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To examine the relationship between intra-access pressures and vascular stenosis, we measured the total (pT) and static (pS) pressures and the severity of stenosis before and after percutaneous transluminal angioplasty (PTA). The dynamic pressure (△p) and static intra-access pressure ratios (SIAPR) were calculated. We analyzed the clinical correlation of △p and SIAPR with the severity and location of stenosis, and searched potential predictive factors for the severity of stenosis using multivariate regression. While SIAPR was significantly decreased only in outflow stenosis after PTA (p < 0.0001), △p was significantly increased in both inflow and in outflow stenosis (p < 0.05). SIAPR was negatively correlated with the severity of stenosis only in outflow stenosis (p < 0.0001), and △p was significantly correlated with both inflow and outflow stenosis (p < 0.05). △p was an independent predictor for the severity of stenosis in both inflow and outflow stenosis (p < 0.05). Thus, our study suggests that △p may be more clinically useful than SIAPR not only in detecting access stenosis regardless of its location, but also providing information about the severity of stenosis.
Complications of vascular access for hemodialysis are major causes of morbidity and mortality in end-stage renal disease patients . The most common complication of hemodialysis access is thrombosis due to flow-limiting stenosis, which eventually leads to access failure [2-4]. Therefore, the National Kidney Foundation-Kidney Disease Outcomes Quality Initiative (NKF-K/DOQI) recommended that accesses should be monitored regularly for the detection of the development of stenosis, and if detected, it should be treated with angioplasty or surgery prior to thrombosis . Despite several proposed surveillance methods such as measurement of access flow, venous pressure, recirculation, or other physiologic parameters [5-9], their clinical use has not been fully established. Although one of those methods could allow salvage of vascular access by early detection of the stenotic lesion, planning an appropriate treatment strategy, and preparing an elective intervention rather than urgent procedures or replacement , several researchers emphasized the importance of physical examination by an experienced physician rather than using such surveillance techniques, especially in autogenous arteriovenous fistula (AVF) [11-13].
Among the surveillance parameters, the static intra-access pressure ratio (SIAPR), a surrogate of access flow rate (Qa), reflects vessel resistance and can be measured at the site of the arterial and venous needle during hemodialysis . The significance of SIAPR is mainly based on a theoretical premise and is defined as static intra-access pressure normalized to the mean arterial pressure (MAP). A high SIAPR usually suggests a low Qa, which also suggests a hemodynamically significant outflow stenosis. Although SIAPR has been thought to be a valuable tool to determine outflow stenosis, its value has been unknown when a hemodialysis access has an inflow stenosis, for example, juxta-anastomosis stenosis, especially in autogenous AVFs. Moreover, the clinical criteria for the intervention to treat inflow stenosis have not yet been established . Thus, in the present study, we examined the clinical significance of intra-access total pressure (pT) and static pressure (ps), and calculated dynamic pressure (Δp) on the basis of Bernoulli's theory. The values were measured and calculated before and after percutaneous transluminal angioplasty (PTA). Simultaneously, the severity of vascular stenosis was measured using radiologic imaging during the procedure. The purpose of this prospective study was to investigate the relationship between the total (pT), static (pS), and dynamic (△p) pressures in the access and the severity of stenosis on radiologic imaging, and to look for potential predictive factors for the severity of stenosis.
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This study provides valuable insights into the effect of intra-access pressures and their indices such as pT, pS, △p, and SIAPR on clinically significant stenosis which causes vascular access dysfunction. To the best of our knowledge, the present report is the first study in the literature examining the association of pT, pS and calculated △p with vascular access function based on Bernoulli's theory.
Among the useful parameters used to detect access dysfunction, SIAPR surveillance is an attractive methodology because it can be performed during dialysis sessions and does not require additional equipment other than a dialysis machine with a digital pressure display. However, the degree of elevated venous pressure, demonstration of actual stenosis, or prediction of underlying thrombosis cannot be demonstrated by SIAPR. Thus, utilization of static intra-access pressure (SIAP) measurement to screen has been reported by Dember et al.  to be a problematic solution for detecting vascular stenosis. Spergel et al.  demonstrated that SIAPR does not correlate with access flow in a prospective multicenter study. They also reported that SIAPR measurement cannot discriminate between two different causes of high-pressure ratio such as low inflow resistance with excellent access function or high outflow resistance with access dysfunction. Thus, we tried to find a new pressure parameter that could be correlated with both inflow and outflow stenosis. On the basis of the physics law of energy conservation, the sum of potential energy represented as pT and kinetic energy represented as pS is always constant. According to the Bernoulli's theory, fluid velocity is determined by the difference of the two pressures (dynamic pressure, △p). Therefore, pressure difference (△p) was chosen as the focus of this study. To estimate the relationship between the severity of vascular stenosis and intra-access pressures and their indices, pre- and post-PTA pressure values were compared. The alterations of intra-access pressures and their indices before and after PTA were presented differently according to the location of stenosis. Although SIAPR significantly decreased in outflow stenosis (p < 0.0001) after PTA, it did not in inflow stenosis (p = 0.291). After PTA, △p significantly increased regardless of stenosis location (i.e., inflow, outflow, or even combined lesions). In this study, angioplasty was performed for significant lesions, which were lesions having stenosis of greater than 50% diameter reduction on the initial fistulography. Unfortunately, this study could not provide any useful data about the alteration of pressures and indices in mild stenosis.
The intra-graft pressure ratio significantly decreased after the procedures in a previous study with 179 cases of graft angioplasty, which rarely included inflow stenosis . Although static venous pressure measurements derived from computerized algorithms have been validated in arteriovenous grafts (not in autogenous AVFs), Tessitore et al.  recently reported that static venous pressure measurement and arterial pressure ratio were poor diagnostic tests for the determination of inflow stenosis.
Some evidence has demonstrated that a certain pressure increased gradually with increasing stenosis severity [20, 21]. In dialysis grafts, Sullivan et al.  determined the relationship between static intra-graft pressures and anatomic stenosis for 34 grafts, and found a positive correlation between graft pressure and the severity of stenosis. This remarkable correlation was present only in arteriovenous grafts and not in autogenous AVFs. Our data demonstrated that SIAPR was not significantly correlated with percent stenosis in inflow stenosis (r = −0.203, p = 0.186), but was significantly correlated with outflow stenosis (r = −0.761, p < 0.0001), compared with a previous report . In particular, △p was significantly correlated in both inflow- and outflow stenosis (r = −0.351, p = 0.019 in inflow, and r = −0.728, p < 0.0001 in outflow) regardless of access type.
Despite the changes in SIAP in vascular access depending on the location of stenosis, the type of vascular access (fistula or graft), and the presence or absence of collateral veins , our study demonstrated that the value of △p was not affected by the type of vascular access. Furthermore, in multivariate analysis including the variables presence of collaterals or grafts, the △p was determined as an independent predictor for the severity of both inflow and outflow stenosis, but SIAPR had statistical significance only in outflow stenosis (Table 3). Therefore, further study should be required to determine the cut off value of access dysfunction.
To calculate △p in the present study, it was required to measure pT by interrupting the blood flow using a catheter balloon. In clinical practice, the method to interrupt blood flow in the access could be applied without a catheter balloon by compressing the superficial fistulated veins or grafts with a tourniquet or the fingers of the examiner.
Our study had two primary limitations. (i) Our study did not contain data about measurement differences by the varied diameters or directions of blood flow in the catheters. (ii) Even though △p is one of the most important determinants of flow rate, it was impossible to build up clinical criteria for access dysfunction because of individual variation in vessel diameter, especially in autogenous AVFs.