Sat27November0556PM 50

A man with a BMI of 35 is being pre-oxygenated prior to rapid sequence induction of anaesthesia.

Which is the most efficient method of pre-oxygenation using a tight-fitting face mask?

(Please select 1 option)

Oxygen 6 litres per minute via a Mapleson A breathing system, with patient sitting up at 30 degrees breathing four vital capacity breaths This is the correct answer

Oxygen 6 litres per minute via a Mapleson A breathing system, with patient supine breathing tidal volume breaths for three minutes

Oxygen 6 litres per minute via a circle breathing system, with patient supine breathing tidal volume breaths for three minutes

Oxygen 6 litres per minute via a Mapleson D breathing system, with patient sitting up at 30 degrees breathing tidal volume breaths for three minutes Incorrect answer selected

Oxygen 6 litres per minute via a Mapleson D breathing system, with patient supine breathing tidal volume breaths for three minutes

Explanation

This patient is morbidly obese and is prone to the development of hypoxia. This will be compounded with the patient in the supine position secondary to:

Reduced functional residual capacity (FRC)

Increased closing capacity (CC)

Diminished tidal volume due to increased resistance of the airway, diminished compliance of the thoracic cage and diminished strength and endurance of respiratory muscles

A tendency towards atelectasis following induction of general anaesthesia and

Increased O2 consumption due to the increased workload of respiratory muscles and to the general increase in metabolism.

In normal circumstances, pre-oxygenation with 100% oxygen via a tight fitting mask can be carried out either using tidal volume breaths for three to five minutes or four vital capacity breaths. This patient is much more likely to be "adequately" pre-oxygenated in the head-up position and maximising the FRC and minimising the CC. The Mapleson A and circle systems are both efficient in spontaneously breathing patients but the Mapleson D requires 160-200 ml/kg/minute to prevent rebreathing.

Answer Statistics

1

38%

2

6%

3

12%

4

43%

5

4%

Times answered: 243