Cannulation of an epidural vessel may occur with either the needle or the catheter during siting of an epidural. The incidence is uncertain since widely varying figures have been quoted (e.g. 5-45%), possibly related to different methods of locating the epidural space, different needles or different definitions. Epidural venous engorgement in pregnant women may increase the likelihood of vessel puncture, and this may be exaggerated further in the sitting position. It is thought to be less likely when the paramedian approach is used, when 5-10 ml fluid is injected before threading the catheter, and possibly when smaller needles and catheters are used.
It is important to recognise bloody tap, because injection of local anaesthetic solution intravenously instead of epidurally may result in systemic toxicity (depending on the drug and dose). In addition, an ineffective epidural block may occur. Continued bleeding from a punctured vessel (e.g. after the epidural has been re-sited) may theoretically lead to an epidural haematoma if coagulation is impaired.
Problems and special considerations
Diagnosis is not usually a problem, especially if the needle has punctured a vessel. Puncture of a vessel by the catheter may be marked by discomfort as the vessel wall is pierced. Blood may then be aspirated from the catheter - although this is not always the case, hence the use of a test dose. Similarly, the absence of a bloody tap does not guarantee correct placement of the catheter. Therefore, it is important to aspirate all epidural catheters before administering any local anaesthetic solution.
Management options
If blood flows from the needle there is no option other than to remove the needle and re-insert it at a different interspace. If blood is obtained again, it may represent a new vascular puncture or blood from the original puncture. If blood is aspirated from the catheter, withdrawing the catheter in 0.5 cm increments, and flushing it with saline after each increment until aspiration is no longer possible, may remove the catheter from the vessel while still leaving enough length in the epidural space for effective anaesthesia. If this is not possible, then it should be re-sited in another interspace.
Key points
• In cases of bloody tap, flushing and incremental withdrawal of the catheter may avoid having to re-site the epidural.
• Bloody tap may not always be present when the catheter is placed intravascularly.