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1
Physiology of labor and pain pathways
Sileshi A.
2 THEORIES:
Direct pressure exerted on cervix by fetus. Progesterone Withdrawal: ↓ progesterone by
placenta & ↑ prostaglandins in chorioamnion results in ↑ uterine contractions.
Oestrogen Stimulation: ↓ progesterone allows oestrogen to ↑ contractile response of uterus.
Fetal Cortisol: Changes biochemistry of fetal membrane: ↓ progesterone & ↑ prostaglandin in placenta.
Distension: uterine muscles stretch causing ↑ prostaglandin.
Amniotic membranes (sac) converts arachidonic acid → Prostaglandin uterine contractility.
3 Premonitory signs of labour: weeks before real labour
Lightening: Fetus settles into pelvic cavity.
Braxton-Hicks: Irregular intermittent
contractions; “false labor”.
Cervical changes: cervix effaces [thins] &
dilates slightly
Baby's head in pelvis pushes against cervix
causing relaxation and effacement.
Cervix in posterior position.
4 Signs True Labor: closer to time of delivery
Uterine Contractions: regular & frequent compared to Braxton-
Hicks
Which becomes stronger with time.
Bloody Show: pink tinged secretions due to softening cervix.(aka
mucous plug)
Rupture of Membranes: (ROM) Labour in 24 hrs. Multiparas sooner.
Clear/odorless.
Green/brown danger sign
Meconium aspiration distress/infection Immediate medical
attention.
5 Difference Between True & False Labor
True Labor Contractions occur at
regular intervals. Intervals (b/n conxn.)
gradually shorten. Intensity gradually
increases. Discomfort is in the back
and abdomen. Cervix dilates. Discomfort is not stopped
by sedation.
False Labor Contractions occur at
irregular intervals. Intervals remain long. Intensity remains
unchanged. Discomfort is chiefly in
the lower abdomen. Cervix does not dilate. Discomfort usually is
relieved by sedation.
6 Stages of labor 3/4
7 First Stage Start of regular uterine contractions until the
completion of cervical dilation(=10cm) ~ 6-18 hrs. primapara; and 2-10 hrs.
multipara.
3 phases : latent, active and transition
Latent phase:- the period between the
onset and the point at which a change in the
slope of cervical dilatation is noted.
Dilation 0-3 cms. Contx.’s mild/irregular.
8 Cont.
Active phase:- phase of a rapid acceleration of cervical dilatation (begins @ 3cm) 4-7 cms. Contx.’s 5-8 min. apart. Lasts 45-
60 sec; moderate - strong intensity. Transitional: Dilation 8-10 cms. Contx.’s 1-2
min. apart; 60 –90 sec.; strong intensity. No pushing until fully dilated.
9 Second stage
Delivery of infant:
up to 1 hr. or ~ 20 contx’s – primip.
20 min. or ~ 10 contx’s in multip. Can last up to 3
hrs.! Esp. in case of EPA
Cardinal movements occur here.
Most difficult & uncomfortable part of labor.
Strong urge to push & bear down as infant passes
through vagina & rectum
10 Third Stage
Delivery of placenta ~ 5 - 30 min.
Separation should be automatic [uterus
contracts & mum bears down]
Manual presses on contracted uterus. “
Crede’s Maneuver”
Syntocinon placenta delivered to avoid
retained placenta.
If no spontaneous delivery of placenta,
manually removed.
Antibiotics
11 Fourth stage
Placenta out; mother recovers.
Lasts ~ 1 hr. unless complications arise.
Then patient is transferred to postnatal
unit.
12 Assessing Progress of Labor
Dilation: 0–10 cm. [opening cervix]
Effacement: 0 –100 % [thinning cervix]
Station: Relationship of presenting part to pelvic ischial
spines -midway in pelvic cavity.
“0 ” station aka “engaged”.
-1 to -5 above “0”
+1 to +5 (outlet) below “0”
+4/+5: baby's head out.
13 Cont.
The progress of labor may be abnormal
and can be classified as a
Slow latent phase,
Arrest of active phase, and
Arrest of descent.
14
15 Mechanism of Labour
Passage of fetus through birth canal involves position
changes called Cardinal Movements of Labour:
Engagement: presenting part enters midpoint of
pelvis at ischial spines.
Descent: downward movement through pelvic inlet
through dilated cervix, reaches posterior vaginal
wall. Mum feels like pushing. Widest part [head]
passes through pelvis.
Flexion: pressure from pelvic floor causes head to
flex towards chest; chin touches chest.
16 Cont. Internal Rotation: occiput in diagonal
position & rotates towards face down
position (OA) (occurs as body parts press
on bony pelvic structures)
Extension: top of head delivered & extends
as face & chin are delivered.
External Rotation: head rotates back to
previous lateral position. Rest of body is
delivered.
17
Factors affecting labour process: 3 Ps
Passenger
Passageway
Powers
18 Passenger: [infant]
A. Fetal head: widest part of body; most
difficult to pass through vaginal canal;
Passage depends on bones, sutures,
fontanelles.
Cranium - 8 bones meet @ suture lines
Cranial bones move & overlap, allows skull to
pass thru birth canal.
Fontanelles: soft spaces created by junctures
of suture lines - covered by membranes;
compress during delivery to aid in passage of
fetus.
“Molding” of infant head.
19 Cont.
Skull widest @ antero-posterior diameter than @
transverse diameter.
Antero-posterior diameter measures differently @
different locations.
Occipitomental diameter- widest - measured from chin
to posterior fontanelle = 13.5 cm
Smallest diameter - lower occiput to anterior
fontanelle (suboccipitobregmatic) = 9.5 cm
Complete flexion allows smallest diameter of fetal
skull to enter pelvis most easily.
20 Cont.
B. Fetal Attitude: degree of flexion of fetal head.
Complete flexion: allows smallest diameter
of skull to pass through pelvic cavity. Best
position!
Moderate flexion: head less flexed making
diameter wider.
Poor flexion: brow or face presentation;
presents skull diameter too wide making
delivery difficult.
21 Cont.
C. Fetal lie: relationship of long axis of fetus
[spine] to long axis of mother:
1. Longitudinal – vertex/breech; vertical in
relation to mum; ~ 99%.
2. Transverse – horizontal in relation to
mum; < 1 %.
3. Oblique - diagonal
22 Cont.
D. Fetal presentation: part of fetal head
enters pelvis;
1. Cephalic 95.5%
2. Breech 3.5%
3. Face 0.3%
4. Shoulder 0.4% [transverse lie]
23 Cont.
E. Fetal position: “occiput is landmark”
Presenting part [occiput, mentum, sacrum]
Landmark is anterior, posterior, transverse in
relation to mother’s spine.
Occiptito-anterior (OA) back of head against
symphysis pubis & face towards spine.
Occipito-posterior (OP) Back of head =
mother’s spine; painful contxs.
Transverse (T) = fetus sideways.
24 Passageway:
Refers to fetus passing through uterus, cervix, vaginal
canal. Single most important determinant to
mechanism of labor.
A. Shape of pelvis:
1. Gynaecoid – 50% of women; rounded, oval
shape; easy vaginal delivery; considered
“normal female pelvis”
25 Cont.
2. Android – 20 % of women; vaginal delivery difficult; prob. C/S; “true male pelvis”
3. Anthropoid – oval; assisted vaginal birth usually with forceps;
20-25%
26 Cont.
4. Platypelloid – < 5 % of women; flattened pelvis; vaginal delivery difficult
27 Cont.
B. Structure of Pelvis
False Pelvis: Outer - broader. Hip bones.
True Pelvis: Internal – narrower. Holds bladder,
rectum, & reproductive Organs.
True pelvis - has 3 parts - inlet, midpelvis,
outlet
[Most important in childbirth]
Contractions of the pelvic inlet, the midpelvis,
the pelvic outlet, or a generally contracted
pelvis Fetopelvic Disproportion
28 Cont.
Powers:
Uterine contx’s: primary force moving fetus
thru maternal pelvis during 1st stage of labor.
Maternal Efforts: woman adds voluntary
pushing force to force of contx.’s during 2nd
stage of labor to propel fetus thru pelvis.
29 Physiology of pain in labor and Neural pathways
Perception of pain by the parturient is dynamic processIt Involves both peripheral and central mechanismsMany factors affect degree of pain experienced by
woman including:-Psychological preparation, Emotional support during labor, Past experiences, The patient's expectations of the birthing process,
and Induction or augmentation of labor with oxytocin.
30Cont.
1st stage of labor – mostly visceral
◦Dilation of the cervix and distention of the
lower uterine segment
◦Dull, aching and poorly localized Slow conducting, visceral C fibers, enter spinal
cord at T10 to L1 to synapse in the dorsal horn. The chemical mediators involved are
bradykinin, leukotrienes, prostaglandins, serotonin, substance P and lactic acid
31Cont.
2nd stage of labor – mostly somatic
◦Distention of the pelvic floor, vagina and
perineum stimulation of pudendal nerve.
◦Sharp, severe and well localized Rapidly conducting A-delta fibers, enter
spinal cord at S2 to S4 impulses pass to dorsal horn cells and finally to the brain via the spino-thalamic tract.
32 Neural pathways
33Physiological response to labor pain Syste
m Response to pain
CVS Pain increases catecholamine level increase in HR, contractility and SVR, all of which increases myocardial oxygen demand
Placenta
Pain increases catecholamine levels vasoconstriction of umbilical vessels and consequently reducing placental blood flow
Respiratory
Pain increases MV maternal hypocapnoea respiratory alkalosis shifts the oxy-hgb disso. Curve to Lt decreased O2 offloading to the fetus
GIT Pain reduces gastric emptying increasing risk of aspiration
34 References
Williams obstetrics 23rd edition Millers anesthesia 7th edition