Nikolai M. Krivitski

Clinical measurement of blood flow in hemodialysis access fistulae and grafts by ultrasound dilution.

Blood flow is a fundamental property of the hemodialysis access device. Periodic monitoring of flow could be useful for detection of impending access failure and prevention of underdialysis, but simple measurements of access flow during hemodialysis are not currently available. Flow in peripheral arteriovenous fistulas and grafts was examined using an indicator dilution technique while the patients blood lines were reversed. The indicator was a bolus of normal saline detected by an ultrasound flow sensor clamped onto the patients blood line. The ultrasound sensor measured blood flow in the tubing using an established transit-time method and simultaneously detected saline dilution of the blood from changes in the average cross sectional velocity of an ultrasound beam that illuminated the blood flowing through the tubing. Access flow was measured 110 times in 25 patients, 16 with loop grafts and 9 with native fistulas. Measured access flow ranged from 125 to 2860 ml/min. The mean error of duplicate measurements within patients was 5.0 +/- 3.8%. To assess the adequacy of saline mixing with the blood, access flow was measured at three dialyzer blood flow rates. In paired studies, no significant difference was observed in access flow measured at two lower dialyzer blood flow rates when compared to flow measured at 350 ml/min. A comparison with access flow measured by a duplex color Doppler technique in seven patients gave a mean error of 9.2 +/- 7.2% in paired studies. These data show that blood flow in peripheral arteriovenous grafts and fistulas can be measured accurately during hemodialysis using ultrasound velocity dilution.

Novel method to measure access flow during hemodialysis by ultrasound velocity dilution technique.

This article presents the theory and bench validation of access flow measurement by ultrasound velocity dilution. Reversal of dialysis lines creates a zone of mixing in the vascular access, allowing the use of dilution technique for access flow measurement. The method of sound velocity dilution sensor calibration was analyzed. A single sensor system was investigated for blood circulation and isothermal injections. A dual sensor system was investigated with heated hypertonic saline circulation for both body temperature (isothermal) and room temperature (cold) injections. The accuracy of in vitro access flow measurement was studied on a two pump bench model, dialysis line flow was varied from 200 to 400 ml/min, access flow was varied from 300 to 2500 ml/min. For the dual sensor system, dilution access measurements tracked a gravimetrically calibrated in-line flow sensor within 3.25 +/- 0.34% for isothermal injections, and within 5.81 +/- 0.43% for cold injections. Measurements were repeatable within 3% independent of the temperature of the injected saline. For the single sensor system, results tracked the in-line flow sensor within 3.83 +/- 0.79%. These data show that access flow can be accurately measured by sound velocity dilution technique.

Hemodialysis access recirculation measured by ultrasound dilution.

The most widely used clinical method for measuring recirculation in the access device is based on urea dilution. The three simultaneous blood samples required during hemodialysis interrupt the treatment, and results of chemical analysis are often delayed for several days. Alternatively, detecting recirculation by dilution of arterial blood caused by a bolus of normal saline injected into the venous blood line has several advantages. In this study, an ultrasound sensor clamped onto the arterial line entering the dialyzer was used to detect such dilution from a reduction in sound velocity observed in the saline diluted blood. Within the target range, the change in ultrasound velocity (ultrasound dilution) is linearly correlated with the dilution of whole blood by normal saline. The same sensor was also used to measure flow in the blood line using an established ultrasound transit-time method. During 34 hemodialyses in 28 patients, only 3 patients had detectable recirculation measured by ultrasound dilution. To further evaluate the sensitivity of the new method the dialysis lines were reversed during hemodialysis in the 25 patients with no recirculation. After this, all had detectable recirculation ranging from 10 to 60%. The mean error of duplicate measurements was 3.9 +/- 2.8%. Recirculation by ultrasound dilution correlated closely with recirculation measured by urea dilution (r = 0.9156, p < 001). The data suggest that the ultrasound dilution method is both sensitive and accurate. Ease of use and immediate availability of results added to the clinical usefulness of this method for evaluating the integrity of the hemodialysis access.

Theory and in vitro validation of a new extracorporeal arteriovenous loop approach for hemodynamic assessment in pediatric and neonatal intensive care unit patients.

Objectives: No simple method exists for repeatedly measuring cardiac output in intensive care pediatric and neonatal patients. The purpose of this study is to present the theory and examine the in vitro accuracy of a new ultrasound dilution cardiac output measurement technology in which an extracorporeal arteriovenous tubing loop is inserted between existing arterial and venous catheters. Design: Laboratory experiments. Setting: Research laboratory. Subjects: None. Interventions: None. Measurements and Main Results: In vitro validations of cardiac output, central blood volume, total end-diastolic volume, and active circulation volume were performed in a model mimicking pediatric (children 2–10 kg) and neonatal (0.5–3 kg) flows and volumes against flows and volumes measured volumetrically. Reusable sensors were clamped onto the arterial and venous limbs of the arteriovenous loop. A peristaltic pump was used to circulate liquid at 6–12 mL/min from the artery to the vein through the arteriovenous loop. Body temperature injections of isotonic saline (0.3–10 mL) were performed. In the pediatric setting, the absolute difference between cardiac output measured by dilution and cardiac output measured volumetrically was 3.97% ± 2.97% (range 212–1200 mL/min); for central blood volume the difference was 4.59% ± 3.14% (range 59–315 mL); for total end-diastolic volume the difference was 4.10% ± 3.08% (range 24–211 mL); and for active circulation volume the difference was 3.30% ± 3.07% (range 247–645 mL). In the neonatal setting the difference for cardiac output was 4.40% ± 4.09% (range 106–370 mL/min); for central blood volume the difference was 4.90% ± 3.69% (range 50–62 mL); and for active circulation volume the difference was 5.39% ± 4.42% (range 104–247 mL). Conclusions: In vitro validation confirmed the ability of the ultrasound dilution technology to accurately measure small flows and volumes required for hemodynamic assessments in small pediatric and neonatal patients. Clinical studies are in progress to assess the reliability of this technology under different clinical situations.

Extracorporeal recording of mouse hemodynamic parameters by ultrasound velocity dilution.

The use of mice as models for cardiovascular studies has traditionally been difficult because of their small size and the lack of appropriate instrumentation to perform fundamental measurements of cardiac output (CO) and total blood volume (TBV). The advent of transgenic techniques to develop mouse strains that mimic human disease makes the development of this instrumentation crucial. The current study outlines a novel technique for the determination of CO and TBV in the mouse using an extracorporeal arteriovenous (A-V) shunt, combined with the measurement of ultrasound dilution after the intravenous administration of small volumes of isotonic saline. The potential sources of error associated with Stewart-Hamilton dilution techniques were addressed by the research. The new techniques were applied in three anesthetized mice (27-36 gm). Isotonic saline (10-80 microl) was injected intravenously while measuring ultrasound dilution in the A-V shunt. The CO ranged from an average of 6.8+/-0.71 to 12.7+/-1.7 ml/min. Heart rates were not significantly altered by the intravenous administration of isotonic saline. The TBV ranged from 4.36+/-0.22 to 5.15+/-1.04 ml/100 gm. These results agree with the literature and suggest that these techniques will prove useful in cardiovascular studies of mice.

Interventional Nephrology and Dialysis: Access Flow Measurement During Surveillance and Percutaneous Transluminal Angioplasty Intervention

The introduction of routine access flow measurement methodology has enabled accurate identification of problematic accesses and provided a means for follow‐up evaluation. These methods have uncovered, in some cases, that interventions are either immediately unsuccessful or that they fail within 3 months to maintain flow above preintervention levels. The purpose of this article is to analyze the main problems that occur at each step in the loop of flow surveillance–intervention–follow‐up and to provide suggestions for improving outcomes. Analysis of published access flow data suggests that the main problems lie in the areas of inadequate analysis of flow surveillance data, lack of objective technology for quantifying intervention effectiveness, and lack of follow‐up flow measurements in the hemodialysis (HD) unit after the intervention. The following three recommendations may improve surveillance outcomes: 1) using a reliable access flow technology combined with analysis of all hemodynamic data (including mean arterial pressure) before referring patients for angiography to decrease surveillance false positives; 2) performing intra‐access blood flow measurement during angioplasty, which may improve outcomes by giving warning of errors before the patient leaves the intervention suite. Success achieved in restoring flow as measured during the intervention usually predicts good immediate outcomes in the HD unit; 3) measuring access flows during the next week after angioplasty. If the results are unsatisfactory, the patient should be further evaluated to avoid a potential thrombotic event.

Determining lung water volume in stable hemodialysis patients

Lung water (LW) reflects the water content of the lung interstitium. Because hemodialysis patients have expanded total body water (TBW) they may also have increased LW. Hypertonic saline promotes a flux of water from lung to blood, which is measured by ultrasound flow probes on hemodialysis tubing. The volume of flux is an indirect measure of LW. Our purpose was to determine the feasibility and reproducibility of LW derived with ultrasound velocity dilution, to determine the effect of ultrafiltration on LW in stable hemodialysis patients, and to compare changes in LW with fluid compartment shifts using bioimpedance. Lung water, cardiac output, total body water, and extracellular and intracellular fluid volumes were measured in 24 stable hemodialysis patients at the beginning of hemodialysis and after ultrafiltration. The LW values at the beginning of hemodialysis (298.8 ± 90.2 ml or 3.67 ± 1.47 ml/kg) fell during hemodialysis (250.8 ± 55.8 ml or 3.12 ± 0.96 ml/kg; p < 0.05), as did TBW and extracellular fluid volumes (p < 0.001). Cardiac output, cardiac index, and central blood volume also decreased significantly with ultrafiltration (p < 0.005, p < 0.005, and p < 0.01, respectively). Results showed that stable hemodialysis patients have higher specific LW values (3.67 ml/kg) than the normal population (2 ml/kg) and ultrafiltration produces a significant decline in LW values.

Volume of extravascular lung fluid determined by blood ultrasound velocity and electrical impedance dilution.

A hypertonic sodium chloride bolus passing through the lung has a sound velocity transient that is biphasic when it reaches the carotid artery. This transient is compatible with water moving into the hypertonic bolus from the lung parenchyma, thereby leaving the lung parenchyma hypertonic. Subsequently, as the bolus leaves the lung vasculature, water passes from the blood into the tissue to return the lung tonicity to baseline, giving a moment when net movement is zero, an instant of osmotic equilibrium. Concurrent measurements of impedance track the sodium chloride transient. A theoretic basis for the calculation of extravascular lung water is derived from the water transferred to the blood, the amount of sodium chloride moved from blood to the lung, and the increase in blood osmolarity measured at the moment of equilibrium. Examples from measurements on sheep suggest that two intravenous injections of hypertonic and isotonic sodium chloride, with observations of sound velocity and electrical impedance in the systemic arterial circulation (which could also provide the cardiac output), provide a basis for calculation of lung permeability, water and salt movements, and extravascular lung water estimation.

Why vascular access trials on flow surveillance failed

Introduction Since the introduction of access flow surveillance technology for routine patient screening in 1995, more than 30 clinical trials have been presented in peer reviewed journals. Despite overall positive outcomes, some trials, including randomized control trials (RCTs), failed to produce positive outcomes for access surveillance. The purpose of this study is to analyze published data related to the main component of access surveillance–-adequate increase of access flow after percutaneous transluminal angioplasty (PTA). Results A total of nine studies for arteriovenous grafts (AVGs) that include 350 accesses and nine studies for arteriovenous fistula (AVF) that included 503 accesses were considered for analysis from 14 publications. Practically, all reference data find high sensitivity (>90%) of access flow measurement to predict 50% stenosis. Mean access flow increase after PTA in AVGs was 319 ml/min (from 238 to 524 ml/min). Mean access flow increase in AVFs was 331 ml/min (from 195 to 402 ml/min). Relative flow increase in AVFs was 1.6 times greater than in AVGs. The authors of failed RCT for AVGs either did not select patients for PTA based on KDOQI guidelines and did not provide/analyze PTA flow results data, or reveal data that obviously show failure of PTA to adequately improve access flow. Summary Access flow surveillance successfully identifies patients with hemodynamically significant stenosis. PTA performed on AVFs produce better hemodynamic results than in AVGs. Inadequate flow increases during PTA and not following KDOQI guidelines are major contributing factors for failed AVG randomized tails. Radiologists should use objective means for flow evaluation during PTA.

Access Blood Flow: Debate Continues: CORRESPONDENCE

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