Obstetric Anaesthesia

Obstetric Anaesthesia

Physiology of Pregnancy

·        Respiratory: 

o  Raised diaphragm ® ¯functional residual volume

o  ­O2 demand: due to ­ maternal metabolism and foetal demands

o  ­Respiration ® respiratory alkalosis (PCO2 approx. 32) with metabolic compensation

o  Þ Becomes apnoeic more quickly

·        Cardiovascular: 

o  ­Blood volume, ­Hb by not so much ® physiological anaemia

o  Blood pressure: if normal pregnancy then small ¯ in systolic, ¯¯ in diastolic due to vasodilation

o  Minimum Alveolar Concentration (MAC) lowers in pregnancy 

o  Volume of CSF reduces ® ¯dose of spinal by 2/3

·        GI: lower oesophageal sphincter less effective ® ­regurgitation

General Anaesthetic in Pregnancy

·        More difficult getting endotracheal tube in: throat is smaller, usually done at night in emergency, etc

·        Mendelssohn‟s syndrome: acid aspiration in surgery when pregnancy ® bigger A-a gradient compared with non-pregnant. Give H2 blockers to ¯stomach acid

·        ­Awareness: inhaled and induction agents cross placenta – so often more cautious dosing. But muscle relaxants don‟t cross placenta. Usual dosing. But can‟t tell if they‟re aware

·        So trend from GA to epidural in pregnancy

·        If doing GA: use rapid sequence induction. Use anaesthetic agent, muscle relaxant and cricoid pressure to occlude oesophagus until tube in

Pathology in Pregnancy

·        Pre-eclampsia: ­BP, proteinuria, oedema

·        Eclampsia: ¯ circulating blood volume, swollen tissues, fits, death

·        Placenta praevia: placenta over os ® big bleed

·        Abruption: placenta separates from uterus ® big bleed

·        Amniotic fluid embolism ® equivalent of PE and DIC 

·        Aorta-caval occlusion/compression. ® faint when they lie down. Uterus compresses IVC ® ¯venous return ® ­HR and vasoconstriction ® ¯perfusion pressure to uterus. Lie on left side, push uterus to left if doing CPR

Analgesic Drugs In Labour

·        Mechanism of pain:

o   Ischaemic pain during contractions is due to ¯O2

o   Contraction against resistance ® colicky pain (like gallbladder)

o   Mechanical: pelvis and perineum

·        Epidural:

o   Very effective

o   Good in eclampsia and where high suspicion of intervention

o   Complications: 

§  ? ­ Rate of subsequent intervention

§  Hypotension

§  Paralysis, infection, haematoma, wrong drug

§  If into subarachnoid ® total spinal overdose

§  If into vein ® cardiotoxic drugs, CNS damage

§  If CNS drains out, positional headache

o   Can‟t have epidural if:

§  Patient refusal

§  Hypovolaemic due to haemorrhage 

§  On anticoagulants, in case puncture epidural vein ® haematoma

§  Septic: could transfer bug from blood to CSF

·        Inhalational – N2O/Nitrous Oxide: variable satisfaction, analgesia and dissociative, 50% vomit, 2 min to peak effect, no further effect once breathed out

·        Opiates (e.g. Pethidine): variable satisfaction, dissociative, safe (midwives can use it), can ® respiratory depression in neonate

·        Psychoprophylaxis: very effective.  If frightened and don‟t know what‟s going to happen, hurts more