Ulrich Moissl

Significance of Interdialytic Weight Gain versus Chronic Volume Overload: Consensus Opinion

Predialysis volume overload is the sum of interdialytic weight gain (IDWG) and residual postdialysis volume overload. It results mostly from failure to achieve an adequate volume status at the end of the dialysis session. Recent developments in bioimpedance spectroscopy and possibly relative plasma volume monitoring permit noninvasive volume status assessment in hemodialysis patients. A large proportion of patients have previously been shown to be chronically volume overloaded predialysis (defined as >15% above ‘normal extracellular fluid volume, equivalent to >2.5 liters on average), and to exhibit a more than twofold increased mortality risk. By contrast, the magnitude of the mortality risk associated with IDWG is much smaller and only evident with very large weight gains. Here we review the available evidence on volume overload and IDWG, and question the use of IDWG as an indicator of ‘nonadherence by describing its association with postdialysis volume depletion. We also demonstrate the relationship between IDWG, volume overload and predialysis serum sodium concentration, and comment on salt intake. Discriminating between volume overload and IDWG will likely lead to a more appropriate management of fluid withdrawal during dialysis. Consensually, the present authors agree that this discrimination should be among the primary goals for dialysis caretakers today. In consequence, we recommend objective measures of volume status beyond mere evaluations of IDWG.

Chronic Fluid Overload and Mortality in ESRD

Sustained fluid overload (FO) is considered a major cause of hypertension, heart failure, and mortality in patients with ESRD on maintenance hemodialysis. However, there has not been a cohort study investigating the relationship between chronic exposure to FO and mortality in this population. We studied the relationship of baseline and cumulative FO exposure over 1 year with mortality in 39,566 patients with incident ESRD in a large dialysis network in 26 countries using whole-body bioimpedance spectroscopy to assess fluid status. Analyses were applied across three discrete systolic BP (syst-BP) categories (<130, 130-160, and >160 mmHg), with nonoverhydrated patients with syst-BP=130-160 mmHg as the reference category; >200,000 FO measurements were performed over follow-up. Baseline FO value predicted excess risk of mortality across syst-BP categories (<130 mmHg: hazard ratio [HR], 1.51; 95% confidence interval [95% CI], 1.38 to 1.65; 130-160 mmHg: HR, 1.25; 95% CI, 1.16 to 1.36; >160 mmHg: HR, 1.30; 95% CI, 1.19 to 1.42; all P<0.001). However, cumulative 1-year FO exposure predicted a higher death risk (P<0.001) across all syst-BP categories (<130 mmHg: HR, 1.94; 95% CI, 1.68 to 2.23; 130-160 mmHg: HR, 1.51; 95% CI, 1.35 to 1.69; >160 mmHg: HR, 1.62; 95% CI, 1.39 to 1.90). In conclusion, chronic exposure to FO in ESRD is a strong risk factor for death across discrete BP categories. Whether treatment policies that account for fluid status monitoring are preferable to policies that account solely for predialysis BP measurements remains to be tested in a clinical trial.

A comparison of different methods to assess phase-locking in auditory neurons

Proposes a method to reconstruct acoustic signals from auditory nerve firings. This is a major step in order to improve cochlear implants by demonstrating what patients perceive with such a device. A computer model of the human cochlea transforms an acoustic test signal into a train of impulses, and the place- and volley-principle of cochlear frequency coding will be evaluated in order to invert this process, i.e. to reconstruct the original signal from the impulses. Phase-locking of the impulses to an acoustic stimulus is believed to be the main information carrier of loudness and frequency in the cochlea. Four different synchronization indices, which are used to assess phase-locking in the sensory receptor cells of the ear, will be presented and compared.

Model-based control of hemodialysis

In 30% of all hemodialysis treatments a sudden hypotension occurs due to excessive fluid removal. Currently the rate of fluid removal (ultrafiltration rate or UFR) is set to predefined values based on expert knowledge of the doctor. This paper is a first step towards closing the open loop control by presenting a hydraulic circulation model of the patient, which can be used in an observer structure in order to keep certain state variables like venous pressure or cardiac output above critical values.

Modellbasierte Analyse von Bioimpedanz-Verfahren (Model-based Analysis of Bioimpedance Methods)

Abstract Diese Arbeit behandelt Möglichkeiten und Grenzen von Impedanz-Verfahren anhand zweier ausgewählter Beispiele: der Bioimpedanzspektroskopie zur Bestimmung von extra- und intrazellulären Flüssigkeitsvolumina, und der Impedanzkardiografie zur Berechnung des Schlagvolumens eines Herzschlages. Schwerpunkt liegt auf der Modellierung der physikalischen Prozesse, die den jeweiligen Verfahren zu Grunde liegen.

Ansätze zur Identifikation von Patientenparametern während der Hämodialysetherapie (Identification of Patient Parameters during Hemodialysis)

Dieser Beitrag beschreibt Ansätze zur Identifikation des Kreislaufzustandes von Dialysepatienten während der Dialysebehandlung. Besonderes Augenmerk wird auf die Beeinträchtigung der Funktion der physiologischen Blutdruckregelung gelegt, die als Hauptursache für das Entstehen von spontanen Blutdruckabfällen angesehen wird. In modernen Dialysesystemen stehen Sensoren zur Bestimmung von Blutvolumen und Bluttemperatur bei der Patientenüberwachung zur Verfügung. Neben diesen Größen kommt der Überwachung der Herzfrequenz besondere Bedeutung zu. Ein entsprechender Ansatz wird in diesem Beitrag vorgestellt. Die Identifikation basiert auf einem dynamischen, geschlossenen Kreislaufmodell mit statisch nichtlinearer Rückkopplung. Das Modell besteht aus drei Kompartimenten, einem nicht pulsatilen Herzmodell, sowie einer Abbildung des Barorezeptor-Systems. Das Modell wird durch eine Parameteroptimierung an jeden Patienten individuell adaptiert. Das Patientenverhalten während der Dialyse wird vom Modell sehr gut simuliert.

Even gold standard methods of body water measurement are not always in perfect agreement.

Dear editor, We read the letter by Milani et al. [1] with interest. Bioimpedance spectroscopy (BIS) is inexpensive, highly reproducible and child-friendly. There will always be a degree of uncertainty in measurements of body water based on whole body BIS because the models used to derive the fluid content of the body have to make a number of assumptions, including the relative proportions of the limbs and trunk in the current pathway and the shape of the limbs. However, this is also the case for gold standard techniques. Total body water (TBW)measurement by deuterium dilution has inherent uncertainty as demonstrated in a study [2] which compared measurements of extracellular water (ECW) using gold standard methods. In this study, TBW was measured by deuterium dilution (TBW_D20), ECW was measured by sodium bromide dilution (ECW_NaBr) and intracellular water (ICW) by total body potassium (ICW_TBK). The comparison of ECW_NaBr with ECW determined from TBW_D2O – ICW_TBK showed good agreement but significant scatter (standard deviations). Comparing these figures with the BIS results, authors found that the scatter of BIS was within the range of the references. The discrepancy between reference methods (D2O, TBK and NaBr) for assessment of ECW and TBW was further demonstrated in a large cohort of 526 participants that included 86 children of diverse age, body weight and ethnicity [3]. These observations suggest that even gold standard methods of assessing body water are not always in perfect agreement. The authors use the TBW results to present the clinical impact of the uncertainty in the measurement; however, it is much more likely that over-hydration (OH), or OH normalised to ECW (OH/ECW) would be used in the management of fluid balance in children. Our results suggest the uncertainty in OH and OH/ECW in children is comparable to results in adults [http://www.bcm-fresenius.de/files/ information_on_reference_ranges.pdf], a cohort in which the link between these parameters and outcomes has been clearly demonstrated. Our aim [4] was to show that when the body composition monitor (BCM) model is applied in children, there is not an additional systematic error related to age. An increase in the discrepancy between the TBWmeasured with BCM and deuterium dilution with decreasing age would suggest a clinically significant change in the percentage of water in normally hydrated tissue with younger age. We would welcome the introduction of a validated method of fluid assessment for use in children that is more accurate than BIS. However, the ease with which BIS measurements can be performed at the bedside means that it can be used to monitor changes in fluid status as part of routine care.

Modell-basierte Erkennung von Mangelernährung mittels Bioimpedanzspektroskopie (Model-based Identification of Malnutrition via Bioimpedance Spectroscopy)

Protein-Energie-Mangelernährung bei Patienten reduziert die Lebensqualität, verlängert die Krankenhausverweildauer und erhöht drastisch die Mortalität. Aktuell existiert kein einfaches und objektives Verfahren zur Bestimmung des Ernährungsstatus und damit zur Erkennung von Mangelernährung. Mithilfe einer Bioimpedanzmessung kann mit einem neu entwickelten Expertensystem Mangelernährung bei Patienten erkannt werden. Die Funktionsweise des Expertensystems orientiert sich an der technischen Fehlerdiagnose. Protein-energy-malnutrition reduces the quality of life, lengthens the time in hospital and increases the mortality dramatically. Currently there is no simple and objective method available for assessing the nutritional status and thus identifying malnutrition. A newly developed expert system detects patients with malnutrition via a bioimpedance measurement. The system is based on the technical fault diagnosis.

Heart Rate Classification Using Support Vector Machines

This contribution describes a classification technique that improves the heart rate estimation during hemodialysis treatments. After the heart rate is estimated from the pressure signal of the dialysis machine, a classifier decides if it is correctly identified and rejects it if necessary. As the classifier employs a support vector machine, special interest is put on the automatic selection of its user parameters. In this context, a comparison between different optimization techniques is presented, including a gradient projection method as latest development.

Filtering respiration in impedance cardiography

Abstract Impedance cardiography is a non invasive method for determination of stroke volume (SV) of the human heart by evaluating heart action induced impedance changes in the thorax. As the inpedance signal is strongly influenced by noise, especially by respiration and movement artifacts, appropriate filtering has to be applied in order to minimize the error in SV calculation. This work compares different filtering methods to the standard high-pass and ensemble average filters.