Question 11

Describe the acid-base changes that occur in acute hypoxaemia

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College Answer

The main points expected for a pass were

  • Definition of hypoxaemia
  • Physiological causes of hypoxaemia
  • Formation of a lactic acidosis
  • Development of hypocarbia secondary to stimulation of respiration from hypoxia and acidosis
  • Explanation of the decreased bicarbonate concentration and negative base excess
  • Extra points were given for more detailed explanation of the lactic acidosis, commenting on the anion gap and some consequences of hypoxaemia.

A common problem was to incorrectly state the relationship between pH and hydrogen ion


As many of these early SAQs, this one has a hidden implied element, where the expected answer clearly looked for components which were not asked about in the actual question stem. If one were to answer the question literally (i.e. as it is written), one would end up with this:

There are two main acid-base effects of hypoxaemia:

  • Respiratory alkalosis, at mild hypoxaemia
  • Metabolic acidosis, with severe hypoxaemia

Mechanism of the respiratory alkalosis with mild hypoxaemia:

  • Acute hypoxaemia (low partial pressure of oxygen at the carotid chemoreceptors) is a potent respiratory stimulant
  • At a PaO2 around 60 mmHg, "hypoxic drive" becomes an important influence on the respiratory control centre, producing an increase in the respiratory rate and tidal volume
  • This increase in minute volume produces an increase in the clearance of PaCO2 
  • The decrease in PaCO2 changes the chemical equilibrium of bicarbonate and carbonic acid, decreasing the concentration of carbonic acid and increasing the pH
  • Alkalosis ensues.

Mechanism of metabolic acidosis with severe hypoxaemia:

  • The oxygen concentration the blood is insufficient to support aerobic metabolism
  • Cellular metabolism switches to anaerobic metabolism and generates lactate
  • Lactate dissociates fully at physiological pH, widening the anion gap and generating hydrogen ions
  • The presence of excess hydrogen ions decreases the pH
  • This decreased pH serves to further stimulate the respiratory centre

However, the college clearly also required the following details for a pass 

Definition of hypoxaemia:  an abnormally low concentration of oxygen in arterial blood, whereas hypoxia is the failure of tissue oxygenation

Causes of hypoxaemia:

  • Decreased alveolar oxygen
    • Alveolar hypoventilation with hypercapnia (eg. opiates, COPD)
    • Decreased atmospheric oxygen tension (eg. high altitude)
  • Decreased oxygen diffusion through the blood-gas barrier
    • Decreased blood-gas barrier surface area (eg. ARDS)
    • Decreased blood-gas barrier permeability (eg. pulmonary fibrosis)
  • Decreased perfusion of gas exchange surfaces
    • V/Q mismatch, eg. pneumonia
    • Shunt (eg. intracardiac right to left shunt)
    • Massively increased dead space (eg. massive PE)
  • Decreased mixed venous oxygen content 
    • Increased O2 consumption (i.e. a relative deficit), due to:
      • an increased metabolic rate, eg. malignant hyperthermia
      • a decreased cardiac output, i.e. increased oxygen extraction ratio

Acid-base changes in hypoxaemia:


Howard, R.P. "Rapid response to: Oxygen transport—1. Basic principles" BMJ 1998; 317 doi: (Published 07 November 1998) 

Treacher, D. F., and R. M. Leach. "Oxygen transport—1. Basic principles." Bmj 317.7168 (1998): 1302-1306.

Tipping, Robert, Robin Berry, and Ian Nesbitt. "Mechanisms of hypoxaemia and the interpretation of arterial blood gases." Surgery (Oxford) 30.10 (2012): 505-511.