Fluctuations in O2 stores and gas exchange with passive changes in posture

Area of Science:

• Physiology
• Respiratory Physiology
• Cardiovascular Physiology

Background:

• Changes in posture can affect oxygen uptake and utilization.
• Understanding the dynamics of oxygen stores is crucial for respiratory and cardiovascular regulation.

Purpose of the Study:

• To investigate the role of lung and blood oxygen stores in oxygen uptake (VO2) fluctuations during postural changes.
• To quantify the changes in oxygen stores when moving from supine to upright and vice versa.

Main Methods:

• Breath-by-breath calculation of oxygen intake (Veo2) and pulmonary capillary oxygen transfer (Vpco2) using a spirometer and mass spectrometer.
• Computation of changes in lung oxygen stores (O2L) and blood oxygen stores (O2b) under the assumption of constant metabolic rate (Vco2).
• Measurements were taken during passive tilt to 60 degrees and upon return to recumbency.

Main Results:

• Transitioning from supine to upright posture led to a rapid increase in O2L and a slow decrease in O2b, resulting in a net oxygen deficit of 130 ml over 10 minutes.
• Upon returning to the supine position, O2L rapidly decreased and O2b increased, creating a net oxygen surplus of 117 ml.
• Changes in O2b were approximately 2.5 times larger than O2L changes and occurred in the opposite direction.
• Shifts in O2b were primarily attributed to changes in blood volume and flow, rather than cardiac output variations.
• The refilling of O2b upon returning to supine, matching the loss during upright posture, induced transient hypoxia and significant hyperpnea.

Conclusions:

• Oxygen stores in the lungs and blood play a significant role in the transient changes in oxygen uptake observed with postural shifts.
• Blood oxygen stores exhibit larger and opposite dynamics compared to lung oxygen stores during postural changes.
• The observed transient hypoxia and hyperpnea upon returning to supine are linked to the refilling of blood oxygen stores.