Cannulation of the impalpable section of radial artery: preliminary clinical and ultrasound observations

Abstract

Editor, Arterial access is generally achieved without difficulty through the radial artery at the wrist. Nevertheless, difficulties may be encountered in patients with low blood pressure, peripheral arterial disease, multiple previous arterial cannulations or variations of normal anatomy. We have used ultrasound to facilitate more proximal forearm radial cannulation in more challenging cases. This report provides preliminary observations on cannulation of the impalpable section of the radial artery as a rescue procedure in cases when traditional puncture at the wrist was not possible or had failed. Ultrasound imaging in our own forearms helped identify potential sites for cannulation. Case series Ultrasound-guided cannulation of the impalpable section of the radial artery was attempted in 15 patients. This followed previous failed attempts elsewhere in 12 patients and an impalpable radial artery pulse in three others. The procedure was successful in 13 of 15 patients. The two unsuccessful cases had pre-existing severe peripheral vascular disease; the vessel was punctured, but the guide wire could not be passed. Observation of the puncture site with ultrasound suggested a flap of intima within the artery, suggesting dissection in one patient and small calcified vessels in the other. The forearm site was abandoned in these patients and a more central site for arterial access chosen and used successfully. All procedures were performed utilizing a Vygon Leadercath 8 cm, 20 g, arterial catheter (Vygon, Cirencester, UK). A 5–10 or 5–13 MHz linear ultrasound probe on a Sonosite iLook, Titan or S nerve ultrasound machine was used with improved resolution of images with each device. Procedures were performed under local anaesthesia if the patient was not already receiving general anaesthesia. The radial artery was visualized in cross-section and the needle introduced in the short axis view of the ultrasound probe. When the white spot depicting the needle tip was seen, it was directed to lie in front of the artery and advanced into it as seen by compression of the vessel. Arterial puncture was verified by pulsatile flow of blood, the guide wire was then passed and the procedure completed in the usual fashion. Discussion We have explored the use of ultrasound to facilitate radial arterial cannulation higher in the forearm. The artery is easily visible with a high-frequency ultrasound probe, which reveals it running a relatively straight course from the division of the brachial artery just below the antecubital crease, to where it is palpable at the wrist. It is seen to pass under various muscle and fascial layers which make it impalpable in most cases from 4–6 cm above the wrist. Anatomical studies and consultation with surgical colleagues who routinely operate in this area suggest that there are no associated vital structures near to the artery; in particular, the radial nerve lies deep in the belly of the brachioradialis muscle rather than in direct contact with the artery. This nerve may be at greater risk with conventional radial arterial puncture just above the wrist. Mid-forearm radial cannulation offers a number of potential advantages over existing sites. It is unlikely to have been used previously by other operators and offers additional sites for rotation of cannulae in long-stay patients. Cannulation in the mid-forearm is on a straight part of the arm, without an underlying joint, so will not be subject to repeated flexor movements, as with conventional radial, ulnar and brachial sites. This may lead to a lesser risk of dislodgement, thrombosis, microbial colonization and loss of the pressure waveform. Similar considerations may apply to the ulnar artery, but its smaller size, proximity to the ulnar nerve and greater depth suggest it is a less attractive option. The slightly deeper position of the artery compared with that at the wrist may make ultrasound examination and guided cannulation easier, as it is recognized that ultrasound imaging of structures immediately beneath the skin (and probe) has limitations unless gel-filled ‘standoff’ is used. We found this route of access to be useful in this small series of challenging patients including those with low blood pressure, failed cannulation at other sites and peripheral oedema. We found that the radial artery can be still adequately visualized and distinguished from veins in patients with a low blood pressure. In those patients in whom we failed to access the artery, ultrasound demonstrated the nature of the problem and encouraged us to move to another site before the artery was damaged further. There were no obvious complications seen in this group of patients and the arterial catheters worked well for pressure measurements and blood sampling in all cases prior to their removal after varying times in situ. In patients who were awake and moving, the consistency of the arterial waveform, which was not affected by wrist position, was noticeable compared with traditional cannulation sites over the flexor aspect of the wrist. The procedure appeared best performed with a large sterile drape with a fenestration over the puncture site. The operator was most comfortable sitting at the side of the patient with the patient's arm resting comfortably on a flat surface, for example an arm board. The artery can also be visualized and punctured from a more dorsal aspect of the forearm with the arm left by the patient's side as shown in Fig. 1.Fig. 1At the junction between the distal and middle third of the forearm, the radial artery is both greater in diameter and more superficial than the ulnar artery, but is not easily palpable. The radial artery is generally easily seen along the whole length of the forearm from the bifurcation of the brachial artery, which was typically 2 cm distal to the skin crease at the elbow. In contrast, the ulnar artery is a smaller and deeper vessel and is more difficult to image, particularly in the proximal one-third of the forearm following the bifurcation of the brachial artery. Paired, easily compressible, venae comitantes were seen on either side of the arteries at most levels (Fig. 2).Fig. 2Peripheral arterial cannulation is typically carried out through the radial artery of the nondominant forearm where it is easily palpable at the wrist in most cases. The presence of the ulnar artery means that the radial artery is generally not an end artery, and if damaged or thrombosed, tissue loss does not usually occur. Classically, the ulnar artery has been regarded as the dominant arterial supply to the forearm and hand, but more recent studies have suggested that the radial artery is more likely to be the dominant vascular supply to the hand [1]. Alternatively, the brachial artery at the elbow may be used, but as it is an end artery, the distal limb is at risk if arterial occlusion occurs. Studies exist of applied radial and ulnar arterial anatomy, including the use of ultrasound assessment, in the context of arterial cannulation [2], arteriovenous fistulae for dialysis [3], and percutaneous coronary intervention [4]. The radial artery is also established as a conduit for coronary artery bypass grafting [5] and a source for free microvascular flap transfers [6]. The detailed anatomy and the relatively high frequency of normal variants may not be well known to many anaesthetists. Anatomical variations of the arteries in the upper limb are common [7,8]. Superficial radial and ulnar arteries may be clinically obvious, but cases of inadvertent cannulation during attempted venous access have been described [9]. Variations in the more proximal arterial supply to the upper arm and forearm are also common, and may not be obvious on palpation. High bifurcation of the brachial artery is a well described variation [10]. In patients with peripheral vascular disease, there will be a small proportion with heavily diseased or blocked radial or ulnar arteries which should be easily identifiable with ultrasound. It is intuitive that such a site should generally be avoided in the case of such findings. Elderly patients tend to have more tortuous ectatic vessels. Ultrasound imaging is not required routinely to cannulate the artery, although it has been used in difficult cases and to reduce the number of attempts required at the wrist. Although not proven, it is likely that the frequency of vascular, tissue or nerve damage and subsequent thrombotic or infectious complications is related in part to the number of needle passes. Ultrasound guidance for arterial access may be useful in patients with low blood pressure, atheromatous vessels, dissections, oedema, obesity, multiple previous cannulations and congenital or acquired variations in anatomy. Newer high-resolution ultrasound probes allow visualization of peripheral radial and ulnar nerves which could be specifically avoided during cannulation attempts. Further study is required to assess the safety and efficacy of this approach compared with traditional cannulation at the wrist. The low frequency of serious complications (e.g. tissue ischaemia and nerve damage), following radial cannulation, makes comparative studies difficult without very large numbers. The increasing use of ultrasound to assist central venous access and nerve blocks means that most anaesthetic departments will already have access to high-resolution ultrasound equipment suitable for assisting arterial access. Conclusion Our series and the results of other reports suggest that ultrasound has a useful role to facilitate arterial access, particularly in the more difficult case. The impalpable sections of the radial artery are potentially suitable sites for cannulation. Further studies are required to ascertain the relative risks and benefits of different sites of cannulation.