Analgesia and Anesthetics during labor Flashcards
Oxygen consumption and maternal metabolism requirement
Maternal tissue: 4ml/min
Fetus+ uterus+ placenta: 12ml/min
At term: oxygen consumption >20ml/min
List normal haematological changes during pregnancy
Physiological haemodilution and anaemia
Thrombocytopenia: due to physiological changes, or pathologies e.g. pre-eclampsia causing HELLP
Pro-coagulative outisde utero-placental unit
Common CVS changes during pregnancy
Sinus tachycardia
JVP: height remain unchanged
Peripheral pulses: ↑volume
Atrial and ventricular ectopic beats, T wave inversion in lead III and aVF
Loud ± split S1, S3, ejection systolic murmur (flow murmur, from 2T onwards and disappear upon delivery)
Explain why pregnant women masks hypotension due to blood loss
Physiological increase in cardiac output masks blood loss and hypotension until 30-35% blood loss
- ↑cardiac output: by 40-50%
Mostly during 1st trimester, can reach 7L/min at rest by 24w
Reaching plateau during 3rd trimester
Further ↑during labor - ↑heart rate: by 10-15bpm cf baseline
- ↑stroke volume: by 10-20mL/beat
Need to differentiate cause of tachycardia during delivery between stress vs blood loss
Non-pharmacological measures for pain relief during labor
One-to-one care in labor from midwife or effective birth partner
Relaxation and breathing exercise
Massage (esp by husband) and acupuncture
Birth ball
Water bath
Intracutaneous water papules
Moxibustion
Low-risk, but difficult to perform RCT and evaluate efficacy
Pharmacological options for pain relief during labor
- Transcutaneous electrical nerve stimulator (TENS)
- Narcotic analgesia - IM pethidine 50-75mg up to Q3-4h
- Inhalation analgesia - self-administration by Entonox facemask (1:1 mixture of N2O:O2)
- Epidural analgesia - mixture of low concentration LA + opioid
Prevention of aortocaval compression
Left uterine displacement by:
- Left tilt with spine board
- Manual displacement and manipulation
Ventilatory parameter changes during pregnancy
Relevance for anaesthesia use
Physiological changes
* * ↑ventilation by 40%
* ↑tidal volume by 40%
* ↓expiratory reserve by 20% but unchanged inspiratory reserve
* ↓lung residual volume
* NO change in respiratory rate
* NO change in vital capacity overall
Pregnant women: prone to develop hypoxaemia
- reduced oxygen reserve and increased demand
- Hyperventilation due to pain/ bear down effort/ anxiety during delivery
- Further decrease FRC by atelectasis, shunting, Increased O2 A-a gradient
- High metabolism and O2 consumption at term
- Quick desaturation, meaning limited time and attempts of intubation allowed
GI changes during pregnancy
Relevance to anaesthesia use
Relevance:
- High risk of aspiration due to decreased gastric motility, increased gastric secretion and acidity, increase intra-gastric pressure
- Intubation is required for GA, airway adjuncts are inadequate
Infant drug exposure dose calculation
Determinants of drug toxicity to infants
Starting feeding before dose may help minimise infant exposure
Poorly developed BBB and liver enzymes in fetus increases risk of toxicity
Parenteral drugs present much higher bioavailability than topical solutions
Physiology of labor pain
First stage:
- Visceral C and A-delta fibers in lower uterine segment and cervix
- Pain signal travels via paracervical ganglion > hypogastric nerve > lumbar sympathetic chain > T10-L1 > higher center
Second stage
- Large A-delta fibers at vagina and perineum > pedendal nerve > S2-S4 > higher center
- Larger fibers require higher concentration of analgesia for second stage
- Non-epidural analgesia might not be able to target pain in both stages
Reason for pain relief during labor
Pain associated:
- Impaired cognitive function
- Predicts PTSD
- Increase postnatal depression
Transcutaneous electrical nerve stimulator (TENS)
- MoA
- C/I
- Procedure
- Use
Narcotic analgesia
- MoA
- S/E
- Disadvantage
Drug of choice: usu IM pethidine 50-75mg up to Q3-4h
Alternative: Intravenous PCAA Remifentanil infusion
S/E:
* N/V in mother → often given tgt with anti-emetics
* Maternal drowsiness and sedation
* Delayed gastric emptying → ↑risk if GA is subsequently required
* Neonatal respiratory depression: esp if ≤2h of delivery
Disadvantage:
- Poor efficacy
- Affect neurobehavioral score of newborn
- Require extensive monitoring e.g. waveform capnography
Inhalation analgesia
- MoA
- Advantage
- S/E
- Use
MoA: self-administration by Entonox facemask (1:1 mixture of N2O:O2)
Advantages: quick onset, short duration of effect, more effective than pethidine
S/E: light-headedness, nausea, inhibition of methionine synthase after prolonged administration (unsure DNA synthesis disruption)
Use: most suitable later in labor or while awaiting epidural analgesia as hyperventilation may result in hypocapnia, dizziness, tetany and fetal hypoxia when used early in labor
Analgesia methods that work vs do not work during labor
Epidural analgesia
Indication
Absolute and relative contraindication
Indication: joint decision between mother, midwife, obstetric team and anesthetist → for effective pain relief esp in prolonged labor, maternal HT, multiple gestation, ↑risk of OT
Absolute C/I: coagulopathy, local or systemic sepsis, hypovolemia, ↑ICP, patient refusal
Relative C/I: difficult anatomy (e.g. scoliosis, previous spinal surgery), pre-existing neurological deficits
Epidural analgesia
Procedure
Care during epidural analgesia
Procedure: prefer in later stages of labor as it limit mobility, but should not defer when requested
- Aseptic technique: cleansing of women’s back, with LA infiltration
- Fine epidural catheter inserted by anesthesiologist at L3-4 interspace until epidural space reached
- Injection of test dose to check position (diluted LA → should NOT have any effect on sensation unless punctured dural space)
- Injection of epidural solution into epidural space: usu a mixture of low concentration LA + opioid → addition of opioid ↓motor blockade and peripheral ANS effects of epidural
Care during the duration of epidural analgesia:
- Pt in lateral or wedged position (avoid aortocaval compression)
- IV drip in situ before and throughout duration of epidural
- Frequent BP monitoring esp after each ‘top-up’ bolus
Epidural analgesia/ Regional analgesia during labor
Possible complications
- Accidental dural puncture (<1%): may result in postural headache due to CSF leakage
- Accidental total SA: severe hypotension, resp failure, unconsciousness, death
- Spinal hematoma and neurological Cx: rare, usu a/w bleeding disorder
- LA toxicity if inadvertent injection into blood vessels, N/V, pruritis, sedation
- Bladder dysfunction: if bladder allowed to overfill → should catheterize bladder during labor
- Hypotension: uncommon, Tx by fluid boluses ± vasopressors
- Neonatal respiratory depression due to opioid content
Epidural analgesia
Advantages
- Gold standard in terms of efficacy
- Versatile: Allow increase dose if operative delivery needed
- Avoids GA and maternal airway manipulation
- Improves maternal haemodynamics: can be used for maternal heart disease/ Pre-eclampsia
- Maintains haemodynamics: Decreases maternal plasma catecholamines, maternal hyperventilation due to painful uterine contractions, improves uterine blood flow with stable plasma CO2 level
Comparison between epidural vs spinal analgesia
Commonly used epidural and spinal mixtures in QMH
Common obstetric operations that require analgesia
Advantages of regional anaesthesia vs GA during labor
General advantages:
- Flexible in operation duration
- Flexible in significant blood loss
- Easier for invasive monitoring
- Airway: avoid airway manupulation and aspiration
- Haemodynamic: avoid surges in BP, avoid uteroplacental blood flow drop with catecholamines
- Neonatal benefits: decrease neonatal depression, neurotoxicity
- Post-op: decrease analgesia use, better neurological monitoring, earlier post-op mobilisation
- Encourages breast feeding
General anaesthesia during labor
Indications
Preparation
Assessment
Indications:
- Crash delivery
- C/I or failed regional anaesthesia
Preparation:
- Supplemental 100% O2 for pre-oxygenation
- IV access
- Standard vitals monitoring
- Sniffing air position for intubation
- Ramping up head position tp open airway
Assessments:
- Thyromental distance, incisor gap, dentition, Head and neck movement, obesity, craniofacial abnormalities, mallampati classification…etc
- Airway edema during labor
