HDCN: Review of abstract by Depner et al. <B>Central blood volume</B>: a new criterion for predicting <B>morbid events during hemodialysis</B> (1996 ASN Meeting)

Depner TA, Krivitski NM
Central blood volume: a new criterion for predicting morbid events during hemodialysis
Am Soc Nephrol
J Am Soc Nephrol (abstract) (Sep) 7:1511 1996

The ultimate cause of hypotension in most dialysis patients is a fall in cardiac output due to reduced cardiac filling. Myocardial dysfunction probably plays only a contributory role. In a minority of patients, a sudden fall in TPR may also contribute. The decrease in cardiac filling is not often associated with a precipitous rise in hematocrit and may be due to sudden dilatation of venous channels in the splanchnic area (e.g., due to adenosine release from tissues rendered ischemic by low perfusion).

Given this information, it is conceivable that monitoring of central blood volume may be of clinical benefit during dialysis. One can argue that one should simply measure the Hct, and look for a "crash-crit" a la CRIT-LINE. We tried to continuously monitor cardiac output using bioimpedance and Hct (Stakisaitis, ASN '95), thinking that, if TPR changes are not a major problem, then changes in CO should reliably predict hypotension (they didn't).

In this abstract, Depner and Krivitski use ultrasound dilution to measure central blood volume. The technique is to inject some saline into the venous bloodline, and see how much this saline dilutes the blood coming into the arterial line over time. Knowing the injected volume and doing some sophisticated time course analysis will yield to a central blood volume (CBV) measurement. One advantage of this method is, that you also get a dilutional measurement of cardiac output (CO).

In 26 patients, measurements of access blood flow (Qac), CO, and CBV were taken "early and late" in dialysis. Reproducibility of measurements was good; CO decreased by an average of 17%, CBV by 13%. Only 7 of the 26 patients had a fall in MAP > 10 mm Hg; this was a relatively stable group.

When they divided the patients into those with (n=10) and without (n=16) hypotensive/ischemic symptoms, CO and CBV fell more in the symptomatic group (-22 and -26% respectively), than in those without symptoms (-11 and -7%, respectively). Changes in CO and CBV correlated well with one another (I think they were measured from the same injection), and also with changes in MAP. Qac also decreased in proportion to changes in MAP. The conclusion of the authors is, that CBV monitoring may be useful in dialysis patients, as changes seem to correlate with changes in MAP and with symptoms.

Comment: This is a nice study utilizing a new hemodynamic measure. They could not make a convincing case that CBV is any more useful than CO, however, as the two measures correlated quite well. I personally think that impedance CO may give equally good results. In our own study, cited above, we chose patients with multiple, repeated sympotmatic hypotensive episodes. It is the latter group that needs to be studied, and such patients, unfortunately, become hypotensive after very small changes in CO and Hct, making detection and prediction of hypotension problematic. Monitoring of CO, CBV, and Hct may be more useful in the less severely ill "stable" patients, with less regular hypotensive/ischemic episodes. (John T. Daugirdas, M.D., University of Illinois at Chicago)

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Am Soc Nephrol
Basic hemodialysis : Complications (acute)