The physiological changes of pregnancy

Introduction Cardiovascular System Respiratory System Gastrointestinal System Haematology Endocrine System Renal Function Central Nervous System

Cardiovascular System Multiple changes, many of which are

compensatory designed to cope with growing fetus, uterus and placenta

Increased cardiac output is main overall result of changes

Changes in haemodynamic parametersCardiac output Increases by 50%

Stroke volume Increases by 30%

Heart rate Increases by 25%

Ejection Fraction Increases by 20%

Left ventricular mass

Increases by 50%

Systemic VR Decreases by 20%

Pulmonary VR Decreases by 34%

Systolic BP Decreases by 6-8%

Diastolic BP Decreases by 20–25%

Haemodynamic changes

Cardiac output - HR / SV

SVR - low resistance vascular beds and hormonal effects

Blood Pressure - decreased

peripheral resistance

Aorto caval compression - positional - can cause severe hypotension

- dependent on gestation

Respiratory system Anatomical changes Lung mechanics and Volume Blood gases Pulmonary circulation

Respiratory system – Anatomical changes

Capillary engorgement in upper airway

Increased circumference of thoracic cage due to flaring of ribs

Upward displacement of diaphragm by enlarging uterus

Respiratory System – Lung Mechanics and Volumes

Tidal volume increases by up to 45% at term FRC is decreased by 20 – 30% at term due to

reduction in expiratory reserve volume and residual volume

Closing capacity can become greater than FRC causing V/Q mismatch and ready occurrence of hypoxia

Minute Volume increased by up to 50%, this is stimulated by high progesterone levels and increased carbon dioxide production

Respiratory System - Blood Gases pCO2 decreases secondary to

alveolar hyperventilation (3.7 – 4.2kPa)

Metabolic compensation reduces bicarbonate to about 18-21mmol/l

pO2 rises to approx 14kPa whilst upright, may fall when supine

Respiratory System – Pulmonary circulation Pulmonary vascular resistance

reduced Pulmonary blood flow increased Pulmonary blood volume increased Oxygen consumption increased by

30 – 60 % due to metabolic demands of fetus, uterus and placenta. (therefore hypoxia can develop rapidly)

Gastrointestinal system Reduced barrier pressure

(LOS pressure – gastric pressure)

Heartburn in 55-80% Reduced gastric secretion Delayed gastric emptying during labour,

particularly if opiates used

Haematology Plasma volume, red cell mass,

and total blood volume all increase

- increased intravascular space

- increased demand for oxygen

- proportionally less increase in red cell mass therefore reduced haemoglobin concentration

Coagulation- Concentration of most

coag factors increase Total plasma protein level falls

Endocrine System Thyroid gland enlarges but mother

remains euthyroid Cortisol and other corticosteroids

increase up to 5 times non pregnant level

Insulin resistance develops, small rise in blood glucose

Many anatomical changes due to production of hCG, oestrogen, progesterone and hPL by placenta

Renal Function Renal blood flow increases, and GFR

increases by about 50%. Clearance of creatinine, urea and amino acids increased

Sodium and water retention due to increased production of renin

Smooth muscle relaxation in renal pelvis, ureters, and bladder, therefore increased risk of urinary stasis and UTI.

Central Nervous System

PARAMETER CHANGE

MAC of volatiles Decrease

Volume of epidural space Decrease

Pressure of epidural space Increase

CSF volume Decrease

CSF pressure Increase

Sensitivity to local anaesthetics Increase

Local anaesthetic dose Decrease

Finally……

Most body systems affected Physiological changes may lead to

different disease presentations Anaesthetic technique must be

altered as appropriate