Tissue oxygen saturation during hyperthermic progressive central hypovolemia.

Area of Science:

• Physiology
• Cardiovascular Research
• Thermoregulation

Background:

• Near-infrared spectroscopy (NIRS)-derived tissue oxygen saturation (So2) reduction indicates central hypovolemia during normothermia.
• Hyperthermia can affect skin blood flow and tolerance to hypovolemia, potentially altering So2 interpretation.

Purpose of the Study:

• To test if maximal reductions in tissue So2 during central hypovolemia to presyncope are similar under normothermic and hyperthermic conditions.
• To investigate the impact of hyperthermia on the interpretation of NIRS-derived tissue So2 during induced hypovolemia.

Main Methods:

• Ten healthy males underwent progressive lower-body negative pressure (LBNP) to presyncope.
• Central hypovolemia was induced under both normothermic (skin temp ≈34°C) and hyperthermic (skin temp ≈39°C) conditions.
• NIRS-derived forearm tissue So2 was measured and analyzed as the change from pre-LBNP.

Main Results:

• Hyperthermia significantly reduced LBNP tolerance by 49%.
• Pre-LBNP tissue So2 levels were similar between normothermic and hyperthermic states.
• The maximal reduction in tissue So2 from pre-LBNP to presyncope was smaller during hyperthermia (-6 ± 5%) compared to normothermia (-10 ± 6%).

Conclusions:

• Contrary to the hypothesis, hyperthermia is associated with a smaller maximal reduction in tissue So2 during central hypovolemia to presyncope.
• These findings suggest that hyperthermia may alter the typical interpretation of NIRS-derived So2 as an indicator of central hypovolemia.