Non-invasive Perioperative Hb Monitoring in Spinal Surgery

The aim of the study, the way it will be conducted, and the complications and possible risks are explained to them. written informed consent is obtained from them. Then, after induction of anaesthesia, an arterial line will be inserted in order to monitor blood pressure and to take blood samples. During the surgery, two rainbow adult ReSposable sensors will be attached to the third or fourth fingers of both hands, and each of sensors will be attached to a monitor. Patients will be randomly assigned to two groups: group A will receive DNB in left hand and group B will receive DNB in right hand. DNB will be performed by injecting 2 ml of 0.25% bupivacaine at the base of medial and lateral sides of the finger attached to the sensor (1 ml on each side of the base).