O2 extraction maintains O2 uptake during submaximal exercise with beta-adrenergic blockade at 4,300 m

Area of Science:

• Exercise Physiology
• Altitude Physiology
• Cardiovascular Physiology

Background:

• Beta-adrenergic blockade preserves whole-body oxygen uptake (VO2) during maximal and submaximal exercise at high altitude despite reduced heart rate.
• Increased stroke volume is hypothesized to maintain systemic oxygen delivery and VO2 at sea-level values.

Purpose of the Study:

• To investigate the effects of nonselective beta-adrenergic blockade on submaximal exercise performance at sea level and high altitude (4,300 m).
• To test the hypothesis that increased stroke volume compensates for reduced heart rate to maintain oxygen delivery.

Main Methods:

• 11 healthy men studied at sea level and 4,300 m.
• Submaximal exercise testing with and without propranolol (beta-blocker) or placebo.
• Measurements included VO2, cardiac output, stroke volume, and arterial/mixed venous blood gases.

Main Results:

• At sea level, propranolol reduced cardiac output and oxygen delivery, with VO2 maintained by increased stroke volume and O2 extraction.
• At 4,300 m, VO2, ventilation, and blood gases were unaffected by beta-blockade.
• Despite increased stroke volume, oxygen delivery remained reduced with propranolol at altitude, with VO2 maintained by further reductions in mixed venous O2 saturation.

Conclusions:

• Beta-adrenergic blockade at high altitude reduces oxygen delivery during submaximal exercise due to incomplete stroke volume compensation for decreased heart rate.
• VO2 is maintained by complex interactions involving mixed venous O2 saturation, arterial O2-carrying capacity, and hemodynamics under hypoxic conditions.
• Propranolol-treated subjects reported higher perceived exertion at altitude despite similar workloads and VO2.