Acid-Base Physiology

Case 5 : An old lady from a motor vehicle crash

Clinical Details

An 80 year old lady (wt 40 kgs) was admitted to the Intensive Care Unit following a motor vehicle accident.

She was the driver and was wearing a seat-belt. She had run off the road in her car and hit a tree. She remembered the accident and was not knocked out. Injuries were a left anterior flail segment, a fractured left patella and facial bruising. She was haemodynamically stable but had respiratory distress with paradoxical movement of her left anterior chest wall. There was no head or neck injury. Recently she had had several unexplained blackouts. Only significant past history was of hypertension for which she took propranolol 120 mgs/day.

She was intubated and ventilated in the Casualty department because of respiratory distress. Initial ventilation was tidal volume 1,000mls at a rate of 10 breaths/min with 100% oxygen. Arterial gases (below) were obtained half an hour later. Peripheral perfusion was good. An intravenous infusion was commenced.

Previous health was good apart from recent 'blackouts'. She was on no regular medication.

Arterial Blood Gases

pH 7.56

pCO2 23 mmHg

pO2 508 mmHg

HCO3 21 mmol/l


First: Initial clinical assessment

History is of an acute disorder. The controlled ventilation settings are delivering a tidal volume of 25mls/kg and a minute volume of 250mls/kg. An acute respiratory alkalosis is very likely.

Second: The acid-base diagnosis

  1. pH: The alkalaemia indicates an alkalosis is present
  2. Pattern: A low pCO2 and a low [HCO3] occurs in a rerspiratory alkalosis and in a metabolic acidosis.
  3. Clues: No other results are presented. It is useful to always have electrolyte results when assessing a blood gas result.
  4. Compensation: The history of sudden onset and short duration indicates an acute rather than chronic disorder. The appropriate rule to assess compensation for an acute respiratory alkalosis is rule 3. The expected [HCO3] according to the ‘2 for 10’ rule (Rule 3) is 20 mmol/l (ie 24 - 4). The difference between the actual and expected [HCO3] is small ( only 1 mmol/l) so there is no evidence of an associated metabolic disorder
  5. Formulation: Acute respiratory alkalosis
  6. Confirmation: No further investigations required

Finally: The Clinical Diagnosis

The final acid-base diagnosis is acute respiratory alkalosis due to mechanical hyperventilation. The cause of her ‘blackouts’ needs appropriate investigation.

This result is consistent with her previous good health, lack of diuretic therapy and good peripheral perfusion. Her gases normalised when minute ventilation was decreased.


The probable sequence of events here was an old lady who had a ‘blackout’ and crashed her car. Because of respiratory difficulty due to the flail chest, she was managed with intubation and controlled ventilation. The initial gas results show a predictable acute respiratory alkalosis due to mechanical over-ventilation. She was receiving a very high tidal volume.

It is not uncommon for intubated patients in an Accident & Emergency Department to be initially over-ventilated as the priority is to ensure adequate oxygenation and to adjust alveolar ventilation later (based on arterial pCO2). In addition, multi-trauma patients often develop a metabolic acidosis and the over-ventilation will mimic the body's compensatory response. Whether this is initially useful is uncertain as the body's compensatory response does take some time to develop. Arterial blood gases should be checked soon after institution of controlled ventilation.