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Mechanical ventilation

Responding to blood gas abnormalities:


Check machine, circuit and airway device. Increase FiO2 to 1.

The response following this depends on the severity of the hypoxaemia and the speed of the deterioration. Always examine the patient. While doing so, arrange CXR and ECG which provide diagnostic information and exclude some treatable causes.

Rapid/ profound hypoxaemia


  • Pneumothorax
  • Bronchospasm
  • Pulmonary oedema
  • Pulmonary thromboembolism
  • Incorrect position of ETT (too far in or too far out)
  • Mucus plugs in large airways

History, targeted examination and investigation should take place rapidly. Treatment depends on the cause, but remember that a pneumothorax developing during positive pressure ventilation may quickly become life threatening.

Gradual worsening of hypoxaemia

SpO2 of 92% is acceptable. In severe respiratory failure, many experts would accept saturations of over 85%.

Clearly the treatment depends on the cause, so reaching a diagnosis is vital. History, examination and investigation should proceed in the usual fashion. Short-term measures which may help are:

  • Increase FiO2
  • Increase mean airway pressure: this can be achieved by
    • increasing PEEP
    • increasing inspiratory pressure or
    • increasing inspiratory time.

The correct response will depend on the circumstances, but keep in mind the principles of ventilator management referred to above (tidal volume, airway pressure, respiratory rate).

Gradual worsening of hypercarbia

Permissive hypercarbia is an accepted part of respiratory management in ICU. The question that should be answered when asked to deal with hypercarbia is: will the treatment required to lower the CO2 cause more damage to the patient than the hypercarbia itself?

If the treatment is to increase airway pressure or tidal volume above the safe values referred to above, the answer is likely to be yes: therefore the hypercarbia should be tolerated. If the treatment is to increase respiratory rate, the answer is likely to be no. If the treatment is to lighten sedation, the answer is likely to be no.

Some experts would prescribe sodium bicarbonate to counteract the respiratory acidosis, but this is not universally accepted.

If a patient is breathing spontaneously, there is a danger that ‘taking over’ a patient’s ventilation only causes a further rise in CO2. Be sure that you can increase the overall minute ventilation, in the face of reduced compliance caused by controlling ventilation, before attempting to do so.


General principles of trouble shooting poor gas exchange for patients on mechanical ventilation

Increase FiO2

History, quick targeted examination of patient and equipment, and CXR and ECG

Definitive treatment if diagnosis made. E.g. bronchodilators for spasm, diuretic for fluid overload, decompression for tension pneumothorax

Improve patient's synchronisation with ventilator. E.g. change to different mode of ventilation or give muscle relaxant

Make sure lungs are well perfused

Try increasing PEEP (max 15 cm H20). This can take a few minutes to work

Increasing RR is worth trying before increasing Vt (if this is > 7 ml/kg)

Call for help (do this first if hypoxia develops quickly and you do not know why)

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