BACKGROUNDIatrogenic injury to the vertebral artery during posterior cervical fusion is a rare and potentially disastrous complication. Differentiating arterial from brisk venous bleeding would be ideal to assist in the intra-operative management. Definitive angiography is typically not feasible during most routine spine surgery.CASE DESCRIPTIONWe describe the case of a patient undergoing an occipitocervical fusion, where brisk bleeding was encountered during dissection of the CB lateral mass. While the dissection was thought to be superficial to critical structures, the nature of the hemorrhage could not be definitely determined by visual inspection by two senior surgeons. The hemorrhage did not readily cease with standard maneuvers such as, the application of various hemostatic agents. Simultaneous blood gas analysis was performed on samples obtained from the patient's radial artery and from the hemorrhage in the operative bed. Comparative analysis concluded that the bleeding encountered in the surgical field was venous in nature.CONCLUSIONBlood gas analysis can be a useful adjunct in determining the nature of hemorrhage from vascular structures in spine surgery when visual inspection is indeterminate.