Increase of LBNP tolerance using elastic compression stockings.

BACKGROUND: Astronauts returning from spaceflight are affected by reduced orthostatic tolerance resulting from exposure to weightlessness. There are some countermeasures currently in use to improve cardiovascular performance of returning astronauts, while there are others that are being tested in flight and in ground-based investigations. This paper presents a study on the use of elastic compression stockings to reduce leg blood capacity (LBC) which is believed to be one of the determinants of orthostatic tolerance. METHODS: The data are from 6 healthy men with a mean age of 36 +/- 5 (SE) yr. Assessment of the effectiveness of stockings in improving orthostatic tolerance is based on a presyncopal-limited lower body negative pressure (LBNP) test, consisting of successive 3 min exposures to negative pressures of -20 hPa (-15 mmHg), -40 hPa (-30 mmHg), and decrements in steps of 10 hPa (7.5 mmHg) from then on until termination of the test. RESULTS: Results show an increase in the maximal level of LBNP tolerated (88 hPa or 66 mmHg for control vs. 108 hPa or 81 mmHg for stockings; p = 0.018) as well as in the cumulative stress index (CSI) (1122 hPa-min or 842 mmHg-min for control vs. 1734 hPa-min or 1300 mmHg-min for stockings; p = 0.029). CONCLUSIONS: The improvement of LBNP tolerance with elastic compression stockings coupled with their ease of use support the need for further experimental studies for evaluating their potential as a countermeasure for astronauts after return from spaceflight.

Simulation of cardiovascular response to lower body negative pressure from 0 to -40 mmHg.

This paper presents a mathematical model for simulation of the human cardiovascular response to lower body negative pressure (LBNP) up to -40 mmHg both under normal conditions and when arterial baroreflex sensitivity or leg blood capacity (LBC) is altered. Development of the model assumes that the LBNP response could be explained solely on the bases of 1) blood volume redistribution, 2) left ventricular end-diastolic filling, 3) interaction between left ventricle and peripheral circulation, and 4) modulations of peripheral resistances and heart rate by arterial and cardiopulmonary baroreflexes. The model reproduced well experimental data obtained both under normal conditions and during complete autonomic blockade; thus it is validated for simulation of the cardiovascular response from 0 to -40 mmHg LBNP. We tested the ability of the model to simulate the changes in LBNP response due to a reduction in LBC. To assess these changes experimentally, six healthy men were subjected to LBNP of -15, -30, and -38 mmHg with and without wearing elastic compression stockings. Stockings significantly reduced LBC (from 3.9 +/- 0.3 to 1.8 +/- 0.4 ml/100 ml tissue at -38 mmHg LBNP; P < 0.01) and attenuated the change in heart rate (from 23 +/- 4 to 8 +/- 3% at -38 mmHg LBNP; P < 0.05). The model accurately reproduced this result. The model is useful for assessing the influence of LBC or other parameters such as arterial baroreflex sensitivity in diminishing the orthostatic tolerance of humans after spaceflight, bed rest, or endurance training.

Orthostatic intolerance during a 13-day bed rest does not result from increased leg compliance.

Increased leg compliance (LC) has been proposed as a mechanism for orthostatic intolerance after spaceflight or bed rest. Using venous occlusion plethysmography with mercury-in-Silastic strain gauge, we evaluated LC before, during, and after a 13-day head-down (-6 degrees) bed rest in 10 men. LC was measured by the relationship between the increased calf areas (in cm2) at thigh cuff occlusions of 20, 30, 50, 70, and 80 mmHg. Orthostatic tolerance was evaluated by a presyncopal-limited lower body negative pressure test (PSL-LBNP) before and after bed rest. The 10 subjects were divided into TOL (n = 5) and INT (n = 5) groups for which the orthostatic tolerance was similar and lower after bed rest, respectively. For TOL (INT) before bed rest, calf area increases were 2.2 +/- 0.5 (SE) (1.3 +/- 0.4), 3.5 +/- 0.7 (2.3 +/- 0.5), 5.0 +/- 0.9 (3.5 +/- 0.6), 5.6 +/- 0.9 (4.4 +/- 0.6), and 6.4 +/- 1.1 (4.7 +/- 0.6) cm2 for thigh occlusion pressures of 20, 30, 50, 70, and 80 mmHg, respectively. Neither for INT nor for TOL were these results significantly changed by bed rest. These results suggest that other mechanisms than increased LC have to be taken into account to explain the decreased orthostatic tolerance induced by this 13-day bed rest.

A mathematical model of the cardiovascular response to +Gz acceleration.

The cardiovascular system is the limiting factor for human tolerance to positive Gz (head-to-foot) acceleration induced during maneuvers of fighter aircrafts. Safe handling of modern fighter aircrafts with higher acceleration capabilities require the use of countermeasures such as a G-suit or breathing at a positive pressure. A better understanding of the mechanisms involved in the cardiovascular response to +Gz acceleration would help improve the design and application of protective measures. This paper presents a simple mathematical model of the cardiovascular system which incorporates arterial and cardiopulmonary baroreflexes, left ventricular-peripheral circulation interaction and decreased venous return. This model is capable of reproducing observed overall cardiovascular response to +Gz acceleration.

Mathematical modeling of human cardiovascular system for simulation of orthostatic response.

This paper deals with the short-term response of the human cardiovascular system to orthostatic stresses in the context of developing a mathematical model of the overall system. It discusses the physiological issues involved and how these issues have been handled in published cardiovascular models for simulation of orthostatic response. Most of the models are stimulus specific with no demonstrated capability for simulating the responses to orthostatic stimuli of different types. A comprehensive model incorporating all known phenomena related to cardiovascular regulation would greatly help to interpret the various orthostatic responses of the system in a consistent manner and to understand the interactions among its elements. This paper provides a framework for future efforts in mathematical modeling of the entire cardiovascular system.