Metallic dilator use for intraoperative outflow vein dilatation to improve dialysis arteriovenous fistula maturation and accelerate cannulation

Metallic dilator  use for intraoperative outflow vein dilatation  to improve dialysis arteriovenous fistula maturation and accelerate cannulation

Ahmed H El-barbary,MD; Amr E Elshaz[y,MD

Department of Vascular Surgery, Tanta University, Tanta, Egypt.

Co"espondence:

Ahmed Albarbary MD: Tel.: 01224176570  - E-mail: albarbary73@yahoo.com

Abstract

Objective: A well-planned and meticulously constructed AVF should mature in 4 to 8 weeks. Creation of a rapidly maturingfistula provides a potential way to diminish the current dependence on central catheter placement.

Aim of the work: To evaluate effect of intraoperative dilatation for small veins using metal dilators on the initial autogenous hemodialysis accesses patency, maturation and cannulation.

Patients and methods: Sixty adult patients with end-stage kidney disease who were scheduled to undergo radiocephalic (RCAVF) or brachiocephalic arteriovenous (BCAVF) dialysis fistula surgery during the period from August 2010 through August 2011. The primary outcomes of the study were fistula maturation and time of successful first cannulation. Vein diameter was assessed intraoperatively by inserting metal dilator heads to measure the approximate diameter. The relationship between vein diameter after metallic dilators passage and maturation, short­ term patency rate and time to first cannulation were assessed.

Results: The patients' age (mean) was 65.9 15 years, range 18-85 years.Preoperative duplex was  performed in  only  15 (25%)  of  the cases.  Mean  duplex  cephalic vein  diameter was

2.23±0.17mm in successful RCAVF patients and 3.44±0.18 mm in BCAVF. Cumulative primary patency at 6 months in RCAVF patients was 75% (30140 fistulae) and 85% (17120 fistulae) for BCAVF. Matured  Fistulas were 49 (81.67%). Eleven fistulas (18.33%)  failed to mature; five underwent revision, mostly  for failure  to mature. Assisted  functional maturity  was 52/60 (86.67%).0verall mean times for fistula maturation and first cannulation  were 32±1.29 days (n=49)  and 40±9.6  days respectively. The more increased  vein diameters  after mechanical dilatation; the more likely for the fistula to mature and the lesser mean time for maturation and cannulation.

Conclusion: Early fistula patency rate was better than that in several reports. We recommend careful preoperative patient evaluation as well as careful anastomosis technique, which includes mechanical dilatation for small veins.

Key words: Arteriovenous fistula, vein dilatation, maturation, cannulation.

Introduction:

Native  arteriovenous fistula  (AVF)  is

preferred than synthetic grafts for hemodialysis vascular access  due to lower  rates  of thrombosis, infection, and longer patency .1.2

The National Kidney Foundation Kidney Disease Outcome Initiative (NKF K/DOQI) guidelines  recommended that   a  wrist

(radiocephalic) fistula is the first choice for native AVF, second is  an  elbow (brachiocephalic) fistula, and third is either a transposed brachiobasilic vein fistula or a forearm arteriovenous graft.3 However, 20-

60% of fistulas fail to mature sufficiently to support  dialysis.4,5 Thus, the high rate of maturation failure  is a major  unresolved

problem. Dilatation of the anastomosed artery and vein are key steps in fistula maturation.6

The clinical data suggest that a well-planned and meticulously constructed AVF  should mature in 4 to 8 weeks.sCreation of a rapidly

maturing  fistula provides a potential  way to diminish the current  dependence on central catheter  placement. A hemodialysis fistula comprises inflow artery, outflow vein and a venous segment suitable for cannulation. All of  these components contribute to the maturation and proper functioning of a fistula. For the fistula  to be functionally mature  it should provide the following: blood flow >600 mL/min through the fistula, a single straight vein >10 em in length or two straight segments at least 4 em in length, the vein diameter should be >6 mm, and located <6 mm under the skin surface. This permits easy placement  of two needles with their tips far enough apart to prevent recirculation.7

Venous outflow lesions are  the  most common lesions found in fistulae that fail to mature. Venous outflow lesions, including the central veins, may  be  due  to  preexisting anomalies, such as anatomically small veins, fibrotic  or stenotic veins,  or  sites  of prior puncture or catheter placement8 The treatment of these lesions concentrates on post-operative management of fistula non-maturation through various techniques of surgical and endovascular modalities, these treatment methods are in part costly in time and expenses  as well as have not  gained  much  acceptance especially in countries with limited resources and late patient referral  for hemodialysis like Egypt where most of patients cannot wait 3-6 months for maturation and cannulation of  his  fistula.

The purpose of this study  is to evaluate

effect  of intraoperative dilatation for small veins  using  metal  dilators on the initial autogenous hemodialysis accesses patency, maturation and cannulation.

Patients and methods:

a) Study design:

We enrolled 60 adult patients with end­ stage kidney disease who were scheduled  to undergo radiocephalic or brachiocephalic arteriovenous dialysis fistula surgery during the period from August 2010 through August

2011. Operations were performed in Vascular Surgery Unit, General Surgery department of Tanta  University Hospitals and  a private dialysis center. Only patients with first time permanent  native hemodialysis access were included in the analysis.Patients who received synthetic vascular access, prior constructed AV  access, brachiobasilic fistulae  were excluded  from the study. The presence  of a temporary dialysis catheter was  not an exclusion criterion. All patients gave written informed consent including relevant details of the procedure and the expected post-operative complications. On the day of surgery, patients underwent preoperative measurement of arterial blood pressure.

- The same vascular surgeons created all fistulas and took all diameter measurements. Pre-anastomosis artery  and  vein diameters were measured with intraluminal probes during surgery.  Dialysis staff and physicians who managed the fistulas and made cannulation decisions were blinded to  diameter measurements.

b) Study endpoints:

The primary outcomes of the study were fistula maturation and time of successful first cannulation. We  defined maturation as successful  and adequate dialysis  through  at least three consecutive sessions within 6 months of surgery.

c) Data and statistical analysis:

Variables were compared by using Fisher's exact test, and  unpaired t tests where appropriate. Data were analyzed by Graphpad prism 5™; a p-value of <0.05 was considered significant.

- Outcome variables included the patients' demographic data, site and type of AV fistula, medical risk  factors as smoking, diabetes mellitus, hypertension, ischemic heart disease, and liver insufficiency. Etiology of renal failure was noted as well as history of prior and recent central catheter placement especially ipsilateral to the planned  site of fistula. Vein diameter was assessed intraoperatively by inserting metal dilator heads to measure the approximate diameter. Intra-and post-operative complications were  recorded such  as vein perforation, thrombosis, failure  of dilator passage or false passage, absence of palpable

thrill, wound  hematoma, hemorrhage and infection. The  relationship between vein diameter after metallic dilators passage and maturation, short-term patency rate and time to frrst cannulation were assessed. Patients were  followed up for a mean  of 6 months (Range 2-11).

d) Patient assessment:

Both upper limbs were assessed for arteries and veins. Arterial assessments included whole limb pulses and blood pressure examination with comparison of both sides, and Allen's test for continuity of the palmar arch. Venous assessments were  done  with  and  without tourniquet placed at about the anterior axillary fold. Duplex ultrasonography (DUS) of arteries and  veins was  an adjunct to physical examination only in patients with uncertain quality and anatomy of the examined vessels. Duplex venous assessments included diameter, compressibility, depth, and continuity.Duplex arterial assessment was for diameter, waveform and flow velocities.

e) Definitions:

Fistula maturation is defined by the determination of both vascular  surgeon  and nephrologist that an access is patent and ready for cannulation based on adequacy  of blood flow through the fistula and the adequacy of vein dilation with respects to length {>IOcm segment), depth (<6mm), and  diameter (>6mm).9 Functional maturation is defined as successful cannulation of the fistula with the ability of the access to deliver a flow rate of

350 to 400 ml/min and maintain dialysis for 4 hours  or  less.10  Inadequate maturation is defined as insufficient access flow to maintain dialysis or the unavailability to cannulate  an AVF,  if required,  at 6 weeks  after surgery. Primary  fistula  failure  is defined as fistula abandonment prior to cannulation  regardless of patency status. This definition includes (1) inadequate  maturation, (2) early thrombosis, (3) failure of first cannulation, and (4) other complications such as ischemia or infection.II Primary patency rate is defined as the time between the initial operation and failure or the need  for reintervention. Assisted primary patency rate is defined as the total life span of the AVF after the initial operation, regardless of whether reintervention was required before

or after initial maturation. Secondary patency rate refers to the life span of an AVF that had become completely occluded but  was successfully reopened by  means of thrombectomy. 12

A uniform  size threshold for acceptable vein diameter was not used during the study period.

f) Operative technique:

All operations were  performed in the operating room under local anesthesia. After venous dissection and venotomy, heparinized saline was instilled into the vein to prevent thrombosis with the forthcoming manipulations. Venous diameter of cephalic vein in forearm or arm was measured by gently inserting  the tip of metallic vascular  dilator (23 em length and 1.5-6 mm caliber), without much distending the vein wall. Vein dilatation began  by gently passing the dilator that approximately mimics the vein internal lumen diameter with successively and gradually increasing dilator diameters. Overstretching was avoided, it is known when a force is needed to introduce dilator  tip with  increased wall transparency. In cases of radiocephalic fistula we dilate the whole forearm cephalic vein till about  the  elbow  crease, in  cases  of  arm brachiocephalic fistulas we dilate the whole arm cephalic vein with the aim of approaching or reaching the cephalic arch with the long dilator as this is a very common area of venous stenosis however, this was done with utmost care and caution as this area is prone for rupture.

Passage at ease  during insertion and withdrawal of the dilator is a must  for completion of dilatation process, then heparinized saline instillation and mild hydropneumatic dilatation was done in order not to overstretch the vein wall. All anastamoses were  performed using  monofilament non­ absorbable polypropylene running sutures. Patients were then seen routinely 7 to 14 days postoperatively and then 4 to 6 weeks thereafter to assess for adequate maturation.

Results:

Demographic data and co-morbidities: The relevant demographic and clinical characteristics of the patients for whom fistula adequacy could be determined are summarized

in Table(l).The patients' age (mean) was 65.9

15 years and ranged between 18 and 85 years. Thirty percent of the fistulas were created in patients 65 years of age or older. Approximately

63.33% of the fistulas were constructed in men (38  patients) and  36.67% in  women (22 patients). Diabetes was present in25 (41.67%) of  the  patients, hypertension was  highly prevalent in  patients with  either  upper  or forearm fistulae 46 (76.67%); Nine patients (15%) were identified as having ischemic heart disease, impairment of liver  function  in 10

{16.67), pulmonary disease in 3 (5%), history ofprevious cerebral stroke in 5 (8.33%) and

7 patients (11.67%) were currently smokers. Approximately (11.67%) ofthe patients were overweight (BMI ?27  kg/m2). Patients  who were receiving dialysis  at the time of their operation were the majority  54 (90%),  and those having ibsilateral central venous catheter were 9 (15%).  Forty forearm  radiocephalic and 20 brachiocephalic fistulas  constructed around the elbow region.

Table (1): Demographic data, co-morbidities and type of AV access.

Results of preoperative duplex examination: Preoperative duplex examination was performed  in only 15 (25%) of the cases. In the  RC AVF  group,  mean  duplex  forearm cephalic vein diameter was 2.23±0.17mm in the successful  fistula patients, and 1.8±0.19 mm in patients  with primary  fistula  failure (P=0.15). In the  BC  AVF  patients, mean antecubital cephalic vein   diameter  was

3.44±0.18 mm.

Procedure related complications:

The forearm cephalic vein was perforated by false passage of the metallic dilator in one patient; fortunately this was near its presumed site  of  anastomosis and  was  managed by elongating the skin incision and more proximal dissection of the vein excluding the perforated segment. In another case of RCAVF, the dilator (No. 2) passed gently and smoothly  but (No.

2.5) stopped midway in spite of good diameter

of vein (3mm); mostly due to outflow stenosis,

we did not try  violent  attempts for dilator passage and completed the procedure but this patient had no post-operative palpable  thrill and fistula  deemed  failed  to mature. Three fistulas  had intra-operative thrombosis that were managed by thrombectomy, that resulted in salvage of2 fistulas and regain of pulsatile flow without thrill in the third one due to small diameter radial artery and weak inflow. One out of the 2 salvaged fistulas were affected by post-operative thrombosis due to hypotension during dialysis.

Mild post-operative complications that were not associated with primary fistula failure are summarized  in Table(2); all were managed conservatively except one case ofBCAVF that developed large wound hematoma then became infected with  thrombosis and  hemorrhage necessitating fistula ligation.

Causes  of primary fistula  fallure:

Primary AVF failure occurred in 13 patients

(21.67%). Early thrombosis was diagnosed in

6 patients (100/o) Table(2), two of these patients had small veins that failed to dilate intra- and post-operatively, one  had  intra-operative thrombosis and thrombectomy was done but again thrombosis ensued postoperatively mostly due to hypotension during dialysis, one had infection, hemorrhage and  thrombosis necessitating ligation and 2 had undergone successful thrombectomy and became mature later on. Inadequate maturation was the underlying problem  in 11.67%  of the cases (n=7), two of which as a result of accessory vein(s) that were revised later on and became mature. Three out of seven  had no palpable thrill either due to small and weak inflow radial artery, or outflow vein stenosis and 2/7 failed to be cannulated one for small sized vein and the other vein was too deep and underwent superficialization and  was successfully cannulated later on.

Table (2):Intra-and post-operative complications.

Maturation, functional maturation and primary fistula  failure rates:

Of 60 access procedures the total number of fistulas that were judged to be matured was

49 (81.67%). Eleven (18.33%)  fistulas were deemed not mature and were either not used, or abandoned at last follow up Table(3). The total number of fistulas deemed ''matureH that actually became  functionally  mature at first time cannulation was 47 out of 49 (95.9%). A total of 5 fistulas underwent revision, mostly for failure to mature. Two of the fistulas in which the vein failed to dilate sufficiently for cannulation underwent accessory vein ligation,

2 had successful thrombectomy, and one was too deep for cannulation and underwent transposition. Thus, after secondary procedmes fistulas achieving assisted functional maturity became 52/60 (86.67%).0verall mean time to fistula maturation  was 32±1.29 days (n=49). For  RCAVf mean   maturation time   was

33.63±4.8  days, while that for BCAVF  was

30.5±4.3 days. Overall mean time from fistula creation to first time cannulation was 40±9.6 days. It was longer in patients with RCAVF (43.22±4.7) days than in patients having BCAVF (38.9±3.6) days Table(4). The more increased vein  diameters after  mechanical dilatation; the more likely  for the fistula  to mature and the lesser mean time for maturation and cannulation.

Access patency:

Cumulative primary patency was reduced in forearm fistulae compared with upper arm fistulae, with  RCAVF having only  75% primary patency at 6 months (30/40 fistulae), compared with 85% at 6 months (17/20 fistulae) for BCAVF;  Similarly, cumulative assisted primary patency was also reduced in RCAVF fistulae compared with  BCAVF; assisted primary patency was  90%  at 6 months  in BCAVF (18/20 fistulae) compared with 85% at 6 months (34/40  fistulae)  in the RCAVF (p=0.036).

Table (3): Vein diameter and its relation to f .stula maturation.

Table (4): Vein diameter and its relation to maturation and cannulation times.

Figure(1):Cephalic vein before dilatation and dilator inside the vein.

Figure(2):Cephalic vein after dilatation and.finished fistula.

Figure(3):Dilator in cephalic vein and.finished anastomosis.

Figure(4):Cephalic vein before and after dilatation.

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Discussion:

In order for a fistula to mature properly, the inflow artery and  outflow vein  should  be unrestricted and free from disease. The diameter of these vessels is important since the resistance to flow through a cylindrical vessel is inversely proportional to the vessel radius.l3 Vein size logically seems  to be a major  predictor of fistula maturation if we base functional maturity on flow. As Pouseuille's law dictates,  flow (Q) is proportional to the product of change in pressure gradient (M') and the fourth power of the vessel radius (r) divided by the viscosity ('11) of blood (Qax r4/ '11). Hence,  larger veins (radius) mean larger flow. However, this does not take into account the arterial factors and normal pulsatile blood flow. Furthermore, venous compliance after fistula creation should be considered.3

In a study by lauvao et al,3 the mean age was 58±1.29,  our patient  population was of slightly higher  age (65±15) years,  and the prevalence  of diabetes was higher than ours (62% vs. 41.67%).  Hypertension, however, was found  in 94% (148/158) ofpatients in their study compared with 76.67% (46/60) in our  study  population. Smokers were  16% similar to our data. Of special importance that our  study  patients were  mostly  on current dialysis (90%) in contrast to most other studies in USA and Europe where the guidelines recommend and  implement planning for dialysis at least 6 months before the presumed date.

There is much discrepancy  between  time of maturation in various reports. For example, the median time to first fistula cannulation differed  between  countries, ranging  from28 days in Japan and Italy to 96 and 98 days in transplantation in UK and US, respectively.l4

In Lauvao  et aJ3 study, among 158 accesses constructed, 47 (29.74%) fistulas were deemed not  mature and  were  either not  used,  or abandoned. Maturation rate  was  111/158 (70%). Functional maturity was 101 out of 111 (91%). Overall mean time to fistula maturation was 82±1.29 days. Mean  time from fistula creation to first time cannulation was 147±19.6 days. Only 80 patients (51%) were already on dialysis.Clearly different results were observed in our study where 11 (18.33%) fistulas were

deemed not mature and were either not used, or abandoned. Overall maturation rate was 49 (81.67%) and functional maturity was 47 out of 49 (95.9%).  Assisted functional maturity after  a secondary intervention was 52/60 (86.67%). Overall mean  time  to fistula maturation was 32±1.29 days (n=49). Overall mean time from fistula creation to first time cannulation  was 40±9.6 days. Nearly similar mean maturation time was observed in Zadeh et  al  study while it was  38.60 days (SD=42.13).15 The Dialysis Outcomes and Practice Patterns Study (DOPPS) reported a mean cannulation time of98 days in the United States.l6 Fitzgerald et all2 reported that overall,

80% fistulas matured, with 23% requiring an intervention to achieve  maturity. The mean time  to   maturation  was    3.8   months.

Several studies have provided insight into

the problem of inadequate maturation however, there is relatively limited  understanding of why  fistulas fail.  Roy-Chaudhury et  all7 summarized the  pathophysiology of early fistula failure. Genetic predisposition, turbulence, low  shear  stress, increases in transmural pressure, differences in compliance between arteries and veins, and injury to vasa vasorum  of mobilized  vein segment all lead to neointimal hyperplasia and harmful vascular remodeling. Failure due to early thrombosis is usually secondary to technical errors, judgment error regarding vessel adequacy, or a period ofhypotension.18 A wide range of early primary fistula failure rates have been reported in the literature. Da Silva et all9 reported a 15% early failure  rate, and Ozkoleli et al 18.8%.20 A range of 12%-29% have been observed in other studies.21,22 Inour study, primary AVF failure occurred in 13 patients (21.67%). Early thrombosis was diagnosed in 6 patients (10%). Overall primary patency was 47/60 (78.33) at

6 months. Also, Ekici et aJ23 reported a 12% early thrombosis rate.Fitzgerald et al12 reported overall primary patency and assisted primary patency rates at 12 months of 50% and 74%, respectively they observed  that 2% of early thrombosis occurred after first use of the AVF.

A recent study by lyem24 reported an early

thrombosis rate  (5.98%) lower  than  that reported in the literature. The author attributed this to the use of mechanical dilatation with a

probe for relieving spasm of both the arteries and veins, and to the use of topical papaverine. He suggested that a proportion of early failure can be attributed to technical inadequacy, which can be avoided  through  this maneuver. He concluded  that mechanical dilatation of the artery and vein,before starting the anastomosis, as well as the use of vasodilatory agents, could decrease early thrombosis of the fistula, and this method has very high early patency. We think that the aim of mechanical  dilatation is to dilate the whole length of the small vein in order to promote maturation and hence cannulation, to  relieve any  spasm  due  to manipulation, to dilate any invisible stenotic or fibrotic areas which would be more evident after vein distension by blood flow causing outflow stenosis and to decrease resistance to blood flow in outflow vein through increasing the  radius. As Corpataux et aFS  found  in RCAVF, vein luminal size can increase  by

86%  in one  week. However, distensibility (compliance) and not size was reported by van der Linden  et al to be predictive of fistula maturation.26

Conclusion:

In this study, the early fistula patency rate was  better  than that  in several  reports, the possible reasons for  this  may  be: careful preoperative evaluation of the patients as well as careful anastomosis technique, which included mechanical dilatation for the vein in each patient and the artery in some cases. Gentle mechanical dilatation of the artery and vein,  before starting the  anastomosis can decrease  the risk of early thrombosis of the fistula.  Smooth  non-violent passage  of the dilator is a must to get benefit instead ofharm from this procedure. Studies are still mandated to elucidate  vein characteristics (early post­ operative  flow, vein size, or compliance) to increase fistula suitability for  dialysis.

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