G Tolerance and the Vasoconstrictor Reserve

Purpose: Because leg arterial stiffness is higher in subjects with high G tolerance, we hypothesized that subjects with high G tolerance would have larger capacity for vasoconstriction.

Methods: Sixteen subjects, eight with high and eight with low G tolerance (H and L group, respectively), were exposed to a cold pressor test (CPT) in supine and upright posture. Heart rate (HR), mean arterial pressure (MAP) and cardiac output (CO) were measured, and total peripheral resistance (TPR) and stroke volume (SV) were calculated.

Results: In the supine position, CPT increased TPR more in the H group; 31 ± 18% than in the L group; 11 ± 7% (p < 0.05). The L group had larger increases in CO than the H group; 17 ± 16 vs. 3.4 ± 7% (p = 0.06). In the upright position, the H group had a larger MAP response to CPT than the L group; 26 ± 14 vs. 14 ± 7% (p = 0.06). The H group, but not the L group, had significant increases in TPR whereas the L group had significant increases in CO and SV.

Conclusions: In response to CPT, the high G tolerance group elevated MAP by increasing TPR, whereas the low G tolerance group showed a dependency on increased CO. The H group seemed to have a larger vasoconstrictor reserve. The results further suggest that vasoconstrictor reserve capacity could constitute the link between the recent finding that indicates a relationship between G tolerance and arterial distensibility in the legs.