Approach to respiratory alkalosis: Clinical sciences

Respiratory alkalosis refers to a decrease in the partial pressure of carbon dioxide or pCO2, resulting in a decrease in the concentration of hydrogen ions in the blood. This is almost always caused by hyperventilation, which increases carbon dioxide removal relative to carbon dioxide production.
Hyperventilation typically occurs in response to triggers such as hypoxia, infection, metabolic acidosis, pain, anxiety, overdose of certain medications, or increased metabolic demand. Respiratory alkalosis is characterized by a pH above 7.45, and a pCO2 below 35 millimeters of mercury. As a reference, the normal pCO2 range is between 35 and 45 millimeters of mercury.

If a patient presents with a chief concern suggesting respiratory alkalosis, perform an ABCDE assessment to determine if your patient is stable or unstable.

If they are unstable, stabilize the airway, breathing, and circulation. Next, obtain IV access and put your patient on continuous vital sign monitoring. Lastly, provide supplemental oxygen, if needed.

Let’s jump back to the ABCDE assessment and talk about stable patients. Your next step here is to obtain a focused history and physical examination and order labs, including an arterial blood gas analysis, or ABG, and BMP.

The history will vary depending on the specific cause, but your patient may report shortness of breath, while the physical exam might show an increased rate and depth of breathing.

ABG typically shows an arterial pH above 7.45, and a pCO2 lower than 35 millimeters of mercury. Finally, BMP will show normal or decreased serum bicarbonate, depending on whether or not there is metabolic compensation; and might show electrolyte imbalances, such as hypokalemia. With these findings, you can diagnose respiratory alkalosis.

Here are some clinical pearls! Once you’ve diagnosed respiratory alkalosis, remember to assess for metabolic compensation by checking the serum bicarbonate level. A compensated respiratory alkalosis is characterized by a normal or slightly increased arterial pH, decreased pCO2, and decreased serum bicarbonate level. This can be seen in chronic conditions in which the kidneys have had time to restore the acid-base balance, by excreting excess bicarbonate. Examples include pregnancy, hyperthyroidism, and chronic liver disease. On the other hand, uncompensated respiratory alkalosis will have an increased arterial pH, a decreased pCO2, and a normal serum bicarbonate level.

A simple way to know if there is metabolic compensation in respiratory alkalosis and acidosis is to use the 1-2-3-4-5 rule. When it comes to acidosis, for every 10 millimeters of mercury rise of pCO2 from the baseline, bicarbonates should increase by 1 in the acute, or by 4 in the chronic respiratory acidosis from their baseline.
On the flip side, in alkalosis, bicarbonate or HCO3 should decrease by 2 for the acute, or 5 for the chronic respiratory alkalosis from the baseline of 24 mmol/L.

Alright, let’s go over causes of respiratory alkalosis, starting with iatrogenic ones. This is often seen in hospitalized patients on respiratory support, like mechanical ventilation. In this case, you should check the ventilator settings. If the respiratory rate or the tidal volume is too high, your patient might be removing excessive amounts of carbon dioxide, resulting in a low pCO2, and eventually alkalosis. At this point, you can diagnose iatrogenic hyperventilation.

Next, let’s take a look at the CNS-related causes. These are conditions that mainly lead to stimulation of the respiratory centers in the brainstem, leading to hyperventilation, then a low pCO2, and ultimately, respiratory alkalosis.

First up is psychogenic hyperventilation. These patients typically present with a sudden onset of shortness of breath and fear, often associated with an anxiety disorder. The physical exam reveals an increased rate and depth of breathing but with no other signs of acute illness. If you see these findings, that’s psychogenic hyperventilation.

Next up are CNS infections. In this case, history reveals fever, headache, photophobia, and with or without neck stiffness, while the physical exam shows altered mental status and possibly nuchal rigidity. If you see this, consider a CNS infection. This could be like meningitis or encephalitis. Next, perform a lumbar puncture to collect cerebrospinal fluid and send it for analysis. If it shows any abnormalities, then the patient's respiratory alkalosis is due to a CNS infection.