Preserving central blood volume: changes in body fluid compartments during hemodialysis.

The understanding of fluid changes during hemodialysis (HD is essential for reducing complications as well as efficacy of the procedure. Bioimpedance spectroscopy provides a non invasive method of measuring total body water (TBW), the distribution of intra (ICF) and extracellular (ECF) fluids, and their changes during HD. Segmental bioimpedance may be used to measure the same fluid shifts but from different body segments; the technique has previously been shown to com pare well with whole body measures. It is possible that fluid shifts occur differently in different body compartments during HD. Based on previous hemodynamic studies we postulated that during HD ultrafiltration (UF) the body attempts to preserve its central blood volume (cardiopulmonary circula tion plus great vessels), and thus fluid shifts would be greater from the periphery than from central compartments. To test this hypothesis, segmental bioimpedance (Xitron Technolo gies, San Diego, CA) was performed on 11 subjects undergoing HD where ECF and ICF values were obtained from the legs, arms and trunk before and after a period of UF. Blood volume change (ABV%) was also followed using an on-line optical hematocrit (Hct) sensor (Crit-Line monitor, In-Line Diagnostics, UT) where deltaBV% = deltaBV% = (1 - Hct1/Hct0) x 100 (Hct0 = baseline Hct; Hct1 = postultrafiltration Hct) The UF of 2.0 L +/- 0.79 L (M +/- SD) over 75 minutes was associated with a deltaBV% of -9.43% +/- 3.6% (M +/- SD), a significant (Student's paired t-test) reduction in total body (TB) ECF (p < 0.02), a weak correlation in reduction in TBW (p = 0.09) but not in TB ICF. The ECF reductions from the trunk, legs, and arms were all significant (minimum p < 0.02); no ICF changes from these compartments were significant. The amount of ECF reduction was greater from the legs (0.7 L +/- 0.6 L) than the arms (0.12 L +/- 0.08 L) and trunk (0.2 L +/- 0.2 L) (all M +/- SD). Multiple regression analysis showed that TB ECF changes correlated strongly with leg (r = 0.94, p < 0.001) and arm (r = 0.72, p = 0.002) ECF changes but not with trunk changes. deltaBV% correlated weakly with leg (r = 0.45, p = 0.08) and arm (r = 0.42, p = 0.10) ECF changes but not with the trunk. As the deltaBV% represents the net volume change between UF and plasma water refilling, thiss indicates that plasma water is being removed more from the peripheral compartments than from the trunk. These data suggest that plasma refilling during HD to preserve central blood volume is more dynamic from the leg ECF than from elsewhere and may, in turn, explain the frequent occurrence of leg cramps during and after hemodialysis.

CQI: hemodialysis vascular access flow monitoring.

One of the main ongoing challenges in nephrology is maintaining a good, well-functioning vascular access. Vascular access problems lead to complications such as access recirculation causing decreased adequacy of dialysis as shown by kinetic modelling and access clotting. Access flow measurement using ultrasound dilution technique is an accurate and better indicator of impending access stenosis than recirculation (urea method). The measurement is non-invasive, the procedure simple, and the monitor accessible at the bedside. The Adam Linton Dialysis Unit of the London Health Sciences Centre, Victoria Campus is currently monitoring access flows (Qa) as a continuous quality initiative using ultrasound dilution technique. Access recirculation (AR) is determined and Qa measurements are done bimonthly on all chronic in-centre and self-care dialysis patients with either arteriovenous fistula or Gore-tex grafts. Qa's of <550 ml/min or 20% decrease in flows are investigated by angiography and early intervention is instituted either by angioplasty or fistula repair. Our unit's goal is to be proactive in our investigation and in our nursing and medical interventions. From our experience, the problem with responding to poor clearances by checking for recirculation after the fact is that valuable time is lost for proactive intervention to preserve the access site and may in fact be too late. In four different patient situations we are able to show how our different interventions have improved Qa's and eliminated AR resulting in increased Kt/V. The intent of this article is to show that Qa measurement can be an ideal way to monitor hemodialysis vascular accesses over time. It provides a means to detect impending access dysfunction before the Qa has decreased enough to have induced AR and/or under-dialysis. By early intervention, optimum dialysis efficiency is achieved and the prescribed Kt/V [urea] is delivered.

On-line optical sensing of blood volume changes to prevent intradialytic hypovolemia.

Intradialytic hypovolemia is a common complication of hemodialysis treatments. Blood volume changes that occur during dialysis can be followed by on-line optical sensing of the patient's hematocrit. Characteristic curves of blood volume changes can be seen in fluid overloaded patients with symptoms of hypovolemia, i.e. cramps, hypotension. The relative blood volume changes in those who have fluid removed to their ideal "dry" weight without symptoms will be shown in comparison. As Phase I of a CQI project, the nephrologist and dialysis nurses at the London Health Sciences Centre performed a cross-sectional study to define the frequency of these curves and their relationship to intradialytic symptoms. The analysis of these curves can be used to re-assess "dry" weights, prevent intradialytic hypovolemia and decrease the use of antihypertensive agents. The correlation of our findings with the results of an independent hypotensive CQI study will be presented.

A comparison of methods for the measurement of hemodialysis access recirculation and access blood flow rate.

The ability to accurately measure access recirculation (AR) is of importance because its presence indicates access dysfunction and may explain why a prescribed Kt/V (urea) has not been delivered. The ability to measure access flow (Qa) allows access monitoring and the detection of impending access dysfunction. AR can be measured by indicator dilution or conductivity tracer techniques. Qa calculation is simple if AR can be detected. The previous techniques are used while the patient's blood lines are reversed to induce AR, and the Krivitski equation gives: Qa = Qb [1-r/r] where Qb = dialyzer blood flow and r = proportion of AR induced. Three methods for AR and Qa measurements were directly compared: 1) ultrasound dilution (Transonics Hemodialysis Monitor, Transonics Systems Incorporated) (TRANS); 2) hematocrit dilution (Crit-Line Monitor, In-Line Diagnostics) (CRIT); and 3) differential conductivity (Hemodynamic Monitor, GAMBRO Healthcare Incorporated) (HDM). Patients were cannulated in a standard fashion and dialysis commenced with lines in normal configuration. A HDM test was performed and, if AR = zero, the lines were reversed to induce AR. HDM, TRANS, and CRIT tests for AR were next done in rapid succession for direct comparison. Each test was repeated three times in succession, the device in random order, to assess test repeatability. Qb was taken from the 1) dialysis machine pump, and 2) directly from TRANS and Qa calculated, using 1) and 2) AR results. In comparison to TRANS, AR results were virtually identical for HDM (TRANS AR = 1.04 HDM-AR + 0.02, r = 0.98, p = 0.0000), and good for CRIT (CRIT-AR = 0.84 TRANS-AR - 0.2, r = 0.81, p = 0.001), but CRIT underestimated the values. Repeatability was assessed by normalizing (%) the SD of repeated measurements; values were 7.5% (HDM), 9.1 % (TRANS), and 17.4% (CRIT). Qa value comparisons were similar (minimal r = 0.83) regardless of Qb source, but CRIT overestimated the value; repeatability data showed 10.6% (HDM), 13.0% (TRANS), and 25.2% (CRIT) (n ranged from 15-64). In summary, TRANS and HDM appear equal as far as accuracy and repeatability of measurements; CRIT results correlated well, but tended to underestimate AR and overestimate Qa, and was less reproducible.