Besarab A, Ross R, El-Ajel F, Deane C, Frinak S, Zasuwa G
The relation of intra-access pressure to intra-access flow
Am Soc Nephrol
J Am Soc Nephrol (abstract)
(Nov) 6:483 1995
Besarab measured access blood flow by Doppler and the access
pressure at zero extracorporeal flow divided by the blood
pressure (VPo/BP) in 86 patients every 6 months. A VPo/BP ration
> 0.45 was cause to perform angiography. In patients with no
access events, flow remained at about 1L/min, VPo/BP at 0.35,
with no change in VPo/BP over time. In 21 patients with
intervention (a self-fulfilling prophecy, as intervention
depended on VPo/BP), VPo/BP had increased by 0.15 to 0.53. Flow
in this time was only 584 ml/min. In 6 patients with thrombosis,
flow was 454 ml/min, VPo/BP was 0.66, and had increased by 0.12
units since baseline.
Comment: A nice study suggesting that one can
monitor either pressure or flow to prevent thrombosis. However,
data by Lavarias et al. suggest that
venous pressure measurements during extracorporeal blood flow are
not very useful. This is logical, as most of the venous
resistance under flowing conditions is found in the venous
needle. It appears that an access flow < 600 ml/min may be a
cause for some concern.
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Am Soc Nephrol
Basic hemodialysis :
Vascular Access: graft/fistula
I think the problem was that I didn't express myself clearly enough.
In my comments section, I meant to say that the failure of Lavarias to
show a predictive value for pressure measurements vs. the positive
findings in this study were due precisely to the fact that pressure measurements were taken
under different conditions. Under conditions of zero flow, the resistance in
the needle is taken out of the equation. Needle resistance is a major determinant of outflow resistance under flowing conditions,
and is a function both of needle size and blood flow rate. Hence the need for different guidelines when 16 gauge or
15 gauge needles are used. Also, not only blood flow but blood viscosity, namely hematocrit, then comes into the
picture. So I strongly believe that pressure measurements under zero flow conditions are an advance, and am not surprised
that they correlated with access flow.
John Daugirdas MD (firstname.lastname@example.org)
Chicago, IL USA-Friday, June 28, 1996 at 13:04:11 (CDT)