We report a case of an infant with a diagnosis of Arnold-Chiari malformation who developed acute cardiovascular collapse during posterior fossa decompression surgery. Haemodynamic manifestations were hypotension and bigeminy characterized by resistance to conventional resuscitation. The aetiology was considered to be due to brainstem compression exerted to control surgical bleeding from an inadvertently lacerated sinus at an unusual site, the rim of the foramen magnum. Restoration of blood pressure and disappearance of arrhythmia immediately followed removal of the brainstem compression rather than volume or pharmacological resuscitation.