This study will begin after obtaining approval from the PGRMC and Institute Ethics Committee (IEC), and registration with the Clinical trials registry of India (CTRI). Standard preanaesthetic evaluation of all participants will be performed. Patients deemed fit for elective surgery and satisfying the inclusion and exclusion criteria will be included in the study after obtaining informed written consent.

Preoperative evaluation and transcranial doppler will be performed on all participants one day before surgery. The evaluation will be done by the primary investigator under the supervision of chief guide. Patient demographics, comorbid illness, glasgow coma score (GCS), medications, and imaging findings including side and location of tumour, midline shift, peri tumoural edema, and mass effect will be noted.

Participants will be explained about transcranial doppler procedure, assured that procedure is non invasive, instructed how to perform breath holding for 30 seconds and hyperventilation. Prior to initiation of TCD, bilateral carotid arteries will be screened for plaques/ stenosis.

The TCD evaluation will be performed for the participants in the Neurosurgical ICU using the (Rimed) transcranial doppler instrument. After positioning the patient supine, non-invasive monitors- NIBP, Pulse oximetry, Etco2 using nasal cannula capnography will be attached and baseline values noted.  Ultrasound coupling gel is applied and A 2MHz TCD probe will be applied over the temporal window after application of liberal amount of gel to insonate the middle cerebral artery (MCA) at a depth of 50-60 mm and is identified by flow towards the doppler probe. Baseline MCA mean flow velocity (V1) will be noted. Baseline heart rate (HR) and mean arterial pressure (MAP), and EtCo2, spo2 will also be recorded at this time point (T0).

Transient hyperemic response test (THRT)

THRT will be done by compressing ipsilateral (to the side of MCA insonation) common carotid artery (CCA) for 10 seconds and looking for changes in MCA mean flow velocity after releasing the compression. (V1) - MCA mean flow velocity before compression of ipsilateral CCA. (V3) - MCA mean flow velocity after release of compression of ipsilateral CCA. Transient hyperemic response ratio (THRR) will be calculated using the formula V3/V1. THRR more than 1.1 will indicate intact autoregulation while less than 1.1 indicate impaired autoregulation. Once MCA flow velocities return to baseline, the test will be repeated, THRR calculated and the average of the 2 readings will be taken for statistical analysis.

Estimation of CO2 reactivity, VMR and CVR.

While MCA velocities return to normal (V1), the patient is asked to continue breathing normally.  Without taking a deep inspiration or sigh, the patient will be asked to hold their breath at the end of expiration and with the lips sealed for a duration of 30 seconds (or until possible). Continuous recording of TCD waveforms will be performed and the mean MCA flow velocity at the end of breath holding (V4) will be noted after which normal breathing will be resumed. Etco2 measurements will be made by attaching nasal cannula capnography to patient and change in Etco2 is noted before and after breath holding/hyperventilation. Then, hyperventilation will be instituted by asking the patient to breathe fast and deep for 30 seconds and mean MCA flow velocity at the end of hyperventilation (V5) will be noted. TCD monitoring will be continued until baseline velocities are reached. The following calculations will be made from the above evaluation:

Absolute CO2 reactivity

Relative CO2 reactivity

Vasomotor reactivity (VMR)

Cerebral vasodilatory reserve (CVR).

The same steps will then repeated on the opposite side and the calculations will be done. All patients will undergo craniotomy for excision of tumour under general anaesthesia. Intraoperative anaesthetic technique will be left to the discretion of the attending anaesthesiologist. Intraoperative details including the anesthetic technique, osmotherapeutic agents used, duration of surgery, fluid and blood transfusion, blood loss, urine output will be noted. Upon dural opening, brain relaxation will be assessed by the operating neurosurgeon and the score noted .Details of complications encountered including brain bulge, venous air embolism, massive blood loss, vasopressor requirement will be recorded. Post-surgery, patients will be extubated on table or will be mechanically ventilated depending on the intraoperative conditions and after discussion with the operating neurosurgeon. THRR will be estimated in intubated patients. Duration of mechanical ventilation, findings of postoperative imaging, and details of postoperative complications (if any) will be noted. Postoperative TCD evaluation of THRR, CO2 reactivity, VMR and CVR will be repeated each day until postoperative day-5. Duration of ICU stay, hospital stay, GCS on discharge and neurological outcome on discharge using GOS-E score will be noted.

GOS-E score at 90 days will be assessed using telephonic interview or during follow-up visits. All data will be recorded on a standard proforma.