Materno-fetal Flashcards
Maternal airway
- Failed intubation 7-10x more common
- Upper airway of ↓volume and friable due to venous engorgement and oedema
- Capillary engorgement and oedema of upper airway (secondary to hormonal changes), exacerbated by fluid overload/ oedema associated with pregnancy-induced HTN + pre-eclampsia
- Incr likelihood of upper airway obstruciton
- Small size ETT may be required
- Incr chest diameter and breast size may make laryngoscopy more difficult
Maternal vertebral column
- Incr lordosis
=> Intercristal line may traverse L3 instead of L4
=> Apex of lordosis more caudal
=> ↓thoracic kyphosis - incr cephalad spread of LA - Ligamentum flavus softens with pregnancy
- Epidural veins engorged late pregnancy - incr risk of venous puncture
- Epidural space may have positive pressure in pregnancy
- Incr fat + venous engorgement in epidural space reduces volume of thecae sac = ↓CSF volume
Anatomy of pain labour
- Nociception via somatic and visceral afferent sensory fibres to spinal cord
First stage (cervical dilation)
- Pain from afferent fibres within lower uterine segment and cervix
- DRG of T10-L1
- Phases of first stage
=> Latent - cervical effacement and slow dilation to 3-4cm
=> Active - rapid increases in cervical dilation until full dilation (10cm)
=> Transitional - foetus begins to descend into pelvic inlet, identified by beginning of somatic pain
Second stage (delivery of foetus)
=> Pudendal nerve carries afferent pain signals from vagina and perinueum
=> DRG of S2-S4
Third stage (delivery of placenta)
- Pain significantly reduced
- Some discomfort with delivery of placenta and contraction of uterus to postpartum size
Pain pathway
Somatic afferent
- 1o afferent - soma in DRG, synapse at dorsal horn (e.g via sacral nerve roots)
- 2o afferent - decussates in anterior comminsure and ascends via spinothalamic tract
- 3o afferent - to cortex (somatic) and sub cortex (visceral, autonomic centres)
General visceral afferent
- 1o afferent - piggybacks onto autonomic efferents
=> With SNS - via white ramus communicans to spinal cord T1-L2
=> With PSNS - via sacral nerves to S2-4 (also cranial)
=> Direct synapse with WDR 2o afferent in deeper layers
=> Convergence onto somatic nociceptive-specific 2o afferent in superficial layers
- 2o afferent - decussation via anterior commissure (incomplete for older pathways) and ascent via multiple spinothalamic tracts
- 3o afferent - from brainstem to diencephalon to higher structures
Physiology of pain in labour
- Labour pain - early is visceral, late is somatic
- Initial pain T11-12, later pain is referred to lager novice-tic field
- Stimulus for pain primarily in first and second stage of labour
Visceral pain
- Mainly C fibres (divide on entering Lissauer’s tract - spread three or more segments above and below point of entry)
- Diffuse and poorly localised
- Felt in abdo between pubis and umbilicus or in the back
- Uterine contractions incr pressure of amniotic fluid causing distension of lower uterine segment and cervix causing pain
- Receptors located in lower uterine segment and cervix - respond mainly to pressure and stretch
- Poorly localised as visceral afferent
Somatic pain
- Mainly A-delta fibres
- Pain in vaginal and rectal areas - may refer to thigh. Pain due to passage of foetus through bony pelvis and birth canal. Stretching and tearing of skin, subcut tissue, fascia and tissues of perineum
- Sharp well localised pain
- Pain in second stage typically increases
Referred pain
- Pain perceived at a location distal to the site of insult
- Due to convergence of somatic and visceral 1o afferent on same WDR projection neurons
Neonatal airway
Size
- Smaller in diameter (easier to obstruct, risk of tracheal stenosis following prolonged intubation, incr difficult for emergency surgical airway)
- Short in length (risk of RMB intubation, easier to accidentally extubatne)
- Superior laryngeal position (C3/4 cf C4/5 in kids cf C5/6 adults, make laryngoscopy more difficult)
Proportions
- Tongue relatively larger (airway obstruction, interferes with laryngoscopy)
- Epiglottis long, stubby, floppy and omega shaped (straight blade to elevate epiglottis)
- Large tonsils and adenoids (obstruction)
Shape
- Larynx more anterior/ cephalad
- Funnel shaped trachea
- Infraglottic narrowing (narrowest point cf glottis in adults)
- Small mandible
- Soft trachea and cricoid
Head
- Large occiput puts head in flexed position
Obligat nasal breathers
CVS + RS changes at birth
At birth changes from parallel to series circulation
- SVR increases
- PVR decreases (lung expands)
- Foramen ovale closes (LAP >RAP -> closure of septum secundum, permanent closure take 4-6weeks)
- Ductus arterioles closes (constricts in response to incr PaO2 at birth, physiological closure 12-24hrs, permanent 2-3weeks)
- Ductus venous closes (functionally closes few hours, anatomically patent for a number of days -> ligament venosum)
Resp
- Bronchial tree developed at 16weeks
- At 28 weeks acing pattern of airways formed
- Type II pneumocytes at 24 weeks - surfactant production beings
At birth
- During delivery 35mL fluid squeeze out of lungs by compression (reabsorbed)
- Negative trans pulmonary pressures with first breath create negative pressure in interstitial space drawing fluid into it and out of lung
- First breath - stimulated by sound, touch, temp. At delivery PaO2 falls and PaCO2 rises, central and peripheral chemoreceptors become more responsive. First breath generates high negative inspiratory pressure -> expand lungs, create FRC
Placental drug transfer
Physical
- Placental surface area
- Placental thickness
- pH of maternal and fetal blood
- Placental metabolism
Uteroplacental blood flow
- Presence of placental drug transporters
Pharmacological
- Molecular weight of drug
- Lipid solubility
- pKa
- Protein binding
- Concentration gradient across placenta
Maternal CVS changes
MAP
- Decrease DBP 20%, SBP 10%, incr PP, nadir 20/40
CO
- Incr 40% by 2/40, 50% 3/40, slight fall from peak at term (incr preload/ VR due to incr volume (oestrogen) and venoD)
HR
- Incr 4/40 - peaks 3/3 25% above normal
SV
- Incr 20-30% predominant 1/3
Directed to uteroplacental flow
- 750ml/min at term
LV hypertrophy and dilatation facilitates incr CO - contractility unchanged.
TPR
- Decr 30% 1/3. VasoD mediated by oestrogen, progesterone, PGs and down regulation of a1
Aortocaval compression
- From 13-16/40
- Compress aorta -> decreased placental perfusion
- Compress IVC -> decr VR and CO -> diverted via collaterals through epidural -> azygous veins
- Can get brady (Bezold-Jarisch) from decr preload
Colloid oncotc pressure falls - incr risk oedema
Blood volume
- Incr, by term 35-40%
- Plasma volume incr 45% (Na + H20 retention, oestrogen stim RAAS)
- RBC volume incr 20-30% (Incr EPO)
- Plasma volume > RBC volume -> 33% decr Hct -> decr viscosity, dilution anaemia
- RBF incr 80%
ECG changes
- Hypertrophy and upward displacement of diaphragm move apex anterior and to the left
- LAD, ST depression, inversion or flattening of T-wave lead III
Labour
- Uterine contraction squeeze 300mL of blood
- Incr BP
- CO incr
- Post delivery CO 60-80% above pre-labour, return to normal 2/52 post delivery
