24.7 Oxygenation Flashcards
a) What physiological factors determine the rate of fall in arterial oxygen saturation in an apnoeic
patient (3 marks),
and which patient groups are most likely to show a rapid fall? (4 marks)
- Reservoir available
(size of the functional residual capacity,
FRC, and its oxygen content). - Rate of oxygen consumption.
- 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 hypercapnoea).
b) How may oxygenation, prior to intubation, be optimised during a rapid sequence induction
(8 marks),
and how can its progress be measured? (1 mark)
> > Explanation to and consent from patient
will assist compliance.
> > Head-up tilt to increase the size of FRC.
> > Manage any issues that are reversibly
causing an impact on FRC,
e.g. aspirate gastric contents in a
patient with small bowel obstruction,
use of non-invasive ventilation
to recruit areas of atelectasis.
> > Tight-fitting mask
(anaesthetic machine circuit) to
ensure no entrainment of room air.
> > 100% FiO2.
> > Gas flow to exceed patient’s minute ventilation
to ensure gas in circuit remains 100%
oxygen whilst patient is still breathing,
and high flow facilitates apnoeic
mass movement after cessation of breathing.
> > 5 minutes.
> > Tidal breathing.
> > Keep mask in situ and maintain
airway patency until ready to intubate.
> > High-flow transnasal humidified
oxygenation after mask removal may
help maintain oxygenation by
apnoeic mass movement and maintenance
of airway patency through positive airway pressure.
Research into the use of such methods instead of standard preoxygenation techniques is ongoing.
> > Standard nasal cannulae delivering
high-flow oxygen may help maintain
oxygenation by mass movement after mask removal.
They can be fixed into place before starting preoxygenation as long as they do not
negatively affect the seal of the face mask.
> > Progress can be measured by monitoring the fraction of expired oxygen (FEO2), aiming for greater than 0.9.
c) What are the possible respiratory complications of prolonged delivery of 100% oxygen? (4 marks)
> > Oxygen leaves the lung and enters the blood,
driven by a concentration gradient
that is generated by the body’s constant
utilisation of oxygen.
When breathing room air,
the collapse of alveoli is prevented by the
ongoing presence of nitrogen within them;
nitrogen is in equilibrium with
its concentration in blood.
However, if breathing 100% oxygen,
alveolar collapse occurs as oxygen leaves.
This results in atelectasis.
> > Areas of perfused lung that are no
longer being ventilated results in shunt.
> > Hypoxic pulmonary vasoconstriction
works to reduce perfusion to unventilated
portions of the lung.
However, this results in raised
pulmonary vascular resistance
and risk of right heart dysfunction and
alveolar capillary leak in susceptible patients.
> > Atelectatic areas of lung are at
risk of development of pneumonia.
> > Efforts to re-recruit atelectatic lungs
may result in volu- and barotrauma.
> > Patients with type 2 respiratory failure
may be dependent on a degree of hypoxaemia
to maintain respiratory drive.
Loss of this drive due to prolonged
administration of 100% oxygen
may result in hypoventilation,
raised arterial carbon dioxide
and consequent obtundation.
Patients most likely to show a rapid fall in oxygen consumption:
Patients most likely to show a rapid fall in oxygen consumption:
1 Patients with low FRC:
> > Poor positioning prior to induction
(FRC is less when supine,
FRC can be improved by head-up tilt).
> > Encroachment on FRC
(obesity, pregnancy, ascites,
bowel obstruction, kyphoscoliosis).
> > Age (lower FRC per unit weight in children).
2 Patients with reduced oxygen content of FRC:
> > Lung disease causing shunt
reduces effectiveness of preoxygenation.
> > Lung disease causing elevated
PaCO2
(according to the alveolar gas
equation, an elevated PaCO2 results
in a reduced PAO2).
3 Patients with increased rate of oxygen consumption:
» Sepsis.
» Thyrotoxicosis.
» Pregnancy.
» Critical illness.
> > Fasciculations secondary to suxamethonium.
> > Childhood –
greater oxygen consumption per unit weight.
