Vascular testing for the surgical resident

TECHNOLOGY Vascular Testing for the Surgical Resident TIMOTHY C. HODGES, MD, AND CHARLES W. VAN WAY III, MD BASIC CONCEPTS IN VASCULAR ULTRASOUND Understanding the tests performed in a vascular laboratory is crucial to the proper education of surgical residents. However, a busy rotation schedule often fails to provide an opportunity for the resident to even visit the vascular lab, much less spend time observing examinations or interpretations. Therefore, on our vascular surgery service we have identified certain concepts and principles that a resident can master in a short period of time and that greatly improve an understanding of the laboratory. PHYSICS Despite a good undergraduate exposure to physics, most surgical residents have forgotten the principles of ultrasound by the time they encounter the vascular lab on their clinical rotations. In addition, the physics used in ultrasound are not obvious. A primer is in order. Ultrasound Ultrasound refers to the use of high-frequency sound waves (in excess of 20,000 Hz) that can penetrate biologic tissue. Sound waves in the range used clinically travel through tissue until they meet a change in tissue density. At this density interface, a portion of the wave energy is reflected and a portion travels deeper into the tissue. How much is reflected depends on the relative densities of the two tissues at the interface. For instance, a change from water to bone produces a strong reflection, whereas a change from skin to muscle tissue will reflect much less energy. Ultrasound scanheads (Fig. 1) consist of two components: an emitter and a receiver. Although they could be separate devices, in practice these consist of the same piezoelectric crystal transmitting roughly 10% of the time and receiving (“listening”) roughly 90% of the time. When an ultrasound pulse is sent into tissue and reflected, the receiver “hears” it. The computer in the ultrasound device can perform 2 mea-

Inquiries to Timothy C. Hodges, MD, Assistant Professor of Surgery, University of Missouri–Kansas City, Medical Plaza One, Suite 308, 4320 Wornall Road, Kansas City, MO 64111; fax: (816) 753-3368.

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surements. By measuring the time from transmission to reception and knowing the speed of sound in tissue, the computer can assign a depth to the reflection level. By measuring the strength of the reflected beam relative to its transmission, the computer assigns a value that is mapped to a pixel brightness level on the TV image display. Modern ultrasound images are composed of 256 gray-scale levels (Fig. 2). Combining a group of crystals along the scanhead surface allows a representation of a section of tissue. This information is then used to construct a 2-dimensional black and white picture known as the B-mode image. Ultrasound energy is progressively absorbed by tissue as it travels through greater depth (ie, the further into the body the sound wave travels, the weaker it gets). It follows that the weaker the sound wave, the weaker the reflection. This effect is termed attenuation of the signal. An unmodified B-mode image would become progressively darker as it shows deeper layers. To compensate for this problem, ultrasound machines have potentiometers. These signal boosters allow the operator to adjust the image so that it is equally bright and well defined at all levels. Doppler Equation The Doppler equation is the expression of the Doppler principle—namely, that measured moving sound wave frequency changes with respect to a stationary observer. The mathematical equation is ⌬f ⫽

2␷f cos ⌰

,

c

which states that the change in frequency is related to the velocity of the moving object (␷), the frequency emitted by the object (f ), the angle of observation (⌰), and the speed of sound in the medium (c). In vascular ultrasound examinations, red blood cells (RBCs) are small moving objects that reflect ultrasound waves; these cells are called specular reflectors. Because the emitted ultrasound frequency and the angle of observation are known, the Doppler equation is solved for the velocity of the RBCs. Although there is no single flow measurement that accurately describes the flow profile, the most commonly recorded

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Figure 1. A linear array ultrasound scanhead.

velocities are the peak systolic velocity and the end diastolic velocity. Fluid Dynamics Fluid flow velocity increases when resistance increases in the face of constant pressure. As an illustration, consider a garden hose. Water flows through the hose relatively slowly, until it reaches the nozzle. At that point it flows through a small opening, which greatly accelerates the flow velocity and allows the stream to travel much farther than if it simply flowed out the end of the hose. In vascular terms, this is comparable to blood flowing through an artery until it reaches a stenosis, where flow velocity increases markedly. This increase is measured using the Doppler principle. Much of the useful information obtained from Doppler ultrasound is based on the finding that increased flow velocity means a tighter stenosis. Fast Fourier Transform Intravascular blood flow is very complex. Not only does flow velocity change during the cardiac cycle, there is a different velocity at the margin of the vessel than in the middle, and there are eddy currents and nonlaminar flow. Each of the thousands of reflected ultrasound waves from all the RBCs in a vessel has a different frequency. The fast

Fourier transform (FFT) is the a mathematical tool used by the computer in the ultrasound instrument to break these complex waveforms into their constituent components. The magnitude and direction of the components can be plotted, producing a Doppler spectrum (Fig. 3). This approach to such a complex problem has practical applications. The FFT can be accomplished quickly, allowing real-time display of Doppler information. The display relates to the quality of blood flow. If most of the RBCs are moving in the same direction at the same time, a relatively clean waveform is produced. The clear area under the curve is called the spectral window. However, if turbulence is present, as is often seen with atherosclerotic stenosis, the movements of the RBCs are more chaotic in speed and direction, and the resulting waveform is not as clean, and is said to have spectral broadening. CONTINUOUS-WAVE DOPPLER Bedside Exam Probably the most common ultrasound examination that a surgical resident is called upon to perform and interpret is the bedside Doppler exam. Typically this exam is performed with a handheld, continuous-wave Doppler device

Figure 2. A 256 gray-scale ultrasound image.

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Figure 3. A Doppler spectrum.

(Fig. 4). The probe, unlike more elaborate machines, usually has 2 separate piezoelectric crystals; one transmitting continuously and one receiving continuously. Pointing the narrow ultrasound beam at a vessel produces an audible signal. Often, the continuous-wave Doppler is used to monitor flow when pulses cannot be palpated. Ankle–Brachial Index (ABI) This simple exam allows a comparison of the ankle blood pressure to the arm blood pressure. Under normal circumstances the ankle pressure should be greater than the arm pressure, so that the normal ABI is 1.0 to 1.2. The idea of the exam is to compare the highest arm pressure to the highest pressure in each limb. When done properly, the exam requires 6 pressure measurements: 1 for each arm, and 1 for each dorsalis pedis and posterior tibial artery. All pressure measurements are performed with the Doppler, including the arm pressures. The highest ankle pressure for each limb is then divided by the highest arm pressure. There is a common misconception that the right leg is always compared to the right arm. But the patient may have subclavian disease as well, and the single highest arm pressure should

be used. One should distinguish between pulses, which are pulsations felt by the examiner’s finger, and signals, which are produced by the Doppler device. The phrase “Doppler pulse” is a misnomer. How much difference in the ABI should be regarded as clinically significant? Based on large intra- and interobserver variability studies, a difference in the ABI of 0.15 is 2 standard deviations from the mean, and so is considered clinically significant. Thus, there is a significant difference between 0.87 and 0.71, but not between 0.87 and 0.82. TROUBLESHOOTING No Signal The absence of the Doppler signal in an extremity is concerning. Before documenting this finding, however, it is important to check with another instrument. Bedside Doppler instruments found on the wards are often old and poorly maintained. Before using any Doppler instrument on a patient, the operator should briefly test it on his or her own

Figure 4. A typical handheld continuouswave Doppler device.

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radial artery. Malfunction can be due to low batteries or a broken device or probe. In addition, “alternative” coupling gels such as electrocardiogram compounds or surgical lubricant work poorly. Their use should be discouraged in favor of proper ultrasound gel. Incompressible Vessels Medial calcinosis is commonly observed in the tibial arteries of diabetic patients. Small vessels may become incompressible by a bedside pressure cuff, even at levels as high as 300 mm Hg. This finding can lead the surgical resident to report the ABI measured at, for instance, 1.6 as “greater than one.” This could be interpreted as a normal finding, but the ABI in this setting may provide limited information, because the patient may indeed have significant limitation of flow that is concealed by the “normal” ABI. DUPLEX EXAMINATIONS The duplex examination is a combination of B-mode imaging with Doppler flow interrogation. The following study types provide examples of the benefits and limitations of the duplex exam. Carotid Duplex examination of the carotid arteries is the most reliable vascular lab examination. A relatively shallow structure is studied, and good images are typically produced. The significant information gathered is the Doppler velocity, and the B-mode image serves to localize the Doppler sample volume. The standard velocity break points for the stenosis categories are 1% to 15%, 16% to 49%, 50% to 79%, 80% to 99%, and 100% (occluded). Venous In contrast to the carotid exam, the information gathered from the venous duplex is based primarily in the B-mode image. The technologist performs sequential compression of the lower extremity venous structures to demonstrate apposition of the vein walls. Lack of wall apposition is an indication that a thrombus is present. For educational purposes, review of a study videotape provides the best demonstration of this process. Visceral Visceral examinations of the renal or mesenteric arteries illustrate principles of abdominal ultrasound for the resident. As with carotid exams, information about stenosis is flow based. Important teaching information includes the technical difficulties associated with studies of deep structures, the nature of renal and mesenteric waveforms, and the renal–aortic ratio. PULSE VOLUME RECORDINGS (PVRs) PVR Examinations Pulse volume–recording examinations are an important tool in the diagnosis of extremity arterial disease. The study provides 2 types of information: pressure measurements and volume changes. The sites of pressures typically measured in the lower extremity are the upper thigh, the lower thigh, the calf, and the ankle. Significant decreases in pressure between sites or between limbs allow the resident to determine the likely location of arterial stenosis. The volume changes require more understanding. A common misconception among residents is that the pressure tracings are

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Figure 5. Pulse volume–recording tracings. The first tracing is relatively normal, whereas the second is moderately dampened.

Doppler waveforms. In fact, the tracings represent the imperceptible changes in limb size with systole and diastole. The well-educated resident should be able to distinguish between normal, moderately dampened, and flat tracings (Fig. 5). By combining the pressure values with the volumetracing characteristics, the resident will be able to predict the appearance of the arteriogram even before it is obtained. Exercise Tolerance Testing (ETT) Lower extremity ETT is analogous to cardiac stress testing: the muscle is stressed for a period of time and its response to that stress is observed. If the resident fully understands Ohm’s law as it applies to vascular systems, he or she can predict the response to stress. In the normal ETT, as calf arterioles dilate in response to increased demand, flow and pressure increase at the ankle. Therefore the ABI measured after exercise in the normal subject increases. After exercise stops, pressure gradually returns to baseline. In the abnormal ETT, due to a fixed proximal stenosis, pressure does not increase in response to exercise—in fact, it decreases as the arteriolar resistance decreases. Following exercise, the ABI gradually returns to normal; the time required to achieve this baseline is proportional to the severity of the underlying arterial disease. CONCLUSION Surgical residents face increasing demands on their time every year. Manpower reductions, funding cuts and increased inpatient acuity reduce the time available for focus on topics such as the vascular laboratory. However, with a brief introduction to some general concepts and exposure to studies in a clinical setting, a resident can gather a large amount of knowledge in a short period of time.

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TIMOTHY C. HODGES, MD CHARLES W. VAN WAY III, MD University of Missouri–Kansas City Kansas City, Missouri •

Volume 56 / Number 9 • November/December 1999