Chapter 12 Vascular Laboratory Testing
Vascular laboratory technology offers many cost-effective applications in the practice of vascular medicine.1 Vascular testing includes both physiological testing and duplex ultrasonography. Physiological testing includes segmental pressure measurements, pulse volume recordings, continuous wave Doppler, and plethysmography. These tests employ sphygmomanometric cuffs, Doppler instruments, and plethysmographic recording devices. Duplex ultrasonography combines gray-scale and Doppler imaging with spectral and color Doppler and is used for the majority of vascular laboratory tests. An ultrasound machine should be equipped with vascular software and two transducers/probes, 5- to 12-MHz transducers for the neck and extremities, and 2.25- to 3.5-MHz transducers for the abdomen.
Limb segmental systolic blood pressure measurements and pulse volume recordings are used to confirm a clinical diagnosis of peripheral artery disease (PAD) and further define the level and extent of the obstruction. Segmental pressures are typically measured in conjunction with segmental limb plethysmography (pulse volume recordings). These techniques are used predominantly in the lower extremities, but are also applicable to the arms. Both procedures are performed using sphygmomanometric cuffs appropriately sized to the diameter of the limb segment under study. The patient rests in the supine position for at least 10 minutes prior to measuring limb pressures. Commercially available machines with automatic cuff inflation are able to digitally store the pressures and waveforms. A continuous-wave (CW) Doppler instrument with a 4- to 8-MHz transducer frequency is used to detect the arterial flow signal. The cuff is quickly inflated to a suprasystolic pressure and then slowly deflated until a flow signal occurs. The cuff pressure at which the flow signal is detected is the systolic pressure in the arterial segment beneath the cuff. For example, if the cuff is on the high thigh and the sensor is over the posterior tibial artery at the ankle, the measured pressure is reflective of the proximal superficial and deep femoral arteries (DFAs) beneath the cuff, as well as any collateral arteries, and not the posterior tibial artery. The Doppler flow signal from an artery at the ankle is typically used for all limb measurements. It is more accurate, although less convenient, to place the Doppler transducer probe close to the cuff being inflated.
Sphygmomanometric cuffs are positioned on each arm above the antecubital fossa, on the upper portion of each thigh (high thigh), on the lower portions of the thighs above the patella (low thigh), on the calves below the tibial tubercle, and on the ankles above the malleoli. Typically, foot pressures are measured by insonating the posterior tibial and anterior tibial arteries at the ankle level. Both arm pressures at the brachial artery are determined. A difference of greater than 20 mmHg between the arm pressures indicates the presence of stenosis on the side of the lower pressure. Pressure measurements are made at the high thigh, low thigh, calf, and ankle levels with a tibial or dorsalis pedis signal selected as the flow indicator. A second method uses one long, contoured thigh cuff rather than two separate thigh cuffs. The lower-extremity pressure evaluation should begin at the ankle level and proceed proximally. Patients who are found to have a normal pressure measurement at rest may require a treadmill exercise test to detect PAD. If disease distal to the ankle is suspected, pedal or digital artery obstruction can be evaluated with cuffs sized appropriately for the toes.
Segmental limb pressures are compared with the highest arm pressure. Ankle pressures are used to calculate the ankle-brachial indices (ABI) for each extremity. This is accomplished by dividing each of the ankle pressures by the higher of the brachial artery pressures.2 A normal ABI is between 1.0 and 1.4, whereas an ABI above 0.9 to 1.0 is borderline abnormal.3 Studies that evaluated the ABI in healthy subjects and patients with PAD confirmed by arteriography found that an ABI of 0.9 or lower was diagnostic of PAD with 79% to 95% specificity and 96% to 100% sensitivity.4 Pressures are compared between levels. A 20-mmHg or greater reduction in pressures from one level to the next is considered significant and indicates stenosis between those two levels. In healthy subjects, the high thigh pressure determined by cuff typically exceeds the brachial artery pressure by approximately 30 mmHg. A thigh/brachial index above 1 is interpreted as normal, and an index of 1 or less indicates stenosis proximal to the thigh (Fig. 12-1). When high thigh pressures are low compared with arm pressure, the site of obstruction could be in the aorta or ipsilateral iliac artery, common femoral artery (CFA), or proximal superficial femoral artery (SFA) (see Fig. 12-1). If only one high thigh pressure is less than the brachial pressure, an ipsilateral iliofemoral artery stenosis is inferred.
Right leg has a pressure drop between low thigh and calf consistent with superficial femoral/popliteal artery stenosis. Left leg has a pressure drop at level of high thigh consistent with iliofemoral artery stenosis. ABI, ankle-brachial index.
In the presence of severe vascular calcification, systolic pressures cannot be determined because the vessels are noncompressible. An index of 1.4 or greater suggests vascular calcification artifact and makes interpretation of the pressure measurement unreliable. Presence or absence of a significant pressure gradient cannot be determined in the presence of vascular calcification artifact. In this setting, the toe brachial index (TBI) is a useful measurement. Toe brachial index is the ratio of the systolic pressure in the toe to the brachial artery systolic pressure. This should be performed in a warm room; cold-induced vasospasm may lower the digital pressure. To perform the procedure, a cuff is placed on a toe. Typically, the great toe is used. The pulse waveform is obtained by photoplethysmography or Doppler. The cuff is inflated to suprasystolic pressure and then deflated. Systolic pressure is determined as the pressure at which the waveform reappears. A normal value for TBI is 0.70.
The same cuffs used to measure segmental pressures may be attached to a plethysmographic instrument and used to record the change in volume of a limb segment with each pulse, designated the pulse volume. Pulse volume waveform evaluation allows assessment of arterial flow in regions of calcified vessels because the test does not rely on cuff occlusion of the calcified artery.5 Each cuff is inflated in sequence to a predetermined reference pressure up to 65 mmHg. The change in volume in the limb segment causes a corresponding change in pressure in the cuff throughout the cardiac cycle. Interpretation of the pulse volume recording (PVR) requires calibration of the amount of air in the cuff.
A pulse volume waveform is recorded for each limb segment. Pulse volume recording analysis is based on evaluation of waveform shape, signal, and amplitude (Fig. 12-2). The configuration of the normal pulse volume waveform resembles the arterial pressure waveform, and is composed of a sharp systolic upstroke followed by a downstroke that contains a prominent dicrotic notch. A hemodynamically significant stenosis manifests as a change in the PVR contour toward a tardus parvus waveform. Both the slope and amplitude decrease when there is more severe disease. Severity of PAD can be defined by the slope of the upstroke and amplitude of the pulse volume (see Fig. 12-2).
Figure 12-2 Pulse volume recording (PVR).
Normal waveform has a sharp upstroke, dicrotic notch, and a period of diastasis. Mildly abnormal waveform has a delay in upstroke and a straightened downslope (blue line). Moderately abnormal waveform has a delay in upstroke (blue line), flat systolic peak, and diminished amplitude. Severely abnormal waveform has a flat systolic peak and very diminished amplitude.
Pulse waveforms can also be obtained using photoplethysmography, recording reflected infrared light. In photoplethysmography, the signal is proportional to the quantity of red blood cells in the cutaneous circulation; it does not measure volume changes. Waveform shape is assessed in a similar fashion in pulse volume and photoplethysmography recordings. Low photoplethysmographic waveforms in the toes identify increased risk of amputation, in addition to the toe pressure.5
Exercise testing is an adjunctive physiological test to evaluate PAD. It is useful to assess functional capacity and determine the distance patients with claudication are able to walk. Moreover, it can be used to clarify whether leg symptoms are related to PAD. This is relevant in patients with symptoms that are atypical for claudication and in those who have a history of intermittent claudication, yet normal ABIs at rest.6 Relative contraindications to treadmill exercise testing for PAD include rest pain in the leg, shortness of breath with minimal exertion, or unstable angina. The test cannot be performed if the patient cannot walk on a treadmill.
Patients are instructed to fast for 12 hours prior to walking on the treadmill. The constant-load treadmill test is performed at a speed of 2 mph and an incline of 12%. Graded exercise protocols increase the grade and/or speed in 2- to 3-minute stages. The Gardner protocol is the most commonly used graded protocol to evaluate walking exercise capacity.7 It begins at a speed of 2 mph and an incline of 0%, and the grade progressively increases by 2% every 2 minutes, allowing for a wider range of responses to be measured. It is often used to determine clinical trial end points such as change in walking time in response to therapy. Other graded exercise protocols, such as the Bruce protocol, are not commonly used because the rapid rate of speed and incline limits assessment of exercise capacity in claudicants.
The treadmill exercise test is terminated when the patient cannot continue owing to leg claudication or chest pain, or is limited by other symptoms such as shortness of breath or fatigue. The patient then immediately lies down on the stretcher. Ankle pressures are obtained starting with the symptomatic leg, followed by the highest brachial pressure. Pressures are repeated approximately every 1 to 2 minutes until they return to baseline. Data recorded from the exercise test should include ankle pressures, length of time the patient was able to walk, time required for pressures to return to baseline, nature and location of the patient’s symptoms, and reason for discontinuing the test. A decrease in ABI of more than 20% immediately following exercise is diagnostic for PAD. The time before ankle pressure returns to normal is increased in more severe disease (e.g., from 1 minute in mild disease to 10 minutes in more severe disease).
By exploiting variations in color absorbance of oxygenated and deoxygenated hemoglobin (Hb), transcutaneous oximetry can determine the state of blood oxygenation. Oximeters use two light frequencies, red at 600 to 750 nm and infrared at 800 to 1050 nm, to differentiate oxygenated and deoxygenated Hb. Deoxygenated blood absorbs more red light, whereas oxygenated blood absorbs more infrared light. Oximeters typically employ both an emitter and receiver. Red and infrared light is emitted and passes through a relatively translucent structure such as the finger or earlobe. A photodetector determines the ratio of red and infrared light received to derive blood oxygenation. When measured continuously, oxygenation peaks with each heartbeat as fresh oxygenated blood arrives in the zone of measurement. Normal values for oxygen tension are from 50 to 75 mmHg. One probe is placed on the chest as a control to ensure that oxygen tension is from 50 to 75 mmHg. A second probe is placed on the limb in the area of interest. Measurements are obtained from the probe, which is sequentially positioned from proximal to distal segments of the limb. Normal limb TcO2 should approximate that of the chest. Transcutaneous oximetry is most often used to determine the level of amputation. A value above 20 mmHg can predict healing at the site with 80% accuracy.7 This measurement is not affected by arterial calcification.
An ultrasound transducer, or probe, emits sound waves in discrete bundles or pulses into the tissue of interest. On encountering a tissue, a portion of the waves is reflected back to the transducer. The fraction of returning waves depends on density and size of the tissue examined. The depth of tissue is determined by the time required for pulse emission and return. Thus, by integrating the number of returning pulses and the time required for return, a B-mode, or gray-scale image may be created. The time for wave reflection decreases with higher ultrasound probe frequencies. Transducer probes with higher frequencies image superficial tissues better than probes with lower frequencies, but lose depth imaging because of attenuation of the returning emitted pulses.
Improvements in technology have permitted band-width widening of vascular transducers, facilitating analysis of harmonics of the fundamental frequency. A harmonic represents a whole-number multiple of the emitted frequency. Because the tissue compresses and expands in response to the application of ultrasound, the fundamental wave may become distorted, impairing image quality. The distortion, however, also creates harmonics of the original frequency that can be detected by the transducer. By detecting only the fundamental frequency and its harmonics, artifact such as speckle and reverberation may be reduced to create a clearer image.
Normal blood flow is laminar in a straight segment of an artery. If thought of as a telescopic series of flow rings, blood moves forward most rapidly in the middle ring, and velocity decreases in the outer rings as blood comes closer to the vessel wall. The cardiac cycle, defined by its pulsatile nature of flow, causes a continual variation in blood flow velocity, highest with systole and lowest with diastole. The concentric or laminar flow of blood may be disturbed at a normal branching point or with abnormal vessel contours, such as those caused by atherosclerotic plaque. Disturbed or turbulent flow causes a much greater loss of pressure than laminar flow.
Determining flow velocity is a mainstay of vascular ultrasonography. Abnormalities in the vessel wall cause changes in flow velocity and permit detection and assessment of stenotic regions within the vessel. Flow in a normal vessel is proportional to the difference of pressure between the proximal and distal end of the vessel. The prime determinant or limitation of flow is the radius of the vessel because volume of blood flow is determined by the fourth power of the radius. For example, a 50% reduction in vessel radius causes a greater than 90% reduction in blood flow. Thus, blood flow represents an example of Poiseuille’s law, which determines flow of a viscous fluid through a tube. Specifically,
where Q denotes volume of flow, ΔP is pressure at inflow minus the pressure at outflow, r is the radius, η is viscosity, and L is tube length. Because blood viscosity, blood vessel length, and pressure remain relatively stable, the most important determinant of blood flow is vessel lumen size.
Vascular ultrasonography can depict flow velocity by taking advantage of Doppler shift frequencies. Frequency will shift either positively or negatively, depending on direction of blood flow. Variables that determine the size of the shift include the speed of sound, speed of the moving object, and angle between the transmitted beam and moving object. Christoph Doppler described this relationship using the following equation:
where Fd is the Doppler frequency shift, Ft is the Doppler frequency transmitted from the probe, V is the velocity of flow, cos is the cosine, θ is the angle between the beam and direction of the moving object, and c is the velocity of sound.
Although a highly reliable imaging modality, ultrasound does suffer from occasional image artifact.8 Dense objects like vessel-wall calcium deposits permit few sound waves to penetrate, resulting in acoustic shadowing and diminishing imaging of deeper tissues. Tissue imaging enhancement may be noted on the far side of echo-free or liquid-filled zones. Tissue interfaces may generate multiple sound wave reflections, causing “additions” to the tissue termed reverberation artifact. Refraction of the sound pulse may cause improper placement of a structure of an image and shadowing at the edge of a large structure. Highly reflective surfaces may create mirror images because the reflecting tissue alters the timing of the returning sound wave. The mirror image should be equidistant from the reflecting surface or tissue.
Ultrasound images are generated using a pulse echo system. The position of the tissue interface is determined by the time between pulse generation and returning echo. Each returning echo is displayed as a gray dot on a video screen using a brightness mode (B mode) in which the brightness of the dot depends on the strength of the reflected wave. A two-dimensional (2D) image is created by sequentially transmitting waves in multiple directions within a single plane and combining the reflected echoes into a single display. The image can be refreshed rapidly, permitting real-time display of the gray-scale image. The surface of interest should be perpendicular to the ultrasound beam to obtain the brightest echo with B-mode imaging. This is readily achieved in vascular imaging because the neck, extremity, and visceral vessels generally lie parallel to the surface of the transducer. Higher-frequency probes are used to image vessels close to the surface, and lower-frequency probes are used to image deeper vessels. Details of the vessel wall can be seen more clearly with the use of harmonics. The wide band width of transducers allows analysis of returning harmonics (whole-number multiples) of the fundamental frequency.
Velocity recordings are obtained with an angle of 60 degrees between the Doppler insonation beam and the flow. In ultrasound practice, the optimal angle of measurement between the beam and blood flow is 60 degrees. Although maximal shift is detected at 0 degrees, this angle cannot be reliably obtained in vascular imaging because the vessels are parallel to the surface of the body. Insonation angles below and above 60 degrees influence the measurement such that small reductions in the insonation angle may alter velocity by 10%, whereas small increases in insonation angle may change flow velocity by 25% (Fig. 12-3). Thus, the sample-volume cursor is placed parallel to the inner wall, and a Doppler9,10 angle from 30 to 60 degrees between the wall and the insonation beam (or flow jet) is used. A normal peripheral artery Doppler waveform consists of a narrow, sharply defined tracing. This indicates that all blood cells are moving at an equivalent speed at any time in the cardiac cycle.11 Waveforms are also characterized as high resistance due to limited flow during diastole (e.g., normal peripheral arterial Doppler velocity waveform), or low resistance with continuous flow during diastole, as when downstream resistance arterioles are widely dilated or there is contiguity with low-resistance circuits (e.g., normal internal carotid artery [ICA] velocity waveform) (Fig. 12-4). The normal high resistance waveform is typically triphasic. The first component is caused by initial high-velocity forward flow during ventricular systole. A range of normal peak systolic velocity (PSV) measurements have been defined12 for each arterial segment, described later in this chapter.
Dashed lines represent different insonation beams. Solid arrow represents direction of flow and position of Doppler sample cursor. Velocity is determined using the Doppler equation, with the cosine (cos) in the denominator. The cos θ degrees = 1, cos 30 degrees = 0.86, cos 60 degrees = 0.5, and cos 90 degrees = 0. c, velocity of sound; Fd, Doppler frequency shift; Ft, transmitted Doppler frequency; V, velocity.
Figure 12-4 High- and low-resistance waveforms.
The second phase of the waveform consists of early diastolic flow reversal as left ventricular (LV) pressure falls below aortic pressure prior to aortic valve closure.13 The final or third component is a small amount of forward flow when there is elastic recoil of vessel walls. Flow is typically not uniform or laminar at bifurcations and sites of stenosis; at these sites flow becomes turbulent. For these locations, the spectral Doppler waveform reflects the fact that blood cells move with varying velocities. Instead of a narrow well-defined tracing (see Fig. 12-4), spectral broadening becomes evident (Fig. 12-5), with partial or complete filling-in of the area under the spectral waveform. This third, or late, diastolic component is usually absent in atherosclerotic vessels that have lost compliance or elasticity.
Color Doppler is the phase or frequency shift information contained in the returning echoes and processed in real time to form a velocity map over the entire imaging field.14 Doppler frequency-shift data are available for every point imaged. This information is then superimposed on the gray-scale image to provide a composite real-time display of both anatomy and flow. When motion is detected, it is assigned a color, typically red or blue, determined by whether the frequency shift is toward or away from the probe. Color assignment is arbitrary and can be altered by the user, but most choose to assign the color red to arteries and blue to veins. With increasing Doppler frequency shifts, the hue and intensity of the color display change, with progressive desaturation of the color and a shift toward white at the highest detectable velocities.
The pulse repetition frequency (velocity) scale determines the degree of color saturation and filling of the vessel lumen. The pulse repetition frequency (radio frequency pulses per second from the probe) is adjusted so that in a normal vessel, laminar flow appears as a homogeneous color. The color appearance changes throughout the cardiac cycle. Increasing flow velocity and turbulence in the region of a stenosis results in production of a high-velocity jet and an abrupt change in color-flow pattern (Fig. 12-6). Color aliasing occurs at the site of stenosis when flow velocity exceeds the Nyquist limit (i.e., when Doppler frequency shift exceeds half the pulse repetition frequency). Aliasing causes the color display to appear as if there is an abrupt reversal in direction of flow (wraparound). This suggests a high-velocity flow jet, requiring confirmation by pulsed-wave Doppler analysis. Color persistence is a continuous flow signal that is the color of the forward direction only, in contrast to the alternating color in normal arteries. There is loss of early diastolic flow reversal. Color persistence corresponds to the monophasic spectral Doppler waveform and is indicative of severe stenosis. Post-stenotic regions display mosaic patterns indicating turbulent flow (see Fig. 12-6). A color bruit in the surrounding soft tissue also indicates flow disturbance. This color artifact is associated with turbulence and occurs with flow disturbances associated with high-velocity jets. The color bruit is particularly useful in locating postcatheterization arteriovenous fistulae (AVF).
Characteristic duplex ultrasound features of a stenosis include elevated systolic velocity, elevated end-diastolic velocity (EDV), color aliasing, color bruit, spectral broadening of the Doppler waveform, post-stenotic flow, and post-stenotic turbulence. An auditory “thump” occurs in the presence of total arterial occlusion. Doppler velocity measurements are the main tools used to evaluate stenosis severity. When flow rate is constant, a decrease in vessel cross-sectional area is balanced by an increase in velocity.13 As blood flow turbulence increases, spectral broadening of the Doppler waveform becomes a clear indicator of turbulent flow seen in the post-stenotic region. The post-stenotic waveform is dampened with a delayed upstroke (see Fig. 12-3). If no post-stenotic turbulence can be identified, inappropriate angle alignment or a tortuous vessel should be suspected.
Power (or amplitude) Doppler is a complementary imaging technique that displays the total strength or amplitude of the returning Doppler signal.15 In comparison with conventional color-flow imaging, color-flow sensitivity is increased by a factor of 3 to 5 times with power Doppler. This enhanced dynamic range can depict very slow flow in the area of a subtotal occlusion that may not be detected by conventional color-flow Doppler. Contrast agents can also help differentiate between occlusion and high-grade stenosis in carotid and renal arteries, especially in cases where multiple renal arteries are present.16
The standard carotid duplex examination includes assessment of the carotid arteries as well as the vertebral, subclavian, and brachiocephalic arteries. Indications for this test include a bruit, transient ischemic attack (TIA), amaurosis fugax, stroke, and surveillance after revascularization.17 The examination begins with a gray-scale survey of the extracranial carotid arteries in transverse and longitudinal views. The operator images the region from the clavicle to the angle of the jaw, in both anterolateral and posterolateral views.18 The common carotid artery (CCA) is typically medial to the internal jugular vein, and the bifurcation is often located near the cricoid cartilage. The ICA is usually posterolateral, with a diameter at its origin greater than that of the anteromedially located external carotid artery (ECA).
Carotid artery stenosis can be focal, and flow patterns can normalize within a short distance. Therefore, the pulse-wave sample volume should be methodically advanced along the length of the vessel; color Doppler may be used for guidance in delineating areas of abnormal flow requiring change in position of the sample volume (Fig. 12-7). Representative velocity measurements should be recorded from the proximal, mid- and distal CCA. The CCA spectral waveform is a combination of the ECA and ICA waveforms, with greater diastolic flow than the ECA but less than the ICA. Atherosclerosis, when present, is usually most evident at the ICA origin, whereas fibromuscular dysplasia may be more evident distally. Using spectral Doppler, the sample volume is advanced throughout the entire ICA. At a minimum, PSV and EDV from the proximal, mid-, and distal ICA segments should be recorded. The vertebral artery is then located posterior to the carotid artery. The vertebral artery and vein lie between the spinous processes. The vertebral artery is followed as far cephalad as possible, sampling the spectral Doppler in the accessible portions of the vertebral artery.
Internal carotid artery (ICA) in each is slightly wider at the origin than external carotid artery (ECA). Red arrow indicates plaque in proximal right ICA. A branch is evident arising from left ECA. In the absence of identified branches, waveforms are necessary to distinguish the ICA from the ECA. CCA, common carotid artery.
Distinguishing between the ICA and ECA is critical to the examination (Fig. 12-8). The ECA is usually smaller, more anteromedial, and has less diastolic flow than the ICA. The ECA will also have branches in the cervical region, whereas the ICA will not. Direct comparison of the waveforms from the two vessels is critical. A velocity waveform obtained from the proximal vessel or the site of maximal velocity should be obtained while intermittently tapping on the preauricular branch of the temporal artery. The intermittent tapping is reflected clearly in the diastolic portion of the ECA waveform, but not in the ICA waveform (Fig. 12-9).