Acid-Base Physiology

Case 27: A man with chest trauma from a car crash

updated 06-Sep-2015

Clinical Details

History: This middle-aged obese man was admitted following a motor vehicle crash. Previous health was good.

Examination: His major injury was chest trauma with a small right pneumothorax and at least five fractured ribs on the right. There was no head or neck injury. Abdomen was soft. He was haemodynamically stable with BP 130/80 and pulse 90/min. Heart sounds were normal. Respiratory distress was present. Respiratory rate was 30/min. Saturation was 95% on high flow oxygen via face mask.

Investigations: Na+ 138, K+ 3.9, Cl- 103, Total CO2 21, glucose 13.6, urea 5.5, creatinine 0.10 (all in mmol/l). CT scan of abdomen was normal. Urinalysis was not reported. Arterial blood gases were collected soon after arrival.

Arterial Blood Gases

pH 7.18

pCO2 73 mmHg

pO2 93 mmHg

HCO3 27 mmol/l


First: Initial clinical assessment

A respiratory acid-base disorder is likely given the trauma and clinical evidence of respiratory distress. There is no indication of a metabolic acid-base disorder. If the patient had been shocked then a lactic acidosis would need to be considered. The patient has not been intubated and ventilated yet. Ventilated patients may be over-ventilated intitially resulting in an iatrogenic respiratory alkalosis.

Second: The acid-base diagnosis

Proceeding systematically:

  1. pH: The acidaemia indicates an acidosis
  2. Pattern: The combination of a high bicarbonate and a high pCO2 (in the presence of a known acidosis) indicates a respiratory acidosis
  3. Clues: The anion gap is 14 (i.e. normal). There is no evidence of renal failure.
  4. Compensation: This is an acute chest injury in a previously healthy person so this is an acute respiratory acidosis. The appropriate rule to assess compensation is (rule 1). The expected HCO3 is [24 + 3.3] which is 27.3 mmol/l. This is almost exactly the actual value; consequently there is no evidence of a co-existing metabolic acid-base disorder.
  5. Formulation: An acute respiratory acidosis. There is no evidence of any other acid-base disorder.
  6. Confirmation: No other tests are required, though a baseline lactate is useful.

Finally: the Clinical Diagnosis

This man has an acute respiratory acidosis due to acute chest trauma. This patient has respiratory failure and requires intubaton and ventilation.


In acute trauma affecting previously healthy patients, the commonest acid-base problem is a respiratory one related to chest or head trauma, pain, or airway obstruction. Shocked patients may develop a lactic acidosis. Resuscitation of patients with large volumes of saline can cause a hyperchloraemic acidosis.

The medical history of acutely injured patients may not be known initially, but pre-existing medical disorders are often present, and indeed could be the cause of the crash (e.g. if the patient had lost consciousness).