Unité d'Hémodialyse, CHU Bretonneau, Tours, France
Sir,
The editorial entitled Vascular access: care and monitoring published in the August 2001 issue [1] was interesting, focusing on the difficulties involved in maintaining patency in arterio-venous accesses (AVA) over a long period and reminding us of the specific measures to achieve optimum survival of AVA.
Unfortunately, some of the messages raised in this editorial are obsolete and have to be questioned.
(i) The distal radio-cephalic native arterio-venous fistula (AVF) remains the gold standard for all patients, regardless of age and clinical situation. This rule has to be applied even in elderly and diabetic patients and those with atheromatous disease [2,3]; a proximal AVA exposes patients to the risk of developing both hand ischaemia and cardiac dysfunction. In most cases construction of distal AVF is possible despite the potentially less good quality of the artery. A brachiocephalic or a brachiobasilic fistula created as a two-stage procedure with delayed superficialization [4], are a better choice to avoid graft placement.
(ii) The preservation of the peripheral venous system has to be instituted as early as possible, i.e. as soon as renal disease is discovered and in diabetics. However there are two upper limbs to be protected. The preservation of only one limb was acceptable some decades ago, but this is no longer the case because life expectancy has dramatically increased in end-stage renal disease patients. Blood samples can easily be taken from the veins of the dorsum of the hands. The strategy of venous preservation has to be maintained even during peritoneal dialysis treatment and transplantation. The choice of peritoneal dialysis or renal transplantation in order to protect peripheral veins is debatable and will be effective only if the strategy of venous puncture on the dorsum of the hands is adopted; otherwise the venous system will be devastated.
(iii) The central veins must be preserved. This is possible even in an emergency. The subclavian vein has to be avoided because the risk of developing stenosis is very high [5] and the femoral route should be the route chosen for catheter placement in emergency, with a soft catheter left in place for several weeks [6], awaiting the creation of a venous access and sparing internal jugular veins.
(iv) Measurement of AVA blood flow by Doppler ultrasound does not depend on the external shape of the fistula because it is measured through the afferent artery; the only difficulty is in obtaining the precise measurement of the inner diameter of the feeding artery.
(v) Early detection of AVA at a high risk of thrombosis depends on clinical examination and close monitoring of circuit pressures, recorded at each dialysis session. A decrease in arterial pressure and an increase in venous pressure in standardized conditions are predictive of arterial side stenosis and venous side stenosis, respectively. Despite its insufficient sensitivity, venous pressure (static or dynamic) and its increase over time have been proved to be predictive of thrombotic events in grafts [7,8]. Intensive surveillance based on venous pressure has proved to be effective in reducing the incidence of thrombosis of grafts [9]. On-line blood flow measurement is more promising to predict AVA failure. Low blood flow and decreased blood flow over time have a high positive predictive value in grafts [10,11] and are also effective in native AVF [12]. Some problems have remained unsolved, explaining the disappointing accuracy reported [8,11]: the optimal frequency of flow measurements and the individual threshold for a given AVA have not been clearly defined. The development of new tools for on-line flow measurement directly incorporated in dialysis machines [13] and permitting more frequent flow measurements might answer some of these questions.
(vi) Surgical revision cannot remain the gold standard for the treatment of access stenosis for two reasons: the results obtained by interventional radiology are similar to those achieved by surgery without the drawbacks of surgery such as invasiveness, consumption of the patient's venous capital, anaesthesia, and hospital stays [1416].
These messages are fundamental to reduce AVA-related morbidity. The difficulties are in the routine application. Treatment requires convinced nephrologists as well as trained surgeons and interventional radiologists.
Notes
Email: pengloan{at}med.univ\|[hyphen]\|tours.fr
References
1 Nephrology Unit, Department of Internal Medicine, University Hospital, Gent, Belgium 2 Nephrology Unit, Department of Internal Medicine, AZ Middelheim Hospital, Antwerp, Belgium
Sir,
I read with interest this second critique by the Tours group of my editorial on vascular access, which was published in the August 2001 issue of Nephrology Dialysis Transplantation [1]. This critique contains a number of remarkable viewpoints. Although it is not my intention to start an everlasting polemic about access, I nevertheless would appreciate the opportunity to answer to some of their remarks.
First of all, I have a semantic problem with the statement that my messages are obsolete. In the Webster's Third New International Dictionary, one can learn that obsolete means: of a kind of style no longer current, outmoded. The literature list that accompanies Dr Pengloan's critique shows that most references that cover her hypotheses have been published at the earliest in 1998. Statement (ii) and (iv) are not covered by a reference, nor is the key sentence of statement (i), about the majority of access systems in diabetics being distal fistulas. Statement (v) confirms to a large extent the position taken in my editorial, since it stresses the importance of access flow as a predictor of access failure. Statement (iii) discusses the choice of central veins for catheter access systems, an item that was not even touched upon in my editorial. Hence, I am convinced that the term obsolete is not at its place here.
Another criticism, suggested by the repeated reactions of this group, which are always amply supported by references not figuring in my text, is that my editorial is covered by an insufficient number of literature citations. A check of the literature on Medline, with the search tools vascular access and haemodialysis/dialysis, reveals that since 1995 at least 630 texts have been published regarding this issue. Hence, it was impossible for me to include all these references, and I guess there will always be colleagues who will not find their preferred articles in my reference list.
Let us now discuss Dr Pengloan's ideas one by one.
(i) The primary access in most diabetics should be a distal AV-fistula. To cover her statement, Dr Pengloan refers to two studies, one by Lin et al. [2] and one by Konner [3]. The study by Lin et al. [2] depicts the successful creation of an AV-fistula in 74 elderly patients and 72 diabetics, in an entirely Chinese population. Of note, primary fistula failure within 1 month in this study was close to 30% in elderly diabetics. Patients in whom it was impossible to create an AV-fistula were excluded from the study. The study contains no indices of body stature in the population under evaluation. In general, the Chinese have a lower body volume than Caucasians, so that it will be easier to create fistulas that offer enough flow to allow adequate dialysis. We also do not know whether these patients were suffering from type I or type II diabetes. In type I diabetes, large and medium sized vessels can still be expected to be patent, which is less probable in type II diabetes. For the time being, most of Caucasian diabetic dialysis patients suffer from type II diabetes. The study by Konner [3], in contrast, was performed in a Caucasian population. Of the 78 diabetic patients, only 20 received a distal fistula, whereas the remaining 58 (74.4%) received an elbow fistula. Dr Konner is to my opinion one of the most careful and experienced vascular access specialists at the moment. If even he prefers an elbow fistula in the majority of his diabetic patients, this publication cited by Dr Pengloan to my opinion contradicts her statement that in most cases construction of a distal fistula is possible. As a clinical nephrologist, my main concern is to have dialysis as adequate as possible, as soon as possible after the start of dialysis. This is perhaps even more correct for diabetics, who are intoxicated both by uraemia and diabetes. My experience is that precious time is often lost with prolonged periods of catheter and/or inadequate dialysis, if first attempts are made to create a distal fistula in diabetics, especially if they are aged. The late referral of many diabetic patients is another confounding factor here [4]. The remark that elbow fistulas cause more steal syndrome and cardiac failure may be correct, but is only supported by case reports. It would be nice to see especially the connection with heart failure proven in a prospective study. Cardiovascular disease in this population, might be at least as much the consequence of uraemic retention [5] as of the access system; in this context, the primary aim of a valuable access should be to offer adequate dialysis as soon as possible. The patients developing steal syndrome are mostly those with concurrent pronounced arterial insufficiency, and hence by definition not the best candidates for a distal fistula. This is why in the editorial, the careful terminology the elbow may be the preferred location was used. The real need is for well-conducted prospective studies, comparing access and patient outcome with distal versus proximal access systems in similar populations.
(ii) The two upper limbs should be preserved for later creation of access systems. In my personal experience, even preservation of one limb is too often forgotten, and I would be very happy if, in every patient, one limb would be preserved. Dr Pengloan's proposal even goes one step further, and is certainly ideal, although, once again, one could wonder whether this is supported by the literature. Practically, this proposal might be difficult to implement. Especially those patients with frequent complications and hospitalizations, needing repeated infusions, are prone to needing early dialysis. One solution is to recur frequently to central vein catheters, but if this tactic is to be applied repeatedly, the central veins are jeopardized, and central veins should be preserved, according to Dr Pengloan's next (and correct) statement. Another solution might be infusions in the foot; however, this has extra risks of infection, especially in diabetics. It might be interesting to have studies that learn about vein preservation tactics, and that give an indication of how often errors are made against the most basic rules.
(iii) The central veins should be preserved. There is no discussion possible about this statement. Because of space limitations, I did not discuss this issue in my editorial, although I did so on earlier occasions [6,7]. To my knowledge we were among the first authors to describe occlusive lesions in conjunction with subclavian catheters [8]. Hence, I disapprove of their use in chronic renal failure without discussion. The prolonged use of soft catheters in the femoral position, as proposed by Dr Pengloan, might be a matter of more debate. Dr Pengloan's statement is based on a descriptive study by Montagnac et al. [9], which contains an interesting set of observations, but which fails to offer comparative data with other insertion positions. The catheters were not tunnelled. Thrombotic complications were seen in 7/55 patients and infections in 14/55, for a mean dwell time of 41 days [9]. It is of note that the DOQI guidelines advise to maintain non-tunnelled femoral catheters for a maximum of 5 days, and only in bed-ridden patients [10,11]. It is difficult to compare Montagnac's data with other studies, because conditions and definitions might be different; in any case, the impression is that the occurrence of reported complications was high in Montagnac's series, compared to studies evaluating similar catheters in other positions [12]. This might be especially the case for infections [12]. One main concern about the prolonged use of the femoral position, apart from the infectious and thrombotic risk, is the risk for laceration of the venous wall, jeopardizing its future use for kidney transplantation. Again, it might be interesting to compare different positions with each other in a prospective study.
(iv) Doppler can obtain reliable fistula flow values; the measurements do not depend on the external shape of the fistula as measurements are performed on the afferent artery. To my knowledge, this measurement is generally performed on the venous section of the access. Sands et al. [13], who performed several studies in this area, measured access flow on the efferent section of the access up to the anastomosis. In the paper by Lin et al. [2], cited by Dr Pengloan to prove the feasibility of distal fistulas in diabetics, fistula flow was measured over the vein. Also in the study by May et al. [14], where the authors compare flows measured by Doppler and ultrasound dilution techniques, access flow is measured at the midstream area of the access (efferent part of the vascular access). They describe a high correlation of flows obtained with both techniques, although they stress the importance of one radiologist performing the Doppler measurements and the importance of the Doppler angle to measure the cross-sectional area, as also pointed out by Dr Pergloan. The measurements were made at a point free of wall irregularities where flow was not turbulent; hence, shape does matter. The need for a continuous presence of one single ultrasonographist, always the same, per dialysis unit, to allow more or less consistent data gathering, has also been stressed by Sands et al. [13]. Measuring flow through the afferent artery might be a valuable alternative, but again, it would be interesting to learn whether studies validate this technique. According to the 1997 as well as 2000 DOQI-guidelines, Doppler had by then not been validated for the measurement of flow in AV-fistulas [10,11].
(v) Clinical examination, venous pressure measurement and on-line blood flow monitoring are predictive for fistula thrombosis. To my knowledge, there are no controlled studies that have shown a significant reduction in thrombosis rate by frequent/routine clinical examination of the vascular access. Clinical signs only develop when a haemodynamically significant stenosis is present for some time; hence, clinical signs are late markers of access dysfunction. Since screening based on access flow has been shown to reduce the thrombosis rate significantly [14,15], this method undoubtedly depicts access dysfunction at an early stage and allows early elective repair. In the presence of the unequalled results of screening based on flow, routine clinical examination is only informative and certainly does not allow early detection as suggested by Dr Pergloan. In my editorial, regarding venous pressure, I state that in the same prospective study that stressed the validity of flow measurements, no predictive value could be attributed to venous pressure measurements [14]. After my editorial had been submitted, Smits et al. [16] published a study demonstrating a predictive value for venous pressure measurements. These data had, however, been obtained only in PTFE-grafts, which is, if we are allowed to use Dr Pengloan's terminology, an obsolete access method. Venous pressures (dynamic and static) are much less relevant in fistulas compared to grafts [15,17]. Also the DOQI-guidelines state that venous pressure measurements are less relevant predictive parameters for AV-fistulas [10,11]. The study by Cayco et al. [18], which is cited by Dr Pengloan, is purely observational, with historical controls. While calculating the sensitivity and specificity, they used a vascular access impairment episode, defined as stenosis or thrombosis, as an endpoint. This means an overestimation of sensitivity, as not every stenosis will cause thrombosis. Compared to screening based on flow, venous pressure is less sensitive and only detects patients with stenosis, not those at risk for thrombosis [19]. In a recent controlled study by MacCarley et al. [15], the validity of sequential flow measurements and the lack of validity of dynamic venous pressure measurements were further substantiated, also in AV-fistulas. Follow-up of flow also turned out to be cost-effective [15]. The only question that remains is whether data obtained in the US, with their worse outcome, even for AV-fistulas, can be extrapolated to Europe. The doubts about the accuracy of flow measurements are based on a publication by Paulson et al. [20]. However, using ROC curves, an AUC of 0.83, as found by Paulson et al., is more than enough for clinical decision making. It is questionable whether many other tests for clinical decision making have such a high sensitivity and specificity. Admittedly, literature offers no uniform consensus at what flows a patient should be referred for angiography, although the different studies with proven benefit almost always use the same referral threshold: <500600 ml/min in fistulas and <600700 ml/min in grafts. For the rest, in the up-to-date literature, there are not so many questions left about access screening based on flow, in contrast to what is suggested by Dr Pengloan. Moreover, all the new tools for access flow measurements are validated against the ultrasound dilution technique. Dr Pengloan suggests then that on-line blood flow measurements might offer a solution for the future, based on the publication by Mercadal et al. [21]. The study by Mercadal et al. is a validation study, not offering a long-term clinical evaluation. In spite of significant correlations, individual differences in registered flow values between this new test method and ultrasound dilution reached values of more than 1 l/min (in one and the same patient 1.02 vs 2.20 l/min, i.e. a difference of more than 100%). This only indicates that a significant correlation not always means individual predictive validity.
(vi) Surgical correction of the fistula is not the gold standard. I already explained the reasons why I named surgery the gold standard, when responding to the letter by Dr Turmel-Rodriguez from the same unit [22], and will not enter into this debate once more. In my opinion, this critique demonstrates that much of the literature that is produced in this area is based on studies that describe a single technique, without comparing them to a standard. What we really need are prospective, controlled, comparative studies. Another point is that the literature in this area is extensive, and that it is easy to find arguments in favour of any hypothesis. There is an urgent need for an unflawed, complete but critical review of the literature.
References