Normothermic central hypovolemia tolerance reflects hyperthermic tolerance

Area of Science:

• Physiology
• Cardiovascular Research
• Environmental Stressors

Background:

• Lower body negative pressure (LBNP) is a model for hypovolemia and cardiovascular stress.
• Hyperthermia can exacerbate physiological responses to stress.
• Understanding tolerance to LBNP under different thermal conditions is crucial for predicting physiological responses.

Purpose of the Study:

• To investigate the relationship between tolerance to LBNP under normothermic and hyperthermic conditions.
• To test if high LBNP tolerance at normal body temperature predicts high tolerance during elevated body temperature.

Main Methods:

• Sixty participants underwent paired normothermic and hyperthermic LBNP tests to pre-syncope.
• LBNP tolerance was quantified using the cumulative stress index (CSI).
• Physiological variables including temperature, heart rate, and arterial pressure were monitored.

Main Results:

• Hyperthermia significantly reduced CSI (P < 0.001), indicating decreased LBNP tolerance.
• A significant positive correlation was found between normothermic and hyperthermic LBNP tolerance (R² = 0.38, P < 0.001).
• Individuals with high normothermic LBNP tolerance maintained significantly higher tolerance during hyperthermia compared to those with low normothermic tolerance.

Conclusions:

• LBNP tolerance during hyperthermia is positively related to tolerance during normothermia.
• Individuals who tolerate LBNP well at normal body temperatures remain relatively tolerant even when hyperthermic.