Vascular access: anatomy, examination, management

Graftula: a composite access consisting of a graft used to repair a dysfunctional hemodialysis fistula

Nonmaturing or dysfunctional hemodialysis fistulas are often repaired with interposition grafts placed either within the fistula (fistula-to-fistula configuration), or connected to another vein (fistula-to-vein configuration). The goal of this study was to compare the survival and usefulness of the composite accesses thus created, which we call "graftulas," with upperarm grafts. This was a retrospective study wherein we determined the survival and thrombosis rates of graftulas (n=24) and upper arm grafts (n=31) placed 1/1/07 through 12/31/09 and followed through 11/30/10. Graftulas resembled grafts as most (96%) were successfully cannulated in 65 ± 43 days. Survival of graftulas was also similar to grafts (58%, 47%, and 32% vs. 56%, 47%, and 39% at 1, 2, and 3 years respectively, p=0.60). However, graftulas had a lower thrombosis rate than grafts (0.5 vs. 1.2 per patient year, p=0.04), and in the fistula-to-fistula configuration, a 2-year thrombosis-free survival of 78%. Total survival of the access site (fistula+graftula) was 92%, 73%, and 42% at 1, 3, and 5 years, respectively. Graftulas possess certain beneficial properties of fistulas and grafts that allows for continued use of the original access site.

Percutaneous Treatment of Dysfunctional Brescia-Cimino Fistulae Through a Radial Arterial Approach

BACKGROUND

Dysfunctional Brescia-Cimino fistulae contribute to significant morbidity in hemodialysis patients. These fistulae normally are treated through a retrograde venous approach. There are no data regarding a transradial approach. Furthermore, measurement of pressure reduction in the radial artery appears to be useful.

METHODS

We retrospectively examined 50 interventions to treat 49 patients (17 men, 32 women; mean age, 61.8 +/- 10.6 years) with Brescia-Cimino fistulae. Inclusion criteria were patients with palpable radial arteries and dysfunctional end-to-side Brescia-Cimino fistulae. Patients with infected fistulae, contrast allergy, upper-arm/synthetic graft/central-vein stenosis, and end-to-end Brescia-Cimino fistulae were excluded from the study. Radial arterial pressures before and after angioplasty were compared as a surrogate of stenosis relief. Anatomic and clinical success rates were calculated.

RESULTS

Sixty-five stenoses and 4 total occlusions were treated through radial access. All radial punctures were successful, except in 1 patient. Most lesions were located in the cephalic vein (87%). Mean length of treated lesions was 4.1 +/- 2.8 cm. Mean pretreatment diameter of lesion stenoses was 76.7% +/- 12.1%. Mean posttreatment diameter stenosis was 22.6% +/- 8.2% (P < 0.001). Systolic, diastolic, and mean blood pressures recorded from the radial artery decreased from 130 +/- 40, 60 +/- 18, and 87 +/- 27 to 88 +/- 40, 43 +/- 18, and 60 +/- 26 mm Hg (P < 0.001, P < 0.001, and P < 0.001), respectively. The anatomic success rate of the transradial approach was 91.3%. The clinical success rate of the transradial approach was 96%.

CONCLUSION

The transradial approach is a feasible and highly effective approach to treat dysfunctional Brescia-Cimino fistulae. Measuring blood pressure reduction through the radial artery appears promising as a hemodynamic evaluation method.

The difficult hemodialysis access extremity: proximal radial arteriovenous fistulas and the role of angioscopy and valvulotomes

BACKGROUND

Native arteriovenous (AV) fistulas (NAVF) offer significantly lower risks than grafts or catheters. Individuals with a difficult access extremity (DAE) are often viewed as unsuitable for NAVFs. The proximal radial artery (PRA) NAVF offers a safe and reliable opportunity for a direct fistula in most patients, and we find it an important surgical option in the DAE.

METHODS

Consecutive vascular access operations were reviewed to find individuals with DAE. We defined the DAE patient group as those individuals where a wrist (Cimino) or upper arm brachiocephalic NAVF was not possible or was predicted to fail.

RESULTS

Preoperative physical and ultrasound examinations identified 58 individuals with DAE. Mean age was 56 years (range 11 to 87), 34 were female, 29 were diabetic, and 27 had previous access surgery. NAVFs were constructed in all patients. No grafts were utilized. Forty-six patients had a PRA NAVF constructed. NAVF patency was 91%. Twenty-three patients required retrograde angioscopy or passage of a valvulotome to gain forearm access. Twenty-one of these 23 individuals maintained an open NAVF segment in the forearm.

CONCLUSION

NAVFs were constructed in all patients. PRA NAVFs play an important role in extending hemodialysis by NAVF for this difficult patient group. Forearm access is often possible in these patients and may be successfully augmented by angioscopy or valvulotomes.

Vascular access as a determinant of adequacy of dialysis

Vascular accesses consist of permanent arteriovenous (AV) accesses (autogenous fistulas and synthetic grafts) and venous accesses (central venous catheters [CVCs]). AV accesses have fewer complications than venous accesses, and are therefore the preferred hemodialysis access. An important additional issue is whether the type of access influences adequacy of dialysis (i.e. Kt/V). Key limiting factors in delivering adequate Kt/V are blood pump speed (Q(B) ), access recirculation, and treatment time. In general, AV accesses support higher Q(B)S with less negative inflow arterial pressures than CVCs. Well-functioning AV accesses are also less likely to exhibit recirculation. Nevertheless, recirculation commonly develops when AV accesses (usually grafts) develop stenosis with decreased access blood flow. Although extension of treatment time can offset the effects of reduced Q(B) and recirculation, this is often impractical and poorly accepted by patients. In conclusion, AV accesses are superior to venous accesses because they are less prone to complications and are more likely to deliver prescribed Kt/V within prescribed treatment time.

Mathematical Model for Pressure Losses in the Hemodialysis Graft Vascular Circuit

Stenosis-induced thrombosis and abandonment of the hemodialysis synthetic graft is an important cause of morbidity and mortality. The graft vascular circuit is a unique low-resistance shunt that has not yet been systematically evaluated. In this study, we developed a mathematical model of this circuit. Pressure losses ΔPs were measured in an in vitro experimental apparatus and compared with losses predicted by equations from the engineering literature. We considered the inflow artery, arterial and venous anastomoses, graft, stenosis, and outflow vein. We found significant differences between equations and experimental results, and attributed these differences to the transitional nature of the flow. Adjustment of the equations led to good agreement with experimental data. The resulting mathematical model predicts relations between stenosis, blood flow, intragraft pressure, and important clinical variables such as mean arterial blood pressure and hematocrit. Application of the model should improve understanding of the hemodynamics of the stenotic graft vascular circuit.

Thrombotic Events of Arteriovenous Fistulae in Hemodialysis Patients Related to the C677T Thermolabile Variant of Methylenetetrahydrofolate Reductase

Background

Hyperhomocysteinemia is a risk factor for thrombosis, a frequent complication of vascular access (VA) in hemodialysis (HD). The enzyme methylenetetrahydrofolate reductase (MTHFR) is necessary for the remethylation of homocysteine (Hcy) to methionine. It has been postulated that patients homozygous and, to a lesser extent, heterozygous for the C677T thermolabile variant of this enzyme present a reduced catalytic activity, with secondary increases in plasmatic Hcy levels (normal: 10 ± 5 μmol/L) and an elevated risk of vascular thromboses.

Methods

Sixty-two patients on chronic HD were divided into two groups: group A (n = 23, 37.1%) was normal for the enzyme (CC); group B (n = 39, 62.9%) was heterozygous (CT). Both groups were not different according to age, sex, time on HD, hematocrits (Hct), baseline levels of Hcy, folic acid and vitamin B12. After the 1st HD session patients were started on folic acid 10 mg/day and 500 μg/week of intravenous (i.v.) methylcobalamin.

Results

Two years later, thrombotic events were not different between the two groups. Group A = 5 (21.7%) vs. group B = 12 (30.7%), Hcy levels were significantly different between final and baseline measurements (group A 21.5 ± 5.2 vs. 16.6 ± 3.9 μmol/L, p = 0.02; group B 22.1 ± 8.9 vs. 16.1 ± 3.9 μmol/L, p = 0.008), folic acid (group A 22.1 vs. 346.9 ng/ml, range (r) = 166–527, p<0.001; group B 19.2 vs. 218.5 ng/ml, r = 138–298, p<0.001) and vitamin B12 (group A 1489 vs. 3192.3 pg/ml, r = 1494–4890, p = 0.01; group B 1086 vs. 1513.8 pg/ml, r = 1092–1934, p = 0.02).

Conclusions

HD patients heterozygous for the C677T variant of the enzyme MTHFR can present a similar risk of thrombotic events in arteriovenous fistulae (AVF) compared to patients normal for the enzyme at a 1-yr follow-up. These results could be explained by an adequate control of Hcy levels after folic acid and methylcobalamin replacement therapy.

In vivo validation of glucose pump test for measurement of hemodialysis access flow

BACKGROUND

The glucose pump test (GPT) is a recently introduced method of measuring hemodialysis access blood flow (Qa). A validation of GPT during dialysis has not yet been done, and performance characteristics of the method have not yet been fully analyzed.

METHODS

The authors studied 33 patients (25 synthetic grafts, 8 autogenous arteriovenous fistulae). Qa measurements by ultrasound dilution (UD) and GPT were done in triplicate during dialysis. In GPT, a baseline blood sample (C(1)) was obtained, followed by infusion of a 10% glucose solution (C(i)) through the arterial needle into the access at 16 mL/min (Q(i)). After 11 seconds, a downstream blood sample (C(2)) was aspirated from the venous needle. C(1) and C(2) glucose were measured by glucometer. Qa was computed by the equation: Qa = Q(i)(C(i) - C(2))/(C(2) - C(1)). A model of the access vascular circuit was used to determine the influence of C(2) aspiration on the Qa measurement.

RESULTS

Mean Qa was 1413 mL/min by UD versus 1,496 mL/min by GPT (P = 0.11). There was a strong linear correlation between the 2 methods (r = 0.905; P <0.001). The pooled coefficient of variation was 6.4% for UD and 9.6% for GPT. The circuit model showed that aspiration of C(2) causes an increase in Qa (DeltaQa) that depends on the aspiration rate (Q(ASP)) and fraction of resistance in the circuit that is downstream to the venous needle: DeltaQa = Q(ASP)(Downstream resistance)/(Total resistance). The model predicts the overestimate is approximately 62 mL/min for grafts and 120 mL/min for fistulae but may vary depending on the balance of resistances upstream and downstream to the venous needle.

CONCLUSION

This study shows that GPT closely correlates with UD, and the method has adequate precision. GPT is an inexpensive method that may help make Qa measurements more widely available than previously possible.