In cases of respiratory alkalosis, what blood gas value is likely to be low?

In respiratory alkalosis, the primary physiological change involves a decrease in carbon dioxide (CO2) levels in the blood, which is reflected as a low partial pressure of carbon dioxide (PaCO2). This condition often occurs when a patient hyperventilates, leading to the excessive exhalation of CO2. As the body loses CO2, the pH of the blood rises, resulting in alkalosis.

The other values mentioned in the question behave differently in this condition. The pH will be elevated, reflecting the alkalotic state. Bicarbonate (HCO3-) levels may be normal or slightly altered, typically decreasing over time as the kidneys attempt to compensate for the alkalosis, but this is not a primary change associated with acute respiratory alkalosis. The partial pressure of oxygen (PaO2) may not be significantly affected directly by the respiratory alkalosis itself and can vary based on oxygenation status.

Therefore, in cases of respiratory alkalosis, the hallmark finding is a low PaCO2, which indicates the underlying mechanism of hyperventilation causing excessive CO2 loss.