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G tolerance and the vasoconstrictor reserve.

Patrik Sundblad1, Roger Kölegård, Ola Eiken

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Individuals with high G tolerance exhibit greater vasoconstriction capacity, unlike those with low G tolerance who rely on increased cardiac output. This suggests a link between vasoconstrictor reserve and G tolerance.

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Area of Science:

  • Cardiovascular Physiology
  • Aerospace Medicine

Background:

  • Leg arterial stiffness is elevated in individuals with high G tolerance.
  • Understanding the physiological mechanisms underlying G tolerance is crucial for aerospace safety and performance.

Purpose of the Study:

  • To investigate the hypothesis that individuals with high G tolerance possess a greater capacity for vasoconstriction compared to those with low G tolerance.
  • To explore the relationship between G tolerance, arterial stiffness, and cardiovascular responses during a cold pressor test (CPT).

Main Methods:

  • Sixteen subjects, divided into high (H) and low (L) G tolerance groups, underwent a cold pressor test (CPT) in supine and upright postures.
  • Measurements included heart rate (HR), mean arterial pressure (MAP), and cardiac output (CO), with calculations for total peripheral resistance (TPR) and stroke volume (SV).

Main Results:

  • In the supine position, the H group showed a significantly greater increase in TPR during CPT compared to the L group (31% vs. 11%).
  • In the upright position, the H group exhibited a larger MAP response to CPT, while the H group increased TPR and the L group increased CO and SV.

Conclusions:

  • High G tolerance is associated with a greater vasoconstrictor reserve, primarily through increased total peripheral resistance.
  • Low G tolerance appears to be linked to a greater reliance on cardiac output adjustments for maintaining blood pressure.
  • Vasoconstrictor reserve capacity may be a key factor connecting G tolerance with arterial distensibility in the legs.