Changes in Capacity of the Leg Veins Studied by a Simple Counter‐pressure Technique

Abstract

The effective transmural pressure in the leg veins may be reduced to zero by appropriate inflation of an anti-g suit applied to the legs only. The central displacement of venous blood consequent upon suit inflation causes consistent physiological changes, particularly an expiratory shift of the end-tidal position recorded by a spirometer. The end-tidal shift is proportional to the expected displacement of blood, and is modified by both the initial venous pressure (varying body posture) and the initial venous tone (varying room temperature and use of pressor amines). It may thus be used as a simple index of the pressure/ volume state of the leg veins before and after administration of pharmacological agents. Inflation of the anti-g suit also produces an increase of respiratory rate and minute volume, but tidal volume is unchanged. Vital capacity is reduced, both inspiratory and expiratory capacity being affected. Pulse rate and pulmonary compliance are also reduced. Each of these changes may be interpreted in terms of central displacement of blood.THE anti-g suit was designed to protect the body against excessive displacement of blood to the lower limbs caused by the sudden accelerations met in high-speed flying. The physiological action of the suit is straightforward. Inflation applies a pressure to the limb surface that counteracts the accelerational force, and the blood content of the capacity vessels thus remains unaltered. If the suit is inflated with the subject under normal resting conditions, the transmural pressure of the leg veins is reduced, and an amount of blood, predictable from the pressure/volume characteristics of the veins, is displaced from the lower limbs to the upper half of the body. Use of the suit for this purpose is a convenient technique in both experimental and clinical studies of cardio-respiratory physiology [Shephard, 1957; Bondurant, Hickam and Isley, 1957;Lewis, Forster and Beckman, 1958]. Some previous workers [Bondurant et al., 1957;Lewis et al., 1958] have applied the suit counterpressure to both abdomen and lower limbs, reporting a fall of lung compliance, a rise of palmonary and systemic arterial blood pressures and an increase of radiographic pulmonary vascular density as a result of inflation. However, interpretation of these findings is complicated by simultaneous displacement of blood and of abdominal viscera. A limited number of observations have also been reported on displacement of blood from the lower limbs only [Shephard, 1957], and attention has been drawn to a possible quantitative relationship between alterations in end-tidal spirometer reading and the volume of blood displaced.The present experiments were undertaken to explore further the nature and magnitude of the respiratory changes that follow inflation of an anti-g suit applied to the legs only, to see whether the respiratory parameters could 175

Simultaneous experimental acclimatization to heat and cold in man

“…Two subjects were studied. The resting tidal volume, respiratory rate, and minute volume were measured by standard spirometric techniques, and the method of anti-g suit inflation (Shephard, 1961) was used to assess the trend of changes in the blood content of the leg veins. The basis of this technique is that sudden compression is applied to the lower limbs, and the change of end-tidal volume caused by the displacement of blood from the limbs to the lungs is measured.…”