Labour Analgesia Flashcards
Where does pain arise from in the first stage of labour?
Pain arises mainly from the uterus due to pressure and dilatation of the lower segment and cervix.
Aδ and C fibres transmit the afferent impulses via sympathetic pathways and the hypogastric plexuses to the lumbar and low thoracic sympathetic chain, then via the posterior roots of T10 to L1 to the spinal cord.
Where is the visceral pain of the first stage of labour referred to?
The cutaneous dermatomes of T10 to L1 (via dorsal rami)
During the late first stage of labour, what does descent of the fetal head do?
Causes pressure on pelvic structures and causes pain with afferents via L2-S1 roots.
What is the pain pathway for the second stage of labour?
Pain of 1st stage continues.
Additional pain arises from dilatation and pressure of lower pelvic and pelvic floor structures
These afferents ( also Aδ and C fibres) are transmitted primarily via the pudendal nerves then to the cord via the S2,3 and 4 roots.
The somatic nerves lead to more localized pain than that arising from uterine contractions.
What physiological effects does pain during labour cause?
Increases in maternal ventilation and cardiac output.
Hyperventilation during contractions can lead to severe hypocapnia, leading to hypoventilation and hypoxaemia between contractions, leading to fetal compromise.
This can be minimized by effective analgesia.
What are the unwanted effects of nitrous oxide?
- dizziness
- sedation
- amnesia
- nausea
- hypoxaemia
- nitrous crosses the placenta, but is removed when the baby breathes
- inhibits methionine synthetase and folate
How efficacious is pethidine as a labour analgesic?
Low efficacy
What are the maternal SEs of pethidine?
- nausea/vomiting
- drowsiness
- amnesia
- dysphoria
What are the fetal SEs of pethidine?
- rapidly crosses placenta
- max fetal concentrations 1-5h after dose
- prolonged sedation, resp depression (increased requirement for naloxone), inhibition of normal feeding behaviour
- this is due to the long half life of the primary metabolite (norpethidine) in the neonate
What is the efficacy of diamorphine in labour?
Poor efficacy
What dose of diamorphine would you give IM in labour?
5-10 mg IM
(dose limited to 1 or 2, due to concern about neonatal respiratory depression with cumulative doses)
What are the maternal SEs of diamorphine?
- reduced incidence of vomiting compared to pethidine
- sedation
- respiratory depression
What are the neonatal SEs of diamorphine?
- diamorphine crosses the placenta rapidly
- both mother and fetus metabolise diamorphine to morphine, and to the primary metabolite of morphine - morphine 6 glucuronide (has a long half life in neonates)
- diamorphine may be associated with better APGAR scores at 1 min than pethidine
What do opioids do to labour?
Inhibit maternal pituitary oxytocin secretion
What is the efficacy of remifentanil in labour?
No significant difference to pethidine.
