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Editorial |
1 From the Department of Radiology, West Los Angeles Veterans Administration Medical Center, 11301 Wilshire Blvd, Los Angeles, CA 90073 (P.Z., E.C., E.G.G.); and Department of Radiological Sciences, UCLA Medical Center, Los Angeles, Calif (P.Z., E.C., S.L.N., N.R., E.G.G.). Received November 6, 2000; revision requested December 23; revision received March 19, 2001; accepted March 22. Address correspondence to P.Z. (e-mail: peterzimmerman@earthlink.net).
Index terms: Arteries, extremities • Arteries, grafts and prostheses, 913.1269, 914.1269 • Arteries, US, 913.12983, 913.12984, 913.12989, 914.12983, 914.12984, 914.12989 • Editorials
Coronary artery bypass graft placement requires an autologous vessel as a conduit. Several options are available, including the saphenous vein, the internal mammary and inferior epigastric arteries, and, with recent increasing frequency, the radial artery. The radial artery has many favorable features as a graft, including a caliber similar to that of the coronary arteries, adequate wall thickness and resistance to serve as a graft, sufficient length for complete revascularization, and a location that allows relative ease of harvesting. Many vascular surgeons are requesting that vascular laboratories perform preoperative noninvasive evaluation of the forearm vessels prior to radial artery harvesting to avoid ischemic complications to the hand and to assess the suitability of the radial artery for graft placement. The purpose of this communication is to familiarize radiologists with the rationale for this examination, discuss the various techniques that have been proposed, and, owing to the lack of consensus about an optimum method, describe how we perform the evaluation in our laboratory.
To the best of our knowledge, radial artery conduits were first used in 1973 (1) but were largely abandoned owing to their propensity for diffuse vasospasm, narrowing, and occlusion. In 1991, however, it was noted that three of the patients originally treated by using these conduits had maintained widely patent radial artery grafts during the 15–18-year interval. Thus, interest in using the radial artery for coronary artery bypass graft placement was rekindled. With modifications of surgical techniques and the use of calcium channel blockers to reduce vasospasm, the long-term patency of radial artery grafts has markedly improved to result in their increasing use during the last few years (2).
Removal of the radial artery poses the risk of ischemic complications to the hand, particularly the thumb and index finger, which could be deprived of flow in individuals with anatomic arterial variations that do not allow adequate collateral flow across the palm. Ischemic complications in radial artery harvesting have been described (3), and although the incidence is probably very low (4), the exact rate is not known. Hand ischemia is a known complication of radial artery cannulation, but the mechanism for this may be distal embolization rather than radial artery occlusion. There are several clinical contraindications to using the radial artery as a graft conduit, including history of arterial trauma, ischemic symptoms in the upper extremity, and Raynaud syndrome. The use of the radial artery in the dominant arm is avoided in many institutions, but this contraindication seems to be lessening in importance when adequate noninvasive examination results confirm that it is appropriate.
To minimize the risk of hand ischemia, many surgeons advocate evaluation of the forearm and palmar circulation prior to radial artery harvesting. This evaluation has two major objectives: (a) to eliminate the possibility of postharvest ischemia of the hand and (b) to ensure that the radial artery is free of atherosclerosis and is the appropriate size. Although upper-extremity arterial atherosclerosis is uncommon, individuals with coronary artery disease are at higher risk.
The anatomic feature that permits harvesting of the radial artery without ischemic complications in the majority of individuals is the presence of anastamoses across the hand between the radial and ulnar arteries, in the form of several arches. The most important pathway is the superficial palmar arch, which typically originates from the ulnar artery and provides the majority of arterial supply to the digits (Fig 1). The deep palmar and dorsal arches usually originate from the radial artery and are generally smaller and less important than the superficial palmar arch and its branches. If the superficial palmar arch is intact, then collateral flow to the radial aspect of the hand should be adequate if the radial artery is removed. However, there are several anatomic variants that may lead to hand ischemia with radial artery harvesting. These include an incomplete superficial palmar arch, radial artery dominance of the superficial palmar arch, and absence or malformation of the ulnar artery. The frequency of an incomplete superficial palmar arch is not certain, but it is reported to range from approximately 6% to 34% (2,5).
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The measurement of oxygen saturation in the digits with radial artery compression is objective, but substantial variations in perfusion do not always result in changes in saturation (4,6). The measurement of systolic blood pressure in the digits also is objective, and although a pressure drop of more than 40 mm Hg has been proposed as an indicator of ischemia, the choice of this value is somewhat arbitrary and without validation. In plethysmography, one records a signal that is reflective of blood flow, but the signal is not quantifiable, and criteria predictive of hand ischemia are not well defined (4). A Doppler US version of the clinical Allen test has been used in many studies, most with continuous-wave Doppler US and one with color and spectral Doppler US (2,3,9). In general, this examination involves Doppler interrogation of the radial aspect of the superficial palmar arch before and after radial artery compression, and assessment for the presence and/or reversal of blood flow as an indicator of an intact arch providing adequate collateral flow (2,3,9,12). These various techniques have had sparse clinical validation.
Three studies (2,3,9) involving the use of the modified US Allen test (with continuous wave Doppler US) included the follow-up of patients who underwent radial artery harvesting, and none of the total of 113 patients who had negative studies (ie, intact palmar arches) had signs of hand ischemia. In a study involving digital oxygen saturation measurements during radial artery compression, 452 patients were followed up after radial artery harvesting, with no cases of hand ischemia.
There is no consensus regarding which examination should be performed as part of a preharvest evaluation. We have designed an examination involving the use of Doppler US to evaluate arterial inflow and the adequacy of collateral flow across the palm. This examination is reasonably comprehensive but does not require an inordinate amount of time (approximately 10 minutes). To date, we have examined 358 patients who underwent coronary artery bypass graft placement with our technique, and in these patients, 53 radial arteries were harvested. According to clinical follow-up data, none of these individuals have experienced symptoms or had signs of hand ischemia.
We begin our examination with a modified US Allen test. If the results of this test indicate that the arch is not intact, the examination is ended. If collateral flow through the ulnar artery is demonstrated, we proceed to evaluate the radial and ulnar arteries for any evidence of obstructive disease or atherosclerosis. Identification of any obstructive lesion in either the radial or ulnar artery is essential, since a diseased radial artery is undesirable for implantation and a diseased ulnar artery may not provide adequate perfusion to the hand after radial artery harvesting.
Although there are no specific velocity criteria to grade stenoses in the upper-extremity arteries, the application of basic findings from other vessels (eg, carotid, lower extremity, and visceral arteries) for the Doppler US–based diagnosis of stenosis seems reasonable. These findings include a focal increase in peak systolic velocity, poststenotic turbulence, and dampening of the waveform distal to the lesion. Since the objective of this examination is to identify a disease-free radial artery as one of several (saphenous vein, internal mammary artery) options for graft placement, the presence of any findings suggestive of stenosis should preclude the harvesting of the vessel. Similarly, gray-scale findings of plaque or calcification also should contraindicate harvesting, although the accuracy of US in the evaluation of these findings in small vessels is unknown. We use color Doppler US with a 7–10-MHz transducer to rapidly survey the vessel for areas of possible stenosis, as manifested by aliasing and vessel narrowing. (The latter finding may be less sensitive owing to color blooming.) Any area of suspicion is analyzed in greater detail by performing spectral Doppler US to determine the peak systolic velocity in the area of possible stenosis and to compare it with the velocity in the contiguous (probably proximal) vessel. We use gray-scale US to assess for any plaque or calcification, and if the vessel is free of disease, the diameters of the proximal, middle, and distal radial arteries are measured on the gray-scale images. Arterial obstructive disease in the upper extremities is rare, although the prevalence is increased in patients with coronary artery disease.
To perform the US Allen test, a 7–10-MHz linear transducer is placed in the crease of the proximal palm at the base of the thumb (Fig 2). At this location, the superficial palmar branch of the radial artery can be identified coursing anteriorly. The approximate position of this vessel generally can be found by drawing a line along the longitudinal axis of the center of the index finger to the point of its intersection with the crease at the base of the thumb (thenar eminence). Flow in this vessel is normally directed toward the transducer and into the superficial palmar arch. The direction of flow in this vessel can be easily determined by using color and/or spectral Doppler US. We generally use color Doppler US to locate the vessel and spectral imaging to assess and document change in flow direction. While the superficial palmar branch is insonated, the radial artery is compressed at the wrist and the operator observes for a reversal (implying the arch is complete) or lack (implying the arch is incomplete) of flow (Fig 3). Reversal of flow implies that the radial artery may be harvested with safety. Complete lack of flow in this vessel with compression essentially precludes the use of the ipsilateral radial artery.
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In this communication, we describe our Doppler US technique for the preoperative evaluation of forearm and hand vessels prior to radial artery harvesting. The technique is relatively easy to perform and rapid, provides anatomic and hemodynamic information, and can be performed with equipment that is available in most radiology departments. It is important to acknowledge that clinical validation data on radial artery mapping techniques, including our own, are scarce and unlikely to be statistically meaningful owing to the low incidence of postharvest hand ischemia. It is also debatable whether the potential for hand ischemia is important enough to warrant any preoperative testing.
Owing to the increasing use of the radial artery for coronary artery bypass graft placement, these are important questions that warrant further investigation. However, optimum study of this topic is difficult because of the low incidence of ischemia and the ethical and philosophical issues regarding the use of a control group that would have to forgo testing that is noninvasive, inexpensive, and theoretically beneficial. Despite these controversies, requests for this examination from our surgical colleagues will probably continue and may be encountered by radiologists who perform vascular US.
REFERENCES
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  D. G. Nezic, A. M. Knezevic, P. S. Milojevic, B. P. Dukanovic, M. D. Jovic, M. D. Borzanovic, and A. N. Neskovic The fate of the radial artery conduit in coronary artery bypass grafting surgery. Eur. J. Cardiothorac. Surg., August 1, 2006; 30(2): 341 - 346. [Abstract] [Full Text] [PDF]
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