The effects of tourniquet deflation on optic nerve sheath diameter in lowerlimb orthopedic surgeries: A prospective observational study.

Pneumatic tourniquets have frequently been used in orthopedic surgeries in order to provide a bloodless surgical area and reduce blood loss via wrapping around the extremities. While there are many advantages to tourniquet applications, hemodynamic or metabolic changes may be observed in relation to the inflation and deflation of the tourniquet. When the tourniquet is inflated, metabolic changes such as increases in carbon dioxide (CO2 ), lactic acid, and potassium and reductions in PaO2 and pH are observed in the ischemic extremity. These ischemic products are then secreted into the general circulation following deflation of the tourniquet1. Increase in PaCO2 leads to cerebral vasodilatation and an increase in intracranial pressure (ICP)2. Although this increase in cerebral blood fow (CBF) is compensated and tolerated in most healthy patients, the increase in cerebral blood volume may cause an increase in ICP in patients with decreased intracranial compliance. Increased intracranial pressure can induce deleterious outcomes, including cerebral ischemia. Therefore, cautious monitoring and thorough management of the ICP is required in patients undergoing lower limb surgery, who have or are at high risk of cerebrovascular disease, using a pneumatic tourniquet3.

 The current gold standard measurement of detecting raised intracranial pressure is through an invasive procedure by placement of intracranial catheter into the ventricles or the cerebral parenchyma. Intracranial pressure can be definitively measured and monitored through devices such as intraparenchymal catheter or an external ventricular drain (EVD) catheter .Bedside sonographic measurement of optic nerve sheath diameter is emerging as a noninvasive technique to detect elevated ICP. Increased ICP is transmitted to the subarachnoid space surrounding the optic nerve, causing optic nerve sheath expansion, and the expansion of this cerebrospinal fluid (CSF) space can easily be detected using ultrasound4.

This study is aimed to investigate alterations in the ICP using ultrasonography to measure the ONSD after defation of the pneumatic tourniquet in patients undergoing lower limb surgery.