There is a good article by Hegewald & Crapo which summarises the respiratory physiology of pregnancy in clear and concise terms. On the basis of this article, I have tried to compile a list of the relevant changes:
|The diaphragm is pushed up||by 4cm|
|Tidal volume increases||by ~ 30-50%|
|Respiratory rate increases||to 15-17|
|Minute volume increases||by 20-50%.|
|Chest wall compliance decreases|
|Lung compliance remains the same|
|pH increases||to 740-7.47|
|PaCO2 decreases||to 30 mmHg|
|PaO2 increases||to 105 mmHg|
|HCO3- decreases||to 20 mmol/L|
|Maternal 2,3-DPG increases|
|p50 remains the same because of alkalosis|
All of this can probably be demonstrated in a big confusing diagram.
Diaphram is pushed up by 4 cm
The diaphram is pushed up by 4cm on average.
Weirdly, not all of this is due to to the effect of the gravid uterus pushing up the abdominal contents.
Apparently the hormone-induced laxity of rib ligaments begins well before the uterus becomes large enough to shove any abdominal contents into the chest cavity. This, however, does not decrease the mechanical effectiveness of the diaphram, and if anything it works more efficiently during pregnancy, with an increase in diaphragmatic excusion by about 2cm.
Tidal volume increases by ~ 30-50%
The increased diaphragmatic excursion changes the tidal volume from (lets say) 500ml in the first trimester to over 700ml in the third trimester.
Respiratory rate increases from 15 to 17 on average; thus, minute volume increases by 20-50%.
This seems to be driven by progesterone, as will be discussed later.
Chest wall compliance decreases (but lung compliance remains the same)
As one might imagine, the workload of the ventilator turbine is somewhat increased by the increased weight of the chest wall, owing particularly to the large turgid breasts heaving around upon it. The changes in ribcage configuration and the upward incursion of the diaphragm also play a role, as the normal "bucket handle" mechanics become disturbed. Lastly, the gravid uterus provides a counterpressure to inspiration, in a manner similar to the distended abdomen of an abdominal compartment syndrome.