The value of low intra-access pressure (IAP) in detecting and excluding low access blood flow.
ASN 30th Annual Meeting, San Antonio
J Am Soc Nephrol (Sep) 8:162A 1997
Most stenosis with AV grafts occurs because of neointimal hyperplasia distal to the anastomotic site. One would predict that, as such stenosis develops, the venous pressure during dialysis would increase. In fact, measures of venous pressure during dialysis have turned out to be rather poor predictors of outlet stenosis, probably because a substantial part of outflow resistance is due to the dialysis needle itself and also to needle placement. Changes in blood viscosity associated with hematocrit also contribute to the variability. Besarab et al. have suggested a new approach: to monitor static venous pressure; e.g., with the extracorporeal circuit connected but with the blood flow off. This pressure can be read from the venous bubble trap transducer, after a machine-specific correction factor is applied to correct for the height of the transducer relative to the patient's heart level. When this corrected static venous pressure is divided by the patient's MAP, values greater than 0.5 have been shown to predict impending access failure rather well for grafts, but not for fistulae, where no pressure measures seem to reliably predict stenosis of the outflow tract or impending failure.
In this abstract, Sherman's group studied this so-called IAP ratio (corrected access pressure divided by MAP) in 48 patients, and correlated these measured to ultrasonically determined access blood flow. They confirmed that an IAP > 0.50 was associated with decreased access flow (in 60%) , but they also found that low values for the IAP ratio, e.g., IAP < 0.35, also were associated with decreased access blood flow (in 25%), when access blood flow < 600 ml/min was the criterion for low flow. Confirming previous work, they found that a high IAP ratio did not identify low flow in AV fistulae, but that a low IAP ratio (< 0.25) was associated with low flow in 4/13 patients with AV fistulas.
Comment: This is an interesting paper. Presumably a low IAP ratio results from either an inadequate anastomosis, or to an obstructive lesion upstream to the venous needle. It would be useful to have such anatomical or radiographic correlates to go along with these data. (John T. Daugirdas, M.D., University of Illinois at Chicago)
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ASN 30th Annual Meeting, San Antonio
Basic hemodialysis : Vascular Access: graft/fistula