Physiological Changes in Pregnancy and Their Anaesthetic Implications

Physiological Changes in Pregnancy and Their Anaesthetic Implications

• Physiological adaptation in pregnancy occurs due to three broad mechanisms (Miller’s Anesthesia, 10th ed.):

• (1) Hormonal changes – oestrogen, progesterone, relaxin, prostaglandins, human placental lactogen, renin–angiotensin–aldosterone activation.
• (2) Mechanical effects of the enlarging gravid uterus (aortocaval compression, diaphragmatic splinting, altered spinal curvature).
• (3) Increased maternal metabolic demand and biochemical changes driven by the fetoplacental unit (increased O2 consumption and CO2 production).
• These adaptations begin as early as the first trimester (well before mechanical effects of the uterus become significant) – important exam point, since many changes (e.g., reduced neuraxial dose requirement, reduced MAC) are biochemical/hormonal in origin, not purely mechanical.
• Nearly every organ system is affected; anaesthetic management must account for altered pharmacokinetics, pharmacodynamics, physiological reserve, and the presence of a second patient (the fetus).

1. Key Quantitative Changes

1.  ECG & Auscultatory Changes
• ECG: shortened PR interval, uncorrected QT interval, small right axis deviation (1st trimester) shifting to left axis deviation (3rd trimester, due to diaphragmatic elevation), transient ST changes, and benign ectopics (atrial/ventricular) and sinus tachycardia are the commonest benign dysrhythmias.
• Auscultation: loud, split S1; systolic ejection murmur (tricuspid annular dilation); S3 (and sometimes S4) audible – of no clinical significance in isolation; apex beat displaced left and up.
• Red flags requiring cardiology work-up: any diastolic murmur, systolic murmur > grade III, severe arrhythmia, or unequivocal cardiomegaly on imaging – these are NOT explained by normal pregnancy physiology.

1. Lung Volumes, Capacities & Ventilation (% change from non-pregnant, at term)

• Oxyhaemoglobin dissociation curve shifts right in the mother (P50 27→30 mm Hg) but lies to the left in the fetus (P50 ~18–19 mm Hg) – this gradient promotes O2 offloading from mother to fetus across the placenta .
• O2 consumption increases 20–60% (Miller: 20–35% at term, further rising in labour to +40% in the first stage and +75% in the second stage) due to fetal/placental/uterine metabolic demand and increased maternal cardiac and respiratory work.
• After 99% denitrogenation, time to SpO2 <90% is ~4 minutes in pregnant patients vs ~7 min 25 sec in non-pregnant – underscoring the importance of thorough pre-oxygenation.

Upper Airway Changes

• Oestrogen-driven increase in airway connective tissue, blood volume, total body water, and interstitial fluid → capillary engorgement, mucosal friability, and oedema of the nasopharynx, oropharynx, larynx and trachea from early in the first trimester.
• Mallampati class tends to worsen as pregnancy/labour progresses (further increased by active pushing in the second stage and by pre-eclampsia).
• Avoid nasal instrumentation/nasotracheal intubation/nasogastric tubes unless essential – risk of brisk epistaxis.
• Use a smaller cuffed endotracheal tube, typically 6.0–7.0 mm ID (commonly quoted as 6.5 mm as a good default), because of airway oedema and narrowing.

Anaesthetic Significance

• Airway management is more challenging: weight gain/breast engorgement hinder laryngoscope insertion; friable mucosa bleeds easily; failed intubation rates are historically quoted as higher in obstetric than general surgical populations.
• Response to anaesthetics: reduced MAC; faster inhalational induction (reduced FRC + increased minute ventilation → faster rise in FA/FI); increased sensitivity to IV induction agents and benzodiazepines → risk of rapid overdose/loss of airway reflexes.

Greater risk of hypoxaemia during apnoea: reduced O2 reserve (↓FRC) + increased O2 consumption + rapid airway obstruction potential.

• Progesterone and oestrogen relax lower oesophageal sphincter (LES) tone → reduced barrier pressure.
• The enlarging uterus elevates and rotates the stomach, shifting the intra-abdominal oesophagus into the thorax and abolishing the protective “pinch-valve” effect at the diaphragmatic hiatus.
• Placental (ectopic) gastrin secretion may increase gastric acidity, although plasma gastrin levels themselves are often reduced/unchanged in studies.
• Gastric emptying of solids/liquids is NOT delayed by pregnancy itself (confirmed on ultrasound studies, including in obesity) – this is a common exam misconception. It IS delayed by: painful uterine contractions/labour, opioids (parenteral or neuraxial, e.g. added fentanyl), and anxiety.
• Epidural analgesia without opioid does not impair gastric emptying, and may even facilitate it.
• Current ASA/consensus position: clear liquids may be consumed during labour by low-risk labouring patients without additional risk factors (obesity, diabetes, difficult airway) – solids should still be avoided.
• Net effect: increased risk of regurgitation and pulmonary aspiration, and increased severity of pulmonary injury if aspiration occurs, due to lower gastric pH and higher volume.

Anaesthetic Significance

• All labouring patients are considered to have a “full stomach”.
• Aspiration prophylaxis (non-particulate antacid ± H2-blocker/PPI ± prokinetic) should be considered before any anaesthetic (neuraxial or general) from the point of view of “worst case” conversion to GA.
• Rapid sequence induction with cricoid pressure (traditionally) and cuffed tracheal tube remains the default technique for general anaesthesia after ~12–18 weeks’ gestation (practice varies by guideline/era – see Section 18 for the evolving cricoid pressure evidence).

• Absolute hepatic blood flow does not change significantly, but the fraction of cardiac output reaching the liver falls (~35% reduction relative to total CO, per WhatsApp/Kaushik notes) even as systemic flow rises – Miller states hepatic blood flow itself is essentially unchanged; know both framings.
• AST, ALT, and bilirubin rise to the upper limit of normal; overt elevation should still prompt work-up.
• Alkaline phosphatase rises up to 2–4× normal due to placental (and fetal) ALP production – this makes ALP unreliable as a marker of hepatobiliary disease in pregnancy.
• Plasma albumin and total protein fall progressively across trimesters (albumin ~4.1–5.3 g/dL non-pregnant → as low as 2.3–4.2 g/dL by the third trimester); albumin:globulin ratio falls from ~1.4 to ~0.9 → more free (unbound) fraction of highly protein-bound drugs.
• Spider naevi, palmar erythema, and oesophageal varices/telangiectasia can occur in up to 60% of normal pregnancies from high oestrogen – do NOT automatically imply liver disease, but caution is still needed with nasogastric tubes/oesophageal temperature probes because of possible varices.
• Plasma cholinesterase (pseudocholinesterase) activity falls ~25–30% antepartum and further to <60% of

non-pregnant values postpartum, but this rarely causes clinically significant prolongation of succinylcholine

block at standard doses.

• Increased incidence of gallbladder disease (incomplete emptying, altered bile composition); acute cholecystitis is the second most common cause of the acute abdomen in pregnancy.

• Among the earliest and most dramatic physiological changes of pregnancy.
• Renal blood flow ↑ 50–80%; kidneys enlarge by up to 30%.
• GFR ↑ ~40–65% (Miller: up to 50% by the third month, sustained until ~3 months postpartum) → creatinine clearance rises.
• Because of the raised GFR, “normal” non-pregnant BUN/creatinine values are relatively elevated for pregnancy: normal pregnant values are approximately creatinine 0.5 mg/dL and BUN 9 mg/dL – values in the “normal” non-pregnant range may reflect significant renal impairment in a pregnant patient.
• Relaxin drives renal vasodilation; progesterone causes smooth-muscle relaxation and dilation of the ureters and renal pelvis – up to 80% of women develop physiological hydronephrosis by mid-pregnancy (more marked on the right due to uterine dextrorotation) – predisposes to urinary stasis and UTI, and can confound imaging interpretation.

Reduced proximal tubular reabsorption with increased filtered load causes physiological glucosuria (up to 1–10 g/day) and mild proteinuria (up to 200–300 mg/day, upper limit of normal often quoted as 300 mg/24h) – both are normal in pregnancy; significant proteinuria (e.g., ≥300 mg/24h with hypertension) raises concern for

pre-eclampsia.

1. Blood Volume & Red Cell Indices
• Plasma volume increases proportionally more than red cell volume → physiological (dilutional) anaemia of pregnancy; typical term haemoglobin ~11.6 g/dL (values below this at any stage should prompt evaluation for true anaemia, e.g. iron deficiency).
• Physiological hypervolaemia serves to: improve nutrient delivery to the fetus, protect against maternal hypotension, buffer delivery blood loss, and (via reduced viscosity) reduce resistance to flow through the uteroplacental bed.
• Driven by oestrogen/progesterone-mediated activation of the renin-angiotensin-aldosterone system (sodium and water retention: ~900 mEq sodium, ~7000 mL total body water retained), and by rising plasma adrenomedullin.
• White cell count rises physiologically (leukocytosis, up to 13,000/mm³ is normal; can rise further in labour, unrelated to infection) – do not over-interpret a mildly raised WCC as sepsis in isolation.

2. Plasma Proteins

• Total plasma protein falls (~7.8 → 7.0 g/dL); albumin falls proportionally more than globulin → falling albumin:globulin ratio.
• Colloid osmotic pressure falls by ~5 mm Hg (WhatsApp note) / 27→22 mm Hg (Miller) – increases susceptibility to pulmonary oedema, particularly relevant in pre-eclampsia and with aggressive fluid therapy.

3. Coagulation – Pregnancy is a Hypercoagulable State

• PT and aPTT are both shortened by ~20%; conventional coagulation tests do NOT reliably detect these changes.
• Thromboelastography (TEG)/ROTEM show a hypercoagulable trace: ↓R, ↓K, ↑α-angle, ↑MA, ↓lysis – changes evident from as early as 10–12 weeks and more marked in labour.
• Platelet count: unchanged, or a mild (~10%) fall in the third trimester from dilution and increased turnover. About 8% of otherwise healthy women have a platelet count <150,000/mm³ – usually gestational thrombocytopenia (a diagnosis of exclusion, rarely <70,000/mm³, not associated with abnormal bleeding). Bleeding time is not altered by normal pregnancy.
• Differential diagnosis of thrombocytopenia in pregnancy: gestational thrombocytopenia, hypertensive disorders of pregnancy (pre-eclampsia/HELLP), and immune (idiopathic) thrombocytopenic purpura.

Anaesthetic Significance

• A routine platelet count is not required before neuraxial anaesthesia in an otherwise healthy, asymptomatic parturient.
• If thrombocytopenia is suspected clinically (pre-eclampsia, easy bruising, known ITP), a platelet count (and clinical bleeding assessment) should be obtained before neuraxial block.
• Despite “normal” intrapartum blood loss being well tolerated, the net hypercoagulable state increases the risk of venous thromboembolism throughout pregnancy and especially postpartum – VTE prophylaxis is an important part of peripartum care.

• Cerebral blood flow increases.
• MAC of volatile anaesthetics is reduced by up to 30–40% (animal studies show up to 40% reduction; human studies ~28% reduction in the first trimester – Miller 10th ed.) and this reduced requirement is present from very

early in gestation.

• Mechanism is not fully clear; likely multifactorial with a probable role for progesterone; interestingly, EEG data suggest the reduction in MAC (immobility response) may act predominantly at the level of the spinal cord rather than a change in cortical/brain anaesthetic sensitivity per se – a nuanced, exam-relevant point from Miller 10th ed.
• Faster inhalational induction: reduced FRC + increased minute ventilation → more rapid rise in alveolar (FA) to inspired (FI) anaesthetic concentration ratio.
• Increased sensitivity to IV induction agents (propofol) and sedatives (benzodiazepines) – reduce doses accordingly.
• Neuraxial local anaesthetic dose requirements fall by ~40% at term (subarachnoid dose reduced by ~25–30%; epidural dose unaltered or only slightly reduced) – local anaesthetic requirements return to normal by 8–24 hours postpartum.
• Two proposed mechanisms for reduced neuraxial requirement:

• Mechanical: aortocaval compression → engorged epidural venous plexus + increased epidural fat → reduced epidural space and CSF volume per spinal segment → greater spread of a given LA dose.
• Biochemical/hormonal: progesterone alters neuronal membrane sodium channel activity/permeability, and CSF specific gravity and acid-base changes occur – this explains why reduced LA requirement is detectable as early as the end of the first trimester, well before any significant mechanical venous engorgement has developed.
• Technical considerations for neuraxial block: increased lumbar lordosis (narrows interspinous gaps, may make landmark palpation harder); apex of thoracic kyphosis sits at a higher level; in the lateral position a head-down tilt naturally occurs due to a wider pelvis (may cause unexpectedly rostral spread).
• Relative resistance to vasopressors is described (altered receptor sensitivity) – but note the opposite is also quoted for chronotropes/vasopressors used during GA – be ready to explain both contexts if asked. 

• Total T3/T4 rise (oestrogen-driven increase in thyroid-binding globulin); free T3/T4 remain unchanged – the patient is clinically euthyroid.
• TSH falls in the first trimester (cross-reactivity of hCG with the TSH receptor) and normalises thereafter.
• Reduced peripheral tissue sensitivity to insulin (placental hormones, e.g. human placental lactogen, are diabetogenic) → higher post-prandial glucose after a carbohydrate load; exaggerated starvation ketosis can occur

due to high fetoplacental glucose consumption.

• Hyperplasia of pituitary lactotrophs → physiological hyperprolactinaemia.
• Active (free) cortisol levels rise ~2.5× above non-pregnant levels (increased production and decreased clearance).

• Relaxin increases joint laxity/mobility throughout pregnancy.
• Progressive lumbar lordosis compensates for the shifting centre of gravity from the enlarging uterus and weight gain → strain on the lower back and pelvic joints, and increased fall risk.