ASN95 Hot Topic -- New methods of measuring access flow and recirculation

Up until recently, measurement of access blood flow was not straightforward. Doppler technology requires that the inside diameter of the vessel be known and constant. Also, the Doppler examination is defeated by turbulence, which makes it difficult to measure the mean blood velocity within the access. Besarab (192/483) solved this problem ingeniously by measuring flow in the brachial artery, which has a constant internal diameter and non-turbulent flow. However, new and exciting technology is now available to measure access flow. The ultrasound dilution method (Transonics, Inc., Ithaca NY), as described by Krivitski in the July issue of Kidney International, is based on injection of saline into the venous needle, and quantifying the dilution effect in the arterial needle, after transiently reversing the lines. A nice in vivo validation of this method was first described by Depner and Krivitski at ASAIO in May (ASAIO J 41:M745, 1995). Koomans' group presented further validation of the Transonics approach by comparing it with magnetic resonance angiography and also with a flow/pressure curve technique of determining flow. Another approach is to measure flow by adding a cold stimulus to the venous blood, and sensing the amount of cold appearing at a sensor on the arterial line (again after crossing the lines). Such a validation was presented by Schneditz using the Fresenius blood temperature monitoring (BTM) module . However, only in vitro data were presented.

The same technology can be used to assess recirculation (the lines are not reversed for this measurement), as first shown by Depner and Krivitski at ASAIO (ASAIO J 41:M749, 1995). George et al used the Transonics technique and found recirculation in only 1/28 patients (mean Qb = 350 ml/min). The results were consistent with urea measurements in most cases. Germaine et al used a modified CRIT-line monitor to measure access recirculation and also found good agreement with urea measurements. Soon there will be at least 4 competing technologies to measure access flow and recirculation: ultrasound dilution (Transonics), optical hematocrit (In-line Diagnostics/B.Braun), thermodilution (Fresenius), and conductivity (Cobe/Hospal).

Now that we can measure access blood flow, what good is it? Lavarias and colleagues found that venous outlet pressures did not predict graft failures. Does measurement of access blood flow do better? Besarab found that an access flow < 450 ml/min was required for recirculation to be seen, and that in patients with thrombosis, a low flow was usually found. However Besarab did find static graft pressure measurements normalized to the blood pressure to be also useful in predicting graft failure. Boereboom found that angiographically proven stenosis was also more frequent in patients with low flowing grafts. (John T. Daugirdas)

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