Human cardiovascular adjustments to acute hypoxaemia.

Area of Science:

• Cardiovascular Physiology
• Respiratory Physiology

Background:

• Cardiovascular adjustments to hypoxemia traditionally involve competing local vasodilation and reflex vasoconstriction.
• Breathing during hypoxemia activates lung mechanoreceptors, potentially altering sympathetic vasomotor activity (SNA).

Purpose of the Study:

• To investigate the effects of moderate to severe hypoxemia on sympathetic vasomotor activity (SNA) and cardiovascular responses at rest and during exercise.
• To determine the mechanisms underlying blood pressure regulation during hypoxemia.

Main Methods:

• Measurements of cardiovascular parameters, plasma norepinephrine (NA) concentration, and sympathetic vasomotor activity (SNA) in humans under normoxic and hypoxemic conditions.
• Infusion of noradrenaline to assess vascular responsiveness during hypoxemia.

Main Results:

• At rest, moderate to severe hypoxemia did not cause splanchnic, cutaneous, or muscle vasoconstriction, suggesting central inhibition of SNA.
• During exercise, moderate hypoxemia markedly increased plasma NA concentration and SNA, but splanchnic vasoconstriction was not observed.
• Blood pressure during maximal oxygen uptake in hypoxemia was maintained by baroreflex-mediated vasoconstriction in active muscles, overriding chemoreflexes.

Conclusions:

• Hypoxemia centrally inhibits SNA at rest beyond a certain threshold.
• During exercise, increased SNA in hypoxemia targets active muscles for blood pressure maintenance via baroreflexes.
• Active muscle vasoconstriction is the primary mechanism for blood pressure regulation during exercise in hypoxemia.