Urine acid-base compensation at simulated moderate altitude

Area of Science:

• Physiology
• Environmental Medicine
• Renal Physiology

Background:

• Acute exposure to high altitude leads to respiratory alkalosis.
• The kidneys play a role in correcting this acid-base imbalance through renal compensation.
• Understanding the time course and extent of renal compensation is crucial for acclimatization.

Purpose of the Study:

• To investigate the time course and magnitude of renal compensation during short-term exposure to moderate simulated altitudes.
• To analyze changes in urine gas tensions and acid-base status in response to varying altitude levels.

Main Methods:

• 48 healthy individuals were exposed to simulated altitudes of 1780, 2085, 2455, and 2800 m for 24 hours each.
• Urine samples were collected anaerobically at sea level and after 6 and 24 hours of altitude exposure.
• Measurements included urine pH, bicarbonate (HCO3-), and partial pressure of carbon dioxide (PCO2).

Main Results:

• Urine pH and bicarbonate levels significantly increased after 6 hours at all altitudes, with pH returning to baseline at lower altitudes but remaining elevated at 2800 m.
• Urine bicarbonate levels returned to near baseline after 24 hours at altitudes up to 2455 m but remained elevated at 2800 m.
• Urine PCO2 increased after 6 hours and normalized by 24 hours across all altitudes.

Conclusions:

• Short-term moderate altitude exposure induces a significant bicarbonate diuresis, potentially due to inhibited renal tubular hydrogen ion secretion.
• Renal bicarbonate compensation is completed within 24 hours at low to moderate altitudes but remains incomplete at higher altitudes (2800 m).