The relationship between cardiac output and access flow during hemodialysis

Satisfactory hemodialysis access flow (Qa) is necessary for dialysis adequa cy. However, high access flows are postulated to increase cardiac output (C O). The relationship between Qa and CO is not well defined. The purpose of this study was to observe the relationship between Qa and CO and to evaluat e the effect of blood volume change (BV Delta) on Qa and CO during hemodial ysis (HD). Measurements of Qa and CO (ultrasound dilution; Transonics Monit or, Ithaca, NY) were performed sequentially at baseline in 18 patients (13 forearm arteriovenous fistulae, 5 Gore-Tex grafts) and after an interventio n involving either HD with attempted zero BV Delta (mean: -0.4%; range: -2. 6 to 1.6%) or a significant BV Delta (mean: -7.3%; range: -3.1 to -11.9%). Measurement of BV Delta was done by hematocrit dilution (Crit-Line Monitor, In-line Diagnostics, Riverdale, UT). The volume ultrafiltered (V-UF) and t he mean arterial pressure (MAP) were recorded at baseline and after interve ntion. In five patients with fistulae, CO was measured after manual occlusi on of the fistula for 1 min. At: baseline, mean (+/- SD) Qa was 1455 +/- 60 0 ml/min, and CO was 6.8 +/- 1.8 L/min. The relationship between Qa and CO was strong, Qa = 0.20 CO + 0.06 (r = 0.62; p = 0.01); this was not signific antly altered with either intervention. Access flow was not changed with ei ther zero BV Delta or significant BV Delta. Cardiac output was not altered when there was no BV Delta; however, CO did decrease by 1.2 +/- 0.6 L/min ( p < 0.001) after BV Delta reduction. The Qa/CO ratio was unchanged after ze ro BV Delta but was increased after BV Delta (p = 0.004). There were no cor relations with MAP change or V-UF. There were no differences in Qa, CO, or Qa/CO by access type. The mean Qa/CO was 21 +/- 6%. Three patients had Qa/C O <15%, and they all had access stenoses. Cardiac output did not decrease a fter transient (1 min) occlusion of the fistula. In conclusion, there is a strong relationship between Qa and CO. With BVA, the Qa is maintained while the CO falls and the Qa/CO increases, perhaps by reflex vasoconstriction o f the systemic circulation. Longitudinal studies are required to determine which is the dependent variable. A low Qa/CO may indicate access dysfunctio n.