Effects of pregnancy on the respiratory system
Two independent changes
Two independent changes: a hormonal one (progesterone) and a mechanical one (the uterus). They affect different variables.
| Variable | Change | Driver |
|---|---|---|
| Tidal volume | inc ~30-50% | Progesterone |
| Minute ventilation | inc ~20-50% | Progesterone |
| Respiratory rate | Unchanged or minimally inc | - |
| FRC | dec ~20% | Uterus |
| ERV | dec ~20-25% | Uterus |
| RV | dec | Uterus |
| TLC | dec slightly (~5%) | Uterus |
| VC, FEV1, FVC, FEV1/FVC, PEF | UNCHANGED | - |
| O2 consumption | inc 20-30% | Fetus, uterus, work of breathing |
- *Spirometry does not change in normal pregnancy -> an abnormal FEV1 or FVC in a pregnant woman is disease, never physiology*
- Dyspnoea of pregnancy affects 60-70% of women, often from the first trimester - i.e. before the uterus is large enough to matter mechanically. It is the progesterone.
Epidemiology
- Physiological dyspnoea in 60-70%, typically first or second trimester, at rest or on mild exertion, not progressive and not associated with hypoxaemia
- Asthma complicates ~8-12% of pregnancies - the commonest chronic disease in pregnancy
- **Rule of thirds: one third worsen, one third improve, one third unchanged**
- Pregnancy accounts for a disproportionate share of maternal deaths from PE, amniotic fluid embolism, and influenza/COVID-19 pneumonitis
- Airway oedema makes failed intubation ~8-10x more likely than in the non-pregnant patient
Hormonal (progesterone) - from early first trimester
- Progesterone is a direct respiratory stimulant and lowers the central chemoreceptor threshold for CO2
- -> inc tidal volume -> inc alveolar ventilation
- -> PaCO2 falls to 28-32 mmHg = a chronic compensated respiratory alkalosis
- -> renal HCO3 excretion -> HCO3 18-22 mmol/L, pH 7.40-7.47
- Consequence: a "normal" PaCO2 of 40 mmHg in a pregnant woman is a serious red flag - it means ventilatory failure, and buffering capacity is already reduced
- Oestrogen -> mucosal oedema and hyperaemia of the upper airway -> rhinitis of pregnancy, epistaxis, difficult intubation (use a smaller endotracheal tube, avoid nasal instrumentation)
Mechanical (uterus) - from mid-second trimester
- Diaphragm rises ~4 cm; subcostal angle widens from ~68 to ~103 degrees; chest circumference inc ~5-7 cm
- dec FRC and dec ERV (diaphragmatic elevation), worse supine
- -> less oxygen reserve -> rapid desaturation with apnoea (also inc O2 consumption)
- -> closing capacity encroaches on tidal volume -> small airway closure -> V/Q mismatch
- Diaphragmatic excursion is actually increased - the diaphragm rises but moves more; ventilation is diaphragm-dependent, so anything splinting it (ascites, obesity, high block) is poorly tolerated
Other relevant changes
- Plasma volume inc 40-50% > red cell mass inc 20-30% -> dilutional anaemia
- Cardiac output inc 30-50%; aortocaval compression supine after 20 weeks -> left lateral tilt
- Hypercoagulable: inc factors VII, VIII, X, fibrinogen and vWF; dec protein S; inc PAI-1/PAI-2 -> VTE risk inc 4-5x, highest postpartum
- Relative immune modulation (Th2 shift) -> more severe influenza, varicella, COVID-19, listeria, malaria
- Oxyhaemoglobin dissociation: maternal curve right-shifts (inc 2,3-DPG), fetal HbF left-shifts -> favours placental O2 transfer
Physiological vs pathological dyspnoea
| Physiological | Pathological | |
|---|---|---|
| Onset | 1st-2nd trimester, gradual | Sudden, or progressive in the 3rd trimester |
| At rest | Common, unchanged with activity | Worse with exertion |
| SpO2 | Normal | Reduced |
| Spirometry | Normal | Abnormal |
| Associated | None | Chest pain, cough, haemoptysis, fever, oedema, orthopnoea |
Blood gases in pregnancy - the reference ranges are different
Blood gases in pregnancy - the reference ranges are different
- PaCO2 28-32 mmHg, HCO3 18-22 mmol/L, pH 7.40-7.47, PaO2 100-105 mmHg
- A PaCO2 >35-40 mmHg in an unwell pregnant woman signals impending respiratory failure
- In an asthma exacerbation: a normal PaCO2 is a late and ominous sign, and it is reached earlier than in the non-pregnant patient
Investigating suspected PE - do not withhold imaging
Investigating suspected PE - do not withhold imaging
- Wells and D-dimer perform poorly (D-dimer rises physiologically)
- Bilateral leg ultrasound first if symptoms of DVT
- Then CXR, and:
- Normal CXR -> V/Q or perfusion-only scan (lower maternal breast dose)
- Abnormal CXR -> CTPA
- Fetal radiation from either is far below the teratogenic threshold (<50 mGy) - the greater risk is missing the diagnosis
- CTPA delivers a higher maternal breast dose; V/Q delivers a slightly higher fetal dose. Either is acceptable.
Asthma in pregnancy
Asthma in pregnancy
- *Undertreated asthma is far more dangerous to the fetus than any asthma medication* - hypoxia causes fetal growth restriction, preterm birth and perinatal loss
- Continue inhaled corticosteroids and long-acting beta-agonists - budesonide and salbutamol have the most safety data
- Treat exacerbations exactly as in the non-pregnant patient: oxygen, salbutamol, systemic corticosteroids, magnesium
- Oral corticosteroids are not withheld for an exacerbation
- Monthly review; written action plan; trigger management and smoking cessation
- Intrapartum: continue inhalers; avoid prostaglandin F2-alpha (carboprost), ergometrine; use oxytocin
Respiratory failure and resuscitation
- Target SpO2 >=95% - the fetus sits on the steep part of the dissociation curve and has little reserve
- Left lateral tilt (15-30 degrees) from 20 weeks to relieve aortocaval compression
- Anticipate difficult airway: senior operator, smaller tube, ramped position, pre-oxygenation (rapid desaturation from a low FRC)
- Perimortem caesarean within 4-5 minutes of maternal cardiac arrest - for maternal resuscitation as much as for the fetus
VTE
- LMWH - does not cross the placenta; warfarin is teratogenic (6-12 weeks) and DOACs are contraindicated
- Continue for the pregnancy and at least 6 weeks postpartum, minimum 3 months total
Prevention
- Influenza vaccine in any trimester, and pertussis (dTpa) at 20-32 weeks - both are strongly recommended and fund-listed in Australia
- COVID-19 vaccination; RSV maternal vaccination per current schedule
- Smoking cessation support
Associations
- Asthma; cystic fibrosis; pulmonary hypertension (*maternal mortality 15-30% - a contraindication to pregnancy*)
- Pulmonary embolism and amniotic fluid embolism
- Pre-eclampsia with pulmonary oedema; peripartum cardiomyopathy; tocolysis-associated pulmonary oedema
- Lymphangioleiomyomatosis (oestrogen-driven, may worsen)
- Sleep-disordered breathing - inc with pregnancy, associated with pre-eclampsia and gestational diabetes
- Rhinitis of pregnancy, epistaxis
- Increased severity of influenza, COVID-19, varicella pneumonitis, coccidioidomycosis, listeriosis
- Obesity - compounds every mechanical change
Postpartum
- All respiratory changes resolve within days to weeks postpartum
- FRC normalises within 1-2 weeks; PaCO2 and HCO3 over several weeks
- Diaphragm position normalises immediately after delivery - hence the rapid relief of dyspnoea
- Asthma control usually returns to the pre-pregnancy pattern within 3 months, and tends to follow the same course in subsequent pregnancies
- VTE risk is highest in the first 6 weeks postpartum, not antenatally
Complications of untreated respiratory disease
- Fetal growth restriction, preterm birth, low birth weight, perinatal mortality
- Maternal ICU admission and death - PE and infection dominate
- The recurring theme: the risk of the disease exceeds the risk of the treatment or the investigation - this is the single most important point in respiratory medicine in pregnancy
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