The most basic assessment of PAD severity can be performed outside the vascular lab in the office setting utilizing appropriately sized blood pressure cuffs and a handheld continuous wave Doppler ultrasound device (4–8 MHz) for ankle/brachial index (ABI) determination. This gives both acoustic real-time flow dynamics of the tibial, peroneal, and pedal arteries along with quantitative data of the cumulative severity of PAD at the level of the ankle. The normal flow pattern has a triphasic characteristic with initial forward flow produced by cardiac systole, a brief phase of reversal in early diastole, and a second low-velocity forward phase in late diastole produced by vascular recoil. This triphasic flow pattern can be affected by decreased peripheral vascular resistance (vasodilation) which abolishes the second phase of flow reversal. Stenosis or occlusion will exhibit downstream changes to the pattern where a single forward phase is observed and the peak systolic flow velocity is blunted with a flattened, rounded waveform morphology.

In addition to the ankle cuff pressure , segmental pressure s with pulse volume wave recordings of the lower limbs can be performed with a standard 4-cuff method in a noninvasive vascular laboratory utilizing high and low thigh cuffs along with calf and ankle cuffs. In a normal patient, a high thigh/brachial index is usually greater than 1.2. An index of 0.8–1.2 indicates aortoiliac stenosis, whereas an index <0.8 indicates occlusion [1]. The difference in systolic pressures between two adjacent levels should be no more than 20 mmHg, and elevated gradients in pressure between levels correspond with occlusive disease within the intervening segment. In addition to the 4-cuff method, a transmetatarsal cuff can be employed, and frequently a great toe pressure and pulse volume wave recording will also be obtained. A normal toe/brachial index is commonly defined as >0.65. Furthermore, the 2007 Trans-Atlantic Inter-Society Consensus II (TASC II) further defined CLI as an absolute ankle pressure of <50 mmHg or a toe pressure <30 mmHg. These pressures correspond to the threshold for impairment of normal repair mechanisms for cellular regeneration in dermal ischemia. Others have suggested an ABI cutoff for CLI defined as <0.35, but this does not take into account the degree of collateralization which may not reflect well in the ABI determination [1].

Alongside pressure recordings, segmental cuffs provide pulse volume wave recordings (PVRs) to detect volume changes at a particular level. These air plethysmographic recordings should not be confused with Doppler waveform patterns, however. A normal PVR should have a short rise time (<160 ms) and a dicrotic notch (Table 19.1) and differences in amplitude observed from the contralateral side (delta usually of >40 %) indicate pathology assuming that the gain is not adjusted. Exercise testing on a treadmill at a standard 2 mph and 12 % grade can be employed to stress the lower extremity arterial circulation in patients with claudication but in CLI, however, this is usually not necessary [1].

While PVR and segmental pressure measurements are helpful for identifying patients with advanced limb ischemia, reliable arterial mapping is required in patients with critical limb ischemia or acute limb ischemia who are considered for intervention or surgery for limb salvage. Imaging should provide an assessment of the anatomic location, morphology of the lesions, and the overall extent of the stenosis or occlusive disease. Digital subtraction angiography is considered the gold standard for arterial imaging. However, computed tomography angiography (CTA), magnetic resonance angiography (MRA), and duplex ultrasound (DU) have similar ability to assist with procedural planning and are frequently the initial mapping tools used in many centers. Duplex ultrasound (DU) combines B-mode ultrasound imaging and pulsed Doppler spectral analysis. It has the ability to provide both anatomic information and hemodynamic information. In carotid stenosis, duplex ultrasound is used for diagnosis as well as preoperative planning and has largely obviated the need for advanced imaging using digital subtraction angiography (DSA) , CTA, or MRA. In peripheral arterial imaging, DU has been validated as a planning and procedural tool and may have advantages over other imaging modalities. DU is routinely used for post-procedural surveillance of venous conduit bypass grafts, and despite the apparent advantages of DU compared to other modalities, DU is not as widely employed for peripheral interventional or surgical planning or as an intervention imaging modality as its more expensive and attractive arteriography counterparts.

More than three decades ago, noninvasive lower extremity arterial duplex mapping was described and validated by Jager and colleagues [2]. Using a mechanically oscillated transducer, they acquired B-mode and Doppler spectral data from the groin to the popliteal artery. Based on the acquired B-mode and Doppler information, waveform contour, peak systolic velocity, and spectral broadening, they described five categories of arterial occlusive disease (Table 19.2). Compared to angiography, duplex had an overall sensitivity of 96 % and a specificity of 81 % for identifying normal versus abnormal segments and a 70 % agreement with regard to the degree of stenosis [2]. In many laboratories, these criteria are still used. Over the next decade, there was a marked improvement in ultrasound technology including the introduction of color-flow Doppler and mechanical transducers. During this time, there was widespread adoption of DU for surgical planning in carotid stenosis and other vascular surgery planning, yet DU was not widely used for evaluation in lower extremity PAD. In 1992, Moneta et al. systematically validated the use of DU in 150 patients, including 96 with limb-threatening ischemia [3]. The overall sensitivity for detecting stenosis >50 % was 89 % in the iliac vessels and 67 % at the level of the popliteal artery. Ultrasound equipment has continued to evolve since the early studies that validated the use of DU for imaging and planning intervention. Despite the improvement in ultrasound technology, in many centers, DU continues to be less commonly utilized than other imaging modalities. In 2005, the ACC/AHA published guidelines on the management of patients with peripheral arterial disease. These guidelines were a collaborative effort and were endorsed by many vascular, interventional radiology and interventional cardiology societies. Using a standardized evaluation of the current literature, they provided classification of the available literature and level of evidence (LoE) available. These guidelines recognized duplex ultrasound as useful to diagnose anatomic location and the degree of stenosis in PAD (class I, LoE: A) and recommended DU for routine surveillance of venous conduits after femoral-popliteal or femoral-tibial-pedal bypass (class I, level of evidence: A) [4]. Recommendations for the use of DU to select patients for endovascular intervention, surgical bypass, and the site of surgical anastomosis carried a class II, LoE: B recommendation. More contemporary data suggests that DU may be used as the sole imaging modality for planning bypass or endovascular revascularization in patients with critical limb ischemia with excellent technical success and long-term outcomes [5]. This study by Sultan et al. reported on 520 patients with CLI undergoing endovascular or bypass revascularization. Although approximately 20 % required adjunctive MRA imaging, DU demonstrated a sensitivity of 97 % and a specificity of 98 % compared to intraoperative DSA for lesion identification. Immediate clinical improvement and 6-year freedom from restenosis, TLR, and amputation-free survival were the same between groups. Using DU for preoperative planning was also estimated to result in cost savings over MRA [5]. Another trial compared the outcome of surgical planning based on DU compared to DSA. Compared to the surgical outcome, the DU-identified surgical plan was correct in 77 % of cases, and arteriography did not change planning for 97 % of procedures. This was similar to the surgical planning based on arteriography in which 79 % of procedures were performed as planned and DU did not change planning in 98 % of cases [6].

While there is an expanding body of literature regarding the use of DU in chronic PAD and critical limb ischemia, the body of knowledge in acute limb ischemia (ALI) is more limited. One retrospective study examined DU for evaluation of 68 patients with ALI where DU was able to successfully identify the most distal patent inflow vessel and the most proximal patent outflow vessel in 99 % of cases [7]. DU was able to identify 100 % of cases that were amendable to thromboembolectomy alone and 94 % of those that required bypass or hybrid procedures. Given the limited literature available, however, it is difficult to ascertain whether DU may play as strong a role in managing ALI as it does in CLI.

Peripheral arterial imaging with DU may be time-consuming when done properly, requiring up to 90 min for a complete bilateral lower extremity examination. The examination should include all arterial segments from the aorta to the ankle or even pedal vessels. In patients with a palpable femoral artery pulse, beginning at the inguinal crease with the common femoral artery may be sufficient. To optimize imaging of the aorta and the iliac arteries, the examination should be performed after an overnight fast in order to avoid signal dropout from overlying bowel gas. B-mode imaging should be performed in longitudinal and transverse orientation to allow adequate imaging of the vessel wall. The addition of color-flow Doppler will allow for identification of areas of turbulence. Power Doppler may be added in areas with slow flow to aid in identifying the lumen. Pulsed Doppler waveform and spectral analysis should be performed systematically at all vessel segments with special attention to areas where B-mode and color or power Doppler identify abnormalities. As described previously, normal peripheral artery Doppler waveforms are triphasic. In the presence of plaque or stenosis, spectral broadening and altered waveforms are noted (Fig. 19.1). Many labs identify stenosis >50 % by a peak systolic velocity (PSV) ratio greater than 2:1 when comparing the velocity at or just beyond the stenotic lesion to the preceding segment (Fig. 19.2). This is in addition to using spectral analysis, the presence or absence of turbulence, and waveform analysis. Some authors have advocated further stratifying the degree of stenosis by using a PSV ratio greater than 4:1 to identify >75 % stenosis and using a PSV ratio greater than 7:1 to identify >90 % stenosis [8, 9]. Other ratio stratification schemes have also been described but are not widely accepted; this may be regarded as a limitation when trying to analyze studies utilizing DU for evaluation and accuracy compared to other imaging modalities.