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Effect of changing the gravity vector on respiratory output and control

¹TBM Lab, Dipartimento di Bioingegneria, Politecnico di Milano, I-20133 Milan; and ²Dipartimento di Medicina Sperimentale, Ambientale e Biotecnologie Mediche, Università di Milano-Bicocca, I-20052 Monza, Italy; and ³Médecine Aerospatiale, Université de Bordeaux, F-33076 Bordeaux, France

Submitted 11 August 2003 ; accepted in final form 13 May 2004

We studied the respiratory output in five subjects exposed to parabolic flights [gravity vector 1, 1.8 and 0 gravity vector in the craniocaudal direction (G_z)] and when switching from sitting to supine (legs bent at the knees). Despite differences in total respiratory compliance (highest at 0 G_z and in supine and minimum at 1.8 G_z), no significant changes in elastic inspiratory work were observed in the various conditions, except when comparing 1.8 G_z with 1 G_z (subjects were in the seated position in all circumstances), although the elastic work had an inverse relationship with total respiratory compliance that was highest at 0 G_z and in supine posture and minimum at 1.8 G_z. Relative to 1 G_z, lung resistance (airways plus lung tissue) increased significantly by 52% in the supine but slightly decreased at 0 G_z. We calculated, for each condition, the tidal volume changes based on the energy available in the preceding phase and concluded that an increase in inspiratory muscle output occurs when respiratory load increases (e.g., going from 0 to 1.8 G_z), whereas a decrease occurs in the opposite case (e.g., from 1.8 to 0 G_z). Despite these immediate changes, ventilation increased, going to 1.8 and 0 G_z (up to 23%), reflecting an increase in mean inspiratory flow rate, tidal volume, and respiratory frequency, while ventilation decreased (approximately –14%), shifting to supine posture (transition time 15 s). These data suggest a remarkable feature in the mechanical arrangement of the respiratory system such that it can maintain the ventilatory output with small changes in inspiratory muscle work in face of considerable changes in configuration and mechanical properties.

microgravity; respiratory mechanics; respiratory control; pulmonary ventilation