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Physiology of labour
This refers to the changes that take place during labor.
This is shortening of muscle fibre. Uterine contractions are involuntary, peristaltic and intermittent. They are regular and painful in nature, enough to distract patient from her normal activities. They are controlled by nervous system and endocrine influence. Normal contractions increase in frequency, strength and duration. They become more painful, rhythmic in nature, starts by occurring every 15-20 minutes in early labor (frequency) and increase to 2-3 minute in second stage, strength increases in intensity lasting 50-60 second in duration at the end of first stage. The pain of contraction has the same characteristic as that of spasmodic dysmenorrhea. It compresses the gestational sac and raises the intrauterine pressure from about 5-10 mmHg to 25mmHg in labor and even increases to 50-75mmHg at the height of contractions in the second stage of labour. As a result pressure is directed towards the less resistant lower uterine segment. This permit the cervix to dilate and the strongly contracting fundus to expel the fetus in the second stage.
Each contraction starts at the fundus near the cornua and spread gradually across downwards to the lower uterine segment, but usually remain stronger and last longer at the upper region of the uterus (fundus), but the peak is reached simultaneously over the whole uterus and the contraction fades from all part together.
Retraction is peculiar to the uterine muscles only, whereby the wave of contraction does not pass off entirely, but the musclefibres retain some of the contractile tone instead of relaxing completely. Retraction assists in the progressive expulsion of the fetus by maintaining the downward pressure. As the upper uterine segment becomes shorter, thicker and the cavity reduces the lower uterine segment becomes longer and thinner.
This is the neuro-muscular harmony that prevails between the upper and the lower uterine segments. The two poles act harmoniously. The upper segment contracts strongly and retracts to expel the fetus while the lower uterine segment contract slightly and stretches to expel the fetus. If polarity is disorganized the progress of labor is inhibited.
By the end of pregnancy the uterus divide into two distinct poles. The lower segment develops from the isthmus, internal Os and the cervical canal. During labor the retracted longitudinal fibres in the upper segment exert a pull on the lower segment making it to stretch. This causes the Os to dilate and become part of the lower segment. The upper segment is concerned with contraction and is getting thicker while the lower segment which is for distention and dilatation is getting thinner aided by the force of the descending head.
Development of retraction Ring: The ridge which formswhere the thick upper segment meet the thin lower segment is known as Retraction Ring. It is a normal ring and should not be felt or seen per abdomen. When it becomes abnormal and can be seen or felt on palpation is known as bandl’s ring . It is a sign of obstructed labour when the upper segment is abnormally thick and the lower segment is abnormally thin. It rises as the upper segment contracts and retracts until full cervical dilatation.
Taking up of the Cervix Effacement: When labour begins themuscle fibres surrounding the internal Os are drawn up by the retraction of the upper uterine segment. The cervix becomes shortened as it merges with the lower uterine segment. It takes place before dilatation in primp but in multip it occurs as the cervixis dilating. In grande multiparae complete effacement may not take place.
Dilatation of the cervix: Is the widening of the external Os. Theretracted muscle fibres of the upper segment exert a pull on the weak lower segment and the cervix, making, it to dilate from a thing closed aperture to an opening large enough to permit passage of the fetal head.
In primigravida the internal Os dilates at the same time the cervix is being taken up, and then the external OS dilates later. In the multiparous woman both the internal and the external OS dilate as the cervix is being taken up. Dilatation of the Os is a gradual process. Full dilatation is 10cm. A well flexed head favours efficient dilatation.
The lost of blood stained mucoid discharge as the cervix dilate. It is from the plug of mucus that guards the cervical canal during pregnancy. It can be seen a few hours before or after labour has started.
When the lower uterine segment stretches, the chorion get detached from it and the increase intra uterine pressure causes the bag of fluid to bulge through the dilating internal OS. The well flexed head which fix neatly into the cervix cut off the fluid in front of the head from the one which surrounds the behind body. This one in front is known as forewaters and the behind is known as Hind – waters
Advantage: It prevents transmission of pressure from the hindwater from being applied on the fore water so keeps the membranes intact during first stage of labour.
General Fluid Pressure:Is the term used when the amniotic fluidequalizes the pressure throughout the uterus during contractions. It ensures adequate supply of oxygen to the fetus during contraction by preventing compression of the placenta by the fetus.
Rupture of Membranes: the amniotic sac should remainintact until the OS is fully dilated (end of the 1st stage) but this is not always the case. In badly fitting presenting part the membranes rupture quite early because the fore waters are not cut off completely from the hind waters so there is transmission of pressure to the fore waters during the intra uterine contractions. In some cases it does happens for no apparent reasons. Sometimes the membranes do not rupture even at 2nd stage and the baby is born with it, this is known as “caul”. Ruptured membran e (RM) may be spontaneous or artificial – ARM. Artificial rupture of membrane (ARM) promotes labour.
Contraction and Retraction continue: During this stage the contraction are more severe, stronger and expulsive, occurring more frequently (about 1-2mins) and of longer duration about 60sec or more. The fetal head press and stretches the vagina which in-turn stimulates uterine action. Also the membranes help the fetus to be in close contact with the cervix, the upper segment gets much shorter and thicker. The placental circulation is much more interfered with than during first stage. The pain suffered during this time is less and different in characteristic from that of first stage because the pain is due to stretching of the vagina, pelvic and perineum. It is therefore felt in the back, pelvis and may radiate down the inner surface of the thigh.
Secondary powers come into play/Accessory muscle: the abdominal muscles and diaphragm now come into play to help with the expulsive contractions to expel the fetus. The woman now has the urge to push “known as bearing do wn”. As the presenting part reaches the pelvic floor and descent it, the pushing becomes involuntary. Secondary powers help to overcome the pelvic floor resistance.
Displacement of the pelvic floor: This is a “swing door action.” The anterior wall of the vagina and the pe lvic floor are pushed upwards and forwards while the posterior wall of the vagina and pelvic floor push downwards and backwards.
The bladder is drawn up into the abdominal cavity. The rectum is compressed by the advancing head, anus bulges, defecation may take place and the anus gapes. The perineal body stretches and thins out lengthen the posterior wall of the birth canal causing the vaginal orifice to be directed upwards.
Expulsion of the Fetus: With each contraction the head descends along the birth canal and recedes in between contractions until it is seen on the vulva. This continues until crowning takes place. When the head no longer recedes between contractions the bi-parietal diameter distend the vaginal orifice and occiput escapes under the symphysis pubis. The head is born by extension. The rest of the body is born by lateral flexion and remaining liquor amni expelled.
Contraction and Retraction Continue: As the upper segment becomes thicker and smaller after the birth of the baby the placenta site also become reduced this makes the placenta to buckle off the uterine wall and separate. The placenta drops into the lower uterine segment or the vagina, followed by the membranes which stripped off the uterine wall by the traction of the descending placenta. The stronger the contraction the sooner the placenta separates (5 mins) about 1/3 seprate with the birth of the baby.
There are two methods of separation and expulsion of the placenta.
Shultze Method: 80%: This is the most common method. Inthis method separation starts from the centre of the placenta and with the aid of Retroplacenta clot the placenta drop into the lower uterine segment or into the vagina. During delivery the fetal surface appears first at the vulva followed by the membranes. There is minimal blood loss with this method. The 3rd stage is neat
Mathews Duncan Method 20%: In this method separationstarts at the edge and slides down sideways. It comes through
the vulva with the lateral border first, like a button through a button hole. The maternal surface is seen first at the vulva. There is trickling of blood throughout the third stage. The third stage .is messier.
Control of bleeding: the Contraction and retraction of theuterine muscle fibers that bring about separation of the placenta also act as “living Ligatures” by compress ing the blood vessels and controlling the bleeding. The clothing mechanism is of little value untill later when contractions are much less. The opposite wall are now in contact and apply further pressure on the placenta site.
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