Average reference recording from the vagal nerve reveals an evoked indirect response

The vagal nerve conveys information from and to many organs, including the heart. The components of the compound action potentials (CAPs) recorded from the vagal nerve give information about the different fiber types present. In this paper, we show that with the average reference recording method an additional component can be measured. This component is missed when using a tripolar recording method. Recordings were made in anesthetized pigs. Results show that there is an indirect component in the CAP of the cervical vagal nerve. This component comes from the periphery most likely from the heart

Extensions and improvements of the electrical conductance method

Continuous monitoring of cardiac output is important in patients who are undergoing intensive care, thoracic surgery or a catheterization for diagnostic reasons. In these patients arterial pressure is routinely determined. In the patients, who are undergoing a catheterization for diagnostic reasons, aortic pressure is detennined. During intensive care and thoracic surgery arterial pressure is determined in both the pulmonary artery and the artery femoralis or radialis. The radial or femoral catheter is a replacement of the pressure catheter in the aorta. To detennine cardiac output continuously from an arterial pressure signal, the aortic pressure was reconstructed from the peripheral pressure [Wesseling et al. 1976, Gratz et al. 1992]. For this continuous cardiac output monitoring from aortic pressure, a model of the circulation is used. A parameter of this model is the compliance of the arterial system, which is the change in volume per unit of length (i.e. segmental volume) over a change in pressure. The compliance is derived from in vitro measurements using a selected group of human aorta's [Langewouters 1984]. Cardiac output can also be detelmined from the pulmonary arterial pressure signal, which is directly measured in this artery. Thus, a reconstruction of this pressure signal is not needed. To calculate right ventricular output, i.e. cardiac output, according to a pulse contour method, we detennined the pulmonary arterial compliance. To determine arterial volume, which was needed to determine compliance we modified the conductance method. We studied the relationship between arterial volume and pressure at a large range of pulmonary arterial pressure. To outline the context in which the research presented in this thesis has been carried out, the function of blood vessels and of large arteries in particular will be described. Next, the anatomy of arteries will be considered. Subsequently, the terms concerning mechanics of blood vessels are explained and fmally the method to determine blood volume in large arteries; the conductance method, will be described

An evoked indirect response in the cervical vagal nerve

The response of fibers in the vagal nerve, evoked by electrical stimulation, has been studied in both animals and humans. These compound action potentials (CAPs) consist of components coming from thick, myelinated fibers to thin, unmyelinated fibers. In our study, the possibility is addressed of an indirect component in the CAP which is involved in reflexive control. By using multiple, consecutive electrode sites along the cervical vagal nerve, both the direction and the velocity along the nerve of the CAPs can be analyzed. This information can be used to distinguish direct from indirect components. In this way an indirect component was revealed

12-segment cuff is superior to ring cuff for sensing and stimulation of cardiac fibers in the vagal trunk

Introduction: The vagal nerve is involved in reflexive control of the heart. We would like to sense vagal activity originating from cardiac sensory neurons, and influence cardiac parameters by electrical stimulation of the vagal nerve. The ability of the 12-segment cuff to influence and sense cardiac fibers in the vagal trunk will be compared with the ring cuff. Methods: Experiments were performed in twelve pigs. At the cervical level three cuff electrode configurations were placed on the left vagal nerve for recording and stimulation. The cuffs were 15 mm long and had three circular Pt/Ir electrode contacts with (Fig. 1b). In seven pigs, the third cuff was replaced with a cuff with 12 segments instead of three rings (Fig. 1a). ECG and left ventricular pressure were also recorded. Sensing In twelve pigs sensing was done with a ring cuff and in seven also with a 12-segment cuff. ENG was band-pass filtered at 100-2000Hz, rectified and low-pass filtered at 10Hz, resulting in the envelope of the ENG. Mean activity was removed to obtain only the variations within a cardiac cycle. An ensemble average was determined from 260 cycles of ENG signal, centered on R-tops of the ECG. A signal was identified as cardiac-modulated if the 95%-CI of the ensemble average had a deviation from zero. Stimulation In five pigs the maximal amplitude (300µs, 50Hz) was determined at which, during stimulation with the ring electrode, no coughing occurred. Next in these five pigs, stimulation with this setting was repeated with the ring cuff and the 12-segment cuff. Results: A significantly cardiac-modulated vagal signal was measured in 6/7 pigs with the 12-segment cuff and in 6/12 pigs with the ring cuff. The effect of electrical stimulation on left ventricular pressure was significantly greater when stimulating with the 12-segment cuff than with the ring cuff (p = 0.03). A significant difference was not reached for R-R (p = 0.2) and left ventricular contractility (p = 0.07). Conclusion: The 12-segment cuff seems superior to the ring cuff, considering selective sensing and stimulation of the cardiac fibers in the cervical vagal trunk

Determination of the mean cross-sectional area of the thoracic aorta using a double indicator dilution technique

A double indicator dilution technique for determining the mean cross-sectional area (CSA) of a blood vessel in vivo is presented. Analogous to the thermodilution method, dilution of hypertonic saline was measured by an electrical conductance technique. Because the change in conductance rather than absolute conductance was used to calculate CSA, pulsatile changes in shear rate of blood and conductance of surrounding tissues had no effect on the data. To calculate CSA from an ion mass balance, cardiac output was needed and estimated from the thermodilution curve using the same 'cold' (hypertonic) saline injection. The mean CSA, obtained from this double indicator dilution method (CSA(GD)), was compared with the CSA obtained from the intravascular ultrasound method (IVUS) in 44 paired observations in six piglets. The regression line is close to the line of identity (CSA(GD) = -1.83 + 1.06 · CSA(IVUS), r = 0.96). The difference between both CSAs was independent of the diameter of the vessel, on average -0.99 mm2 ± 2.64 mm2 (mean CSA(GD) = 46.84 ± 8.21 mm2, mean CSA(IVUS) = 47.82 ± 9.08 mm2) and not significant. The results show that the double indicator dilution method is a reliable technique for estimating the CSA of blood vessels in vivo

AMBER/VLTI observations of 5 giant stars

While the search for exoplanets around main sequence stars more massive than the Sun have found relatively few such objects, surveys performed around giant stars have led to the discovery of more than 30 new exoplanets. The interest in studying planet hosting giant stars resides in the possibility of investigating planet formation around stars more massive than the Sun. Masses of isolated giant stars up to now were only estimated from evolutionary tracks, which led to different results depending on the physics considered. To calibrate the theory, it is therefore important to measure a large number of giant star diameters and masses as much as possible independent of physical models. We aim in the determination of diameters and effective temperatures of 5 giant stars, one of which is known to host a planet. AMBER/VLTI observations with the ATs were executed in low resolution mode on 5 giant stars. In order to measure high accurate calibrated squared visibilities, a calibrator-star-calibrator observational sequence was performed. We measured the uniform disk and limb-darkened angular diameters of 4 giant stars. The effective temperatures were also derived by combining the bolometric luminosities and the interferometric diameters. Lower effective temperatures were found when compared to spectroscopic measurements. The giant star HD12438 was found to have an unknown companion star at an angular separation of ~ 12 mas. Radial velocity measurements present in the literature confirm the presence of a companion with a very long orbital period (P ~ 11.4 years).}Comment: accepted for publication in A&

The structure of cool accretion disc in semidetached binaries

We present the results of qualitative consideration of possible changes occurring during the transition from the hot accretion disc to the cool one. We argue the possible existence of one more type of spiral density waves in the inner part of the disc where gasdynamical perturbations are negligible. The mechanism of formation of such a wave as well as its parameters are considered. We also present the results of 3D gasdynamical simulation of cool accretion discs. These results confirm the hypothesis of possible formation of the spiral wave of a new, "precessional" type in the inner regions of the disc. Possible observational manifestations of this wave are discussed.Comment: LaTeX, 16 pages, 8 figures, to be published in Astron. Z

Wall shear stress as measured in vivo: consequences for the design of the arterial system

Based upon theory, wall shear stress (WSS), an important determinant of endothelial function and gene expression, has been assumed to be constant along the arterial tree and the same in a particular artery across species. In vivo measurements of WSS, however, have shown that these assumptions are far from valid. In this survey we will discuss the assessment of WSS in the arterial system in vivo and present the results obtained in large arteries and arterioles. In vivo WSS can be estimated from wall shear rate, as derived from non-invasively recorded velocity profiles, and whole blood viscosity in large arteries and plasma viscosity in arterioles, avoiding theoretical assumptions. In large arteries velocity profiles can be recorded by means of a specially designed ultrasound system and in arterioles via optical techniques using fluorescent flow velocity tracers. It is shown that in humans mean WSS is substantially higher in the carotid artery (1.1–1.3 Pa) than in the brachial (0.4–0.5 Pa) and femoral (0.3–0.5 Pa) arteries. Also in animals mean WSS varies substantially along the arterial tree. Mean WSS in arterioles varies between about 1.0 and 5.0 Pa in the various studies and is dependent on the site of measurement in these vessels. Across species mean WSS in a particular artery decreases linearly with body mass, e.g., in the infra-renal aorta from 8.8 Pa in mice to 0.5 Pa in humans. The observation that mean WSS is far from constant along the arterial tree implies that Murray’s cube law on flow-diameter relations cannot be applied to the whole arterial system. Because blood flow velocity is not constant along the arterial tree either, a square law also does not hold. The exponent in the power law likely varies along the arterial system, probably from 2 in large arteries near the heart to 3 in arterioles. The in vivo findings also imply that in in vitro studies no average shear stress value can be taken to study effects on endothelial cells derived from different vascular areas or from the same artery in different species. The cells have to be studied under the shear stress conditions they are exposed to in real life

The HARPS search for southern extra-solar planets XI. Super-Earths (5 & 8 M_Earth) in a 3-planet system

This Letter reports on the detection of two super-Earth planets in the Gl581 system, already known to harbour a hot Neptune. One of the planets has a mass of 5 M_Earth and resides at the ``warm'' edge of the habitable zone of the star. It is thus the known exoplanet which most resembles our own Earth. The other planet has a 7.7 M_Earth mass and orbits at 0.25 AU from the star, close to the ``cold'' edge of the habitable zone. These two new light planets around an M3 dwarf further confirm the formerly tentative statistical trend for i) many more very low-mass planets being found around M dwarfs than around solar-type stars and ii) low-mass planets outnumbering Jovian planets around M dwarfs.Comment: Revised version resubmitted to A&A Letters, 5 pages, 4 figure

Planet Populations as a Function of Stellar Properties

Exoplanets around different types of stars provide a window into the diverse environments in which planets form. This chapter describes the observed relations between exoplanet populations and stellar properties and how they connect to planet formation in protoplanetary disks. Giant planets occur more frequently around more metal-rich and more massive stars. These findings support the core accretion theory of planet formation, in which the cores of giant planets form more rapidly in more metal-rich and more massive protoplanetary disks. Smaller planets, those with sizes roughly between Earth and Neptune, exhibit different scaling relations with stellar properties. These planets are found around stars with a wide range of metallicities and occur more frequently around lower mass stars. This indicates that planet formation takes place in a wide range of environments, yet it is not clear why planets form more efficiently around low mass stars. Going forward, exoplanet surveys targeting M dwarfs will characterize the exoplanet population around the lowest mass stars. In combination with ongoing stellar characterization, this will help us understand the formation of planets in a large range of environments.Comment: Accepted for Publication in the Handbook of Exoplanet