The noninvasive vascular laboratory assists with the diagnosis of peripheral arterial disease.
In general, two basic approaches are used: (1) indirect measures that characterize the func- tional severity of the disease, such as segmental pressures, Doppler waveform analyses, plethys- mography, and skin perfusion pressures; and (2) direct measures that characterize the anatomy of the disease using color duplex imaging. These studies are used to accurately diagnose the loca- tion and extent of peripheral arterial disease, assist with the planning of therapeutic options for the disease, and follow the outcomes of peripheral vascular interventions.
Indirect Evaluation of Arterial Disease
Ankle-Brachial Index
In patients without palpable pedal pulses, the next step in the clinical evaluation is to perform a Doppler analysis. This is usually performed in the clinic or at the bedside using a handheld Doppler device. The probe is placed at an angle (Fig. 3.1) over the dorsalis pedis and posterior tibial arteries to determine if a signal can be obtained. The complete absence of Doppler signals suggests significant peripheral arterial disease. Approximately 10% of the normal pop- ulation does not have a dorsalis pedis artery;
hence, the absence of a dorsalis pedis signal alone is not a significant finding. It is important
to place the probe directly over the artery but at a 45- to 60-degree angle to obtain the best signal. Doppler signals can be used to assess the severity of the disease. A blood pressure cuff is inflated just above the ankle and a Doppler signal is listened for while the cuff is deflated.
The highest pressure in which a signal is heard (dorsalis pedis compared to posterior tibial) is the ankle index.A similar approach with a blood pressure cuff on the upper arm and a Doppler probe on the brachial artery is used to deter- mine the brachial index. The ratio of the ankle to the brachial index represents the ankle–brachial index (ABI). The ABI is usually 1 or greater. An ABI of 0.5 to 0.8 is consistent with claudication, and less than 0.4 is consistent with critical limb ischemia. Diabetic patients often develop medial calcinosis. Simply stated, the medial wall of the vessels become calcified (“bone-like”) and cannot be compressed by a blood pressure cuff. Thus, the ABI in a diabetic patient may be falsely elevated, and other studies are needed to accurately assess the peripheral vascular status. In general, however, the ABI is a useful screening test for peripheral vascular disease.
Segmental Limb Pressures
Segmental limb pressures assist with determin- ing the location of disease by measuring the pressure in the upper thigh, lower thigh, below the knee, just above the ankle, and at the trans- metatarsal level (Figs. 3.2 and 3.3). Again using
Noninvasive Vascular Examination
Colleen M. Brophy
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a Doppler, an arterial signal is found at the dor- salis pedis or posterior tibial artery, the cuff is inflated until no signal is heard, and then deflated until the signal resumes. The systolic pressure is recorded for each cuff at each loca- tion. A gradient (decrease) in pressure greater than 20 mm Hg between adjacent levels suggests arterial occlusive disease in the vessel between the two cuffs (Fig. 3.3). An arterial pressure in the thigh that is less than that in the arm sug- gests aortoiliac occlusive disease. A drop in pressure between the upper thigh and lower thigh cuffs suggests superficial femoral artery
disease. It is important to make a distinction between aortoiliac and infrainguinal disease because the overall approach is somewhat different.
An additional useful measurement, particu- larly in diabetic patients, is to measure the digital pressures. The digital arteries are less likely to be affected by medial calcification and hence provide useful information when the ABI is falsely elevated. Digital pressures are meas- ured by placing a pneumatic cuff around the digit and measuring a plethysmographic arte- rial waveform using a photoelectrode on the end of the digit. In general, a normal toe pres- sure is 80% to 90% of the brachial pressure (normal toe–brachial index is 0.8 to 0.9). A toe pressure less than 38 mm Hg has been correlated with impaired forefoot wound healing (Vitti et al., 1994).
In patients with symptoms consistent with claudication but relatively normal indices, exer- cise testing should be performed. A standard exercise test involves measuring the ABI at rest followed by walking on a treadmill at 2 miles/
hour at a 10% to 15% incline for 5 minutes.
The ABI is recorded 1 minute after exercise.
Normally, after exercise, there is a slight in- crease or no difference in the ABI. If the ABI decreases after exercise, this is consistent with claudication.
Pulse Volume Recording
Pulse volume recordings (PVRs) are a plethys- mographic measure of limb perfusion.
Figure 3.1. Doppler signals are best obtained by holding the Doppler probe at an angle over the artery that is being insonated.
Figure 3.2. Segmental limb pressures are measured by placing the cuffs along the extremity.
Figure 3.3. Segmental limb pressures show a low high-thigh pressure, indicating iliac disease, and a drop-off between the high-thigh and low-thigh pressures, indicating superficial femoral artery disease on the left side. On the right side there is a drop-off between the pressures in the low thigh and popliteal, indicating distal superficial femoral artery/popliteal artery disease. ABI, ankle–brachial index; Dorsaus Pedis (DP), Posterior Tibial (PT), prothrombin time; PVR, pulse volume recording.
Plethysmography is essentially a measurement of volume. In general, the same cuffs are used to obtain segmental limb pressures and PVRs. The cuff is inflated and the volume shift that occurs with the cardiac cycle in that limb segment is recorded as a waveform. The PVRs do not provide quantitative data but can be analyzed in a qualitative manner. In normal patients there is a brisk upstroke, rapid decline, and dicrotic notch in the waveform. In the presence of peripheral arterial disease, the waveform becomes broader with a decreased amplitude. A PVR tracing that is flat at the forefoot (trans- metatarsal) level is consistent with significant peripheral vascular diseases and suggests that revascularization will be required for any fore- foot lesion to heal.
Doppler Waveform Analysis
Similar to PVRs, the Doppler waveform can be analyzed to determine if there is disease. Instead of an audio signal, a digital signal is converted to a tracing. Normally the waveform is triphasic (Fig. 3.4). With moderate disease the reverse flow component is lost and the waveform is monophasic. With severe disease, the waveform is blunted and/or absent.
Skin Perfusion Pressure
The skin perfusion pressure (SPP) is an addi- tional noninvasive vascular evaluation that is useful to determine which foot ulcers will heal with local wound care or minor amputation and which will require revascularization or major amputation. An SPP of less than 30 mm Hg can predict failure of forefoot wound healing with
Figure 3.4. Normal triphasic Doppler waveform.
an accuracy of 80% (Castronuovo et al., 1997).
Skin perfusion pressure has largely replaced transcutaneous oximetry.
What Wounds Will Heal?
The aggregate measurements that are consistent with impaired wound healing are listed in Table 3.1. However, it is ultimately the clinical response to wound care that is the most impor- tant factor. If debridement and wound care leads to consistently unhealthy appearing wounds, angiography and revascularization should be considered. If good wound care leads to healthy granulation tissue, further evaluation is likely unnecessary.
Direct Evaluation of Arterial Disease
Color duplex imaging or “duplex” imaging incorporates real-time B-mode imaging and pulsed Doppler spectral analysis (Fig. 3.5). B- mode imaging directly views the blood vessel and provides anatomical detail of the vessel.
This modality can analyze plaque characteris- tics, identify thrombus and the intimal flaps, and measure vessel diameter (to detect aneurysms and pseudoaneurysms).
B-mode imaging is also used to localize areas of stenosis so that Doppler velocities can be determined. Velocity analyses are an indirect measure of the degree of stenosis. For carotid artery analyses, the degree of stenosis is esti- mated based on the peak systolic velocity (PSV), end-diastolic velocity (EDV), and the ratio of the PSV at the stenosis to the velocity in the common carotid artery (Vr). In general a Vr
greater than 4 is indicative of clinically significant stenosis (Table 3.2).
Infrainguinal graft surveillance can be per- formed with duplex imaging. Because most
abnormalities occur in the first 2 years after implantation, graft surveillance is most impor- tant in this time frame. A PSV greater than 150 to 200 cm/second is considered abnormal. In addition, the ratio of the velocity at the stenosis to the normal proximal velocity is used to determine the degree of stenosis (Vr). In addi- tion, high EDVs (>100 cm/second) are also sug- gestive of high-grade stenoses. Although the high-velocity criteria (PSV, EDV, Vr) are the most accurate to assess the risk of graft throm- bosis, low-velocity criteria are also helpful. The graft flow velocity (GFV) should normally exceed 45 cm/second. The velocities are used to stratify the risk of thrombosis (Table 3.3) (Mills, 2001). Prophylactic graft revision should be considered in grafts with a high risk of thrombosis.
Duplex examination is also useful for the diagnosis of popliteal artery aneurysm. This condition should be suspected if a widened popliteal pulse is palpated. An arterial diameter Table 3.1. Noninvasive measurements consistent with
impaired wound healing
Ankle–brachial index (ABI) <0.3–0.4
Toe pressure <35–40mmHg
Skin perfusion pressure <30mmHg Pulse volume recording (PVR) Flat forefoot tracing
Figure 3.5. Color duplex image of a high-grade carotid stenosis with a peak systolic velocity of 319 and end-diastolic velocity of 123.
Table 3.2. Doppler velocity criteria for carotid stenosis Stenosis (%) PSV EDV (cm/sec) Vr
Normal <123 <140 <4.0
1–15 <123 <140 <4.0
16–49 <123 <140 <4.0
50–79 >123 <140 <4.0
80–99 >123 >140 >4.0
EDV, end-diastolic velocity; PSV, peak systolic velocity; Vr, velocity ratio.
greater than 10 mm is considered abnormal.
Bilateral aneurysms are seen in about 50% of cases, and proximal aneurysms occur in approx- imately 30% to 50% of patients. Consequently, it is imperative that aneurysms at other locations be excluded, with abdominal aortic aneurysms and femoral aneurysms the most common.
Conclusion
Noninvasive vascular laboratory tests comple- ment the clinical evaluation in determining the appropriate therapeutic approaches to patients with peripheral vascular disease. The specific laboratory tests should be tailored to the individual patient’s diagnoses. The labora- tory provides noninvasive information on both functional and anatomical aspects of vascular disease.
References
Castronuovo JJ Jr, Adera HM, Smiell JM, Price RM. (1997) J Vasc Surg 26:629–37.
Mills JL Sr. (2001) Semin Vasc Surg 14:169–76.
Vitti MJ, Robinson DV, Hauer-Jensen M, et al. (1994) Ann Vasc Surg 8:99–106.
Table 3.3. Risk stratification for graft thrombosis
Category PSV Vr EDV GFV
(cm/sec) (cm/sec) (cm/sec) Highest risk >300 >3.5 >100 <45 High risk >300 >3.5 <100 >45 Intermediate 180–300 >2.0 <100 >45
risk
Low risk <180 <2.0 <100 >45 EDV, end-diastolic velocity; GFV, graft flow velocity; PSV, peak systolic velocity; Vr, velocity ratio.
