OBJECTIVES: The purpose of this study was to explore the physiology underlying the beat-to-beat variations of ventricular function during atrial fibrillation (AF). METHODS: Left ventricular pressure, and its first derivative (LVdP/dtmax, an index of contractility, and aortic blood velocity (and its integral AVI, an ejection index), were recorded using cathetermounted transducers in 15 patients with AF during cardiac catheterisation. Transfer function modelling was used to examine the influence of preceding intervals on LVdP/dtmax, and of LVdP/dtmax on AVI. The technique also allowed simulation of the behaviour of LVdP/dtmax in response to specific manipulations of interval. RESULTS: The variations in LVdP/dtmax recorded from the AF patients were shown to be dependent on up to six preceding intervals; a maximum of 91% of the variation was explicable in this way. The influences of mechanical restitution (MR, the relationship between preceding interval and contractility), postextrasystolic potentiation (PESP, the inverse relationship between pre-preceding interval and contractility) and the decay of that potentiation were all demonstrated. These influences collectively appeared to be powerful determinants of AVI. Simulations of LVdP/dtmax, following single interval perturbations, were entirely consistent with these interval force effects. CONCLUSIONS: The cardiac interval force relationship in man is an important determinant of the beat-to-beat variations of contractile and ejection function during AF: the beat-to-beat variations in contractile (or inotropic) function are independent of changes in ventricular filling or fibre-length.

Aged↗

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Some comments on ventricular afterload.

In this issue of Basic Research in Cardiology a paper by Timisjärvi et al. is presented in which the problem of afterload, generated by the resistance against which the left ventricle is ejecting, is discussed. These authors have performed experiments in which they examine the role and validity of the so-called mean outflow resistance (MOR) of the left ventricle. In this editorial, the opportunity is taken to present some thoughts on the different possibilities of expressing the interrelation between the heart and the arterial system. It is intended to show how, from the first ideas about ventricular-arterial interaction bei Ph. Broemser (1935; 1) different concepts about simple descriptions of the functioning of the heart, the arterial system and the matching between both have evolved. In particular, the concept by Sunagawa et al. seems worthy of mention. An examination of this, and several other concepts, prove that the peripheral resistance and, particularly, the MOR, are important parameters that determine the influence of the arterial system on the performance of the heart. However, since MOR by definition is closely related to the total peripheral resistance, there is no essential need to introduce this resistance as a new concept of ventricular afterload. Calculations based on this definition of MOR demonstrate an agreement between theory and experimental results.

Animals↗

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A place for theoretical cardiology.

Explore the source record for details and available documents.

Academies and Institutes↗

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Blood flow and blood volume determinations in aorta and in coronary circulation by density dilution.

Continuous blood mass-density measurements were performed in anesthetized dogs and injections of 0.7-1.4 ml/kg isotonic saline solution were applied. The resulting density dilution curves were used to compute blood volume, total flow in the aorta and local flow in the coronary circulation. Blood volume calculations were compared with blood volume determined by Evans blue injections and a close agreement was found. Blood flow determined by density dilution was independent from the investigated sites of injection or sampling. We conclude from these results that small volume injections of isotonic saline solution can be used to determine blood volume and flow by density dilution. In addition to these findings, a marked retention of the injected fluid was observed. Possible mechanisms to explain this retention include albumin deposition in the endothelial pores and/or variations of blood viscosity and capillary pressure.

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Influence of tonicity on the viscoelastic properties of blood during isovolemic dilution.

The influence of isotonic or hypotonic dilution on viscoelastic properties of blood is examined. The viscous, as well as the elastic, properties of blood samples diluted with isotonic saline or pure water, respectively, and of undiluted whole blood samples are compared by means of a dynamic capillary viscosimeter (OCR-D, A. Paar K.G., Austria). The dilution (approximately 17% of the total blood volume) was performed isovolemically and retained the same rbc count. The rbc swelling observed as a consequence of changes in plasma osmolarity was tracked by the high resolution density measurement, according to the mechanical oscillator technique. Since no significant rbc swelling was found in the dilution with isotonic saline, viscoelastic resistance of blood was efficiently reduced in the observed range of shear rates (2 s-1-100 s-1). This decrease is due to reduced plasma protein concentration, which also lowers plasma viscosity by approximately 18%. Although plasma viscosity is significantly decreased in hypotonic dilutions (-12%), flow properties of the rbc suspensions are, in general, significantly impaired. This is due to the osmotic rbc swelling (hematocrit = +8%), which increases viscous resistance within the suspending fluid, as well as elastic resistance of the rbc due to a loss in rbc deformability. It can be concluded that isotonic dilution leads to a decrease in the viscosity of blood, whereas hypotonic dilution--in an order of magnitude which may occur during resorption of water--leads to increased viscous and elastic resistance of the blood.

Blood Viscosity↗

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Numerical blood flow analysis: arterial bifurcation with a saccular aneurysm.

The flow pattern and the paths of fluid particles in a saccular aneurysm located at the bifurcation of an intracranial arterial segment are investigated with a numerical method. A normal physiological flow pattern was assumed as input to the studied segment. The theoretical study is carried out for two different Reynolds numbers and two different geometries of the aneurysm. The governing equations for incompressible Newtonian fluid flow are solved using the finite element method. The results show the disturbed blood flow in the pathologically altered bifurcation and the flow activity in the aneurysms. It is particularly important that blood particles can circulate in a whirl within the aneurysm for a time which seems long enough to permit the generation of cell aggregates or/and blood clots.

Blood Flow Velocity↗

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Arterial blood pressure and its measurement.

Pressure is defined as force per unit area. The actual value of the recorded pressure depends on dynamic influences, like acceleration and friction in a moving fluid, and on static influences like the effect of a gravitational field. In the arterial system, four mechanisms exert the main influence on the measured pressure: (1) cardiac output and peripheral resistance, on which the mean value of the pressure depends; (2) the flow amplitude and the characteristic impedance of the artery, on which the pressure amplitude depends; (3) the orthostatic pressure component; (4) the local velocity of flow, on which the recorded pressure depends, and is described by the Bernoulli equation. Under normal conditions, this influence is rather small. The measurement of arterial pressure can be done either invasively or with one of the following noninvasive methods: occlusive cuff method, vascular unloading, arterial tonometry, or measurement of the arterial pulse wave velocity. Each of these methods has certain advantages and disadvantages. It seems necessary to point out the fact that due to wave reflections in the arterial system, pressure values depend on local influences and differ from location to location. Most markedly, the influence of wave reflection can be demonstrated proximally, as well as distally, to the occlusive cuff in the Riva Rocci method. Once the blood pressure has been measured in a person, the problem of the assessment and interpretation of the measured values has to be considered. According to the decision of the WHO, certain limits are defined which permit a distinction between normotension and hypertension. In each case, of course, a physician has to consider the importance of individual conditions.

Blood Pressure↗

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A statistical approach to the analysis of phenomena of frequency potentiation of isolated myocardial strips.

To analyze the influence of the pattern of stimulation on contractility, isolated myocardial strips of guinea pigs have been stimulated in a pseudo-random sequence of intervals. By this stimulation method, which is analogous to the RR-interval sequence during cardiac arrhythmia, it is possible to get a large variety of different contractions. Using methods of correlation and regression analysis, and especially methods of time series analysis, it is possible to study the relations between properties of succeeding contractions. To describe the time course of the isometric force, a multiple regression model was established. The model parameters are estimated from a pseudo-random sequence of contractions, and in an economical parameter selection procedure the input variables, which have a substantial influence on the properties of one contraction, are identified. As a result of this procedure we found that at least properties of 4 to 6 preceding contractions have a substantial influence on one contraction. It is not possible to find indicators for frequency potentiation by free eye in the pseudo-random sequence of contractions. We can show that these effects are present when the behaviour of the contractility is simulated using the regression model, whose input variables are the intervals for different stimulation frequencies. This indicates that processes lasting longer than one contraction are responsible for the effects of frequency potentiation.

Animals↗

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Platelet activation in normo- and hyperlipoproteinemias.

In the last few years it became obvious that platelets are involved in the development of atherosclerotic diseases. This involvement of platelets has been taken into account in the "response to injury" hypothesis of atherosclerosis. The hypothesis is based on the assumption that atherosclerotic lesions result from endothelial injury, followed by the interaction of vessel wall constituents with lipoproteins, macrophages, and platelets. In the first part of this review, general aspects of platelet activation are summarized and the pathways of platelet aggregation as well as their involvement in blood coagulation are discussed. The second part of this paper describes the influence of cholesterol, lipoproteins, and apolipoproteins upon the activation and metabolic behavior of platelets. Physiological and pathophysiological processes particularly occurring in different types of hyperlipoproteinemias and atherosclerotic disorders are discussed in this context.

Adenosine Diphosphate↗

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Determination of cardiac output and of transcapillary fluid exchange by continuous recording of blood density.

We have tested the application of the continuous measurement of the density of the arterial blood for the calculation of the cardiac output and for the determination of the transcapillary fluid exchange. The density of arterial blood was continuously recorded in anesthetized dogs with a mechanical oscillator device (DMA 602 MW built by A. Paar KG, Graz). The time resolution of this device is less than 1 sec, the accuracy is 10(-6) g/ml. Simultaneously the arterial blood temperature was recorded with a thermistor probe. The intravenous injection of isotonic solutions yields temperature and density transients which are of a similar shape and proportional to the injected volume in amplitude. There is a good agreement between the cardiac output calculated from thermodilution transients and from density dilution transients. The injection of hypertonic solutions, e.g. 5% NaCl, 20% mannit or 14.4% urea, into a vein yields arterial density dilution transients which show marked differences and very characteristic features. They can be explained by the assumption of osmotic fluid shifts in the microcirculation of the lung. During the transient of a bolus of hypertonic NaCl or mannit, a transcapillary influx of about 0.170 X 10(-3) ml/sec per mosmols/l and per g wet tissue weight is generated. The effect of hypertonic urea is less by a factor of about 1/2, which is in agreement with the fact that urea diffuses rapidly into the erythrocytes. We conclude that the method of density dilution allows to record and quantify cardiac output and osmotic fluid shifts through the microcirculation of the lung of intact dogs.

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