hg, requirement of inotrope, elevated Troponin-T or any incidence of percutaneous coronary intervention

NEUROLOGICAL COMPLICATIONS Stroke ( neurological deficit attributed to an acute focal injury of the central nervous system (CNS) by a vascular cause, including cerebral infarction, intracerebral hemorrhage (ICH), and subarachnoid hemorrhage (SAH)) ,transient ischemic attack ( transient episode of neurologic dysfunction due to the focal brain, spinal cord, or retinal ischemia, without acute infarction or tissue injury), delirium (transient, usually reversible, cause of mental dysfunction and manifests clinically with a wide range of neuropsychiatric abnormalities guided by MOCA score)

PULMONARY COMPLICATIONS Respiratory failure (room air oxygen saturation <90%, supplemental oxygen requirement to maintain saturation >94%, mechanical ventilation, reintubation) pulmonary edema, any chest radiograph evidence of consolidation, ARDS satisfying Berlin Criteria

RENAL COMPLICATIONS Any rise in creatinine more than 0.3mg/dl or 50%from baseline and requirement of dialysis or renal replacement therapy

GASTROINTESTINAL COMPLICATIONS Evidence of postoperative abdominal distension, Delayed return of bowel sounds (postoperative days), wound infection

QUALITY OF LIFE Quality of life will be assessed based on EQ5D3L scale before hospital discharge and after 3months either in person or over telephone