LBNP: past protocols and technical considerations for experimental design.

INTRODUCTION Lower body negative pressure (LBNP) has been used for decades to simulate orthostatic stress and the effects of blood loss in humans. Since the definitive review of LBNP in 1974, new applications have been developed and research has revealed conflicting cardiovascular and neurohormonal responses during and after LBNP. METHODS A search of the literature was conducted for 1964-2007 using the Web of Science and the search terms "cardiovascular system," "orthostasis," "spaceflight," and "methodologies" to identify publications in English that describe human studies where LBNP was used to simulate orthostasis. Publications cited in the earlier review were excluded, leaving a total of 215 articles for consideration. RESULTS We divided the reported protocols into eight categories based on the pressure, pattern, and duration of the stimulus: 1) mild, constant, short; 2) mild, constant, long; 3) mild, ramp, short; 4) mild, ramp, long; 5) moderate-to-strong, constant, short; 6) moderate, constant, long; 7) moderate-to-strong, ramp, short; and 8) strong, ramp, long. The review showed that these protocols stimulate different reflexes and can be used to produce particular responses. DISCUSSION Based on the review, we developed guidelines for using LBNP in a predictable and reproducible manner. Variables that must be controlled include subject characteristics, procedures, and environmental conditions as well as specifications for the LBNP chamber and seal positioning. An understanding of the many technical details of such experiments and the nature of elicited cardiovascular and neurohormonal responses is required to design optimal protocols to address specific research questions.