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Letters to the Editor |
,
Pierre Bourquelot, MD,
,
Alain Raynaud, MD,*,§,
Bernard Beyssen, MD,* and
Marc Sapoval, MD*
Department of Cardiovascular Radiology, Hôpital Broussais, Paris, France*; Department of Cardiovascular Radiology, Clinique Saint-Gatien, 8 Place de la Cathédrale, F-37000 Tours, France; Department of Surgery, Clinique Jouvenet, Paris, France; Department of Cardiovascular Radiology, Clinique Labrouste, Paris, France§
Editor:
In the December 1998 issue of Radiology, Dr Menegazzo and colleagues (1) reported on their promising preliminary experience concerning the value of magnetic resonance (MR) imaging in the assessment of the upper limb veins prior to creation of hemodialysis fistulas. There are, however, incorrect statements in the article, and the authors did not fully discuss the limitations of their study.
1. In the introduction, there is an incorrect description of the strategy for creation of vascular access; Dr Menegazzo and colleagues completely overlooked the possibility of creating a transposed brachiobasilic fistula, which is even more regrettable because preoperative imaging is especially indicated to assess the patency of the basilic vein. In Europe, these transposed brachiobasilic fistulas are used more often than brachioaxillary grafts, and they are placed at the same level as grafts found in the guidelines of the American Dialysis Outcomes Quality Initiative (2).
2. With regard to the indications, Dr Menegazzo and colleagues simply mentioned that preoperative mapping was indicated "except in cases where superficial veins are clinically well developed and are judged to be clearly patent" (1). They overlooked the second indication, which is the evaluation of central veins, especially in patients in whom central catheters had previously been placed while they were being treated in intensive care units or to initiate emergency dialysis. As a consequence, they failed to discuss how to evaluate these central veins. Venography remains the most reliable examination in such cases since it is not possible to study the intrathoracic veins with the use of ultrasonography (US).
3. The comments against alternative techniques—namely, venography and US—are not fair. Nowadays, there is no contraindication for venography. This examination has been routinely performed (especially in the Paris, France, area) for years with the use of carbon dioxide and not iodine in cases where residual renal function needs to be preserved, a fact of which the authors cannot be unaware. US also has been proved reliable for years when performed by experienced physicians, and it was scientifically confirmed in a recent article by Silva et al (3).
4. The nonvisualization of the upper arm veins above their lower third is a major drawback of the MR imaging technique reported by Dr Menegazzo and colleagues, since it means that the quality of the basilic vein and the final arch of the cephalic vein cannot be assessed. This concern, with regard to central veins in general, was not addressed in the discussion.
5. When they described the venographic technique, Dr Menegazzo and colleagues explained that a tourniquet was placed around the arm "to allow overall opacification of both the superficial and the deep veins." Deep forearm or upper arm veins are of no value in the creation of a fistula, and no one aims to image them. In upper limb venography, the tourniquet is placed to dilate the superficial forearm veins, and the distention is even increased by the injection of the contrast medium, which allows appreciation of the actual size of the veins. When veins remain spastic despite the tourniquet, additional local injection of nitrogen monoxide–releasing vasodilators is necessary. It is probably because they used a suboptimal venographic technique that Dr Menegazzo and colleagues found no difference in venous diameter between MR imaging performed without a tourniquet and venography performed with a tourniquet dilating the veins.
6. In the Results section, Dr Menegazzo and colleagues focused on the diameter of the veins, whereas it is more important to appreciate the absence of stenosis or occlusion. The quality of the maximum intensity projection (MIP) reconstruction images is often questionable in this issue (compared with that of the cephalic vein in fig 3b of the article). If small-diameter veins really were a contraindication, wrist fistulas could not be created in children, but they are (4).
In conclusion, this article on preoperative mapping with the use of MR imaging gives the findings of an interesting preliminary study. However, given the limited information the article provides, the place of MR imaging alongside noninvasive US techniques and minimally invasive venography, which is more informative with regard to central veins, is questionable.
References
,
Béatrice Viron, MD, and
David Menegazzo, MD*
Departments of Radiology* and Nephrology, Hôpital Bichat, 46 rue Henri Huchard, F-75018 Paris, France
An upper arm or forearm is not dedicated per se to creation of an arteriovenous hemodialysis fistula. Consequently, the role of the physician is primarily to offer the best site for creation of an arteriovenous fistula, to help maintain a normal vascular flow of the upper limb where the arteriovenous fistula has been created, and to maintain further patency of vascular veins if the initial arteriovenous fistula is subject to complications.
In the assessment of these veins, Dr Turmel-Rodrigues and colleagues state that it is probably because we used a suboptimal venographic technique (1) that we found no difference in venous diameter between MR imaging performed without a tourniquet and venography performed with a tourniquet dilating the veins. We do not think that this is the right explanation. In a paper on a further study to be submitted for publication (Laissy JP, unpublished data, 1999), the ability of superficial forearm veins to dilate during forearm and hand exercises was observed in only four of 10 subjects (40%). The average percentage of dilation (± SD) was 4% ± 19 overall. There was no difference in diameter increase, whatever the vein diameter at rest. In converse, exercise involved deep veins, of which a greater number was seen. Since the principle of time-of-flight sequences is to obtain high signal from flowing blood, the use of a tourniquet resulted in stationary blood with no signal within veins at MR venography, thus precluding assessment of availability and accurate measurements of venous diameters.
Among the other noninvasive preoperative techniques, we agree that US studies are useful (2), but they fail to provide a vascular map. This technique has been widely used at our institution.
A great deal has been done with regard to carbon dioxide venography, especially in patients in whom residual renal function needs to be spared. Carbon dioxide venography has been reported to be a safe, feasible, and potentially useful method in treating patients with contraindications to iodinated contrast material or with unsatisfactory images from examinations with iodinated contrast material (3), especially in patients at risk for nephrotoxicity due to reduced nephronic function. Unfortunately, in our experience, carbon dioxide venography was of limited value in a nonnegligible proportion of patients and was responsible for a cerebral gas embolism in one of them. Moreover, to our knowledge, few centers in Paris are able to perform carbon dioxide venography. Consequently, at our institution, MR venography is preferred over carbon dioxide venography.
We are not in accordance with the criticisms of Dr Turmel-Rodrigues and colleagues when they state that our article failed to discuss how to evaluate central veins. In our article, an entire paragraph is dedicated to this problem: "The volume of upper limb imaged with the surface coil was limited to the forearm, elbow, and distal arm; hence, the central veins were not evaluated. In current practice, it is common to aggressively image these vessels preoperatively because many patients have had central or subclavian catheters inserted, which result in significant and silent stenoses" (1, p 727). This is an important drawback that probably will be corrected with the ongoing development of surface coils.
Irregularities of veins are at least as important as their diameter, because they can hamper the further patency of an arteriovenous fistula, as underlined by Dr Turmel-Rodrigues and colleagues. This is the reason why MIP reconstructions are complementary to source images. These MIP reconstructions were altered in our study because, with our machine, MR venography could be performed only by using nonoverlapping two-dimensional acquisitions.
Flow conditions within fistulas in pediatric patients are different when they are compared with those in adults, especially because of children's capabilities of venous growth, and stenosis raises specific problems. Indeed, children experience a high rate of stenosis (4). We do not think, consequently, that this problem could be extrapolated to adult patients in whom superficial proximal veins with small diameters are considered unsuitable for hemodialysis creation.
One major concern is that since our study (1), our vascular surgeons do not request iodinated contrast material–enhanced or carbon dioxide x-ray venography as frequently as before, since hemodialysis fistulas are often created on the sole basis of color Doppler US findings, MR venographic findings, or both.
In conclusion, in our study we attempted to demonstrate the feasibility and added diagnostic value of MR venography as a replacement for contrast-enhanced venography. Despite the criticisms of Dr Turmel-Rodrigues and colleagues regarding the nonassessment of central veins, we do consider it true that this noninvasive method, even when associated with others, will be able to be substituted for iodinated contrast–enhanced or carbon dioxide venography in the near future.
References
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