Volume-controlled ventilation (VCV) and pressure-controlled ventilation (PCV) are two traditional ventilation modes in operating room, with different respiratory mechanics.  The two ventilation modes have different influences on intrathoracic pressure and consequently venous  return.During general anaesthesia, changing from supine to prone position may have adverse effects on oxygenation, ventilation and circulation.

         Prone position interferes with lung mechanics by decreasing the pulmonary       compliance and increasing the airway pressure^1-3. High airway pressure may in turn impair venous return to heart, decrease cardiac output and increase the systemic venous pressure. Different modes of mechanical ventilation are used to balance respiratory mechanics, gas exchange and hemodynamics.

        Volume-controlled ventilation (VCV) has been widely used as a positive-pressure ventilation (PPV) method during anesthesia, but this method does increase airway pressure when chest compliance is decreased as occurs in during prone position surgeries. This increased airway pressure will be reflected as increased epidural venous pressure leading to increased surgical bleeding. On the other hand, pressure-controlled ventilation (PCV) limits the peak airway pressure (P_peak) and reduces intrathoracic pressure thereby improving venous return and cardiac output.⁴

    However, advantage of PCV over VCV in improvement of oxygenation and ventilation has shown inconsistent results. To surgeons, the major problems encountered performing spine surgery are those of exposure and bleeding. To anaesthesiologists, the major problems noted in those patients are difficulties with ventilation and with cardiac dysfunction if the abdomen and chest are restricted.⁵

The present study aims to compare the effects of PCV with VCV on lung mechanics, oxygenation, ventilation and hemodynamics.