How much LBNP is equivalent to 1G, 2G and 3G? Does "equivalent" mean that the two activate the same cardiovascular response intensity or is there a defined objective way of converting one to the other?
Thank you!
Lower body parts from iliac crest (waist), when enclosed inside a metallic chamber and subjected to negative pressure, causes a pulling of blood to the dependent parts of the body i.e. in the lower limbs. It has been reported that -30 to -40 mm Hg of LBNP is equivalent to standing upright at +1Gz acceleratory force, which is nothing but standing on the earth. Upright posture pulls around 300-800 ml of blood to the lower limbs. However, the equivalence between +2Gz and +3Gz acceleratory forces with LBNP is difficult to establish and also not physiologically correct in the sense that G-force always involves the neurovestibular system, unlike LBNP where vestibular system involvement is virtually nil. Blood pulling to the lower limbs in LBNP causes deactivation of low pressure and high-pressure receptor resulting in sympathetic activation and parasympathetic augmentation. But, in G exposure, the dynamics of autonomic cardiovascular control of heart rate and blood pressure work differently with an involvement of neurovestibular system along with unloading followed by deactivation of baroreceptor resulting in sympathetic activation and parasympathetic enhancement. However, the intricate details of the physiological regulatory mechanisms in G exposure and LBNP exposure are very complex. For further details, you may follow any textbook on aerospace physiology/medicine.
Regds
Dr (Prof) Biswajit Sinha
Scientist
Bangalore
IAM