Respiratory Physiology in O&G
Clinical Overview
Start with one sentence that the whole chapter hangs from: the lung exists to load oxygen into blood and unload carbon dioxide out of it, and pregnancy reorganises that machine to serve two people while leaving the mother with less room for error. Everything that follows — rapid desaturation at induction, why a "normal" adult PaCO2 can be ominous in pregnancy, why pneumonia and sepsis harm the fetus through maternal oxygen delivery, why pulmonary oedema appears easily in pre-eclampsia, why ARDS is a shunt-and-compliance problem, and why cord gases are the fetal end of maternal ventilation and placental exchange — is a consequence of that single job done under pregnancy conditions.
Respiratory physiology in O&G is therefore not a list of pregnancy changes to memorise. It is the science of how a borderline oxygen and acid-base economy behaves when disease, anaesthesia or obstetric catastrophe pushes against it.
Pregnancy creates a respiratory system with higher ventilation, lower carbon dioxide, lower oxygen reserve and less tolerance for delay. Minute ventilation rises mainly through tidal volume. Functional residual capacity falls. Oxygen consumption rises. The airway becomes more vascular and oedematous. The baseline arterial blood gas (ABG) shifts toward a compensated respiratory alkalosis. These changes are normal, but they leave little margin when disease adds hypoxaemia, hypercapnia, airway difficulty, pulmonary oedema, sepsis or anaesthetic apnoea.
The useful Primary model is:
progesterone-driven hyperventilation -> lower PaCO2 and bicarbonate -> easier maternal-fetal CO2 transfer -> reduced FRC plus increased oxygen consumption -> rapid desaturation when ventilation stops
Core Knowledge
The Respiratory System Has Three Jobs
The lung must ventilate alveoli, exchange gases and match ventilation to perfusion. The cardiovascular system then carries oxygen and carbon dioxide between lung, tissues and placenta.
| Function | Mechanism | O&G relevance |
|---|---|---|
| Ventilation | Movement of gas in and out of alveoli | Determines PaCO2 and pH |
| Oxygenation | Transfer of oxygen from alveolus to blood | Determines maternal and fetal oxygen supply |
| CO2 elimination | Transfer of CO2 from blood to alveolus | Determines respiratory acid-base status |
| V/Q matching | Matching air flow to blood flow | Explains shunt, PE, pneumonia and ARDS |
| Airway protection | Upper airway reflexes and swallowing coordination | Aspiration risk during anaesthesia, seizures, reduced consciousness |
The key split is:
ventilation is about CO2; oxygenation is about alveoli, V/Q, diffusion, haemoglobin and cardiac output
A patient can oxygenate poorly with normal CO2 early in disease if she compensates by increasing ventilation. A rising PaCO2 in a tachypnoeic pregnant patient is therefore a late and dangerous sign.
How a Breath Physically Happens
Before any gas can be exchanged it has to move, and movement obeys mechanics. This is the foundation the rest of the chapter rests on.
Inspiration is active. The diaphragm (supplied by the phrenic nerve from cervical roots C3 to C5) and the intercostal muscles contract, the chest enlarges, and the pressure in the pleural space falls below atmospheric. That negative pressure pulls the lung open against its own elastic recoil, so gas flows in down a pressure gradient. Expiration at rest is passive: the muscles relax and the lung's elastic recoil deflates it. Because the phrenic nerve arises so high in the cervical cord, diaphragmatic breathing survives a thoracic-level spinal injury but not a high cervical one — a fact that matters in obstetric trauma and high regional block.
Two mechanical properties decide how hard that work is:
- Compliance is how easily the lung stretches for a given pressure (change in volume per change in pressure). A stiff, low-compliance lung needs more pressure for the same breath, so the work of breathing rises.
- Surface tension at the air-liquid interface inside each alveolus tends to collapse it. By the law of Laplace, the collapsing pressure rises as the alveolus gets smaller, so without help, small alveoli would empty into large ones.
Surfactant — made by alveolar type II pneumocytes — is the lung's answer. It lowers surface tension, and lowers it most in the smallest alveoli, which keeps them open and keeps compliance high. This single molecule links three apparently separate topics in O&G: fetal lung maturation (surfactant appears late in gestation, which is why preterm infants develop respiratory distress and why antenatal corticosteroids are given), neonatal resuscitation, and adult ARDS (where injured pneumocytes stop making functional surfactant, so alveoli collapse and the lung stiffens).
| Mechanical property | What it means | O&G relevance |
|---|---|---|
| Compliance | Volume change per pressure change; lung stretchiness | Falls in pulmonary oedema, ARDS, late pregnancy chest-wall loading |
| Elastic recoil | The lung's tendency to deflate | Drives passive expiration; opposed by surfactant and chest wall |
| Surface tension / Laplace | Pressure that collapses small alveoli | Why surfactant exists; why preterm lungs and ARDS lungs collapse |
| Surfactant | Type II pneumocyte product that lowers surface tension | Fetal lung maturity, neonatal RDS, ARDS pathophysiology |
| Airway resistance | Opposition to gas flow in conducting airways | Asthma, bronchospasm; measured as FEV1/FVC |
Airway Smooth Muscle Mediators
Airway tone is controlled by autonomic, inflammatory and prostaglandin pathways. This is why some O&G medicines are respiratory problems before they are obstetric solutions.
| Mediator or pathway | Usual airway effect | O&G relevance |
|---|---|---|
| Beta-2 stimulation | bronchodilation | salbutamol physiology; beta-agonist side effects |
| Histamine and leukotrienes | bronchoconstriction and oedema | asthma, allergy and anaphylaxis |
| Prostaglandin E2 | often bronchodilator or protective in airways | prostaglandin biology is receptor-specific |
| Prostaglandin F2-alpha | bronchoconstriction risk | carboprost can trigger bronchospasm; avoid/caution in asthma |
| Nitric oxide | smooth-muscle relaxation and vascular effects | pulmonary vascular physiology and gas-signalling concept |
The exam discriminator is receptor biology. "A prostaglandin" is not one effect. PGE and PGF pathways differ, so a drug used for uterine atony can be dangerous in a patient with reactive airways.
In normal pregnancy these influences happen to cancel out. Progesterone and prostaglandin E2 relax airway smooth muscle, while prostaglandin F2-alpha and the lower resting lung volume tend to narrow the airways. The net effect is that overall airway resistance, FEV1 and peak flow do not change in healthy pregnancy. The clinical message is therefore the inverse: measurable airflow obstruction is never a feature of normal pregnancy, so spirometric obstruction in a pregnant patient is disease until proven otherwise.
Minute Ventilation, Tidal Volume and Dead Space
Minute ventilation is the total gas moved each minute:
minute ventilation = tidal volume x respiratory rate
Tidal volume is the amount of gas moved with each breath. Respiratory rate is breaths per minute. Pregnancy increases minute ventilation mainly by increasing tidal volume, not by dramatically increasing respiratory rate.
The more useful equation is alveolar ventilation:
alveolar ventilation = (tidal volume - dead space) x respiratory rate
Dead space is ventilated gas that does not exchange CO2 with blood. Anatomical dead space is the conducting airway. Physiological dead space includes alveoli that are ventilated but poorly perfused, such as in pulmonary embolism or low cardiac output.
| Variable | If it increases | Physiological consequence |
|---|---|---|
| Tidal volume | More alveolar gas per breath | PaCO2 falls if CO2 production unchanged |
| Respiratory rate | More breaths per minute | PaCO2 falls until fatigue or dead-space limits appear |
| Dead space | More wasted ventilation | PaCO2 rises unless minute ventilation increases |
| CO2 production | Fever, sepsis, labour, shivering | Ventilation must rise to keep PaCO2 controlled |
High-yield chain:
sepsis -> high CO2 production plus increased work of breathing -> tachypnoea -> fatigue -> falling alveolar ventilation -> PaCO2 rises -> impending respiratory failure
A sense of scale helps. Resting oxygen consumption in a non-pregnant adult is roughly 250 mL/min, met by delivering about 1 L/min of oxygen to the tissues and extracting around a quarter of it. Pregnancy raises oxygen consumption by only a modest amount — on the order of 50 mL/min extra near term — yet minute ventilation rises by around 40 percent, far more than the rise in oxygen demand. That mismatch is the whole point: the extra ventilation is not there to meet a large oxygen requirement, it is there to drive PaCO2 down and create the maternal-fetal carbon dioxide gradient. The small absolute reserve also explains why a labouring, febrile or septic woman, whose oxygen consumption climbs steeply, can outrun her supply quickly.