24.1 PreOxygenation

Question 1

a) What factors determine how long oxygen saturation is maintained in an apnoeic patient? (30%)

Answer 1

This depends upon the balance between how
quickly the oxygen is being used,
how much of a reservoir there is to use,
and whether there is a way to
keep topping up that reservoir.

The reservoir is the product of the functional
residual capacity and its oxygen content.

Remember the alveolar gas equation?
PAO2 = PiO2 - PaCO2 / RQ

It demonstrates that increasing PiO2 increases PAO2 and how a raised
PaCO2 has a negative impact on PAO2.

1. Reservoir available –
   product of the functional residual capacity
   (FRC) and its oxygen content:

Size of FRC dependent on:

> > Posture
(FRC is less when supine,
can be improved by head-up tilt).

> > Anaesthesia (reduces FRC).

> > Encroachment on FRC
(obesity, pregnancy, ascites,
bowel obstruction, kyphoscoliosis).

> > Age
(lower FRC per unit weight in children).

Oxygen content dependent on:

> > Fraction of inspired oxygen preceding apnoea
(the alveolar gas equation determines that breathing 100% oxygen increases the available reservoir
by a factor of nearly 5,
compared to room air).

> > Lung disease
(shunt reduces effectiveness of preoxygenation).

> > PaCO2
(according to the alveolar gas equation,
an elevated PaCO2 results in a reduced PAO2).

Rate of oxygen consumption:
Increased in:
» Sepsis.
» Thyrotoxicosis.
» Pregnancy.
» Critical illness.
» Fasciculations secondary to suxamethonium.
» Childhood – greater oxygen consumption per unit weight.

Patency of airway:

At normal steady state,
oxygen is removed from the lungs
at the rate of its consumption
(approximately 250 ml/min in a textbook adult).

Carbon dioxide delivery to the lungs is 80% of this,
as determined by a respiratory quotient of 0.8.

After apnoea, oxygen removal from the lungs
persists at the same rate.

However, as the partial pressure of carbon dioxide in the alveoli starts to rise,
the concentration gradient between blood and alveoli
reduces,

negatively impacting on further movement of carbon dioxide into the lungs.

Lung volume consequently falls.

If the airway is patent, this results
in apnoeic mass movement of gas
(oxygen, if anaesthetic mask is still firmly
held in place) into the lungs,
significantly extending the time to desaturation
(although not addressing acidosis and hypercapnia).

Question 2

b) How may pre-oxygenation be performed and its progress assessed? (30%)

Answer 2

> > Explanation to and consent from patient,
improves compliance.

> > Head-up tilt to increase the size of FRC.

> > Tight-fitting mask
(anaesthetic machine circuit) avoids entrainment of
room air.

> > 100% FiO2.

> > Gas flow to exceed patient’s minute ventilation to ensure gas in circuit remains 100% oxygen whilst the patient is still breathing,
and high-flow oxygen facilitates apnoeic mass movement after cessation of breathing.

> > Five minutes.

> > Tidal breathing.

> > Monitor fraction of expired oxygen (FEO2),
to target greater than 0.9.

Question 3

c) What are the clinical advantages and disadvantages of pre-oxygenating a fit adult? (40%)

Answer 3

Advantages

1. Difficult to predict difficult intubation,
   therefore provides a margin of safety
   in unpredicted difficulties.
2. Cannot predict severe laryngospasm.

Disadvantages

1. Risk of respiratory incident during induction in
   fit patients is low, so may be unnecessary.
2. Prolongs induction by five minutes.
3. Intolerance of tight-fitting mask,
   sense of claustrophobia.
4. Increases alveolar collapse at induction
   resulting in risk of atelectasis and
   postoperative hypoxia.