Changes in the respiratory system

The effect of pregnancy on the respiratory system is less than the ef­fect on the cardiovascular system. Tidal volume is the volume of gas inspired or expired in each respiration; in the non-pregnant state this is approximately 500 mL, increasing to 700 mL in late pregnancy (34, 35).

Table 8.2 Cardiac chamber dimensions (measured by echocardiography) during pregnancy and postpartum in pregnant women (n=18)

LA, left atrial dimension; LVd, left ventricular diastolic dimension; RA, right atrial dimension; RVd, right ventricular diastolic dimension.

Reproduced from Campos O., Doppler Echocardiography During Pregnancy: Physiological and Abnormal Findings, Echocardiography, Vol 13, No 2, pp.135-146, 1996, with permission from John Wiley and Sons.

These changes are stimulated by progesterone which decreases the threshold and sensitizes the medulla oblongata to carbon di­oxide. This causes overbreathing which begins in each luteal phase of the menstrual cycle and is maintained if conception occurs (36). Pregnancy does not change the resting respiratory rate, which re­mains constant at approximately 14-15 breaths per minute (34). Minute ventilation is a measure of tidal volume and respiratory rate which is increased in pregnancy by about 40% from 7.5 L/min to 10.5 L/min due to the increase in tidal volume. Therefore, the resting pregnant woman increases her ventilation by breathing more deeply and not more frequently.

Progesterone also increases erythrocyte carbonic anhydrase con­centration in red cells causing carbon dioxide levels to be lower in pregnancy than non-pregnancy. Arterial pH remains constant due to an associated decrease in plasma bicarbonate concentration from 28 to 22 mmol/L (37). This contributes to a decrease in plasma osmolality. These changes are shown in Table 8.3. The low maternal carbon dioxide levels allow efficient placental transfer of fetal carbon dioxide which increases during the third trimester due to increased fetal metabolism.

Alveolar ventilation is increased which causes a 5% in­crease in maternal oxygen concentration. The increased levels of 2,3-diphosphoglycerate (DPG) in the red cells causes a rightward shift of the maternal oxyhaemoglobin dissociation curve. This facili­tates oxygen transfer to the fetus which has a lower oxygen concen­tration than the mother. This lower sensitivity of fetal haemoglobin to 2,3-DPG causes a marked leftward shift of the oxyhaemoglobin dissociation curve. Oxygen consumption increases by about 16% from 220 to 255 mL/min in pregnancy due to the increased demands of fetal and maternal tissues.

Forced expiratory volume (FEVi) describes the volume of air that can be forcibly expired in 1 second. This is approximately 80-85% in non-pregnant adults and this is not changed in pregnancy. Peak expiratory flow rate is similarly unchanged (38, 39). This lack of

Table 8.3 The influence of pregnancy on respiratory variables

PCO₂, partial pressure of carbon dioxide; PO₂, partial pressure of oxygen.

Source data from Broughton Pipkin P Maternal physiology In: Edmonds KD (ed), Dewhurst's Textbook of Obstetrics & Gynaecology, 8th edn, pp. 5-15. Chichester: John Wiley & Sons; 2012.

change is likely due to the net effect of bronchodilation in response to prostaglandin E and progesterone and bronchoconstriction due to prostaglandin F2α.

Clinical considerations

• Prostaglandin E is commonly used for induction of labour. Prostaglandin F2α is commonly used for severe postpartum haemorrhage (e.g. carboprost); its effects on bronchoconstriction can be severe in women with asthma.

• Dyspnoea is a common symptom experienced in pregnancy. It cannot be attributed to airway restriction given resistance is actu­ally reduced. It is therefore attributed to the level of effective reflex stimulation of overbreathing in the medulla.

• Spinal anaesthesia is commonly used for analgesia in labour and caesarean section. However, effective spinal anaesthesia is associ­ated with a significant reduction in FEVi, vital capacity, and peak flow and this is significantly greater in women with a body mass index (BMI) greater than 30 kg/m² (40).