Introduction

Hypovolemia and hypotension are commonest perioperative hemodynamic disturbance.Fluid administration is the first line of treatment in these patients. Majority of anaesthetized patients undergoing surgery have functional intravascular volume deficit even before surgery that may be due to fasting and bowel preparations. Pre-emptive hemodynamic monitoring and therapy has shown to reduce postoperative morbidity and mortality. Major surgeries causes significant tissue trauma and are associated with systemic inflammatory response leading to vasodilatation, capillary leakage and may be associated with severe haemorrhage, major fluid shifts with third space fluid loss and variable insensible losses that are difficult to estimate leading to inadequately corrected hypovolemia and inappropriate use of vasopressors  even though the blood pressure may be normal. In addition anaesthetic drugs may cause variable degree of vasodilatory and myocardial depressing effects leading to reduced effective intravascular circulating volume and hypotension. All these factors leave patients undergoing surgery at risk of hemodynamic instability and tissue hypoperfusion.

Fluids are commonly administered to prevent and treat hypovolemia. The practice of intraoperative fluid therapy has changed from routine standard therapy measuring and replacing estimated various losses to restrictive fluid strategy which is becoming routine during perioperative period where third space losses are not replaced.  Perioperative fluid approach and goal directed therapy also aim to keep neutral balance has shown to improve patient outcome.Perioperative hypovolemia affects tissue oxygenation leading to organ dysfunction and on the other hand excessive fluid loading is associated with poor wound healing, postoperative wound infection, anastomotic leakage and cardiopulmonary complications. Avoiding fluid overload by restricting fluids has shown early return of bowel function, less length of stay and with fewer complications.Thus the major goal is to maintain adequate volume status and perfusion pressure, avoiding hypovolemia and hypervolemia since both inadequate and excess fluid during intraoperative period affects patient outcome. However, how much fluid to give the patient is unclear and arbitrary fluid therapy may eventually lead to fluid excess or deficit.

Optimizing the fluid balance is of great concern before, during and after surgery. Fluid optimization is effective in preventing subsequent mortality and morbidity only when it is given at an early stage, before organ failure has developed. Delayed circulatory optimization after the establishment of organ failure is probably ineffective and may be harmful⁶. It is recommended that fluid therapy should be considered only when there is both need to increase cardiac output and patients are fluid responsive.Therefore good hemodynamic assessment is required to identify fluid responders i.e. patients who will benefit by giving fluids. Traditionally used static hemodynamic parameters like central venous pressure (CVP), pulmonary capillary wedge pressure (PCWP) are of limited value in predicting fluid responsiveness.Various dynamic indices which depend on cardiopulmonary interactions like stroke volume variation (SVV) and pulse pressure variation (PPV) are superior to static indices and shown to accurately predict fluid responsiveness in both ICUand surgical patients. Dynamic goal directed therapy (GDTdyn) that used dynamic indices to guide fluid therapy in surgical patients has shown reduced length of stay and post surgical morbidity.

Dynamic indices have their limitations for use with low tidal ventilation i.e. Vt ≤6 mL/kg predicted body weight (PBW) as major limitation precluding its routine use.Low tidal volume ventilation is commonly used in ICU today, it is recommended not only in patients with adult respiratory distress syndrome (ARDS) but also in patients at risk of ARDS, with septic shock, potential organ donors, and patients undergoing high risk intra- abdominal and thoracic surgerie. The traditional use of standard high tidal volume (10mL/kg PBW) during surgery to prevent hypoxia and atelectasis has shown to cause lung injury and more postoperative pulmonary complications leading to worse outcomes. Reducing tidal volume to 6ml/kg PBW during major surgery has shown to reduce postoperative pulmonary complications.Thus low tidal volume ventilation during intraoperative period is currently recommended and commonly practiced.

During low Vt ventilation the magnitude of change in airway driving pressure may not be enough to produce an adequate change in intra-cardiac pressure. Thus PPV and SVV may be low even in fluid responders. This may preclude the use of PPV and SVV during low tidal volume ventilation. To overcome this limitation with the use of PPV and SVV during low tidal volume in critically ill patients we developed a new test called the “tidal volume challenge" This test involves transiently increasing the tidal volume from 6mL/kg predicted body weight (PBW) to 8mL /kg PBW for one minute and thereafter reducing the tidal volume back to 6mL/kg PBW. The change in PPV and SVV following the “tidal volume challenge” (i.e. ΔPPV_6-8 and ΔSVV_6-8) reliably predicts fluid responsiveness in patients ventilated at low Vt with cut off values of 3.5% and 2.5 % respectively.

Other tests proposed to overcome the limitation of low tidal volume are passive leg raise test (PLRT) and end expiratory occlusion test (EEOT) and lung recruitment maneuver. EEOT is performed by interrupting the tidal ventilation at end expiration for 15 seconds using automatic button in ventilator. This test is not feasible on the anesthesia machines. PLRT requires passive movement of limbs and torso which is not feasible during surgery. Another recent test that used lung recruitment manoeuvre (30 cmH2O Positive end expiratory pressure - PEEP for 30seconds) in operating room patients showed that a 30% decrease in stroke volume (SV) after recruitment manoeuvre reliably predicted fluid responsiveness. Tidal volume challenge is an easy bedside test that can be done at any part during surgery procedure and may work in a similar way when used with SV.Thus, we would like to test the utility of “tidal volume challenge” test in improving the reliability of changes in PPV, SVV and SV in predicting fluid responsiveness in patients ventilated using low tidal volume during intraoperative period, which is a major limitation for the reliable use of these tests during surgery.

OBJECTIVE: