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

1. Elevation of the diaphragm (main cause)
2. 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