R. W. de Boer

Relationships between short-term blood-pressure fluctuations and heart-rate variability in resting subjects I: a spectral analysis approach

A method to attribute the short-term variability of blood pressure and heart rate of resting subjects to their various causes, using spectral techniques, is presented. Power spectra and cross-spectra are calculated for beat-to-beat values of R-R interval and blood pressure from subjects who were seated in a comfortable chair. Interval values as well as systolic, mean and pulse pressures show variations linked to respiration and to the so-called 10 s rhythm. The diastolic pressure values are scarcely influenced by respiration in the normal respiratory range (0·20–0·35 Hz), but do show 10 s variability. Relationships between pressure and interval variability which indicate that the 10 s variability in systolic pressure leads the interval variation by two to three beats become manifest in cross-spectra; however, no such lag is found between the respiration-linked variations in systolic pressure and intervals. It is argued that the technique presented provides a critical test for models of the fast regulation of the cardiovascular system.

Electrical properties of platinum electrodes: Impedance measurements and time-domain analysis

The impedance of small platinum electrodes has been measured as a function of frequency. In the range of 0·1 Hz to 10 kHz, this impedance Z can be described accurately as K/(i2πf)m. The value of K is about 2·5 Mω for a surface are of 0·05 mm2. The values for m are in the order of 0·75. Various step responses are computed based on this expression and are compared to recorded wave forms. All step responses are shown to result directly and consistently from the same expression. The consequences of these results for various models and equivalent circuits of the electrode-medium interface are discussed.SommaireTout en préparant une mise au point expérimentale sur les mesures de résistance spécifique du tissue cariaque par une méthode à quatre électrodes (Schwan, 1963; les resultats devant être publiés), il fut nécessaire d’obtenir une connaissance plus approfondie de l’impédance électrique des électrodes métalliques. Celles-ci sont largement utilisées dans la recherche mèdicale et biologique. Elles possèdent des avantages spécifiques sur les microélectrodes en verre pour l’enregistrement de signaux rapides à cause de leur basse impédance à haute fréquence. Toutefois, elles ne conviennent pas du tout pour les mesures de potentiels (Gesteland,et al., 1959). Il existe une litérature approfondie sur électrodes métalliques (voir la revue deDymond, 1976) mais on n’a pu trouver aucune référence sur les mesures d’impédance à bande de fréquences inférieures à 10 Hz. Dans ce résumé, nous publions les résultats de l’impédance d’électrodes en platine entre 0,1 Hz et 10 kHz et nouse établirons avec ces renseignements les relations entre ces résultats et ceux de théories différentes sur l’impédance d’électrodes publiées antérieurement (Warburg, 1899, 1901;Jaronet al., 1968;Polack 1974a, b, c). En outre, nous donnons un analyse des caractéristiques de durée des électrodes en platine.ZusammenfassungIm Zuge der Vorbereitung eines Versuchsaufbaus zur Messung des spezifischen Widerstands von Herzgewebe mittels eines Vier-Elektroden-Verfahrens (Schwan, 1963; Veröffentlichung der Ergebnisse steht bevor), wurden genauere Kenntnisse über den elektrischen Widertand der von uns verwendeten Metallektroden erforderlich. Metallelektroden finden in der biologischen und medizinischen Forschung verbreiteten Einsatz. Sie haben gegenüber den Glas-Mikroelektroden besonders den Vorteil, daß sich damit schnelle Signale registrieren lassen, da ihr Hochfrequenz-Widerstand sehr niedrig ist. Sie sind jedoch keineswegs für die Messung von GS-Potentialen geeignet (Gestelandet al., 1959). Über Metallelektroden liegt eine umfangreiche Literatur vor (siehe Überblick vonDymond, 1976), es konnten jedoch keinerlei Hinweise für Widerstandsmessungen für den über 10 Hz hinausgehenden Frequenzbereich gefunden werden. In dieser Arbeit legen wir unsere Ergebnisse für Messungen des Widerstands von Platinelektroden zwischen 0.1 Hz und 10 kHz dar, und befassen uns auf der Grundlage dieser Daten mit einem Vergleich unserer Ergebnisse und den verschiedenen bereits veröffentlichten theoretischen Modellen zum Thema Elektroden-widerstand (Warburg, 1899, 1901;Jaronet al., 1968;Pollak, 1974a,b,c). Außerdem liefern wir eine Analyse der Zeitbereichscharakteristiken von Platinelektroden.

Relationships between short-term blood-pressure fluctuations and heart-rate variability in resting subjects. II: A simple model

A simple model of the beat-to-beat properties of the cardiovascular system is used to interpret the results of spectral analysis of blood-pressure and interval data. The model consists of two equations, one representing the fast regulation of interval by the systolic pressure (baroreflex), the other one representing a Windkessel approximation of the systemic arterial system. The model, when applied to interval and blood-pressure data from resting subjects, explains the lack of respiratory variability in the diastolic pressure values. The baroreflex equation seems to describe the data only in the region of respiratory frequencies. The shape of the phase spectrum of systolic pressures against intervals is modelled by difference equations, but no physiological interpretation of these equations is given.

Intramural resistivity of cardiac tissue.

The macroscopic resistivity of canine ventricular tissue has been measuredin vivo. Using a tetrapolar intramural electrode, values have been obtained for transmural resistivity as a function of depth, phase of the cardiac cycle and of the frequency of the applied current. The time course of resistivity owing to induced local ischaemia has been recorded. Using epicardial electrodes, the amount of anisotropy of the macroscopic resistivity has been estimated. The results obtained put the transmural resistivity at about 410 Ωcm and show it to be independent of the cardiac phase, independent of frequency and independent of depth. Induced local ischaemia was found to be followed by a substantial increase in the resistivity with time. The amount of anisotropy observed epicardially shows the ratio of transverse to longitudinal resistivity to be of the order of 2. These results are discussed in relation to a cylindrical model of the underlying fibre structure.

Spectrum of a series of point events, generated by the integral pulse frequency modulation model

The paper deals with the relationship between the spectra of the input signal and the output signal of the integral pulse frequency modulation (IPFM) model. The IPFM model is a physiologically attractive device for the conversion of a continuous input signal into an output signal, consisting of a series of events (e.g. nerve spikes, heart beats). Two different spectra are used in the analysis of a series of events; the interval spectrum and the spectrum of counts. The latter spectrum is known analytically for the event series belonging to a sinusoidal input signal. An approximation to the interval spectrum of this series is presented. Using data from a simulated IPFM model, it is shown that, for an input signal consisting of the sum of two sinusoids, terms at sum and difference frequencies appear in the interval spectrum but not in the spectrum of counts. However, the spectrum of counts is contaminated by sidebands of the mean repetition frequency. It is concluded that in general the spectral properties of the input signal cannot be recovered fully from the interval spectrum, nor from the spectrum of counts, the more so as physiological series of events will seldom be generated by an ideal IPFM model.

The control of speed in elite female speed skaters

From ten participants in the World Championships Speed Skating for Ladies 1983 a number of selected mechanical parameters were measured and correlated with speed and external power. The parameters were derived by means of video and film analysis of strokes at the four distances: 500 m, 1500 m, 3000 m and 5000 m. The results show that these speed skaters control the different speeds at different distances mainly by changing their stroke frequency and not by changing the amount of work per stroke. However, at the same distance the relatively small interindividual differences in performance level appeared not to be correlated to differences in stroke frequency but were correlated to differences in push-off mechanics. Better performers gain some potential energy during the gliding phase and show a more horizontally directed push-off in the frontal plane. Maximal knee extension velocity did not show any correlation with performance. The fact that this might be connected to the absence of a plantar flexion during push-off is discussed.

Physiological and biomechanical comparison of roller skating and speed skating on ice

SummaryEight well trained marathon skaters performed all-out exercise tests during speed skating on ice and roller skating. To compare these skating activities in relation to the concept of training specificity, relevant physiological (VO2,VE, RER and heart rate) and biomechanical variables (derived from film and video analysis) were measured. There were no significant differences between oxygen uptake (50.5±8.0 and 53.3±6.7 ml·min−1·kg−1), ventilation (102.4±11.2 and 116.0±11.1 l·min−1) or heart rate (174±12.2 and 176±14.5 min−1) between speed and roller skating. In roller skating a higher RER (1.16±0.1 cf. 1.05±0.1) was found. Power, work per stroke and stroke frequency were equal. Due to a higher coefficient of friction the maximal roller skating speed was lower. The effectiveness of push-off and parameters concerning the skating techniques showed no differences. In roller skating a 7.5% higher angle of the upper leg in the gliding phase occurred. It is speculated that the blood flow through the extensor muscles might be higher in roller skating. It is concluded that roller skating can be considered as a specific training method which may be used by trained speed skaters in the summer period.

Applied physiology of speed skating

Speed skating exercise can be better understood by taking account of physiological and biomechanical considerations. Comparison with other sports shows the unique and peculiar way of skating propulsion. The relatively long lasting isometric muscle contractions during the gliding phase, alternated with high power output push-offs, place unusual demands on the (local) energy delivering systems. The short and explosive push-off needs a specific pattern of motor unit recruitment. Some mixture of slow twitch (to sustain skating posture) and fast twitch fibres (to effect push off) in the hip and knee extensors seems necessary for optimal skating performance.

Supramaximal test results of male and female speed skaters with particular reference to methodological problems.

SummarySix male and six female elite speed skaters were tested during two bicycle ergometer tests: a 30 s sprint test and a 2.5 min supra maximal test. During the 2.5 min test oxygen consumption was measured every 30 s. The males showed 30–31% higher mean power output values both during the sprint test (1103 versus 769 Watt) and during the 2.5 min test (570 versus 390 Watt). Maximal oxygen consumption was 31% higher for the males than for the females (5.10 versus 3.50 l · min−1). However, when expressed per kilogram lean body weight (LBM), power output and oxygen consumption was equal for both sexes. Differences between present and previous results are most likely due to methodological problems with the estimation of load during the supra maximal test. Subjects appear to experience difficulties in distributing their power output over the 2.5 min if they are tested for the first time. For experienced skaters and cyclists, fixed levels of 19 W · kgLBM−1 as initial load setting for the sprint test and 8 W · kg LBM−1 for the 2.5 min test are recommended.

A mathematical model of the double-sucrose-gap voltage-clamp method

Abstract A simple mathematical model of a double-sucrose-gap voltage-clamp setup is presented. The model is used to calculate the difference between an assumed current-voltage relationship of a preparation, and the one measured by the double-sucrose-gap technique, taking into account the cable-like structure of the preparation. It is demonstrated that the abovementioned difference may be important for the early peak current, as well as for the late outward current. It is concluded that quantitative results obtained bythe double- sucrose-gap technique should be considered with care.