• SOM 4th Yr Curriculum

• CLERKSHIP INFORMATION
• Introduction / Goals and Objectives
• Rotations
  • HMC
  • UWMC
  • VAMC
• Seminars
• Faculty Only
• PULMONARY EDUCATION
• Pulmonary Med Library
• Arterial Blood Gases
• Cardiopulmonary Exercise Testing
• Mechanical Ventilation
• Pulmonary Function Testing
• Pleural Effusion
• Radiology

Arterial Blood Gas

Case 6 Answers

A climber is coming down from the summit of Mt. Everest. At an altitude of 8,400 m (P_B ~ 272 mmHg), he has a blood gas drawn while breathing ambient air as part of a research project. The blood gas reveals pH 7.55, PCO₂ 12, PO₂ 30 and HCO₃- 10.5

Acid-base status:

• The patient has a high pH (alkalemia)
• The PCO₂ is low (respiratory alkalosis) and the bicarbonate is low (metabolic acidosis). The high pH in conjunction with the low PCO₂ tells us that the respiratory alkalosis is the primary process.
• The metabolic acidosis is the compensatory process
• Summary: Primary respiratory alkalosis with metabolic compensation.

Alveolar-arterial oxygen difference:

The alveolar-arterial oxygen difference is only 2.3!  When you get to very low barometric pressures and low alveolar oxygen tensions like the values seen in this case, the alveolar-arterial oxygen difference can be very small. This is a normal value at this elevation. Hypoxemia with a normal alveolar-arterial oxygen difference is due to either hypoventilation or a low inspired PO₂ . In this particular case, it is the low inspired PO₂ that is responsible for the profound—but well-tolerated—hypoxemia.

Explanation for the clinical picture:

The patient is climbing at extremely high altitude where the low oxygen tensions in the arterial blood trigger the hypoxic ventilatory response. This response is, in turn, responsible for the respiratory alkalosis. As the patient spends increasing time at these high altitudes, metabolic compensation occurs and allows the ventilatory response to increase even further in magnitude. It is important to note that the person could not achieve this degree of minute ventilation on acute exposure to this altitude and, as a result, would not be able to maintain an adequate PaO₂ to support life. It is only with long periods spent acclimatizing to the environment that the person can achieve sufficient levels of minute ventilation and maintain adequate levels or oxygenation.

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