High Altitude Diagram

When you’re young and you’re striving, it’s all uphill, and it’s easier to climb. Then, when you get and look around, you sort of say, ‘Wow, the altitude’s kinda thin up here!’

Ron Howard.

High Altitude Diagram

Enter Your Altitude & Altitude units
(Max. 5000 meters or 16500 feet).
Then select PCO2 units and Submit.

Below 1500 meters or 4,920 ft
diagram defaults to Sea-Level.

High Altitude Diagram. Enter the Altitude and PCO2 Units. For the selected High Altitude the Interactive Diagram provides the blood’s acidity (pH), the respiratory component (PCO2), metabolic component (Base Excess) and a text description. It is based on the Sea Level Diagram but instead of a Standard Base Excess (SBE), it provides an Altitude Base Excess (ABE).

Hypoxemic Threshold: Specific receptors in the Carotid Body detect low PaO2 levels (hypoxemia) to stimulate ventilation and lower the PCO2. The threshold varies with individuals but above 1500 meters (4920 feet)1, people are normally Altitude-Adapted with a low PCO2. This is the threshold altitude chosen for this high altitude diagram.

Diagram Explanation:

Altitude Base Excess is a useful concept corresponding to Sea-Level Standard Base Excess. It measures the metabolic component for that altitude. Zero means normality with no indication for urgent metabolic correction.

A pH of 7.4 with a PCO2 of 27 mmHg (3.6 kPa) is normally reported as respiratory alkalosis fully compensated by metabolic acidosis (Standard Base Excess of -7 mMol/L). However, for an Altitude-Adapted person living at 4,500 meters (14,760 ft) the same values are normal.

Observe both diagrams. The abnormality seen at Sea Level is normal on the High Altitude version. The normal values provide the correct target for therapy. The relationship to all of the abnormal zones is now preserved which helps when recognizing compensation or additional abnormalities.

A pH of 7.4 with a PCO2 of 27 mmHg (3.6 kPa)


Adaptation: In 1925 Barcroft claimed that around 4500 meters (or 14,800 ft): “All dwellers at high altitude are persons of impaired physical and mental powers”.2 However, this claim was disputed and more than 200 million people live at altitudes above 2,500 meters.3 Indeed, at moderately high altitude people are normally Altitude-Adapted and healthy with a decreased risk of postoperative pneumonia4 and a prolonged life-expectancy.5,6

Normal Values: The pH remains normal (pH = 7.4) because Metabolic acidosis balances the low PCO2 (Respiratory alkalosis).  Both are appropriate for the altitude and neither of them requires treatment, i.e., Altitude Base Excess is normal (0 mMol/L).7 Because the exact values depend on altitude, the appropriate altitude diagram is essential when assessing a patient, planning the treatment, and observing progress. The original Sea-Level Zones8  have been relocated to suit the altitude.“Base Excess” is an appropriate part of the name because it is familiar when evaluating metabolic progress and treatment.


I am deeply indebted to Iván Solarte MD, MHPE. Full Professor, School of Medicine, Pontificia Universidad Javeriana, Respiratory Medicine Unit, Hospital Universitario San Ignacio. Bogotá Colombia. He persuaded me to modify the original sea-level version. Neither of us appreciated the magnitude of this undertaking; it occupied me for several weeks. In addition to his stimulus, he provided diagrams and equations, as well as several helpful ideas and corrections. Our work is now published as Correspondence in Anesthesiology10.

The author is also extremely grateful to Peter Grogono, B.A., M.A., M.Comp.Sc., Ph.D. (Department of Computer Science and Software Engineering, Concordia University, Montreal, Quebec, Canada) for the introduction to animating diagrams using JavaScript to create the basic Sea Level Version of this diagram.


1 Zubieta-Calleja G,Jr, Paulev PE, Mehrishi JN, and Zubieta-Calleja G,Sr. Extremely high altitude hypoxic conditions during Mount Everest expeditions, residence at South Pole stations, in Tibet and among the Andes: Van Slyke equation modification is crucially important for acid–base measurements. Journal of Biological Physics and Chemistry 2012; 12:103-112.

2 Barcroft J. The Respiratory function of the blood. Part I. Lessons from high altitudes. 1925; Cambridge University Press, Cambridge, UK.

3 Zubieta-Calleja G, Zubieta-Calleja L, Ardaya-Zubieta G, and Paulev PE. Do Over 200 Million Healthy Altitude Residents Really Suffer from Chronic Acid–Base Disorders? Indian J Clin Biochem 2011; 26(1):62–65.

4 West JB. High Life. Association of Hospital Altitude and Postoperative Infectious Complications After Major Operations. Aasen DM, Wiedel C, Maroni P, Cohen ME, Meng X, and Meguid RA. High Altitude Medicine & Biology 2019; Dec:421-426. (http://doi.org/10.1089/ham.2019.0062)

5 Ezzati M, Horwitz MEM, Thomas DSK, Friedman AB, Roach R, Clark T, Murray CJL, Honigman B. Altitude, life expectancy and mortality from ischaemic heart disease, stroke, COPD and cancers: national population-based analysis of US counties. J Epidemiol Community Health 2012; 66:e17–e17.

6 Zubieta-Calleja GR, Zubieta-DeUrioste NA. Extended longevity at high altitude: Benefits of exposure to chronic hypoxia. BLDE University Journal of Health Sciences 2017; 2:2:80-90.

7 Acevado LE, Solarte I. Gasimetria arterial en adultos jovenes a nivel de Bogata. Acta Médica Colombiana 1984; 9:1-14.

8 Schlichtig R, Grogono AW, Severinghaus JW. “Human PaCO2 and standard base excess compensation for acid-base imbalance.” Critical Care Medicine 1998; 26:1173-1179.

9 Paulev PE, and Zubieta-Calleja GR. “Essentials in the Diagnosis of Acid-Base Disorders and their High Altitude Application”. J Physiol Pharmacol 2005; 56 Suppl 4:155-70.

10 Grogono AW, Solarte, I. “Correcting Acid Base Interpretation for High Altitudes”. https://doi.org/10.1097/ALN.0000000000003581