A 47-year-old female was admitted due to acute SAH (Hunt Hess 2, Modified Fisher Grade 4). Initial cerebral angiography identified a ruptured left PICA aneurysm, treated by coil embolization. To manage obstructive hydrocephalus, an external ventricular drain was inserted. Despite systemic milrinone per Montreal Protocol, transcranial doppler (TCD) on day 6 revealed critical vasospasm in bilateral middle cerebral arteries (MCAs) and basilar arteries. Subsequent cerebral angiography confirmed diffuse vasospasm, and intra-arterial verapamil treatment resulted in radiographic resolution. After transient episodes of right-sided hemiparesis, TCD and CT Angiogram on days 7 and 8 confirmed vasospasm in bilateral MCAs. Due to elevated ICPs, administering a bolus dose of IT milrinone or repeating cerebral angiography was not possible. An IRRAflow catheter was placed to facilitate cerebrospinal fluid (CSF) drainage and continuous IT milrinone administration. Continuous infusion was calculated based on a pharmacokinetic study completed to target CSF drug levels with maximal TCD response. Follow-up TCDs from days 8-12 exhibited significant improvement in vasospasm with normalization of ICPs and a non-focal neurologic examination allowing for extubation. The patient underwent a gradual wean off continuous intrathecal milrinone therapy over 5 days, and the IRRAflow system was removed on day 14. No complications arose, and the patient was discharged to acute rehabilitation.