3 Factors That Influence Surveillance Outcomes
Transonic ultrasound dilution technology revolutionized comprehensive vascular access patency management by enabling routine surveillance to detect decreasing access flows that foreshadow access thrombosis failure. However, the value of surveillance continues to be scrutinized. In assessing the value of surveillance one must not consider surveillance alone, but in the context of the three factors that influence surveillance outcomes. They are:
- Accuracy of the technology used for routine surveillance;
- The guidelines used for referring a patient for an intervention (percutaneous transluminal angiography, PTA);
- The outcomes of PTA.
First, consider the accuracy of the technology used for routine surveillance. Indicator dilution measurements with the Transonic Hemodialysis Monitor are the recognized Gold Standard for vascular access measurements. Its accuracy and reproducibility are proven and unequalled.1-3
“Ultrasound dilution (Krivitski Method): Very high reproducibility and the negligible impact of changes in blood flow on the accuracy of vascular access flow measurement justifies its current status as the reference method for vascular access flow evaluation.”
Only thermodilution (BTM, Fresenius) is considered comparable 1,2
Second, it is critical not to rely on one single measurement to determine if a patient should be referred to PTA but to follow established surveillance guidelines to diagnose the onset of stenosis. KDOQI Guidelines suggest that for native fistulas, the threshold for the critical flow threshold is >500 mL/min. 4 European Guidelines set the flow threshold of >300 mL/min in forearm fistulas as an indication for preemptive intervention.5 For vascular access prosthetic grafts, both KDOQI and European Guidelines set the Critical Flow Threshold at >600 mL/min or access flow of less than 1000 mL/min if flow drops 25% (European Guidelines: 20%) or more over four months.
Finally, positive outcomes for extending a fistula or graft’s life depends on successful PTA. Surveillance only identifies a potential problem. It must be coupled with a successful intervention for ultimate patency success. A 2003 study by Tessitore5 evaluated the effect of PTA on functioning fistula survival. Sixty-two stenotic, functioning AVF patients were enrolled: 30 were allocated to control and 32 to PTA. Analysis showed that PTA improved AVF functional failure-free survival rates with a fourfold increase in median survival and a 2.87-fold decrease in risk of failure. PTA induced an increase in access blood flow rate by 323 (236 to 445) ml/min suggesting that improved AVF survival is the result of increased flow. The conclusion was,
“That prophylactic PTA of stenosis in functioning forearm fistulas improves access survival and decreases access-related morbidity, thus supporting the usefulness of preventive correction of stenosis before the development of access dysfunction. It also strongly supports surveillance program for early detection of stenosis.”
In summary, the value of surveillance must be judged only in accordance with the the three factors listed above that influence its outcomes.