Physiology of pregnancy

Regarding normal physiological changes in a healthy pregnancy, which one of the following changes would NOT be consistent with expected changes?

a) 10% increase in heart rate by 12 weeks gestation

b) 20% increase in stroke volume by 12 weeks gestation

c) 20% increase in red cell volume by 28 weeks gestation

d) 20% increase in anatomical dead space by 28 weeks gestation

e) 50% increase in glomerular filtration rate by 12 weeks gestation

Answer: d

Explanation

Virtually every organ system exhibits physiological change during pregnancy to

compensate for the increased demands of sustaining the mother and growing foetus

and in preparation for the enormous physiological challenge of parturition. Causing

some frustration to candidates is that authoritative sources occasionally differ in their

account of some of the variations. An example of this is the calibre, volume and

resistance of large conducting airways. Some argue that capillary engorgement and

oedema reduce calibre whereas others propose that prostaglandin-mediated smooth

muscle relaxation causes dilation of conducting airways resulting in a marked reduction

in resistance and an increase in anatomical dead space by up to 45% at the third

trimester. Either way, Option (d) stands out as incorrect when compared to the correctness

of the other four options. The 20% increase in red cell volume is tempered by the

50% increase in plasma volume in the same period giving rise to the ‘physiological

anaemia of pregnancy’. Total blood volume increases by 40%. Other changes are not

listed here but it aids recall if they are categorised into cardiovascular, respiratory,

gastrointestinal (and hepatic), renal, haematological (including volumes), neurological

and those other changes secondary to the placenta as an endocrine organ.

Reference

Power I, Kam P. Maternal and neonatal physiology. In: Principles of Physiology for the

Anaesthetist. London: Arnold, 2001; pp. 345–52.