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Biomedical subjects

M Hexamer

Publications and source records attributed to M Hexamer.

18 recordsLinked to original sources
PubMed2002

New sensor based on fibre optics for measurement of heart movement.

Innovative fibre-optic sensor technology for measuring the movement of the myocardial walls, and from this the heart chamber volumes, was developed. An optical fibre, with a mirror at its end, is inserted into a catheter located in the heart. An opto-electrical control unit positioned outside the heart contains both the light source and the signal receiver. It generates and couples the light into the fibre and transforms and analyses the reflected signal. With such a system, the movement of the cardiac wall can be continuously measured during each cycle, because the fibre moves synchronously with the heart, and this movement bends the fibre, changing the optical attenuation. Experiments where the fibres were wound around metal cylinders of different diameters revealed a maximum sensitivity of 4% mm(-1), diameter. The noise signal corresponded to about 1% of the diameter. First tests in a working pig heart showed a high correspondence of the fibre signal with cardiac parameters. Although these tests are promising, further long-term, extensive experiments in preclinical test devices, and later in clinical tests, must be carried out before the new sensor is used in clinical practice. The fibre-optic technique could be used in monitoring devices, assist devices, pacemaker systems or cardioverter defibrillators.

Animals↗
PubMed2002

[Biventricular defibrillation using transvenous electrodes].

This study investigated the feasibility of transvenous biventricular defibrillation using an electrode in a left ventricular vein. The standard lead configuration with a coil in the right ventricle (RV) and a coil in the superior vena cava (SVC) was compared with an additional unipolar coil in an accessible epicardial vein. Only biphasic shocks were used with different shocking modes between the coils in the RV, LV, SVC and the ICD-generator (CAN). Shocks were applied starting with 30 J, decreasing till the DFT was reached. As a result there is a lower DFT when defibrillation is performed including the left ventricular electrode. The impedance did not show a significant increase after more then 20 consecutive shocks. It is a feasible and workable application that might help to reduce the energy demand and increase the safety of such a system.

Animals↗
PubMed2002

[Rate-responsive pacemaker concept: parametric system identification].

The focus of this paper is the design of a rate-responsive pacemaker which uses the atrio-ventricular conduction time (AVCT) as an exertion related sensor signal. AVCT can be easily obtained from an intracardial electrogram. During atrial pacing AVCT shortens with exercise but increases with the stimulation rate. Hence, an AVCT based pacemaker always establishes a closed-loop system. General design rules for an AVCT pacemaker have been developed from our experimental results and a system-theoretical treatment. Topics addressed were the controller gain, uncertainties concerning the dynamics of the AVCT, the attenuation of disturbances, the closed-loop bandwidth and stability.

Algorithms↗
PubMed2002

[Blood gas regulation in isolated beating pig hearts].

Continuous perfusion with oxygenated blood is required to supply isolated beating pig hearts. To meet physiological conditions the blood gas parameters pH, pO2 and pCO2 should correspond with values usually found in pigs, respectively. Controlling these parameters manually is a very time consuming task and is therefore done best by a control system consisting of gas blender, oxygenator and blood gas sensors. As this control system should be used with an transportable perfusion apparatus, this paper describes an approach where the gassing is realized only with air and oxygen. In this approach pO2 and pH are measured on blood side so that the control system can change oxygen concentration and gas flow adequately on the gas side.

Acid-Base Equilibrium↗
PubMed2000

Assessment of the chronotropic response at the anaerobic threshold: an objective measure of chronotropic function.

The evaluation of the heart rate response to exercise is important for the diagnosis of chronotropic incompetence and the assessment of a rate responsive algorithm of sensor-controlled pacemakers. The aim of the present study was to examine a classification of the chronotropic response at an individually moderate exercise level. Sixteen pacemaker patients (patient group, age 62.9 +/- 7.6 years) with sick sinus syndrome and 15 age-matched healthy subjects (control group, age 57.6 +/- 9.4 years) underwent a maximum cardiopulmonary exercise test on a treadmill after a protocol with individually selected incremental steps. To analyze the patients' intrinsic heart rate response, the rate responsive mode of the pacemaker was switched off. Chronotropic incompetence was diagnosed in eight patients whose maximal heart rate was < 80% of the age-predicted heart rate. The heart rate at the anaerobic threshold was significantly lower in the chronotropically incompetent subgroup than in the chronotropically competent patients and the healthy subjects (85.9 +/- 6.6 beats/min vs 100.3 +/- 9.9 beats/min and 112.9 +/- 11.7 beats/min, respectively). The chronotropic slope of the heart rate reserve as a function of the metabolic reserve was significantly higher in the control group than in the patient groups with either mild or severe chronotropic incompetence (0.94 +/- 0.17 vs 0.64 +/- 0.08 and 0.43 +/- 0.14, respectively). Furthermore, the chronotropically incompetent response could be divided into a linear type with and without a threshold, an exponential, and a logarithmic type. The anaerobic threshold was an objectively detectable breakpoint at an individually moderate exercise level that could be used for characterization of chronotropic function. At the anaerobic threshold, a physiological heart rate response was about 220--age--50 beats/min. A deviation of more than 10 beats/min below this physiological value characterized chronotropic incompetence.

Algorithms↗
PubMed1999

Multi-loop control of liquid cooling garment systems.

In this study the effects of multi-loop control of liquid cooling garments (LCGs) under exercise heat stress conditions were investigated by experiments and theoretical analysis. A triple-loop LCG, by which the torso, arms and legs could be independently cooled, was used in the two series of experiments carried out in a hot environment (35 degrees C/40% RH). The experiment consisted of rest, exercise on an ergometer at 70 W and exercise at 110 W. In the first experiment, each water inlet temperature (TWI) was adjusted according to the local thermal sensation. In the second experiment, TWI for the torso including arms and TWI for the legs were regulated by a skin temperature controller with set-point adjustment via heart rate. The experiments showed that a multi-loop LCG is more effective than a single-loop LCG in providing thermal sensation and comfort adjusted to the requirements of the different parts of the body, and that a skin temperature controller could be applied to a multi-loop system. The theoretical analysis was carried out using a mathematical model of thermoregulation. The results showed that a strong cooling of the surface over the working muscles (legs) provided the greatest thermoregulatory advantage during low body exercise, because most of the heat generated within the working muscles can be removed directly by heat conduction to the skin. Optimization of a human/LCG system could be attained by an optimal configuration and control. However, an optimal configuration always depends on the application purpose of an LCG system.

Adult↗
PubMed1999

Restoration of cardio-circulatory regulation by rate-adaptive pacemaker systems: the bioengineering view of a clinical problem.

In the past, the development of rate-adaptive (sensor-controlled) pacemaker systems seems to have been determined primarily by the availability, compatibility and other properties of the technical sensor. This paper, however, focuses on the system-physiological aspect in an attempt to answer the question to what extent physiological cardiovascular control is restored by the pacemaker system. This is a question which should be asked before attempting to design a sensor-controlled system and especially before designing multisensor systems with infinite combinations. Four categories are defined: direct bridging ("shunting"), open loop systems, closed systems using cardiorespiratory or metabolic coupling and those using cardiac signals. Further subdivisions are shown. From the bioengineering as well as from the physiological viewpoint a system should preferably not combine sensors from one and the same of these categories. At present direct bridging is available only for the atrioventricular (AV)-block, so that for sick-sinus-syndrome (SSS) patients feedback control via cardiac signals ("inotropic" pacemaker) comes nearest the goal without, however, ideally bridging the gap. Open-loop systems should no longer be developed as single-sensor systems. A well developed activity sensor, however, which quickly pinpoints the most prominent stressor of cardiovascular control is best suited to complement another sensory system achieving closed-loop control. New and promising concepts orientated toward direct bridging are the analysis of monophasic action potentials and the "dromotropic" concept, both of which seek direct correlation with the "chronotropic" information not available in SSS patients.

Adaptation, Physiological↗
PubMed1999

Influence of the pacing rate on the atrioventricular conduction time during aerobic and anaerobic exercise: basic concepts for a dromotropically controlled rate responsive pacemaker.

The dromotropic pacemaker concept needs a rate responsive algorithm in which the pacing rate is controlled by the atrioventricular conduction time (AVCT). To develop basic concepts for such a rate responsive algorithm, the influence of the pacing rate on the AVCT was investigated. Seven patients (62 +/- 7.8 years) with sick sinus syndrome and intact atrioventricular conduction underwent two cardiopulmonary exercise tests (CPX) on a treadmill. According to the determination of the anaerobic threshold (AT) and the patients maximum capacity in the first incremental CPX the work rate for two exercise levels below and above the AT were chosen for the second constant workload CPX. The calculation of the optimal pacing rate (HRopt) was based on the oxygen uptake (VO2) during exercise after reaching steady-state conditions. According to the increase of the VO2 from 14.8 +/- 2.3 mL/min per kilogram during aerobic work (38.3 +/- 16.0 W) to 19.4 +/- 4.7 mL/min per kilogram during anaerobic work (80.6 +/- 32.3 W), the HRopt was calculated to be 98.6 +/- 6.9 beats/min and 116.4 +/- 4.7 beats/min. Starting from HRopt, the pacing rate was increased (overpacing) and decreased (underpacing) by about 5 beats/min every minute. At optimal pacing rate the AVCT decreased significantly from 233.0 +/- 30.5 ms during aerobic work and to 226.4 +/- 27.3 ms during anaerobic work (P < 0.05). Whereas overpacing induced a significant prolongation of the AVCT during aerobic work (4.17 +/- 1.78 ms per 10 beats/min) and anaerobic work (3.84 +/- 1.60 ms per 10 beats/min), underpacing yielded a significant shortening of the AVCT by about 4.49 +/- 2.64 ms per 10 beats/min during aerobic work and 4.75 +/- 1.87 ms per 10 beats/min during anaerobic work (P < 0.01). The slopes of the regression lines of the relationship between AVCT and pacing rate were not significantly. different. Based on the reciprocal relationship of heart rate (HR) and AVCT, basic concepts may be established for a dromotropic rate responsive algorithm.

Aged↗
PubMed1999

Relationship between atrioventricular delay and oxygen consumption in patients with sick sinus syndrome: relevance to rate responsive pacing.

To develop a dromotropic-controlled rate adaptive algorithm for patients with sick sinus syndrome (SSS) and intact AV conduction, 14 pace-maker patients with SSS underwent cardiopulmonary exercise testing (CPX). During exercise, the pace-maker was programmed in an AAT mode without rate adaptation, whereby 3 patients developed supraventricular arrhythmia and 11 patients kept sinus rhythm. Chronotropic incompetence (CI) at heart rate (HR) < 95 beats/min at the anaerobic threshold (AT) was found in five patients. In patients with chronotropic competence (CC), the HR increase was significantly greater than in CI patients (rest: 73.2 +/- 12.6 vs. 64.2 +/- 4.0 beats/min;AT:101.2 +/- 6.2 vs. 82.0 +/- 5.1 beats/min;peak: 135.2 +/- 10.7 vs. 103.2 +/- 10.9 beats/min). There was no significant difference in the AVD between CC and CI patients (rest: 167.7 +/- 38.6 vs. 170.8 +/- 22.5 ms, AT: 156.2 +/- 30.7 vs. 163.6 +/- 21.6 ms, peak: 144.7 +/- 29.0 vs. 152.4 +/- 15.0 ms). The correlation coefficient between HR increase and VO2 was +1.0 and between AVD decrease and VO2 - 1.0 in both groups. An increase in pacing rate from 75 beats/min to 120 beats/min without exercise (overpacing) led to a prolongation of the AV interval of about 30.6 +/- 14.2 ms. Based on this closed loop control with negative feedback, a dromotropic rate adaptive algorithm for patients with SSS and intact AV conduction could be developed.

Aged↗
PubMed1997

Control of liquid cooling garments: technical control of mean skin temperature and its adjustments to exercise.

This paper describes an automatic control concept for liquid cooling garments. The concept consists of an own controller for mean skin temperature whose setpoint is either fixed or adjusted to the metabolic heat production by means of the heart rate signal. The controller for mean skin temperature included both a proportional and an integral signal path (PI-type), the latter being able to eliminate any load error within the control loop. This means that the actual skin temperature will always match the given setpoint irrespective of the amount and the origin of the heat gain at the body shell. Experiments were carried out to test the operation of the skin temperature controller. There the setpoint was fixed while metabolic heat production was changing. After a transient period with deviations, the load error was always eliminated by the skin temperature controller. With this result one can also imagine the controllers ability to compensate changing heat gains from the environment. Despite this behaviour, the amount of heat removal was not high enough to prevent sweating and warm discomfort during all exercise levels. Therefore we draw the conclusion that, in addition, the setpoint of the skin temperature should be adjusted to the metabolic rate/heat production. A convenient physiological signal that reflects the current level of metabolic rate is the heart rate signal. After being filtered the heart rate signal was used during some experiments to change the setpoint of the skin temperature controller. The reason for this filtering (lowpass, time constant = 10 min) was, firstly, the necessity of attenuating the consequences of short-term psychological effects on the heart rate and secondly, the avoidance of vasoconstriction due to too fast changes of the exercise/heart rate induced cooling rate. In the following experiments it became clear that the adjustment of the skin temperature setpoint to the exercise level was an improvement as there was less sweating and the subjects felt more comfortable.

Automation↗
PubMed1996

Control of liquid cooling garments: technical control of body heat storage.

This paper describes a concept of how liquid cooling garments (LCG) can be automatically controlled by the objective physiological state. The technically controlled parameter was mean body temperature which was calculated from the measured rectal and mean skin temperature. This was motivated by the fact that mean body temperature is the basis for estimating body heat storage, a commonly used measure of thermal strain. Here the setpoint of mean body temperature was the individual value taken in a preceeding resting period and it was the task of the technical controller to keep the actual value of mean body temperature as close as possible to the setpoint. The most important tuning parameters of the controller were the weighting coefficients for rectal and mean skin temperature in the calculation for mean body temperature. The ratio of these two coefficients determined the degree of compensation for any rectal temperature shift by changing mean skin temperature. Test experiments were carried out (n = 5) in which the controller was able to clamp mean body temperature to the setpoint thereby preventing heat storage. Although exercise rate (75 W) was the same, sweating and warm discomfort occurred in some cases due to the individual rectal temperature rise. Another source of discomfort were delays or paradoxical time courses of rectal temperature at the start or end of exercise which were responsible for a delayed onset of cooling or heating. To avoid these effects, the oxygen consumption signal, which is very fast and directly correlated to the exercise rate, was added to the control loop. Each increase of this parameter above its resting level lowered suit temperature. As heat storage should not be completely rejected by this new signal pathway, the controller for mean body temperature still remained active. The repetition of the experiments showed that the load error in the control loop was smaller and the comfort level in transient phases higher. For a further improvement of this concept it is recommended that the weighting coefficients be tuned to the individual requirements.

Adult↗
PubMed1995

Control of liquid cooling garments: subjective versus technical control of thermal comfort.

Liquid cooled garments (LCG) are a powerful tool for alleviating heat strain during work in hot conditions. However, the potential advantage of an LCG depends on a more or less proper control of the cooling liquid's temperature. To gain more knowledge on this subject two experimental studies concerning manual control by the wearer were carried out. In both studies the subjects had to exercise in a warm environment. In the first study the subjects (n = 5) were asked to control the temperature directly. They used different strategies, ranging from gradual changes to oscillations of the suit temperature. This was accompanied by large differences of the chosen inlet temperature and the comfort level attained, although by repetition of the experiment it was possible to increase the subjective thermal comfort. In the second study a technical 'comfort' controller changed the inlet temperature of the LCG in the appropriate direction whenever the personal assessment of thermal sensation differed from the neutral state. The subjects (n = 6) had no information about the control mode. The experiments showed that it was possible to maintain a comfortably neutral thermal sensation throughout nearly the whole experiment. This result, however, was partly in contrast to the objective thermal state of the subjects. Two of them had problems with their heat balance, expressed by the fact that their rectal temperature did not reach a steady state. The results of both studies indicate that manual or subjective control is not optimal and should therefore be replaced by an objective control of the thermal state.

Body Temperature↗
PubMed1993

Computer supported analysis of cardiovascular parameters by impedance cardiography and plethysmography.

A fully automated system for computer supported analysis of cardiovascular parameters on the basis of impedance technique, which is confined to the analysis of relative changes of stroke volume and cardiac output, has been developed and tested. The evaluation showed good agreement between automatic and manual calculation. As the whole computational procedure is done by software, which is written in a common language (BASIC), a maximum of flexibility and portability is guaranteed. Estimation of relative changes of stroke volume and cardiac output as well as peripheral blood flow is possible without any manual interaction. Extension from physiological measurement of healthy individuals to clinical application has been started.

Cardiac Output↗