When a patient is positioned in the prone posture under general anaesthesia, abdominal pressure exerts force on the internal organs, causing the diaphragm to shift towards the cephalad direction. Additionally, the weight of the trunk compresses the thoracic cavity, reducing the anterior-posterior  chest diameter. These alterations in body mechanics lead to a 20-30 percent reduction in respiratory compliance and an increase in peak inspiratory pressure. Consequently, the observed rise in PIP during prone positioning is primarily attributed to the diminished respiratory compliance resulting from these mechanical factors.

In conditions of reduced pulmonary compliance, achieving adequate ventilation may necessitate the use of elevated airway pressures. These elevated pressures can compromise venous return to the heart, leading to a reduction in cardiac output and an elevation in systemic venous pressure. Increased venous pressure in the epidural venous plexus is a recognized contributor to excessive surgical bleeding. Furthermore, elevated spinal venous pressure can decrease spinal cord perfusion pressure, calculated as the difference between mean arterial pressure and spinal venous pressure, thereby increasing the risk of neurological complications.

To mitigate these variations, a bolster is traditionally placed beneath the chest and pelvis to reduce intra-abdominal pressure and its associated physiological alterations. Spine surgeons commonly employ these bolsters to achieve optimal patient positioning during spinal surgeries. However, in percutaneous nephrolithotomy procedures, urologists typically avoid the use of such supports, as they can obstruct surgical access to the targeted area

Thus, this study will compare the changes in Dynamic compliance of the respiratory system in the prone position with and without bolsters. In addition, changes in Peak airway pressure, mean airway pressure, EtCO2, heart rate, mean arterial pressure and Oxygen saturation will also be studied.