We also identified 45 instances among 8 patients being dialyzed with the smaller dialyzer surface area (1.9 m2), where all the data required had been recorded simultaneously and identified only 2 instances with AR. The ROC curve constructed for the EID/Qb ratio for these patients had an area under the curve of 0.721% ± 0.259% (95% CI 0.213–1.000). Among these patients, the EID/Qb ratio of 50% had a sensitivity of 50% and specificity of 83.7% (Youden index of 0.34), a positive likelihood ratio of 3.1, and a negative likelihood ratio of 0.6, resulting in a positive predictive value of 12.5% and a negative predictive value of 97.3%. The highest Youden index of 0.47 was noted at an EID/Qb ratio of 56.6% with a sensitivity of 100% and specificity of 47.1%.
A solitary instance of AR (among 18 instances available with all the data) in a patient being dialyzed with an even smaller dialyzer (1.7 m2, KoA 1103 ml/min) was noted incidentally. The EID/Qb ratio in this instance was 49%—a level that would have been detected by our recommended threshold of ≤50%.
Linear regression analysis showed excellent correlation between the flow rate measured by the Gambro Phoenix machines' compensated flow rate and the Transonic HD-02 monitor (y = 117.48 + 0.74x, r = 0.79, p < 0.0001; Figure 4). The compensated blood flow overestimated the Transonic blood flow by 14.2 ml/min on average (95% CI 10.96–16.1 ml/min), and the agreement between these two measures of pumped blood flow rate through the dialyzer is shown using a Bland-Altman plot (Figure 4).
Vascular access function and patency are essential for optimal management of HD patients. Vascular access dysfunction is associated with an increase in morbidity and mortality. Arteriovenous fistulae are the preferred choice access for HD patients.3 They have the lowest rate of thrombosis, infection, cost of implantation, and maintenance, as well as morbidity and mortality when compared with other types of permanent vascular access.3 As a result, there is an ongoing effort to increase the prevalence of AVF among HD patients in the United States. In the era of high-flux, high-flow HD, a functioning AVF capable of providing high pump blood flow rate without AR is required to ensure maximal efficiency of HD.
Arteriovenous fistulae develop stenoses over time, which if detected and corrected early, can help to prevent inadequate dialysis and other complications related to access dysfunction.5,17 Arteriovenous fistula recirculation occurs in the presence of hemodynamically significant stenoses or when the pumped blood flow rate through the dialyzer (Qb) exceeds the access flow rate (Qa).3,21,27 However, the monitoring of access flow alone is not an adequate measure of access function as significant recirculation can occur with adequate access flow as a result of inappropriately close positioning of needles, inadvertent line reversal, or cardiopulmonary recirculation and in fistulae with branches and collaterals.13,14,28
Dialysis adequacy is adversely affected by recirculation and is reflected in a lower EID. The dialyzer surface area, dialysate flow rate (Qd), and the blood flow rate (Qb) also influence the EID.16 As a result, a low EID is also seen in situations that are associated with low Qb in the absence of access dysfunction such as in patients with a new AVF. The EID/Qb ratio ≤50% identified AR among patients who received dialysis using dialyzers with a range of surface areas and KoAs. Use of the EID/Qb ratio instead of EID alone prevents the inadvertent identification of cases of low EID resulting from a low Qb as cases of AR. This simple ratio can be readily calculated at the chairside and can be used to identify patients with inadvertent line reversal, needle malposition, a smaller dialyzer surface area, or true AVF dysfunction when used early during a dialysis session. As a result, once inadvertent line reversal and needle malposition have been ruled out, EID/Qb ratio ≤50% can be used to identify patients who need further evaluation of their AVF.
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