RESPIRATORY CHANGES IN PREGNANCY

Changes in the respiratory system may be categorised as
anatomical and physiological. The anatomical changes
make upper airway obstruction and bleeding more likely
during mask anaesthesia and may make tracheal intubation
more difficult. There is approximately a 7-fold increase in
failed intubations in parturients at term. The anatomical
changes include capillary engorgement and oedema of the
upper airway, pharynx, false cords, glottis and arytenoids.
There is also an increase in chest diameter, to allow
increased minute ventilation, and an enlargement of the
breasts, which can make laryngoscopy with a standard
Macintosh blade more difficult.
The gravid uterus progressively displaces the diaphragm
cranially reducing diaphragmatic movement in late
pregnancy, particularly in the supine position. Inspiratory
reserve volume is increased but vital capacity, total lung
volume and FEV1 remain unchanged. A decrease in both
residual and expiratory reserve volumes causes a 20%
reduction in functional residual capacity, which in turn
causes airway closure in 50% of parturients at term in the
supine position. Thus, pre-oxygenation is less effective in
the term parturient and desaturation is likely to occur much
faster than in the non-pregnant patient. A pre-oxygenation
period of 3 - 5min is the standard recommendation. Some
of the changes to respiratory physiology are illustrated in
Figure 2.
Bronchial and tracheal smooth muscle relaxation are a
result of increased progesterone concentrations. This often
causes the symptoms of asthma to lessen in pregnancy.
PaCO
2
falls and then levels off at 4.1kPa (31mmHg) by
the end of the first trimester. This is caused by a 10%
increase in the respiratory rate, secondary to progesterone
mediated hypersensitivity to CO
2
, and an increase in
alveolar and minute ventilation, secondary to increased
respiratory rate and tidal volume. PaO
2
rises to 14 kPa
(105mmHg) during the 3rd trimester but then falls to
less than 13.5 kPa (101mmHg) at term because increased
oxygen consumption is no longer fully compensated for
by the rise in cardiac output. Thus, the alveolar arterial
oxygen gradient increases. In some parturients this may
be worsened by aortocaval compression and closure of
dependant airways. At term (40 weeks gestation), oxygen
consumption and carbon dioxide production are increased
by 60% above non-pregnant values