The HAPTO study will recruit adult patients with aSAH due to a burst aneurysm. These patients must be scheduled to have their aneurysm treated surgically to prevent further bleeds, and need an external ventricular drain for clinical reasons (to drain fluid and relieve pressure on the brain). At the end of their surgery for their aneurysm, a further drain will be left at the site of the surgery (which is in the basal cisterns) and they will additionally have a drain sited in their lumbar spine. Vancomycin will be given through these drains. Additionally, these drains will allow the fluid in the brain to be collected to measure how haemoglobin levels and vancomycin levels differ between compartments and change over time. Patients will participate in the study over a period from recruitment at three days after aSAH to a maximum of ten days after aSAH. The data will be analysed to determine the relationship in haemoglobin concentrations between different areas of the brain and spine after aSAH, and how vancomycin distribution is related to its route of administration.