Shifts in blood volume alter the perception of posture

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Abstract

Recent experiments have shown that somatic graviceptors exist in humans. Traditionally, extravestibular gravity information has been thought to originate from mechanoreceptors in the joints, muscles and skin. Experiments with normal, paraplegic and nephrectomized subjects revealed that the kidneys and the cardiovascular system are involved in providing truncal gravity information. The present study intends to determine the influence of shifts in body fluid, especially of the distribution of blood along the subjects' spinal (Z-) axis, on the perception of posture. To this end, the distribution of body fluids was altered by means of the technique of lower body negative and positive pressure (LBNP and LBPP). LBNP leads to venous pooling of blood in the legs, whereas LBPP prevents venous blood from pooling, increasing central volume. Changes in blood distribution were measured by segmental impedance cardiography for four body segments: the upper torso (thoracic cavity), lower torso (abdominal and pelvic region), thigh and calf. Seventeen healthy subjects (mean age: 27.3 years) participated in the experiment. They were positioned on the side (right-ear-down head position) on a tilt table which the subjects and the experimenter could tilt via remote control around an axis parallel to the subjects' visual (X-) axis. The experimenter set the initial tilt in total darkness to arbitrary angles, while strictly alternating between head-up and head-down tilts. Subjects were then asked to rotate the board until they felt they were in a horizontal posture. Means and variances of eight pairs of settings were taken as a measure of the subjective horizontal posture (SHP). During LBNP (−30 mmHg), subjects perceived being tilted head-up, whereas LBPP (+30 mmHg) led them to feel tilted head-down. The results corroborate the hypothesis of an effect of the blood's mass on graviception and also indicate supplementary contributions of other visceral afferences. © 1997 Elsevier Science B.V.

Introduction

Perception of orientation in three-dimensional space is one of the most important prerequisites for accurately performed motor behavior and efficient interaction with the environment. Even slight disturbances in locomotor equilibrium and perception of body posture may lead to serious clinical symptoms, such as vertigo and motion sickness. The present experiment was designed to provide further knowledge on the sensory-integrative processes involved in orientation perception in three-dimensional space, particularly on man's perception of body posture. It is widely acknowledged that vestibular and visual information plays the major role in controlling locomotor equilibrium and posture. But extravestibular mechanoreceptors may also contribute to it. This is suggested by observations in patients who lost all vestibular afferents (bilaterally neuromectomized subjects). These patients are able to keep an upright posture virtually as well as normal subjects, even in the absence of visual information, but fall when information on ankle position or sole pressure is missing or disturbed experimentally (Horak et al., 1992). Traditionally this information has been thought to stem from proprioceptors in joints, tendons, muscles and skin, that is, from sources distributed all over the body. However, psychophysical experiments with normal and labyrinthine defective subjects had shown that the perception of posture is to a great extent controlled by hitherto unknown graviceptors in the human trunk rather than by proprioceptive afferents (Mittelstaedt and Fricke, 1988). Subsequent experiments yielded information on the location of the relevant receptors and the pathways that convey the afferent signals from the visceral organs to the brain. Evidence from studies with paraplegic and nephrectomized subjects suggested the involvement of the kidneys and the vascular system (Mittelstaedt, 1992; Mittelstaedt, 1996).

The present study was designed to further investigate the effects of the cardiovascular system on the subjects' perception of their posture. This empirical question can be answered if the following conditions are fulfilled: first, the vestibular and visual information must be eliminated, easily accomplished by specific body position and complete darkness in the experimental room; second, changes in blood distribution have to be produced by means other than changes in body position, because changes in posture obviously stimulate the vestibular organ and are fed back by neck flexion; third, the amount of shifting blood volume during supine position should at least equal the amount of blood shifting caudad (i.e. venous pooling) or craniad (i.e. fluid shift towards the thoracic cavity) during upright or head-down posture. To fulfil the second and third condition, a special technique is required which allows for moving blood volume systematically within physiological boundaries. In aviation- and space-medicine, as well as in studies on orthostatic tolerance, a widely used method exists, called Lower Body Negative Pressure (LBNP). It consists of a pressure chamber in which negative pressure is exerted upon the subject's lower body (from the pelvic girdle downwards) resulting in venous pooling of blood in the legs, as usually occurs during orthostasis. By this device it is also possible to exert positive pressure upon subject's lower body which then results in the opposite effects of LBNP, that is, prevention of venous pooling and increase of blood within the thoracic cavity (i.e. increase of central volume). This, however, requires technical modification of the pressure chamber, to be described below. In experiments on the effect of LBNP on the circulation, Musgrave et al. (1971)had incidentally experienced sensations of tilt, particularly during changes of pressure, yet failed to measure the phenomenon. These methods should enable us to determine the impact of blood volume shifts upon the subjective horizontal position quantitatively. The relations between these two variables may reveal how the cardiovascular system affects gravity perception.

Section snippets

Subjects

Seventeen volunteers (14 males) participated in the study. Their mean age was 27.3 years, ranging from 23 to 54 years. A medical examination (by F.B.) prior to the study ensured that the subjects were in good health. None of them was on prescribed medication. Subjects with cardiovascular disorders (hypertension, cardiac arrhythmia, orthostatic intolerance) were excluded. After explanation of the goal of the study and the procedures used, subjects signed an informed consent form. The subjects

Results

First, changes in blood volume produced by LBPP and LBNP in comparison to the two baseline phases are reported (a), followed by presentation of changes in SHP induced by volume shifts (b).

(a) Inspection of the impedance measures revealed that systematic changes in volume shift occurred only in the upper torso (thoracic cavity), whereas corresponding changes in the other bodily segments were difficult to identify due either to their small size or enhanced variance. Therefore the results reported

Discussion

The present study should be seen within the framework of recent results on somatic graviception in normal, neuromectomized, paraplegic and nephrectomized subjects (Mittelstaedt, 1992, Mittelstaedt, 1996). Briefly: evaluation of the residual faculties of these patients led to the conclusion that somatic graviception be mediated by two distinctly localized inputs, the first entering the spinal cord at the 11th thoracic segment, the second reaching the brain cranial of the 6th cervical segment,

Acknowledgements

This work was supported by a grant from the Deutsche Forschungsgemeinschaft (German Research Society) to the first and second author (# Va. 37/19-1). Thanks are due to the members of the group at the Max-Planck Insitute in Seewiesen, especially to Evi Fricke and Willy Jensen and to the LBPP/LBNP Developing Team at the Center for Psychobiology and Behavioral Medicine at the University of Giessen (director: D. Vaitl), especially to Walter Kirchner, Karlheinz Ruhl, Dr. Rudolf Stark, Hartmut

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