Periodic vascular access monitoring and surveillance can predict the presence of vascular access stenosis and subsequent thrombosis in patients undergoing hemodialysis (HD) [1]. Preemptive intervention to correct stenosis of the failing access prevents thrombosis and extends the life span of the vascular access in comparison with an attempted repair after a thrombotic event [2], [3], although this issue has become controversial [4]. National Kidney Foundation Kidney Disease Outcome Quality Initiative (NKF-KDOQI) [5] and European best practice guidelines [6] recommend prospective surveillance of fistulae and grafts together with physical examination on a regular basis, and also recommend angiography and prompt preemptive intervention to correct stenosis, which may improve patency rates and may decrease the incidence of thrombosis. In addition, both guidelines recommend monthly measurement of access flow as the preferred tool for surveillance of fistulae and grafts. A decreased rate of access flow has been shown to be a significant predictor of vascular stenosis and subsequent thrombosis for both fistulae and grafts [7], [8], [9].

Measurement of access flow using the ultrasound dilution technique has been described by Krivitski and is a reliable method for screening for access dysfunction [10]. Its advantages over Doppler ultrasound are its reproducibility, low cost, operator-independence, and ability to provide rapid feedback before and after corrective intervention [11], [12].

A clinically controversial issue that has been raised in regard to the measurement of access flow by ultrasound dilution is the variation in access flow according to the time of measurement during HD. The NKF-KDOQI guidelines recommend that the flow assessment should be performed during the first 1.5 hours of the treatment to eliminate error caused by decrease in cardiac output or blood pressure related to ultrafiltration and/or hypotension [5]. Restriction of the measurement time to within the first 1.5 hours of the treatment limits the number of measurements that can be performed by one operator, which is a significant issue in clinical practice, considering the number of patients who require prospective monthly measurement of access flow. However, whether the ultrafiltration volume significantly alters the access flow remains controversial [13].

The aim of this study was to evaluate whether access flow could be significantly reduced when measured late during HD. In addition, we evaluated whether high ultrafiltration volume or decrease in blood pressure at the end of HD could significantly reduce access flow. Overall, we evaluated whether access flow should be measured early during HD, or it could be measured at anytime during HD. The effect of patients' demographic parameters on the access flow was also evaluated.

In this study, we found the variation in access flow during HD to be relatively small (6.1%). The NKF-KDOQI guidelines on access flow surveillance clearly recommend prospective trend analysis based on monthly measurement rather than responding to a single isolated abnormal value. Furthermore, a patient should be referred for angiography when the access flow in the graft has decreased by >25% over a 4-month period [5]. As clinical decisions are usually based on reductions of access flow by >25%, the 6.1% variation in access flow during HD is relatively small and clinically insignificant.

The change in MAP during HD did not correlate with the variation in access flow in the total group of sessions, because the MAP measured before HD, and 30 minutes, 120 minutes, and 240 minutes after the start of the HD session was not different. We next compared the access flow between the sessions in which the MAP measured after 240 min had decreased by ≥5% or 10% compared with pre-HD MAP and sessions in which the MAP did not decrease by ≥5% or 10%. We found that a≥5% decrease in MAP during HD reduced access flow after 240 min by an average of 12.3%, and this reduction was statistically significant. A≥10% decrease in MAP during HD also significantly reduced access flow. Based on our results, we would recommend that any significantly decreased measurement of access flow in a patient in whom MAP had decreased by ≥5% should be rechecked at the next session. Krivitski [14] similarly recommended that if a significant decrease in MAP had been observed, the clinician should confirm any reduction in access flow by repeating the measurement at the next session before referring the patient for angiography. Rehman et al. measured access flow serially 30 minutes, 90 min, and 150 min after the start of HD and found a strong correlation between access flow and the MAP 90 min after the initiation of HD, with each 10% decrease in MAP resulting in an 8% decrease in access flow [15]. They concluded that access flow could be measured up to 2–2.5 h after the start of HD and suggested postponing measurement in patients in whom MAP had decreased more than 15%. By contrast, Agharazii et al. [13] found that the measurements of access flow made within the first and last 30 min of HD in 50 patients did not differ. The access flow decreased by 11.7% over the course of the HD session, but this reduction dropped to 4.9% when the access flows were corrected for a MAP of 100 mmHg using the following equation: Qac'=Qac (100/MAP), where Qac' is the corrected access flow and MAP is the actual mean arterial pressure at the time of measurement. They concluded that the variation in access flow during HD was relatively small, especially when the values were corrected for MAP. Therefore, access flow can be measured by the ultrasound dilution method at any time during HD.

In our study, the ultrafiltration volume did not affect access flow during HD. In addition, it was not correlated with the change in MAP. A net reduction in the effective circulating plasma volume due to an imbalance between the ultrafiltration rate and the plasma refilling rate has been shown to induce intradialytic hypotension during HD. However, intradialytic hypotension is caused not only by the reduced circulating plasma volume but also by the impaired physiologic adaptation to the reduced plasma volume by means of constriction of resistance vessels, increased heart rate and myocardial contractility, and constriction of capacitance vessels [16], [17]. Recently, an automatic biofeedback system for controlling blood volume changes during HD has been studied in hypotension-prone HD patients. This system is based on the concept of blood volume tracking to prevent hypovolemia and works by continuous modification of the ultrafiltration rate and dialysate conductivity. In the studies, the use of blood volume controlled HD reduced the frequency of intradialytic hypotensive episodes by 30–50% in comparison with conventional HD [18], [19], [20]. These results clearly demonstrate that ultrafiltration volume is not the only cause of hypotension during HD. Therefore, ultrafiltration volume might not be an independent risk factor for either the decrease in blood pressure or the variation in access flow during HD.

Measurement of access flow by ultrasound dilution takes at least 10 min. As we are well aware of the importance of monthly vascular access surveillance, measuring the access flow only within the initial 1.5 h of HD, in accordance with the NKF-KDOQI guidelines, requires a greater number medical personnel to measure access flow in all patients and results in higher medical expenses.

The limitations of this study include the small number of patients. Because this was prospective observational study, we did not decide the number of patients based on the statistical sample size evaluation. Thirty patients were not enough to confirm the effect of the parameters measured in this study on the variation in the access flow during HD.

In conclusion, access flow measurement by the ultrasound dilution method is very reproducible, and the variation in access flow during HD (6.1%) is relatively small, as clinical decisions for vascular access dysfunction requiring angiography are usually based on reductions of access flow by >25%. Because decreased blood pressure during HD (MAP reduction by >5%) is a risk factor for reduced access flow, any clinically significant reduction in access flow in patients in whom MAP has also decreased should be confirmed at the next HD session. However, in most patients whose blood pressures remain stable during HD, access flow can be measured at any time during the HD treatment.

All contributing authors declare no conflict of interest.