Apnoea and pre oxygenation

Apnoea and pre-oxygenation
Andrew Biffen* and Richard Hughes
*Correspondence email: abiffen@nhs.uk
INTRODUCTION
The purpose of pre-oxygenation is to increase
physiological stores of oxygen in order to prolong
the time to desaturation during a period of apnoea,
a situation that is usual following induction of
anaesthesia. This is particularly the case during a
rapid sequence induction, when positive pressure
ventilation is avoided prior to intubation of the
trachea. Pre-oxygenation can also be thought of as
denitrogenation – highlighting the fact that it is the
nitrogen within the lungs that is being displaced by a
high inspired oxygen concentration.
The rate of oxygen desaturation is influenced by the
balance between oxygen stores and consumption.
Oxygen is stored in the body within the lungs, blood
and tissues. In the context of pre-oxygenation, the
greatest increase in oxygen store is within the lungs;
more specifically, the functional residual capacity
(FRC),seeFigure1.Lungoxygenreservesareaproduct
of the fractional concentration of oxygen within the
alveoli (which we estimate by measuring the oxygen
fraction in expired gas – FeO2
) and FRC. Ventilation-
perfusion (V/Q) mismatch, particularly shunt, is an
additional factor that affects the oxygen content of
blood. This may be influenced by the relationship
between the FRC and the closing capacity (the lung
volume at which airway closure first occurs during
expiration). Oxygen consumption is influenced by
metabolic rate. Shorter time to desaturation occurs
in certain clinical scenarios including obesity, sepsis,
pregnancy and in children.
FUNCTIONAL RESIDUAL CAPACITY
The FRC is the volume of gas remaining in the lungs
at the end of a normal tidal expiration and reflects
the balance between the tendencies for the chest
wall to expand outwards (due to tone in the chest
wall muscles) and the lungs to collapse inwards.
The spirometry diagram in Figure 1 depicts the
FRC and other lung volumes. In a healthy adult
the FRC amounts to 30ml.kg-1
, totalling 2100ml
in a 70kg adult. However, many patients presenting
for surgery have a reduced FRC, which in turn
reduces the lung’s oxygen store. Reasons for this
include obesity, pregnancy, anaesthesia (with or
Summary
Pre-oxygenation is:
safe
simple
cheap
effective
well-tolerated
This article provides a
compelling argument in
favour of pre-oxygenation
prior to all general
anaesthesia.
Andrew Biffen
Anaesthetic Trainee
Richard Hughes
Consultant Anaesthetist
Torbay Hospital
Torquay
UK
ClinicalOverviewArticles
without neuromuscular blockade) and lung disease.
Nonetheless, pre-oxygenation is still beneficial in
comparison to breathing room air.
Calculating oxygen reserves
It is possible to calculate oxygen delivery and
consumption, to demonstrate the effects of pre-
oxygenation:
page 20
Figure 1. Spirometry trace depicting lung volumes and
capacities. IRV = Inspiratory reserve volume, TV = Tidal
volume, ERV = Expiratory reserve volume, RC = Residual
capacity, VC = Vital capacity, FRC = Functional residual
capacity, TLC = Total lung capacity
TLC
(6000ml)
TV
(500ml)
IRV
(3000ml)
VC
(5000ml)
FRC
(2500ml)
ERV
(1500ml)
RV
(1000ml)
Time
Volume
The alveolar gas equation is used to calculate
PAO2
:
PAO2
= PIO2
– [PACO2
/R]
When breathing air (21% O2
):
PAO2
= 0.21 x (101.3 – 6.7) – 5.3/0.8 = 13.2kPa
This is equivalent to 13% (273ml) of oxygen
in an FRC of 2100ml, the remaining contents
being 75% nitrogen, and the 7% water vapour
and 5% carbon dioxide used in the alveolar
gas equation calculation. For the purposes
of calculation, oxygen consumption at rest is
considered to be 3.5ml.kg-1
.min-1
.
Update in Anaesthesia | www.wfsahq.org/resources/update-in-anaesthesia
This article was originally published as Anaesthesia Tutorial of the Week 297 (2013)

If the gas in the upper airway has a low fraction of oxygen (e.g.
air), then nitrogen will build up in the lungs, effectively being
concentrated, resulting in loss of the pressure gradient and cessation
of flow. If the airway is obstructed during this time, there will be rapid
development of negative intrapulmonary pressure. Not only will this
result in missing the benefits of the oxygen elevator, but may cause
airway collapse and pulmonary oedema.
This reinforces the benefits of maintenance of a patent airway and
application of 100% oxygen as good practice during apnoea at
induction of anaesthesia.
PRACTICALITIES
Various methods have been described to achieve the process of pre-
oxygenation. A consistent feature is the requirement for a tightly
fitting mask, with the avoidance of any leak which would allow
entrainment of room air, and therefore nitrogen. Selection of an
appropriate size of mask is important.
Difficulty achieving a good seal may be found with bearded or
edentulous patients in particular. In situations where you are unable
to prevent leaks or the patient is phobic of the mask, an alternative is
to ask the patient to form a seal around the catheter-mount with the
mask removed, ensuring that they do not breathe through the nose
(consider the use of a nose clip). This can be useful for patients who
suffer from claustrophobia (fear of enclosed spaces).
Timing
The necessary duration of pre-oxygenation has been debated and
studied extensively, with options including three minutes of tidal
breathing, four vital capacity breaths in 30 seconds or eight vital
capacity breaths in 60 seconds. To some extent these fixed regimens
are unnecessary in the presence of end tidal oxygen monitoring
(ETO2
). If this monitoring is available it is possible to observe the rise
in ETO2
on a breath-by-breath basis, with an endpoint of achieving
an ETO2
> 85% (100% is not achievable due to the presence of CO2
and water vapour). The actual time required varies between patients;
it may be achieved more quickly than three minutes, especially if a
patient has a relatively small FRC, while in certain circumstances it
can take longer. In the absence of ETO2
monitoring, either 3 minutes
of tidal breathing or eight vital capacity breaths in 60 seconds are
recommended. Use of four vital capacity breaths in 30 seconds has
been found to be inferior to the other two methods. With either
method, it is advantageous for the patient to exhale completely
(down to residual volume) prior to the start of pre-oxygenation.
The filling of the FRC with oxygen can be described by a wash-in
curve and the contrasting process of de-nitrogenation is represented
by a wash-out curve. Both processes are negatively exponential and
allow for an understanding of the methods for pre-oxygenation
suggested.
Time constants
The nitrogen wash-out curve corresponds to the formula:
y = a.e-kt
SHUNT
As well as providing a smaller reservoir of oxygen, the relevance of
a reduced FRC extends to V/Q mismatch. The closing capacity is
the lung volume at which small airways will close. If the FRC falls
within the closing capacity, airways will close during tidal breathing,
resulting in alveoli that are perfused, but not ventilated. This is known
as shunt, a phenomenon that is not improved by the administration
of higher concentrations of oxygen.
APNOEA
During the apnoeic period, oxygen continues to be taken up into
the blood from the lungs. The uptake of oxygen from the lungs far
exceeds the return of carbon dioxide from the blood to the alveoli,
due to the body’s extensive buffering systems which absorb large
quantities of CO2
. This net loss of volume leads to development of a
negative pressure in the lungs.
If the upper airway is patent, gas will be continually drawn via the
trachea into the lungs, equalising the pressure gradient. If the gas
in the upper airway is 100% oxygen, this pressure gradient can be
maintained for a long period - theoretical modelling suggests that
this time may exceed 30 minutes. This phenomenon, also referred
to as the oxygen elevator, can significantly prolong the time until
desaturation. It should be noted that carbon dioxide is not transferred
out of the lung during this process, so there will be a gradual rise in
PaCO2
.
page 21
Continuing with the example of a 70 kg adult, approximately
250ml.min-1
of oxygen is consumed. Thus, in this model, the
FRC provides a reservoir of oxygen equivalent to 70 seconds
worth of oxygen consumption. Not all of this oxygen can be
extracted from the alveoli; once the PAO2
falls below 6kPa,
there will be little concentration gradient to maintain flux of
oxygen to haemoglobin. The amount of useable oxygen in
this reservoir is therefore likely to be only around 150ml. Actual
time to desaturation depends on a complex set of factors as
described above.
Pre-oxygenation is a highly efficacious way of extending the
time to exhaustion of oxygen reserves and desaturation.
When breathing 100% oxygen*:
PAO2
= (101.3 – 6.7) – [5.3/0.8] = 88kPa*
This is equivalent to approximately 88% (1800ml) of oxygen
in the FRC – equivalent to more than seven minutes worth
of oxygen consumption, or around ten times the amount of
useable oxygen compared to breathing room air.
This demonstrates that replacing the nitrogen in the FRC with
oxygen greatly increases available reserves.
* Note that this is a theoretical figure. Achieving perfect pre-
oxygenation is often not possible; aiming for an ETO2
> 85% is a
good goal. This will still give over 1500ml of pulmonary oxygen in
this example.

The time constant of an exponential process relates to the time
taken for the value of the exponential (in this case the amount
of nitrogen in the FRC) to fall to 37% of its previous value. As a
result, after four time constants the process will be 98% complete.
The time constant relates to the ratio of volume and flow, i.e. FRC
and alveolar minute ventilation (VA). For a 70kg patient with a tidal
volume of 490ml (7ml.kg-1
), a respiratory rate of 12 breaths per
minute and anatomical dead space of 140ml (2ml.kg-1
), VA would be
4200ml.min-1
. As noted above, the FRC is 2100ml. Consequently, a
time constant of 0.5 minutes is obtained.
From the calculations above, it is evident that after two minutes (four
time constants), 98% of the nitrogen in the FRC will have been
washed out and replaced with oxygen. A three minute period ensures
a safe margin to account for inter-patient variability.
Breathing system
The breathing system employed during pre-oxygenation should be
taken into consideration. When using a circle system it is necessary
to ensure an oxygen flow greater than minute ventilation (MV);
i.e. at least 6L.min-1
in a 70kg patient, in order to maintain 100%
oxygen within the circuit. Higher flows (15L.min-1
) are required if
vital capacity breaths, rather than tidal breathing, are taking place
(due to the increased MV). With a Mapleson D breathing system
(Bain circuit) high oxygen flows (2-3 x MV) are required to prevent
re-breathing of expired nitrogen and carbon dioxide.
(consumption). Consequently, their rate of desaturation is notably
greater than the non-obese patient. Coupled with a higher rate of
difficult bag mask ventilation and difficult intubation, there is a key
role for pre-oxygenation in the obese patient in order to maximise
the PAO2
(store). A further adjustment to the process is to sit such
patients up. This improves matters by increasing the FRC compared
to the supine position, with a 25 degree elevation having been shown
to significantly reduce the rate of desaturation in obese patients.
Pregnancy
Pre-oxygenation has an important role to play in the anaesthetic
management of the pregnant patient. The enlarging uterus causes
elevation of the diaphragm and a reduced FRC, within which the
closing capacity may fall. Metabolic demand increases due to the
growing foetus and placenta. Therefore, desaturation occurs more
rapidly. Furthermore, airway management in obstetric patients is
known to be difficult more frequently than in the general surgical
population. Pre-oxygenation provides an added margin of safety if
efforts at establishing an airway become prolonged. Unlike obesity
the 25 degree head up position has not been shown to reduce the rate
of desaturation of pregnant patients.
Sepsis
In critically ill and septic patients the time taken to desaturate can be
greatly reduced. Factors that influence this are an increase in oxygen
demand and cardiac output, and reduced tissue oxygen extraction
associated with sepsis. It is likely that V/Q mismatch is increased,
further compounding the problem of rapid desaturation. In such
patients achieving ideal oxygen saturations approaching 100% can
be difficult, even with the administration of 100% oxygen. However,
good quality de-nitrogenation of the patient’s FRC prior to intubation
(and associated apnoea) will still help to delay a precipitous fall in
oxygen saturation.
Paediatrics
Children may be less likely to tolerate the process of pre-oxygenation.
However, its use should be carefully considered as children have a
higher metabolic rate than adults and de-saturate more quickly as
a result. Many children will cooperate with the process when it is
explained to them and efforts should be made to do this in individuals
at high risk of desaturation.
Tracheal extubation
Much of the preceding text has referred to the use of pre-oxygenation
prior to induction of anaesthesia. It should be noted that the same
principles of increasing oxygen stores within the FRC are of use prior
to tracheal extubation, providing additional oxygen stores in the
event of an airway complication at this time. Correlation with ETO2
is of use to ensure adequate de-nitrogenation.
CAUTIONS
One deleterious effect of the administration of 100% oxygen is
atelectasis. This results from oxygen uptake from poorly-ventilated
alveoli leading to alveolar collapse. However, this problem may be
easily remedied through the use of recruitment manoeuvres and
page 22
Time (minutes)
Nitrogen(%)
10
20
30
40
50
60
70
80
0.5 1.0 1.5 2.0 2.5 3.0 3.5
Editor’s notes
Where oxygen concentrators are used, the oxygen flow is
generally limited to 10L.min-1
, making effective delivery
of 100% oxygen through a circle or drawover system
impossible. One alternative is to fill high capacity bin bag
with oxygen and use this as a reservoir attached to the
open end of the the drawover system.
OTHER CONSIDERATIONS
Obesity
As noted above, the balance between oxygen stores and oxygen
consumption determines the rate of desaturation of a patient. Obese
patients have a reduced FRC (store) and a greater metabolic rate
Figure 2. The nitrogen wash-out curve during pre-oxygenation

should not be seen as a contraindication to the appropriate use of
pre-oxygenation. Once a secure airway has been obtained, the FIO2
may be reduced to an appropriate level for that particular patient.
Rare circumstances where the risk-benefit balance may not be in
favour of the use of 100% oxygen are in patients receiving bleomycin
treatment and those with conditions in which the pulmonary
vasculature is sensitive to changes in the FIO2
. The latter are a special
case where high FIO2
may need to be avoided. Expert advice should
be sought. Bleomycin is asssociated with a pneumonitis, which can
be potentiated by a high FIO2
. The short periods of time for which
100% oxygen is used in the context of pre-oxygenation are deemed
to be safe when balanced with a lower maintenance FIO2
. Further
detail about this is beyond the scope of this article.
SUMMARY
When properly performed, pre-oxygenation prolongs the time until
desaturation when apnoea occurs. Maintenance of a patent airway
with continued application of 100% oxygen during apnoea further
prolongs the time to desaturation.
REFERENCES
1. Tanoubi I, Drolet P, Donati F. Optimizing pre-oxygenation in adults. Can J
Anesth 2009; 56: 449–66.
2. Hardman JG, Wills JS, Aitkenhead AR. Factors determining the onset and
course of hypoxaemia during apnoea: an investigation using physiological
modelling. Anesth Analg 2000; 90: 619–24.
3. Wills J, Sirian R. Physiology of apnoea and the benefits of pre-oxygenation.
Continuing Education in Anaesthesia, Critical Care & Pain (2009) Volume 9
Number 4: 105 -8.
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