Diffusive Clearance of Small and Middle-Sized Molecules in Combined Dialyzer Flow Configurations

Clearance of low (LMW) and middle molecular weight (MMW) solutes was investigated in vitro for different dialyzer configurations and mutual flow directions. Single pass tests were performed with two low flux Fresenius F6HPS hemodialyzers placed in series (12 tests) and in parallel (6 tests), and results were compared with those for one single dialyzer (2 tests). Either high concentrated (45mS/cm) bicarbonate dialysis fluid (surrogate LMW) or trisodiumphosphate (surrogate MMW) concentration (31mS/cm) was used as blood substitution fluid. Standard blood and dialysate flows of 250 and 500ml/min, respectively, were prescribed. Clearance was derived from conductivity measurements in blood and dialysate compartment, correcting for the overall ultrafiltration rate of 0.1–0.5l/h. In a single dialyzer, changing the counter current flow to co-current deteriorates diffusive clearance by 14% (LMW) and 18% (MMW). Compared to one single dialyzer using counter current flow, clearance increases by 3 to 8% (LMW) and by 15 to 18% (MMW) using two dialyzers in parallel and in series, respectively. As a consequence, the benefit by using a second dialyzer is more prominent for larger molecules. Moreover, pressure profiles drawn for the different configurations show the impact of limited convection on diffusive clearance.

CITRATE ANTICOAGULATION AND ADVERSE EVENTS

Several patients with heparin intolerance were dialysed with tri-sodium citrate as anticoagulant without acute clinical problems (good tolerance). After some weeks however problems arose. In all patients an alkalosis developed: the pre dialysis bicarbonate level rose progressively from 27 mmol/l to 40 mmol/l. This could be tempered by lowering the dialysis fluid bicarbonate concentration from 37 mmol/l to 25 mmol/l. A second problem was a progressive rise in pre dialysis sodium level from a mean of 136 mmol/l to 150 mmol/l. Adapting the dialysis fluid sodium concentration from 140 mmol/l towards 132 mmol/l could solve this. The third problem was a progressive rise in serum aluminium level in patients from 3 microg/l to 38 microg/l. After excluding water, concentrate, dialysis fluid, drug intake, etc... as possible sources, we controlled the aluminium level in the glass bottle containing tri-sodium citrate. We noted the very high value of 35,300 microg/l. After replacing the glass bottles with polyvinylchloride bags with a negligible aluminium content, the serum aluminium levels returned back to normal. It is known that citrate chelates the aluminium present in the glass of bottles or vials.

Quality monitoring of care: ask the patient

In 1998 and 1999, we organised a questionnaire to be filled out by chronic haemodialysis patients. The first part dealt with general information data on the specific patient and the second part dealt with quality assessment data given as experienced by the individual patient. This second part included a ranking of preference by the patients and aspects patients wanted to see improved in the future. As we implemented the EDTNA/ERCA Clinical Standards and Core Curriculum, the monitoring tool, as described in this paper, was developed in order to monitor the effect of implementation. As a result of this two-year monitoring process of quality of care, we can follow up the care provided by our multidisciplinary team. It has proven a very useful tool to find out what is to stay as it is and what is to be improved. Simply closing the loop!

Heparinisation in chronic haemodialysis treatment: bolus injection or continuous homogeneous infusion?

After several incidents where patients either bled after dialysis or suffered severe anaphylactic reactions caused by intravenous heparin injections, we decided to administer heparin as a continuous homogeneous intravenous infusion. This had been usual practice in our unit in the past. Following the shocking experience of the death of a patient due to an anaphylactic reaction during his first dialysis, we wanted to warn colleagues for the potential dangers of administering intravenous heparin as a bolus injection.

Continuous monitoring of the spent dialysate urea level using a disposable biosensor. Clinical evaluation

The first real-time, online measurements obtained using a screen-printed planar sensor to continually monitor urea in the spent dialysate are described. Evaluation of the sensor performance in the real clinical situation involves an extensive comparison of pre and post dialysis characteristics. Steady-state response and calibration curves show excellent reproducibility (typically within 5%; maximum deviation, 15%). The real-time results show that the characteristics predicted from the predialysis characterization are preserved during and after the prolonged period of continuous contact with the dialysate without needing recalibration. An operational stability of more than 10 hours of continuous use has been observed. Stability including intermittent wet-storage was found to be more than 5 days. Zero-baseline stability greater than 0.2 mmol/l (1.2 Og/dl) during the 4 hr treatment period and a resolution of 0.2 mmol/l were achieved, with a full-scale range of 15 mmol/l (90 mg/dl). A response time of less than 3 minutes allows dialysate urea levels to be monitored as a function of dialysis parameters such as varying blood flow. Parallel analysis of dialysate samples taken at 15 min intervals shows good correlation with the recorded data, although correspondence of the absolute values requires that the real-time data be rescaled. Because no degradation of the sensor performance was observed in vitro, this deviation can be explained only in terms of a dialysate based interference, probably caused by changes in pH and buffer.

Fibrin sheet covering subclavian or femoral dialysis catheters

The existence of a sheet around a single lumen dialysis catheter tip, which provokes a valve mechanism, is proved by the observation that several times during the replacement procedure of a dialysis catheter, a sheet surrounding the surface of the catheter is removed with the dialysis catheter. This sheet is grey, approximately 1 mm thick and 30 mm long and consists of fibrin and thrombocytes. Bacteriological examinations were always negative. The existence of the sheet in vivo is demonstrated by digitalized angiography during the removal procedure for single lumen dialysis catheters. Rarely, only the sheet is removed with the catheter. It all other instances, the sheet is stripped off and remains in the subcutaneous tunnel or in the vascular bed without causing much clinical discomfort in most patients. Occasionally an episode of cough, dyspnea, hypotension, retrosternal oppression or hemoptae after removing the single lumen dialysis catheter, suggest pulmonary embolism or lung infarction.

Severe electrolyte disturbances in lethal subclavian catheter dialysis

Syncope, irreversible shock, and respiratory and circulatory arrest developed in a 54-year-old diabetic man chronically dialyzed with a subclavian catheter (SC) minutes after the end of a dialysis session. Resuscitation attempts remained unsuccessful. During the resuscitation attempt, a blood analysis showed severe hyponatremia, acidosis, and hypochloremia. Respiratory and cardiac arrest developed during dialysis in a 64-year-old woman on chronic SC dialysis. Resuscitation was unsuccessful; chloremia levels were 79 mEq/L, and calcemia levels were 20 mg%. Both patients were dialyzed with a standard dialysate solution. The reasons for the electrolyte disturbances could not be explained technically. The autopsy showed myocardial perforation by the SC and accumulation in the pericardium of the fluids administered during the resuscitation attempt (e.g., glucose 5%, bicarbonate, Ca gluconate, human albumines), thus explaining the erroneous electrolyte results. The reason for the perforation was a too-rigid central femoral vein catheter, erroneously labeled a subclavian catheter by the supplying firm. Because of a syndrome of progressive vena subclavia and vena cava sclerosis with insufficient arterial phase flow and venous-phase bleeding around the puncture site during single-needle dialysis, the original SC had to be replaced by a longer one with the tip located in the atrium (this SC was actually a femoral catheter). Analysis of the fluid aspirated through the SC can determine the diagnosis in sudden death of SC dialysis patients.

A disposable urea sensor for continuous monitoring of hemodialysis efficiency

A new sensor for the on-line measurement of urea in the dialysate output is described. The sensor is based on a differential measurement of conductivity changes induced by the urease-catalyzed hydrolysis of urea. The use of screen printing for the batch-fabrication of the basic transducers results in cheap disposable devices. In addition, the sensor has been designed to fit into a standard male luer-adapter, and can be plugged directly into the dialysate line. The in vitro response is linear to urea concentrations exceeding 6620mM. A resolution of 20020 microM has been achieved with a baseline stability of 50 microM/hr. Interferences caused by fluctuations in the ionic strength and the consequent conductivity changes are effectively suppressed by the differential sensor pair. The efficiency of this suppression is expressed in a common mode rejection ratio of typically 40 to 50. Preliminary ex vivo results show the feasibility of the concept. The sensor principle is not restricted to urea but can be extended to other molecules of interest for hemodialysis monitoring, such as creatinine and L- and D-amino acids.

Comparison of a new and a standard single-needle dialysis system using a mathematical model

Two single-needle dialysis systems are compared for their hydrodynamic and kinetic behavior, the classic double-headed pump system and a newer bidirectional blood pump. First, discharges and pressures are calculated in a time-space grid to learn about the flow characteristics of the single-needle systems. These parameters are important for they control ultrafiltration and clearance rates in the artificial kidney. Also, the influence of different artificial kidneys and expansion chambers on the flow parameters is studied. Second, starting from these results, the removal of urea and creatinine is simulated using a double-pool model for both a normal dialysis session and an ultrashort daily autodialysis. As a result, the use of a single marker molecule to study or characterize solute removal is questioned. The new blood pump is able to perform dialysis filtering equivalently to the classic system but has its own specific advantages.

A mysterious extrafraction in the gamma area of protein electrophoresis in dialysis patients is found to be fibrinogen

The appearance of a heretofore unnoticed extrafraction in the gamma protein electrophoresis of 17 dialyzed patients was linked to the modified Shaldon dialysis catheter. Medication interference, infection, and immunological or hematological disease as causative factors were ruled out. After initial contradictory results, the extrafraction was determined to be fibrinogen, present because of the slow release of heparin contained in the catheter, so that electrophoresis was performed on plasma instead of on serum.