The Allen test helps determine the integrity of the palmar arch, which is quite variable, connecting the radial and ulnar artery supply (Fig. 10.1). The value of this information is uncertain for fistula creation at the wrist, but an incomplete arch poses a risk of hand ischemia if the radial artery is ligated or inadvertently occluded. On physical examination, the test is performed by occluding the radial and ulnar arteries by compression while the patient’s fist is clenched and then released. The hand becomes blanched and should rapidly become hyperemic on release of the radial artery (Fig. 10.2). Radial insufficiency is indicated if the palm remains blanched for at least 5 seconds after release of radial compression [4, 5].

A more quantitative assessment can be made using a continuous-wave Doppler placed over the palmar arch, while the radial and then the ulnar artery are occluded. Decrease in strength of the Doppler signal with radial or ulnar occlusion is an indication of an incomplete palmar arch. The hyperemic response may also be used to assess adequacy of flow. To test this response, the hand is clenched for 2–3 min and then released. If inflow is normal, hyperemia results in increased diastolic flow. Reactive hyperemia can be quantified by calculating the resistive index:
where PSV is the peak systolic velocity and EDV the end-diastolic velocity. In one study, the arterial resistive index during reactive hyperemia was significantly lower in successful fistulas than in those that failed within 24 h (0.5 ± 0.1 versus 0.7 ± 0.2) [6]. Another approach is measurement of digital pressures with and without radial compression. A digital pressure less than 60 % of systemic pressure indicates an increased risk of symptomatic steal when a fistula is placed. Some laboratories use photoplethysmography (PPG) to assess the vasculature of the hand. PPG detects the amount of blood in the skin. The waveform is nearly identical that of arterial pressure. For preoperative testing, the PPG is used with a digital cuff to measure blood pressure in the thumb (Fig. 10.3) [4]. The normal waveform has a rapid upstroke with a sharp peak and dicrotic notch in the downslope. Thumb pressure should be above 80 mmHg and should not drop more than 30 % with radial compression. A dampened waveform and low pressures indicate an abnormal ulnar artery or palmar arch.

In a study of 287 patients undergoing cardiac surgery, 85 % had a normal Allen test as assessed by simple compression and observation. The remaining 43 underwent duplex scanning of the radial and ulnar arteries; only five were abnormal (2 % of the total group). All of these patients had their radial artery harvested with no adverse consequence to the hand [7]. Although renal failure patients are likely to have more diffuse and calcific disease of their upper extremity arteries than patients undergoing cardiac surgery, this study suggests that a simple Allen test in conjunction with duplex scanning can safely identify patients who will tolerate loss of radial artery perfusion to the hand.

Normal, healthy upper extremity arteries can supply fistula flow while continuing to adequately perfuse the hand. Steal occurs when the inflow is diminished due to central arterial stenosis or stenosis of the brachial or forearm arteries, which is particularly prominent in diabetic patients with renal failure. If there is uncertainty regarding the arterial inflow or the presence of stenosis of distal arteries that will be used for fistula creation, an arteriogram should be obtained.

Normal venous anatomy of the upper extremity is shown in Fig. 10.4. When considering potential fistula sites, it is useful to keep in mind that a successful fistula needs to fulfill the rule of sixes at maturity: the vein should be at least 6 millimeters in diameter (generally 2.5 mm or better at creation); vein depth should be no more than 6 mm; and flow should be at least 600 cc/min. In addition, there should be a length of at least 10 cm of accessible vein for ease of access and adequate separation between the inflow and outflow needles to prevent recirculation. Therefore, the examiner needs to determine if the vein is of adequate diameter, is either not too deep or transposable to a superficial location, and is of adequate length.

Physical examination of the veins should be done with the patient in a comfortable environment and preferably well hydrated. Note should be made of prior venipunctures, which may cause synechiae in the vein or scaring of the wall that prevents adequate vessel dilation. Prominent veins on the chest wall, neck, or shoulder should raise suspicion for central venous obstruction, as does hand or forearm edema. Veins may be particularly difficult to examine if the patient has recently finished dialysis and is relatively volume depleted. The extremity is examined in a dependent position. Veins that are not prominent may be dilated by application of a tourniquet and repeated clenching and relaxation of the fist. Ultrasound vein mapping may not be necessary if the examination reveals a 4 mm or larger superficial cephalic or basilic vein in the forearm that is collapsible and can be traced to the elbow or a similar cephalic vein in the upper arm that extends from the elbow to the shoulder. Even in these cases, however, the office examination is enhanced by duplex ultrasound, which confirms vein diameter, patency, lack of thrombosis or stenosis, communication with more central veins, and adequate size and quality of the proposed inflow artery [8]. Occasionally, veins that were not adequate when measured preoperatively become large and dilated in the operating room following regional anesthesia. These veins may be prone to spasm and can sometimes disappoint when used as a fistula. The examiner should not overlook the basilic vein in the forearm, which is often preserved and can be used as a transposition fistula.