14. Effects of Anaesthesia on Lung Function

Question 1

Can you describe the effects of
general anaesthesia on lung
function?

Answer 1

General anaesthesia has multiple important effects on lung function. The easiest way to think of the changes is to divide them into categories:

1. Respiratory control
2. Lung mechanics
3. Gas exchange

All the changes described are exaggerated
in those patients with lung disease.

Smokers should be counselled to
stop smoking at least 6 weeks prior to
their operations to allow their

mucociliary and inflammatory cell function
to return to somewhere approaching normal.

Failing this, they should definitely
not smoke for at least 12 hours prior to surgery,
as the half-life of carbon monoxide is 4 hours.

In all patients, the effects of anaesthesia
on lung function will last a
few hours post-operatively.

After major surgery, or in those with
lung disease, the
effects may last many days.

Question 2

1. Respiratory control

Answer 2

> The patient’s basal metabolic rate
drops by 15%
following induction of anaesthesia,
due to thalamic inhibition.

> The response to hypercapnia is blunted 
and the acute responses to
acidosis 
and 
hypoxia 
are almost entirely abolished.

Question 3

Lung mechanics

Answer 3

1
> Following induction,
functional residual capacity (FRC)
falls by 15–20%

because there is a
loss of muscle tone.

This decreased tone reduces the
bucket handle action of the rib cage,

which consequently moves less;

breathing is more dependent
on the movement of the diaphragm,

which is similarly weakened.

Phasic activity develops in the
expiratory muscles,
which are normally silent.

2
> In spontaneous,
awake ventilation,
expiration is a passive movement;

under anaesthesia it becomes an active one.

3
> Lung compliance is reduced
and airway resistance increases slightly;
these combine to increase the work of breathing.

4
> Mucociliary transport mechanisms
are reduced,
which can cause retention of secretions.

Question 4

3 Gas exchange

Answer 4

> The changes described
above can cause atelectasis
and inhibit hypoxic pulmonary vasoconstriction.

Logically, the application of PEEP should
improve the situation as it
helps to ‘splint’ the alveoli open.

However, applying PEEP will also
reduce blood flow to the splinted areas,

by altering the pressure across
the alveoli and therefore capillary walls

(remember Starling’s resistors
and West’s zones of the lung).

So PEEP may actually 
increase V./Q. mismatching 
and 
should be applied with
care in unstable patients. 

As with all things, this is a balance of risk.

The application of PEEP may also
destabilise the cardiovascular system
in a particularly sick patient.

Without the application of any PEEP,
around 10% of pulmonary blood is
shunted or perfuses areas
with low V /Q

> Alveolar dead space rises from
0 to 70 mL

and

physiological dead space
from 150 to 220 mL.

Intubation decreases dead space,
but this effect is reduced by connectors etc.

Question 5

If the patient is placed on their side on the table, how will their position affect the flow of gases
into the lungs?

Answer 5

> The flow of gases is dependent
upon whether the patient is
spontaneously breathing or
being ventilated with positive pressure.

> I f the patient is spontaneously ventilating,
gas will be drawn into the
dependent lung,
i.e. the lowermost lung.

The situation is reversed,
however,
when the patient is ventilated.

Now gas will follow the path of
least resistance into
the non-dependent lung,

whose total compliance
would be greater as it does
not have the weight of the thorax pressing
down on it.

As blood is preferentially
distributed to the lower lung, this
can cause V /Q mismatching and desaturation.

It is easy to remember which way round 
this is as it is obvious that blood
will preferentially perfuse 
the dependent lung 
under the influence of gravity.

In our natural state,
e.g. asleep in bed on our side,
nature would not invent
a system that would worsen oxygenation –
so the air must also flow into
the dependent lung to increase V /Q matching.

A practical application of this
principle can be seen in ITU when 
we see patients improve/desaturate as
they are turned and the 
good/bad lung is preferentially ventilated.

Question 6

Table 14.1 Summary of distribution (%) of ventilation under anaesthesia in the
lateral position:

Answer 6

Dependent Lung Non-dependent Lung

Awake, spontaneously breathing 60% 40%

GA, spontaneously breathing 45% 55%

GA, IPPV 40% 60%

Thoracotomy 30% 70%