3.6 Apnoea Physiology

Question 1

What are the preconditions for brainstem testing?

Answer 1

1. • Identifiable pathology causing irremediable brain damage
2. • Coma with exclusion of hypothermia,
     depressant drugs,
     reversible circulatory, metabolic and endocrine disturbances
3. • Apnoea, needing mechanical ventilation

Question 2

What is the basic neurological principle of apnoea test component of brainstem death testing?

Answer 2

The intact respiratory centre will initiate breathing if the threshold PaCo2 is
reached, which is usually 6.65 kPa. In brainstem death the respiratory centre
is destroyed and apnoea persists above this threshold.

Question 3

How is oxygenation maintained during the apnoea test?

Answer 3

By apnoeic mass transfer of oxygen.

Question 4

Can you explain the physiology of apnoeic oxygenation?

How does O2 transfer occur in apnoea

Answer 4

1. The oxygen consumption (Vo2) remains fairly constant at ∼250 mL/min.
2. This is delivered to the tissues by haemoglobin,
   whose oxygen is then replenished,
   on return to the pulmonary circulation.
3. In an apnoeic patient,
   approximately 250 mL/minute of oxygen will move
   from the alveoli into the bloodstream;
4. only 8 to 20 mL/minute of carbon dioxide
   moves into the alveoli,
   with the remainder being buffered in the bloodstream.
5. The end result is that the net pressure in the alveoli
   becomes slightly subatmospheric,

generating a mass flow of gas from pharynx to alveoli.

6. This process where the alveoli continue to take up oxygen
   even without diaphragmatic movements or
   lung expansion is called the mass transfer of
   oxygen or apnoeic oxygenation.
7. In healthy people under ideal circumstances, Pao2 can be maintained at
   > 100 mm Hg for up to 100 minutes without a single breath, although the
   lack of ventilation will eventually cause marked hypercapnia and significant
   acidosis.

Question 5

Can you explain the physiology of apnoeic oxygenation?
What happens when you add nasal o2

Answer 5

Apneic oxygenation with nasal cannulae works because the pharynx is filled
with high Fio2 gas and functions as an oxygen reservoir.

The discrepancy between the 10 mL Co2
entering the alveolar space
and the 250 mL o2 leaving it

causes an influx of gas from the airway above the
alveolar space.

If it is open and filled with 100% o2
(pre-oxygenation and catheter with o2),

then 240 mL is drawn into alveolar space.

This is only 10 mL o2 less than requirement;
therefore, Pao2 falls at 0.5 kPa/min.

Question 6

What are the various factors that significantly influence the time period from the onset of apnoea to critical hypoxia?

Answer 6

1. • Functional residual capacity (FRC)
2. • Preoxygenation
3. • Maintenance of patent airway
4. • Haemoglobin level
5. • Basic metabolic demand (VO2)

Question 7

Functional residual capacity (FRC)

Answer 7

• Functional residual capacity (FRC)

° Conditions where there is a decreased FRC

such as obesity,
lung disease,
kyphoscoliosis,
pregnancy, and
children,

critical hypoxia is reached more rapidly

Question 8

Preoxygenation

Answer 8

° Denitrogenation due to preoxygenation

greatly increases the time for

hypoxia after apnoea

Question 9

• Maintenance of patent airway

Answer 9

° Closed airway:

In closed airway, apnoea commences with an
intrathoracic pressure equal to ambient pressure.

The extraction of oxygen results
in subatmospheric intrathoracic pressure

and alveolar collapse almost immediately,
thereby dangerously reducing the
alveolar partial pressure of oxygen

° Patent airway:

An open airway will allow oxygen
to diffuse into the apnoeic lung,

which has been shown in animal

and simulated human studies to maintain oxygen
saturation for up to 100 min

Question 10

• Haemoglobin level

Answer 10

° Anaemia will cause a small reduction

in the time to critical hypoxia,

although this effect will be more noticeable in

patients who also have a reduced FRC

Question 11

• Basic metabolic demand (VO2)

Answer 11

The more the demand, the quicker the hypoxia