Factors associated with access blood flow in native vessel arteriovenous fistulae.

BACKGROUND: Access blood flow (Qa) identifies stenosis in patients with native vessel AV fistulae (AVF), but data on factors that are associated with Qa in normally functioning accesses are sparse. Such factors could be used in conjunction with Qa to improve the diagnostic performance of screening. We examined the relationship between Qa and certain clinical characteristics in a large group of patients with AVF. METHODS: This was a retrospective study of incident and prevalent haemodialysis patients treated at a single institution, all of whom had a functioning AVF during the study period. Qa was measured bimonthly using ultrasound dilution in all subjects. Mixed models were used to explore the relationship between Qa and a group of independent variables, including systolic blood pressure (SBP), diastolic blood pressure (DBP), mean arterial pressure (MAP), diabetes mellitus, patient age, sex, height, body mass index (BMI) and AVF location (forearm vs upper arm). RESULTS: A total of 4084 Qa measurements was made in 294 patients. Univariate analysis found that younger patient age, non-diabetic status, higher blood pressure (SBP, DBP, MAP, all at the time of Qa measurement), upper arm AVF location and overweight status (BMI >/=25) were significantly associated with Qa. SBP appeared to be more strongly associated with Qa than either DBP or MAP. Patient sex, height and interval between access creation and Qa measurement were not significantly associated with Qa. Tests for interaction suggested that the association between SBP and age and Qa varied significantly by access location. In a multivariate model, SBP, overweight status and diabetic status were independently associated with Qa. The strength of the association between these characteristics and Qa appeared to be clinically relevant. CONCLUSIONS: Our findings suggest that a single Qa threshold for angiography in all patients may be simplistic, and that the optimal threshold might vary by patient subgroup. The strong association between SBP and Qa suggests that adjusting Qa for SBP may improve the specificity of access screening. Further work is required to determine whether such modifications to current practice would improve the predictive power of Qa measurements for detection of stenosis in AVF.

Best threshold for diagnosis of stenosis or thrombosis within six months of access flow measurement in arteriovenous fistulae.

Canadian clinical practice guidelines recommend performing angiography when access blood flow (Qa) is <500 ml/min in native vessel arteriovenous fistulae (AVF), but data on the value of Qa that best predicts stenosis are sparse. Because correction of stenosis in AVF improves patency rates, this issue seems worthy of investigation. Receiver-operating characteristic curves were constructed to examine the relationship between different threshold values of Qa and stenosis in 340 patients with AVF. Stenosis was defined by the composite outcome of access failure or angiographic stenosis occurring within 6 mo of the first Qa measurement. The Qa value was then classified as true negative, true positive, false negative, or false positive for stenosis. An additional analysis was performed in which Qa was corrected for systolic BP before assigning it to one of the four diagnostic categories. The area under the curve for the composite definition of stenosis was 0.86. Graphically, Qa thresholds of <500 and <600 ml/min had similar efficacy for detecting stenosis or access failure within 6 mo, and both seemed superior to <400 ml/min. However, the frequency of the composite definition of stenosis among AVF with Qa between 500 and 600 ml/min was only 6 (25%) of 24, as compared with 58 (76%) of 76 when Qa was <500 ml/min. This suggests that most lesions that would be found using a threshold of <600 ml/min occurred in AVF with Qa <500 ml/min and that the small gain in sensitivity associated with the <600-ml/min threshold would be outweighed by the reduced specificity compared with <500 ml/min. Correcting Qa for BP did not improve diagnostic performance or change these results, which were consistent in several sensitivity analyses. Qa measurements seemed to predict stenosis or incipient access failure equally well in groups defined by diabetic status, gender, and AVF location. In conclusion, it was found that Qa <500 ml/min seems to be the most appropriate threshold for performing angiography in patients with native vessel AVF. It is recommended that clinicians arrange angiography when Qa is <500 ml/min in AVF.

Access flow monitoring of patients with native vessel arteriovenous fistulae and previous angioplasty.

Screening strategies based on measurement of access blood flow (Qa) allow detection and angioplasty of subclinical stenosis in native vessel arteriovenous (AV) fistulae. However, little is known about the efficacy of Qa measurements for detecting recurrent stenoses in fistulae and that of angioplasty for correcting them. A total of 303 patients were studied over 30 mo; 69 (23%) of these had stenoses, of whom 53 underwent angioplasty. Of those undergoing angioplasty, 30 patients had 46 episodes of recurrent positive studies and underwent repeat fistulography. In 31 of these episodes (19 patients), stenosis was again identified and treated successfully with angioplasty. Overall positive predictive values for stenosis were similar in first and subsequent episodes of stenosis (71% versus 67%), and angioplasty was associated with sustained increases in Qa for both first and subsequent episodes. Assisted patency in fistulae that required repeat angioplasty was 87% (median follow-up 10 mo after the second angioplasty). In conclusion, Qa is effective for detecting first and subsequent lesions in patients with AV fistulae, and angioplasty of first or subsequent lesions is associated with sustained increments in Qa. Continued screening after correction of first stenoses appears reasonable, because of both the frequency of recurrent stenosis and the success of repeat intervention.