Respiratory changes in pregnancy
[Ref: PK1:p349-350]
Changes to anatomy
Diaphragm
- Diaphragm is displaced upwards by about 4cm
* Contraction is NOT marked restricted
Thoracic cage
- Anteroposterior and transverse diameters increase by 2-3 cm
* Due to lower ribs flare out and increase in subcostal angle (from 68 to 103 degrees)
- Circumference increase by 5-7cm
These changes are due to relaxin
* Secreted by corpus luteum
* Relaxes ligament attachments of the ribs
Other changes
- Capillary engorgement throughout respiratory tract
--> Vocal cord may be swollen/oedematous
- According to [AA4:p630]
* Difficult intubation in term pregnant women is 1 in 300, compared with 1 in 2200 in non-pregnant population
* Tonge and epiglottis also increase enlarged
- Large airway dilated
--> Decreased airway resistance by 35%
Changes to lung volume
Significant changes in lung volume occurs from the 20th week
ERV and RV
- ERV and RV gradually decrease
- 20% less at term (than non-pregnancy level)
Causes of ERV and RV change
- Elevation of the diaphragm (main cause)
- Increase in pulmonary blood volume
FRC
- Decreases by 20% at term
- In supine, FRC is about 70% of that in erect position
Tidal volume
- Tidal volume begins to increase in the first trimester
--> 40% above non-pregnant level at term
NB:
- In [PK1:p349], both 28% increase and 40% increase in tidal volume were quoted
- [JN5:p320, KB1:p249] tidal volume increase by 40%
Capacities
At term
- Inspiratory capacity (IRV) increases 10%
- Expiratory capacity (ERV) decreases 20%
- Total lung capacity decrease by 5%
- Vital capacity unchanged
NB:
According to [KB2:p248, AA5:p326]
* IRV is unchanged
Other changes
Compliance
- Lung compliance unchanged
- Chest wall compliance decreases
--> Total lung compliance decreases by 20%
NB:
- Cause: elevation of the diaphragm
Minute ventilation
- Minute ventilation starts to increase in early weeks
- Maximal hyperventilation occurs as early as week 8-10
- Minute ventilation increases to 50% above non-pregnant level at term
Component
- 40% increase in tidal volume
- 10% increase in respiratory rate
NB:
- [KB2:p249] RR increase by 15%
- [JN5:p320] RR unchanged
Cause
- Stimulation of the respiratory centres by progesterone
- [JN5:p320] Progesterone sensitise central chemoreceptors
--> Increase the slope of pCO2/ventilation response curve by 3 fold
- [JN5:p320] Hypoxic ventilatory response is also increased by 2 fold
Result
At term, (with full renal compensation)
- pCO2 = 32mmHg
- pO2 increase very slightly due to hyperventilation
- Lower bicarbonate level (18-21mmol/L)
- pH normal
- Increase in 2,3 DPG
Overall,
- Oxygen dissociation curve stays unchanged
Other notes
- [JN5:p320] Posture makes little difference in oxygenation
Oxygen flux
According to [KB2:p249],
- Cardiac output increase by 30%
- Blood oxygen content decrease due to fall in [Hb]
Overall,
- Oxygen flux at term is about 10% above non-pregnant level
Changes during and after labour
During labour
- Uterine contractions increase O2 consumption by 60%
- Minute ventilation increases by 70% due to pain
--> Hypocapnia
--> Transient hypoventilatory period between contractions
--> Brief desaturation of O2
After delivery
- FRC and RV returns to normal within 48 hours
- Tidal volume declines to normal within 5 days
Anaesthetic implication
- Decreased FRC and higher O2 consumption
--> Reduce the O2 reserve
- Anatomical changes in upper airway
--> More difficult endotracheal intubation