Effects of systemic hemodynamics on flow within vascular accesses used for hemodialysis.

Absolute value of access flow (QA) and change in flow (deltaQA) over time are major determinants of access patency. However, QA may change in response to variation in systemic hemodynamics among dialysis sessions. We examined the effect of mean arterial pressure (MAP), cardiac output (CO), and segmental resistances (R) on QA. Access flow and CO (L/min) were determined by Transonic ultrasound dilution. Static intra-access pressures (mm Hg) at the arterial segment (AS) and venous segment (VS) were determined with the access unoccluded. During access occlusion (O), the AS pressure was equated to arterial pressure (MAPo), whereas the VS pressure reflected venous pressure (VP). Total and segmental vascular resistances (mm Hg-min/L) were calculated as deltaP/Q. We studied 58 arteriovenous (AV) grafts and 35 autologous AV fistulae (AVF) with measurements on two or more occasions in 43 grafts and 25 AVF. MAPC differed from MAPo by >20 mm Hg in 22% of patients. AS (58 +/- 2 vs. 31 +/- 2) and VS (40 +/- 1 vs. 25 +/- 2) were greater in grafts than in AVF, whereas VP was equal. Access flow (0.91 +/- 0.03 vs. 0.91 +/- 0.05 L/min), cardiac output (5.1 +/- 0.1 vs. 5.5 +/- 0.2 L/min), and total access resistance (115 +/- 5 vs. 11 +/- 6) were equal in grafts and AVF, but non-access systemic R was lower in patients with AVF that those with grafts (26 +/- 1 vs. 30 +/- 1). AS and VS resistances were greater in AVF than grafts (87 +/- 6 vs. 54 +/- 3 and 37 +/- 3 vs. 16 +/- 3). Multivariate analysis indicated that CO and ipsilateral MAPo affected flow in both access types. In grafts, all three access resistance elements, AS, VS, and total independently influenced flow, whereas in AVF, the VS did not. Unexpectedly, the ratio of systemic to access resistance also influenced access flow. The pressure in the venous system draining the access affected access flow in AVF but not grafts. We conclude that the hemodynamics of grafts and AVF differ. Cardiac output, MAP, and the arterial segment resistance influence QA in both access types and need to be considered when evaluating QA as part of the trend analysis for detecting access dysfunction.