Correcting Acid Base Interpretation for High Altitudes

Acid–base results normally report arterial values: overall acidity (pH), Paco₂, and Standard Base Excess (SBE). Although numerical values are customary, an interactive diagram (http://www.acid-base.com/interactive.php, accessed September 25, 2020) helps recognition by displaying results over zones radiating out from a normal center based on a meta-analysis of relevant papers (figs. 1 and 2).¹  The numerical values and the text are appropriate for managing patients at or near sea level.²  However, as altitude increases, the ventilatory response to hypoxia induces chronic respiratory alkalosis, leading to a reduction in serum bicarbonate and thus progressively misleading results.

One author (I.S.) recognized the critical need for altitude-appropriate values, diagram, and text interpretations. He provided the equation relating altitude to acid–base diagrams, as well as the concept that in healthy altitude-adapted individuals, the metabolic component—Altitude Base Excess (ABE)—must be regarded as 0 mEq/l, i.e., requiring no treatment.^3,4  AG created the resulting interactive acid–base diagram (https://www.acid-base.com/altitude.php, accessed September 25, 2020). The local high altitude from 4,920 feet up to 16,500 feet is inserted to obtain a diagram for adapted residents.

The scale values for the Paco₂ on the high-altitude diagram (fig. 2) vary with the altitude. Moving the mouse to the patient’s pH and Paco₂ provides the ABE. The various zones all provide their customary interpretation just as they did at sea level but now altitude appropriate. This provides relevant text descriptions with a target for therapy, facilitates tracking a patient’s progress, and makes abnormalities recognizable.

These clinical examples are for patients living in Bogota, Colombia (altitude, 8,660 feet). They illustrate the critical value of the high-altitude diagram:

On the sea-level diagram (fig. 1), patient A appears to have normal acid base results (pH 7.4, Paco₂ level of 40 mmHg). However, he has chronic respiratory acidosis with metabolic alkalosis (compensation)—obvious on the high-altitude diagram (fig. 2).

On the sea-level diagram (fig. 1), patient B (pH 7.32, Paco₂ level of 40mmHg) apparently has pure metabolic acidosis, whereas he actually has pure respiratory acidosis due to drug overdose—obvious on the high-altitude diagram (fig. 2).

Patient C has chronic obstructive pulmonary disease, pulmonary hypertension, and diuretic treatment with mild hypokalemia. Her arterial blood gases (pH 7.48, Paco₂ level of 37 mmHg) on the sea level diagram appear to show a mixed alkalemia (metabolic and respiratory), clearly wrong for the clinical scenario (fig. 1) but obvious using the high-altitude diagram: metabolic alkalosis with respiratory compensation (fig. 2).

Even at this altitude (8,660 feet) conventional acid–base results provide significant errors. The interactive diagram described accepts any altitude up to 16,500 feet. As the altitude increases, so does the critical importance of obtaining acid–base values and text descriptions for that altitude.

The authors declare no competing interests.