Lobar
Def: the process by which regular painful uterine contractions bring about effacement & dilatation of the cervix & descent of presenting part, leading to expulsion of the fetus & the placenta from the mother.The physiology of lobar:
The mechanism of initiation of labor is still unknown, both the mother & fetus make contributions toward the onset of lobar.
The myometarium:
a-myometarial cells contain actin & myosin, interaction between them lead to contraction in response to increase in intracellular free calcium ions under the effect of prostaglandin & oxytocin.
b-separation of the actin & myosin lead to relaxations but here unlike in any muscle in the body there is degree of shortening , this progressive shortening of muscle cells called "retraction"
c-the result of this retraction process is development of the thicker actively contracting "upper segment" & the lower segment of uterus become thinner & more stretched. Then the cervix is taken up into lower segment & its efface then dilated.
d-myometarial cells laid down in collagen mesh. There is cell to cell communication by means of gap junctions, which facilitate the passages of various products of metabolism & electrical current between cells. This gap junctions are absent for most of the pregnancy but increase in numbers toward term & in lobar. Prostaglandins stimulate their formations.
e-a uterine pacemaker from which contractions originate does exist but not demonstrate histologically.
The cervix:
It is contain muscle cells & fibroblasts separated by ground substance, interactions between collagen, fibronectin & dermatan sulphate during the earlier stages of pregnancy keep the cervix rigid & closed. Contractions at this point do not bring about effacement or dilatations. Under the influence of prostaglandins, there is an increase in proteolytic activity & reduction in collagen & elastin. Interleukins bring about a pro inflammatory change with invasion by neutrophiles. Dermatan is replace by the more hydrophilic hyaluronic acid, so increase in the water content of the cervix. This causes cervical softening or ripening so that contractions when begin bring about the processes of effacement & dilatations.Hormonal factors:
a-progesterone maintains uterine quiescence by suppressing prostaglandins release, prevent connection between myometrial cells & inhibit oxytocin release. Oesterogen opposed the action of progesterone, prior to lobar progesterone receptor reduce & Oesterogen conce, increase.
b-prostaglandins synthesis by the chorion & the decidua is enhanced so increase calcium influx into the cells.
c- the production of CRH from the placenta increases to ward term & potentiate the action of prostaglandin & oxytocin .
d-the fetal pituitary gland produce oxytocin & the fetal adrenal gland produce cortisol which stimulate conversion of progesterone to Oesterogen .
e-fergusson reflex here pressure from fetal presenting part against the cervix is relayed via a reflex arc involving the spinal cord & result in increased oxytocin release from maternal pituitary glands.
The onset of labor :
1-It is characterized by regular painful uterine contractions that comes every (2-4 minutes) & lasted from (30-60 second) or longer. The intensity of intrauterine pressure generated with each contraction averages between (30-60 mmHg).
2-loss of show (a blood stained plug of mucus passed from cervix ).
3-spontaneous rupture of membrane.
The duration of labor :
The morale of most women starts to deteriorate after 6 hours in labor,& after 12 hours the rate of deterioration increase, also there is risk of fetal hypoxia & need for operative delivery.
It's difficult to define prolonged lobar but its reasonable that lobar lasting more than 12 hours in nulliparous & 8 hours in multiparous should regarded as prolonged.
The average duration of first lobar is about 8 hours & subsequent lobar 5 hours. First lobar rarely last more than 18 hours, & subsequent lobar not more than 12 hours.
The mechanism of lobar :
it is series of changes in position &attitude that the fetus undergoes during its passage through birth canal .
It is described here for the vertex presentation & gynaecoid pelvis. The relation of the fetal head & body to the maternal pelvis changes as the fetus descends through the pelvis. This is essential so that the optimal diameters of the fetal skull are present at each stages of descent.
Engagement
a-the widest diameter (biparietal diameter) of the presenting part has passed through pelvic inletb-It occur prior to labor ( 36 weeks ) in majority of nulliparous but not in multiparous which occurs at time of labor
c-If 2/5 of the head is palpable abdominally so the head is engaged
d-Normally the head enter the pelvis in the transverse position [ occipito – transverse ]so the anterior parietal bones slides over symphysis pubis followed by posterior parietal bones so that sagittal suture stay synclitic i.e. midway between symphysis & sacral promontory
e- if the sagittal suture approach more the promontory of the sacrum so more of ant. Parietal bone present itself to examing fingers so called ant. Asynclitism if suture lie close to the symphysis pubis more of post. Parietal bone will present and this called post. Asynclitism.
Descent
a-In nulliparous engagement occur before onset of labor and further descent not occur until onset of 2nd stage of labor. In multiparous descent begin with engagement
b-Descent is brought by one or more of 4 forces
Pressure of amniotic fluid
Direct pressure of the fundus upon breech
Contraction of abdominal muscles
Extension & straighten of fetal body
Flexion
a-As the descending head meet resistance from cervix, Wall of pelvis, pelvic floor, flexion of the head result.
b-The chin is brought into more contact with fetal thorax and shorter( sub occipito-bregmatic ) diameter substitute longer (occipito- frontal) diameter.
Internal rotation
a-It is turning of head in a manner that occipit moves gradually from original position anteriorlly towards symphysis pubis or less common towards sacrum(occipito posterior). It is essential for completion of labor.
b-Internal rotation is always associated with descent of presenting part and it is not occur until head is reach level of spines( i.e. engaged ).
Extension
a-When head reached vulva after flexion it under goes extension
b-Extension brings the base of occipit into direct contact with inferior margin of symphysis.
c-the vulvar outlet is directed upward and forward so that extension must occur before the head pass through vulvar outlet if flexed head not extend it would impinge upon posterior portion of perineum.
*Two forces come into play:
1.Forces excreted by uterus act more posteriorly
2.Force supplied by resistant pelvic floor & symphysis act more anteriorly.
The result is the direction of vulvar outlet
The head is borne by further extension as the occipit, bregma , forehead , nose , mouth & finally chin
Immediately after birth the head drop down ward so the chin lies over anal region of mother
Restitution
When the head is delivering, the occipit is directed anteriorly . as soon as it escape from the vulva, the head aligns itself with shoulders, which have enter the pelvis in the oblique position. The slight rotation of the occipit through 1/8th of circle is called Restitution
External rotation
For the shoulder to be delivered it should rotate into direct anterior- posterior plane. When this occur the occipit rotate through further 1/8th of circle to transverse position called External rotation.
Shoulder delivery
After restitution & external rotation the shoulder will be in the anterior – posterior position. The anterior shoulder is under symphysis pubis delivered first than posterior shoulder.
This lecture by Dr-Nadia AL-Assady
CABOG & FIBOG