It is important prior to any cannulation that the circulation of the hand is assessed: temperature, capillary refill and the colour of the hand (RCN [381]). The radial artery is the preferred access site because of adequate flow through collateral supply, which prevents ischaemia should either the radial or ulnar artery be interrupted (Handlogten et al. [200]). Although not fully reliable, the Allen test should be completed prior to radial arterial cannulation in order to establish whether there is adequate blood supply to the hand (Figure 17.44). The Allen test consists of simultaneously compressing both the radial and the ulnar arteries for approximately 1 minute (Tegtmeyer et al. [435], Tiru et al. [443]). During this time, conscious patients rapidly open and close their hand to promote local exsanguination. Approximately 5 seconds after release of one of the arteries (usually the ulnar), the extended hand should blush owing to capillary refilling. If blanching occurs, palmar arch circulation is inadequate and a radial cannula could lead to ischaemia of the hand (Pierce [374]).

Due to the unreliability of the Allen test, ultrasound can be used to assess vessel patency and size (Bodenham et al. [43], Moussa Pacha et al. [339]). Furthermore, Adam et al. ([1]) recommend evaluation by a Doppler probe as the most reliable alternative. Shiloh et al. ([412]) suggest that using ultrasound guidance may improve arterial cannulation. Indeed, Gu et al. ([187]) advocate that the use of ultrasound reduces the incidence of first‐attempt failure of arterial cannulation when compared to palpation alone. However, ultrasound is still not commonly used during arterial cannulation.