Editor, Arterial cannulation is a commonly performed invasive procedure for continuous blood pressure monitoring in the critically ill and those undergoing major surgery. It is generally viewed as a safe procedure, but complications do occur. Whereas haematoma and temporary arterial occlusion are relatively common, more severe complications such as local infection, sepsis, nerve damage, profuse bleeding, pseudoaneurysm formation and distal ischaemia seem to be rare.1,2 In a retrospective study of 17 840 arterial cannulations, major complications were identified in just 0.084%.3 We report an unusual but severe and avoidable complication of radial artery cannulation. A 38-year-old woman developed severe bleeding during caesarean section and was admitted to the intensive care unit. An arterial cannula (20 gauge with Floswitch; BD, Franklin Lakes, New Jersey, USA) was inserted into her left radial artery. When the cannula was removed 2 days later, the patient described significant pain and a jet of blood, which was difficult to stop. Following this, the patient repeatedly complained about a painful lump on her left wrist, which initially was regarded as a haematoma. Two months later, an ultrasound examination revealed a foreign object resembling a cannula inside the artery (Fig. 1). Allen's test demonstrated total occlusion of the radial artery with perfusion of the hand by the ulnar artery only. The distal 3 cm of a polyurethane cannula was surgically removed from the artery (Fig. 2). The proximal part of the retained cannula appeared to be stretched and was cut at an angle. Fibrosis of the artery was noticed proximal to the location of the cannula, which had migrated distally to occlude the deep branch as well as the superficial palmar arch. The intima layer had suffered irreversible damage making reconstruction of the artery impossible (Fig. 3).Fig. 1: No captions available.Fig. 2: No captions available.Fig. 3: No captions available.Informed consent was obtained from the patient for publication of data from this case, including photographs. A retained cannula fragment in the radial artery can cause serious long-term complications. If collateral blood flow through the ulnar artery and palmar arches is insufficient, it could lead to claudication of the hand or even acute ischaemia due to thrombosis or embolisation.1 Existing reports of retained cannula fragments inside the radial artery point to precipitating factors that include stress on the cannula because of poor fixation, malfunctioning of the cannula itself due to poor manufacture, damage to the cannula when sewing the catheter to the skin or during removal of the securing stitch.4–7 The fact that the cannula fragment in our case was cut at an angle proximally has led us to believe that it had been damaged during placement. This can occur if the needle is re-inserted into the cannula either by accident or in order to re-position the cannula. Upon removal, the cut edge would act as a retention grip against the arterial wall, which would explain the pain and profuse bleeding as well as stretching of the cannula fragment. The retained fragment was not recognised until 2 months after its presumed removal. The intima layer was permanently damaged not just from the arterial tear but also along the course of the cannula due to foreign body reaction. At that stage, simple arterial reconstruction was not possible, something that might have had serious consequences if ischaemic symptoms had developed. The lesson to emphasise here is that reinsertion of a needle into a cannula should not be attempted when the cannula is already in the patient. A Seldinger technique either with a separate or integrated guide-wire would potentially help to avoid such complications. Furthermore, we stress that integrity of the cannula should always be confirmed following removal. Early intervention is essential to allow arterial reconstruction. Ultrasonography is suited for detection of foreign bodies and may reveal other conditions such as haematoma or pseudoaneurysm formation. We suggest that ultrasound examination should be considered if integrity of the removed cannula is uncertain or if patients experience continuous pain and discomfort from the cannulation site. Any invasive procedure carries a potential risk for the patient and to ensure patient safety in everyday procedures, such as radial artery cannulation, clinicians should be aware of the range of problems that can occur. To improve the quality and safety of treatment, an international register of complications due to inserted arterial catheters would provide useful information regarding patient risks as well as implant failure. Acknowledgements relating to this article Assistance with the letter: none. Financial support and sponsorship: none. Conflicts of interest: none. Presentation: none.