Biomedical subjects

C G Birk

Publications and source records attributed to C G Birk.

4 recordsLinked to original sources

PubMed2001

Hemodynamic reproducibility during blood flow measurements of hemodialysis synthetic grafts.

We have previously shown that graft blood flow (Qa) has a poor accuracy in predicting graft thrombosis. In this study, we determined whether hemodynamic variation helps explain this poor predictive accuracy. We also determined whether standardized timing of Qa measurements, which is widely recommended, will promote measurement reproducibility. We analyzed variations in mean arterial pressure (MAP) in seven consecutive dialysis sessions for 51 patients and determined the influence of MAP on Qa (by ultrasound dilution). We used a pooled coefficient of variation (CV) to summarize MAP variation within individual patients (computed as +/-2 CVs). MAPs from the seven sessions varied widely, and most variation was present with the first MAPs at the beginning of the sessions. These first MAPs varied by +/-23%, whereas variation for the entire session was +/-28%. The influence of MAP on Qa was determined by measuring the two together during consecutive thirds of a single session. The percentage of change in MAP (DeltaMAP) and Qa (DeltaQa) from the first to middle or last thirds of the session varied over wide ranges: -37% to 86% and -43% to 78%, respectively. The DeltaQa versus DeltaMAP correlation was relatively strong for changes between the first and middle thirds (r = 0.666) and first and last thirds (r = 0.646) of the session (both P: < 0.01). We conclude that MAP varies far more widely during dialysis than previously recognized. This variation is associated with large changes in Qa that may impair accuracy in predicting thrombosis. This wide MAP variation also indicates hemodynamic reproducibility is not feasible when measuring Qa. Thus, we do not recommend standardized timing of Qa measurements during dialysis. A practical method of addressing poor Qa reproducibility may be to take frequent measurements so that trends can be recognized before thrombosis occurs.

Blood Pressure Determination↗

PubMed2000

Accuracy of decrease in blood flow in predicting hemodialysis graft thrombosis.

We recently showed that a single low graft blood-flow measurement (Qa) does not accurately predict graft thrombosis. In this study, we prospectively determined whether percentage of decrease in Qa (DeltaQa) or adjustment of Qa for mean arterial pressure (Qa/MAP; Delta(Qa/MAP)) provides greater predictive accuracy than a single Qa. We monitored 83 grafts from 80 patients for thrombosis over periods up to 12 months. Qa (by ultrasound dilution) and MAP were measured monthly during the study. Receiver operating characteristic curves were used to determine whether Qa, DeltaQa, Qa/MAP, or Delta(Qa/MAP) provided the combination of high sensitivity (>80%) and low false-positive rate (FPR; <20%) needed for clinical use. This level of predictive accuracy requires an area under the curve (AUC) of approximately 0.90. We analyzed the four predictors by a number of criteria and found that all AUCs were less than 0.90 and adjustment for MAP reduced the AUC. In predicting thrombosis within 1 month, for example, AUCs for Qa and net DeltaQa (over 3 months) were 0.84 and 0.82, respectively, whereas AUCs for Qa/MAP and net Delta(Qa/MAP) were 0.78 and 0.75, respectively. At a sensitivity of 80%, FPRs for all predictors were at least 30%. Thus, a high sensitivity always required a high FPR. These results show that DeltaQa and adjustment for MAP are not more accurate than a single low Qa in predicting thrombosis. None of these predictors provide enough predictive accuracy to be the sole criterion for clinical decision making. A successful monitoring and intervention program will likely require the inclusion of other predictors that, together with Qa, may provide the needed accuracy.

Area Under Curve↗

PubMed1999

Does blood flow accurately predict thrombosis or failure of hemodialysis synthetic grafts? A meta-analysis.

A number of studies have reported that a single low blood flow (Qa) measurement in synthetic hemodialysis grafts predicts thrombosis or failure. In a meta-analysis of these studies, we computed receiver operating characteristic (ROC) curves that evaluated the predictive accuracy of a Qa measurement. The ROC curves plotted sensitivity versus false-positive rate for predicting thrombosis or failure at different Qa thresholds. A perfect predictor has an area under the curve (AUC) of 1.0, whereas a predictor with no discriminative ability has an AUC of 0.5. We identified studies through a literature search and included our own unpublished data. A random-effects model was used to combine the ROC curves from different studies. Of 19 identified studies, 12 were suitable for computing binormal ROC curves (6 predicted thrombosis; 6 predicted failure). The studies measured Qa and then observed outcome during periods of 1.5 to more than 6 months. The combined AUCs from these studies indicate Qa was a relatively poor predictor, with 0.70 +/- 0. 04 (range, 0.61 to 0.84) for thrombosis and 0.76 +/- 0.07 (range, 0. 62 to 0.90) for failure. The wide range of AUCs also shows there was much heterogeneity between studies. We conclude that a single Qa measurement does not appear to have enough accuracy to be a clinically useful predictor of graft thrombosis or failure. Serial Qa measurements and identification of factors that caused heterogeneity between studies may be needed to achieve sufficient accuracy.

Blood Flow Velocity↗

PubMed1998

Accuracy and reproducibility of urea recirculation in detecting haemodialysis access stenosis.

BACKGROUND: There is wide disagreement among studies that have evaluated the accuracy of urea recirculation (UR) in detecting vascular access stenosis. The 3-site method (UR3) has been discredited and replaced by the 2-site method (UR2), but few studies have evaluated UR2. METHODS: We compared the accuracies of UR2 and UR3 in detecting stenosis in 59 haemodialysis patients during a 12-month period. All patients were studied without regard to clinical suspicion of stenosis. Stenosis (> or = 50% luminal narrowing) was diagnosed by duplex ultrasound and confirmed by angiography. The reproducibility of UR2 was determined by computing its total standard deviation (SDTOT) from measurements during three dialysis sessions over a 15-day period. RESULTS: Stenosis was found in 32% of 124 access studies (mean luminal narrowing = 58%, range = 50%-83%). The mean UR values of stenotic accesses were only slightly higher than non-stenotic accesses for both UR2 (5.6% vs 2.9%, P < 0.01) and UR3 (13.1% vs 11.2%, P = 0.22). An increase in blood pump speed from 300 to 425 ml/min did not improve detection of stenosis by UR2. There were no UR thresholds that could adequately separate the presence of stenosis from its absence. The SDTOT of UR2 was 3.8%, indicating that a patient's UR2 measurement may vary over a range of 16% (+/- 2SDTOT = +/- 8%). CONCLUSION: Stenosis of the haemodialysis access does not predictably cause recirculation, and the reproducibility of the UR2 measurement is poor.

Humans↗