Fig. 22.1
Normal arterial velocity tracing (multiphasic). (a = systolic component; b = early diastolic component; c = late diastolic component)
Therefore, there are several types of Doppler velocimetry:
- 1.
Auditory: this processes the Doppler signal as sound. It has the advantage of containing all Doppler frequencies with the exception of those extreme frequencies removed by filtering. A trained technician or physician can easily distinguish normal signals from those received proximal to, within, or distal to a stenosis or occlusion. A higher-pitched signal can mean that the probe angle is very acute to the vessel angle or it can indicate a significant arterial occlusion.
- 2.
Analog wave tracing: this method employs a zero-crossing frequency meter to display the signals graphically on a strip chart recorder. It has an acceptable overall accuracy, but it is not as sensitive as the spectral analysis, and it also has the following drawbacks, noise and under- or overestimation of high and low velocities, respectively.
- 3.
Spectral analysis: this method displays frequency on the vertical axis, time on the horizontal axis, and the amplitude of backscattered signals at any frequency and time (Fig. 22.2). It has the advantage of displaying the amplitudes at all frequencies, but it is free of many of the disadvantages that were previously described for the analog wave tracing.
Fig. 22.2
A spectral analysis of the right common femoral artery. This method displays frequency on the vertical axis, time on the horizontal axis, and the amplitude of backscattered signals at any frequency and time. (This picture was taken by a color duplex ultrasound machine)
Indications for Testing
The arterial lower extremity Doppler examination is a useful tool in many aspects of peripheral vascular medicine. It validates the diagnosis of the presence, location, and severity of arterial occlusive disease, helping to differentiate true vascular claudication from pseudoclaudication that arises from neurologic or musculoskeletal disorders. Therefore, this test is indicated for patients with symptoms and signs of arterial occlusive disease, which vary from claudication and rest pain to skin changes suggestive of arterial insufficiency, e.g., nonhealing ulcers [3–7]. The arterial lower extremity Doppler study is also helpful in determining the level of leg amputation and the benefit from lumbar sympathectomy [8–10]. In addition, the Doppler examination is useful in screening patients with Raynaud’s disease or syndrome [11], or arteriovenous (AV) fistula [12], and to rapidly assess patients who have suffered possible arterial trauma.
In the case of iatrogenic arterial injury, Doppler ultrasound is suitable for assessing post-catheterization arterial obstruction following femoral or brachial cardiac catheterization or peripheral arteriography. Similarly, any complication following insertion of indwelling arterial monitoring catheters can be readily screened with the Doppler detector. It is also helpful in patients with shock.
Intraoperative measurement of ankle pressures after completion of aortofemoral bypass or aortoiliac endarterectomy can be used to predict the results of the procedure. The determination of segmental pressure measurements in the postoperative period aids in quantitatively assessing the results of aortofemoral bypass (see below). Ankle to brachial indices can be used to detect progression of lower extremity arterial disease after surgical intervention, which can present as worsening native arterial disease or a failing bypass conduit. These studies can be conducted formally in the vascular lab or rapidly at bedside or the postoperative recovery area using a continuous-wave Doppler and a handheld sphygmomanometer. A drop of the ankle to brachial index of more than 0.15 carries a sensitivity of 41% and a specificity of 84% in detecting the progression of lower extremity arterial disease [13].
Indications for Arterial Doppler Examination
- 1.
Calf pain while walking (claudication)
- 2.
Leg pain at rest, suggestive of ischemia
- 3.
Skin changes suggestive of arterial insufficiency
- 4.
Nonhealing ulcers
- 5.
Previous vascular reconstructive procedures—follow-up
- 6.
Intraoperative application
- 7.
Determination of the level of amputation and the response after lumbar sympathectomy
- 8.
Assistance in the diagnosis of Raynaud’s disease or phenomenon and arteriovenous fistula
- 9.
Detection of pulses in shock states or in trauma
Methods and Interpretations
The complete arterial lower extremity Doppler examination consists of three components: (1) analysis of the arterial analog wave tracing, (2) measurement of the segmental systolic limb pressures, and (3) calculation of the ankle-brachial index (ABI).
After the history is taken, the patient is allowed to rest in the supine position on the examining table for 10–15 min to ensure the measurement of pressures in the resting state. The patient is placed in the supine position with the extremities at the level of the heart. The head of the bed can be elevated slightly, and the patient’s head can rest on a pillow. The patient’s hip is generally externally rotated with the knee slightly bent to facilitate the lower extremity evaluation. Alternative positions for the Doppler lower extremity examination include right or left lateral decubitus (patient on his or her side) or the prone position for access to the popliteal artery.
Gornik et al. recently validated a method of determining the ankle to brachial index in the seated position [14]. This could broaden the availability of peripheral arterial disease testing in patients compromised by musculoskeletal or cardiopulmonary conditions that limit their ability to tolerate the supine position.
The Doppler probe (transducer) must be positioned on the long axis of the vessel. An angle of insonation of approximately 45–60° is usually used for this study. The leg pulses (femoral, popliteal, dorsalis pedis, and posterior tibial) are evaluated by palpation and by audible Doppler signals. The pulses are graded as II, I, or 0, and the Doppler signals are graded as normal (biphasic), abnormal (monophasic), or absent.
Qualitative Doppler Waveform Analysis
For the lower extremities, Doppler velocity waveforms are recorded from the following arteries bilaterally: (1) common femoral artery at the groin level, (2) superficial femoral artery, (3) popliteal artery, (4) posterior tibial artery (at the level of the medial malleolus), (5) dorsalis pedis artery (at the dorsum of the foot), and (6) occasionally the peroneal artery (at the level of the lateral malleolus). Auditory signals are obtained. If the examiner is using a headset, the right earphone provides forward (antegrade) flow signals, while the left earphone provides reverse (retrograde) flow signals. The qualities of the auditory signals and the waveforms are observed and analyzed.
The normal arterial velocity signal is multiphasic. That is, it is characterized by one systolic and one or more diastolic components (Fig. 22.1). In the major peripheral arteries, the systolic component is a large positive deflection indicative of a high net forward flow velocity. This is followed by a brief period of net flow reversal. This flow reversal is then followed immediately by another positive deflection, the diastolic forward flow component. The brief period of flow reversal characteristic of the major peripheral arterial velocity signal is a function of the generally high resistance of the extremity vascular bed. Lowering resistance, via vasodilation, can eliminate the net flow reversal. The normal arterial velocity signal is also pulsatile, i.e., it cycles with each heartbeat. Thus, the normal nonpulsatile, phasic, low-pitch venous signal is easily differentiated from the pulsatile, multiphasic arterial signal.
Abnormal signals are generally monophasic (Fig. 22.3), nonpulsatile, or absent. Biphasic signals can also be considered abnormal (Fig. 22.3). It is imperative to observe for deterioration of the waveform, e.g., triphasic to biphasic or triphasic to monophasic of the Doppler signal quality from one level to the next level. A monophasic and dampened signal can be obtained proximal to an obstruction as well as distal to it. In the absence of additional obstructions, the distal signal can normalize.
Fig. 22.3
An abnormal arterial tracing of the lower extremity in a patient with stenosis of the common femoral artery. The upper tracing was recorded from the popliteal artery distal to the obstruction, and the lower tracing was taken at the level of the posterior tibial artery. These signals are monophasic
The arterial velocity signal produced just before an occlusion is characteristically of short duration, i.e., a slapping signal of low amplitude. However, the arterial signal produced over a stenotic segment is a high-pitched signal with less prominent diastolic components. The signal from just beyond the stenotic segment is also characterized by dampened systolic and absent diastolic components, but it is not as high pitched as the stenotic signal. The signal beyond an occluded arterial segment is like a post-stenotic signal, although the systolic component may be of even lower amplitude. The signal produced by the prominent collateral arterial signal is high pitched and almost continuous. These similarities among the abnormal arterial velocity signals make the differentiation difficult. Therefore, if there is difficulty in interpreting these signals, they can be called either normal or abnormal for practical purposes (Fig. 22.3).
As noted in the abnormal wave tracing , a Doppler signal obtained from a common femoral artery that is diseased shows the poor quality of the signal (poor upslope and downslope, with a somewhat rounded peak) (Fig. 22.3). A similar waveform can be obtained from the common femoral artery distal to a proximal iliac artery obstruction. Similarly, the Doppler signals obtained from the posterior tibial artery at the level of the ankle distal to that occlusion are somewhat continuous with low-pressure resistance secondary to a vasodilated arterial bed in the presence of proximal arterial obstruction.
Quantitative Interpretation Criteria of Doppler Waveform
Pulsatility Index (PI)
PI is calculated by dividing the peak-to-peak frequency by the mean (average) frequency [15] signal as seen in Fig. 22.4. This ratio is independent of the beam-to-vessel Doppler angle when using handheld Doppler equipment. As seen in Fig. 22.4, the pulsatility index equals P1 to P2 divided by the mean frequency. Normally, the values of the PI increase from the central to peripheral arteries. A PI of >5.5 is normal for the common femoral artery, while a normal PI for the popliteal artery is approximately 8.0. These values decrease in the presence of proximal occlusive disease, e.g., a PI of <4 or 5 in the common femoral artery with a patent superficial femoral artery (SFA) indicates proximal aortoiliac occlusive disease. However, the same reduced PI is not diagnostic if the SFA is occluded.
Fig. 22.4
The method for calculating the pulsatility index
Inverse Damping Factor
This is calculated by dividing the distal PI by the proximal PI of an arterial segment. It indicates the degree to which the wave is dampened as it moves through an arterial segment [15], e.g., severe stenosis or occlusion of the SFA is usually present when the inverse femoral-popliteal damping factor is less than 0.9 (a normal value = 0.9–1.1).
Transient Time
Systole should be simultaneously evident at a specific site bilaterally. Delay on one side may indicate a more proximal occlusive disease. You must compare the signals bilaterally at the same site.
Acceleration Time or Index
This differentiates inflow from outflow disease. It is based on the principle that arterial obstruction proximal to the site of the Doppler probe prolongs the time between the onset of systolic flow to the point of maximum peak in waveforms at the probe site (Fig. 22.5). Figure 22.5a shows a normal common femoral artery tracing. There is a quick systolic upslope representing a normal acceleration time, in contrast to Fig. 22.5b, which shows a slower upslope from the onset of systole to maximum peak from an abnormal common femoral artery tracing. Acceleration time is not prolonged when there is disease distal to the probe. It is applied to those signals evaluated by spectral analysis because it is necessary to maximize sensitivity and minimize artifacts. Generally, an acceleration time of equal to or less than 133 ms suggests the absence of significant aortoiliac disease. False-positive results can occur with technical errors, e.g., a Doppler angle = 70°, which may dampen the Doppler signal qualities, and in the presence of poor cardiac output since the Doppler flow signal will be attenuated with the waveform detecting slow upstroke, rounded peak, and slow downslope.
Fig. 22.5
A normal common femoral artery tracing with a normal acceleration time (a) and an abnormal common femoral artery tracing with an abnormal acceleration time (b)
Doppler-Derived Maximal Systolic Acceleration
Van Tongeren et al. recently evaluated the Doppler-derived maximal systolic acceleration in determining peripheral arterial occlusive disease in patients with diabetes mellitus [16]. These patients are subject to falsely elevated ankle to brachial indices secondary to medial calcification. They found that a maximal systolic acceleration of >10 m/s2 was highly predictive for the exclusion of peripheral arterial occlusive disease (negative predictive value 95%), whereas a maximal systolic acceleration of <65 m/s2 was highly predictive for the detection of peripheral arterial occlusive disease (positive predictive value 99%).
Limitations of the Analog Wave Tracing Analysis
The Doppler waveforms may be affected by (1) ambient temperature, (2) uncompensated congestive heart failure resulting in dampened waveforms following exercise, (3) an inability to distinguish stenosis from occlusion, (4) an inability to precisely localize the occlusion, (5) and an inability to be applied on patients with casts or extensive bandages that cannot be removed. It is technologist dependent, and the result can vary with the Doppler angle used.
Segmental Doppler Pressures
After completion of the examination and analysis of the arterial analog tracings, the second component of the Doppler examination is started, i.e., determinations of the segmental systolic limb pressures. Doppler segmental pressures have the same capabilities of analog wave tracing , i.e., to help in identifying the presence and severity of arterial occlusive disease, to provide an objective baseline to follow the progression of peripheral vascular disease of the lower extremity and/or the postoperative course, and to somewhat evaluate the treatment plan. The results of this testing are usually combined with the Doppler velocity waveform analysis. The patient preparation and positioning are similar to those of the Doppler velocity waveform analysis.
Technique for Segmental Doppler Pressures
The brachial artery Doppler systolic pressures are measured in each arm. Cuffs of appropriate size (bladder dimension 12 × 40 cm) are placed on each arm. The brachial artery is palpated in the antecubital fossa, and a small amount of acoustic gel is applied to the skin over the artery. The arterial signal is found using the Doppler probe, and then the cuff is inflated until the signal disappears (20–30 mmHg beyond the last audible Doppler signal). The cuff is slowly deflated until the arterial signal is again audible, at which time the pressure is recorded. Unlike the standard stethoscope, as the cuff is further deflated, the velocity signal will not disappear, so the diastolic pressure cannot be determined.
Four 12 × 40 cm pneumatic cuffs are applied at various levels on each leg: as high on the thigh as possible, just above the knee, just below the knee, and above the ankle (Fig. 22.6a). The examiner then listens to the posterior tibial and the dorsalis pedis arterial signals (Fig. 22.6b). The posterior tibial artery is found just posterior to the medial malleolus, and the dorsalis pedis artery is found on the dorsum of the foot. Occasionally, the peroneal (lateral tarsal) artery is examined (found just anterior to the lateral malleolus). Of these vessels the one with the strongest Doppler signal is chosen for the ankle pressure. If none of the vessels can be located with the ultrasound probe, the popliteal artery signal is identified in the popliteal fossa. High-thigh, above-knee, below-knee, and ankle pressure readings are taken. An automatic cuff inflator may be used to save time. An alternative method involves using only three cuffs with a single, relatively wide cuff at the mid-thigh.
Fig. 22.6
(a) Technique for measuring the segmental Doppler pressures using the four-cuff method. (b) The application of the Doppler probe on the dorsalis pedis artery
Several important facts concerning cuff characteristics should be noted. It is most important that the pneumatic bladder of the cuff completely encircle the limb. The bladder of the cuff should be placed over the artery. This is especially important when the bladder does not encircle the limb. Just as bladder length affects the pressure determination, bladder width must also be related to the limb diameter. For the most accurate measurement of blood pressure, the width of the pneumatic cuff should be 20% greater than the diameter of the limb [3]. For all practical purposes, this means that larger arms require wider cuffs. A cuff that is too narrow, relative to the limb diameter, results in an erroneously high pressure (30–90 mmHg greater than arm pressure).
The four cuffs used in this test to determine the segmental pressures at different levels of the lower limb are all the same width (12 × 40 cm), making the pressures at the widest part of the limb (high thigh) erroneously high. Some laboratories use a large (19 × 40 cm) thigh cuff to comply with the recommended width/girth relationship and thereby give a more accurate thigh pressure. However, the cuff is so wide that only one can be placed on the thigh. The three-cuff technique utilizes one large cuff placed as high as possible on the thigh. With this technique, a more accurate thigh pressure is obtained (a thigh pressure that is very similar to the higher brachial pressure).
Segmental Doppler pressures of the lower extremity are obtained bilaterally at the following sites and in this order using a handheld or machine sphygmomanometer with automatic display: ankle pressure (using the posterior tibial artery and dorsalis pedis artery); below-knee pressure (calf pressure), using the best signal of the posterior tibial artery or the dorsalis pedis artery; above-knee pressure (same as below-knee pressure, although the popliteal artery can be used if the ankle Doppler signals are difficult to obtain); and high-thigh pressure (the same as above-knee pressure). If a pressure measurement needs to be repeated, the cuff should be fully deflated for about 1 min prior to repeat inflation.
Barnes [4] used a narrower cuff (12 × 40 cm) for measuring the proximal and distal thigh pressures and accepted the artificially high values obtained. This technique allows an approximation of the common femoral (inflow) artery pressure by the proximal cuff and the superficial femoral artery pressure by the above-knee cuff. When only one large cuff is used on the thigh, the single thigh pressure measured does not differentiate aortoiliac from superficial femoral artery occlusive disease. For convenience, the aneroid manometer is used rather than the mercury manometer. The aneroid manometer has the advantage of being portable, inexpensive, easily exchanged from cuff to cuff, and an accurate pressure registering system.
For the analog wave recording technique , diastolic pressure is taken as the pressure at which there is continuous forward flow during diastole. But this point of return of continuous forward flow in diastole would be difficult to determine in the vasoconstricted or high-resistant limb, because the tracing would be constantly crossing to zero during the period of net flow reversal. The problem is overcome by purposely inducing a state of reactive hyperemia in the vasoconstricted limb. This hyperemia is a state of vasodilation.
Interpretations
After determination of the segmental systolic limb pressures, analysis of the various segment pressures is done. Normally, the proximal thigh pressure should be 20–30 mmHg higher than that of the arm, and the pressure gradient between adjacent levels of measurement in the leg should be no greater than 20–30 mmHg. A low proximal thigh pressure signifies aortoiliac or common femoral occlusive disease. An abnormal gradient between the proximal thigh and the above- or below-knee cuff is indicative of superficial femoral or popliteal artery occlusive disease. An abnormal gradient between the below-knee and ankle cuffs indicates tibioperoneal disease. Figure 22.7 shows a patient with occlusion of the left superficial femoral artery as indicated by the pressure differential between the high-thigh and above-knee readings (160–116 mmHg, respectively). A horizontal difference of 20–30 mmHg or more suggests significant disease at or above the level of the leg with the lower pressure. Figure 22.8 shows a patient with significant stenosis or occlusion at the aortoiliac level as indicated by low high-thigh pressures bilaterally.
Fig. 22.7
Segmental systolic limb pressures of a patient with severe stenosis of the left superficial femoral artery
Fig. 22.8
A patient with significant stenosis or occlusion at the aortoiliac level as indicated by low high-thigh pressures bilaterally
Thigh Pressure Indexes
Thigh pressure/higher brachial pressures are normally greater than 1.2, while 0.8–1.2 suggests aortoiliac occlusive disease, and less than 0.8 indicates that proximal occlusion is likely. When the high-thigh pressure is low compared with the brachial artery pressure, the level of obstruction could be beneath the cuff as well [17]. Thus, the site of obstruction could be the aorta, iliac artery, common femoral artery, or even the proximal superficial femoral artery. With the three-cuff technique, the large, single thigh cuff segmental pressure is normally similar to the brachial pressure.
Ankle-Brachial Index
Another component of the arterial lower extremity Doppler examination is the calculation of the ankle-brachial index (ABI) . From the ankle and brachial systolic pressures, a ratio is obtained that is helpful in determining the presence and magnitude of occlusive disease.
Since normal lower limbs have ankle pressures equal to or greater than their ipsilateral arm pressures (recorded in a supine position), a ratio of 1.0 or greater is taken as normal. However, mild to moderate atherosclerotic disease may not affect resting ankle pressures significantly, so all persons having an ABI of 1.0 or greater will probably benefit from stress testing, e.g., treadmill exercise as described in detail later.
Numerous methods of calculating the ankle to brachial index have been described based on variances in the numerator taken in the ABI equation [18]. The current method recommended by the American College of Cardiology/American Heart Association uses the higher of the two ankle systolic arterial pressures, termed the high ankle pressure (HAP), as the numerator in the ABI equation [19]. A second method uses the lower of the two ankle systolic arterial pressures, termed the low ankle pressure (LAP) as the numerator when calculating the ABI [20]. Some studies use the average of the two ankle systolic pressures as the numerator [21]. Some studies have used the posterior tibial systolic pressure to calculate the ABI [22].
It is generally agreed upon that an ABI of 0.9–1.0 signifies normalcy or minimal arterial occlusive disease, an ABI of 0.5–0.9 signifies a claudication level, less than 0.5 signifies the presence of ischemic rest pain or severe arterial occlusive disease, and less than 0.3 is compatible with trophic changes of the lower extremity. Some believe that an absolute ankle pressure of less than 50 mmHg, rather than an ABI of 0.5, is better at predicting symptoms at rest. It has also been suggested that an ABI of equal to or more than 0.5 represents single segment involvement and that lower values are more indicative of multilevel disease [5].
Technique for Toe Doppler Systolic Pressure
The digital study is often done in combination with a physiological lower extremity arterial test, usually a segmental Doppler pressure with or without Doppler waveform analysis. An appropriately sized cuff, the width of which should be at least 1.2 times that of the toe, is applied to the base of the toe (s). Two 2.5-cm cuffs are usually used for fingers and a 2.5- to 3-cm cuff for the great toe. The digital pulse can be examined using the usual Doppler probe, and a similar technique is applied to measure the Doppler toe pressure. In situations where the first toe pressure cannot be assessed (i.e., severe ulceration of the first toe precluding practical cuff placement or a previous first toe amputation), Bhamidipaty et al. found reliable results measuring the systolic pressure in the adjacent second toe [23].
Normal toe pressures vary from 60% to 80% of the ankle pressures. Values significantly less than this signify digital arterial occlusive disease. The exception to this criteria is when the ankle pressure is artificially high (arterial calcinosis), in which case the toe pressure may be much lower than 80% of the ankle pressure in the absence of digital artery disease. It is generally believed that there is little difference between the toe pressures in diabetics and nondiabetics, which makes toe pressure determination very helpful in patients with very artificially high segmental Doppler pressures at the ankle level [24].
Since toe systolic pressures and/or TBIs are likely to be less affected by medial calcification, false-positive results are rare [25, 26]. While authorities define critical forefoot ischemia as an absolute toe pressure <30 mmHg [27], measurements can be inaccurate in the presence of vasoconstriction due to a cold environment or patients with underlying vasospastic pathologies [28]. Thus, a diminished toe pressure can be due to either peripheral arterial disease or vasoconstriction. A toe pressure of >55 mmHg has been correlated with the ability to heal a foot ulcer in diabetic patients [29].
A toe-to-brachial index (TBI) >0.75 is generally considered as normal, whereas a TBI <0.25 is indicative of significant peripheral arterial disease [30]. Caution must be used, however, in attempting to use an absolute TBI cutoff value for determining the presence of distal small vessel disease when applying normal versus abnormal TBI results. For example, if the normal TBI cutoff value of 0.75 is chosen, a lesser result of 0.55 may only reflect more advanced proximal arterial disease and not the condition of the digital vessels themselves. Some recent studies have questioned the overall applicability of the TBI, noting ambiguous diagnostic criteria and the lack of evidence-based guidelines for a definitive cutoff value [31–33]. Yet others maintain that the TBI remains an effective measurement for detecting underlying PAD and is often preferred in specific patient populations [34, 35]. Certainly, the TBI can be used to augment the accuracy of ABI results in patients where ABI values may be presumed suspect [36].
Limitations and Sources of Error in Doppler Segmental Pressure Determination
- 1.
Media sclerosis: This may cause falsely elevated Doppler pressures in those patients with calcified vessels, e.g., patients with diabetes or end-stage renal disease. Toursarkissian et al. [24] reported the results of a retrospective review of 101 diabetic patients without aortoiliac disease to analyze the ability of various noninvasive tests to predict the level of >50% significant stenosis of infrainguinal arterial disease. Patients were studied with ABI, toe-brachial indices (TBI), segmental pulse volume recording (pulse volume recording), segmental pressures, segmental Doppler waveforms, and arteriography. Their findings showed that as a single test, the Doppler waveform appears to have the best angiographic correlation, although the summed diagnosis of combined Doppler waveform and pulse volume recording data was superior in distinguishing multilevel disease from isolated tibial disease. It was also concluded that segmental Doppler pressures were of limited value in patients with diabetes mellitus, even in multimodality testing.
- 2.
Hypertension: When the systemic pressure is elevated, the absolute post-stenotic values are also erroneously high. Since there is no linear relationship between the change in the systemic pressure and the peripheral pressure, the measurement should always be repeated after the systemic pressure has normalized.
- 3.
In patients with multilevel occlusive disease, it is difficult to interpret segmental pressures.
- 4.
Measurement of pressure post exercise: Two examiners should carry out the examination simultaneously after physical exertion to evaluate both extremities; otherwise an adequate rest period between the measurement of the right and left sides is needed. The lower extremity that has the lower resting pressure should be measured first, because the recovery time, post exercise, is otherwise too long in pathological cases.
- 5.
Edema: In solid edema, especially lipedema, adequate arterial compression may fail, causing erroneously high pressure values.
- 6.
Patients with uncompensated congestive heart failure may show decreased ankle-brachial indexes after exercise.
- 7.
This test cannot distinguish between stenosis and occlusion and cannot precisely localize the area of occlusion, although it can identify a general location. Similarly, it cannot distinguish between common femoral artery disease and proximal external iliac artery disease.
- 8.
Resting period: An adequate resting period of 10–20 min before measurements are taken must be observed. Where there are poorly compensated flow obstructions, the resting period should be longer, in order to avoid measuring erroneously low-pressure values.
- 9.
Deflation errors: Releasing the cuff pressure too quickly (above 5 mmHg/s) causes erroneously low values. Therefore, a deflation velocity of around 2 mmHg/s should be maintained.
- 10.
Arm-leg measurement intervals: The time difference between Doppler pressure measurements should not be too long. Intraindividual systemic blood pressure fluctuations can occur and affect the results.
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