Respiratory Alkalosis

Overview
  • Respiratory Alkalosis is a pathophysiological category of alkalosis and refers to those caused by primary disturbances of ventilation. Although ventilatory defects can cause significant increases in the Blood pH, renal compensatory mechanisms can largely correct the pH over several days.
Pathophysiology
  • The fundamental cause of respiratory alkalosis is excessive Alveolar Ventilation, resulting in a decrease in the partial pressure of arterial carbon dioxide (PaCO2). Decreased PaCO2 results in an misalignment of the Henderson-Hasselbalch Equation for the bicarbonate buffer which largely determines the pH of the extracellular fluid. Mathematically, the increased ECF pH results from a decrease in the ratio between PaCO2 relative to the ECF concentration of bicarbonate ([HCO3-]). More colloquially, excessive alveolar ventilation results in the "Breathing Off" of gaseous CO2 which represents the weak acid form of the bicarbonate buffer. The deficiency of acid results in an increase in the total extracellular fluid pH.
Compensation
  • Respiratory alkaloses can be compensated by the actions of the kidneys which serve to realign the bicarbonate buffer Henderson-Hasselbalch Equation over a period of several days. As described in Renal Response to Acid-Base Imbalance, the kidneys respond to alkalosis by excreting bicarbonate, thus reducing the ECF bicarbonate concentration. The decreased bicarbonate concentration realigns the Henderson-Hasselbalch Equation for the bicarbonate buffer and thus largely corrects the ECF pH. Consequently, a renally-compensated respiratory alkalosis is characterized by decreased levels of PaCO2 (caused by the primary ventilatory disturbance) as well as decreased levels of ECF bicarbonate (caused by the renal compensation). However, it is important to point out that renal compensation cannot completely correct the ECF pH and thus the ECF will still remain slightly alkalotic even after compensation.
Etiologies
  • Respiratory alkaloses are relatively rare entities and are caused by hyperventilation. Hyperventilation may be voluntary, caused by poorly adjusted ventilators, or may occur in High Altitude as a response to the low levels of inspired oxygen.
Diagnosis
  • An uncompensated respiratory alkalosis is characterized by a blood pH far above 7.45, decreased PaCO2, and a largely normal blood bicarbonate. A renally-compensated respiratory acidosis is characterized by a blood pH only slightly above 7.45, decreased PaCO2, and a decreased blood bicarbonate concentration.