Carotid sinus pressure changes during push-pull maneuvers

The push-pull maneuver (PPM) is defined as a reduction in G-tolerance when positive acceleration (+Gz) immediately follows negative acceleration (-Gz) exposure, with the carotid baroreceptors presumably playing a dominant role in the ensuing BP (SBP) responses. The objective of this study was to determine whether application of neck pressure (NP) during the preceding -Gz phase maintains +Gz tolerance during subsequent +Gz.

There were 10 experienced men who were exposed to 3 centrifuge run types using a multi-axis centrifuge: a relaxed control run from +1.4 baseline to visual tolerance; a relaxed control PPM run (PPM-C) consisting of 5 s of -1 Gz followed by 15 s of +Gz to visual tolerance; and an experimental PPM run performed with pressurized neck (PPM-NP) consisting of -1 Gz for 5 s followed by 15 s of +Gz at the previous PPM-C G-tolerance level.

Relaxed control G tolerance (3.6 _ 0.26 Gz) was greater vs. the PPM-C (3.0 +/- 0.21 Gz) and PPM-NP (3.1 +/- 0.20 Gz) conditions, but the two PPM conditions did not differ significantly. During -Gz, mean R-R interval for PPM-NP was significantly shorter than in the PPM-C from second 1 to second 3. During the +Gz phase, however, R-R interval responses between PPM-C and PPM-NP differed only at seconds 8 and 9. There were no differences in carotid sinus SBP between PPM-C and PPM-NP during -Gr. During +Gz, carotid sinus SBP was significantly depressed in PPM-NP and PPM-C conditions vs. Control.

Application of NP during the -Gz phase, despite altering R-R interval, did not ameliorate SBP responses or reductions in G tolerance during subsequent +Gz exposure. Despite neck compression counteracting increased carotid hydrostatic pressure during -Gz, the carotid baroreceptor response is likely opposed by the aortic or other baroreceptors.