The incidence of instrumental vaginal deliveries (IVD) varies from 6 to 12% and depends on the institution and the population. Commonest indications are delay in the second stage of labour, poor maternal effort and fetal distress including cord prolapse in the second stage of labour. Maternal indications include severe cardiac, respiratory or hypertensive disease or intracranial pathology where bearing down effort may be detrimental for her health.
Prolonged second stage may be due to inadequate uterine contractions, poor expulsive efforts by the mother, minor disproportion or malposition. The incidence of IVD is slightly more with the use of epidural analgesia and this may be due to inadequate uterine activity secondary to abolition of Ferguson's reflex due to the absence of reflex release of oxytocin due to stretching of the upper vagina . Inadequate uterine activity in these women may be improved by oxytocin infusion to reduce IVD .
Certain prerequisites need to be fulfilled prior to performing an instrumental vaginal delivery. The condition of the mother and the fetus and the clinical situation should be considered carefully. The medical personal should introduce themselves to the woman and her partner and explain the reason for IVD. Assessment should be with a chaperone. The findings and the plan of action and the procedure that is to follow should be explained. Verbal or written consent should be taken based on the protocol after explaining the indication, advantages and disadvantages which should be recorded. It is a distressing time for the mother and her partner and sensitive explanation and counselling is needed.
General examination should include condition of the mother, pain relief and hydration. Analgesia in the form of pudendal block and local perineal infiltration (20 ml of 1% plain lignocaine) may be adequate for low forceps or ventouse deliveries. For midcavity instrumental deliveries an epidural and for a trial of instrumental delivery, spinal anaesthesia may be more suitable. Fetal condition should be evaluated based on clinical information and the degree of normality or otherwise of the auscultation or cardiotocographic findings. In cases of cord prolapse, antepartum bleeding or prolonged deceleration there is some urgency to deliver and actions should proceed with a brisk speed.
Abdominal examination is important to assess the size of the fetus, the fifth of head palpable and the adequacy of uterine contractions. Oxytocin infusion should be considered if uterine contractions are inadequate (less than four in ten minutes each lasting >40 s) in the absence of signs of fetal compromise. Bladder should not be palpable suggesting that it is empty. If not, it should be emptied by catheterization. If the fetus is felt to be large (depends on the size of the mother but should be considered large if the estimate suggests it to be >4.5 kg) extra caution need to be taken to avoid prolonged period of traction and to be prepared for possible shoulder dystocia.
Vaginal examination should confirm the cervix to be fully dilated with absent membranes. The colour and quantity of amniotic fluid should be noted. The presentation should be vertex. Excess caput (soft tissue swelling) or moulding may suggest the possibility of some disproportion. Overlapping of skull bones and inability to reduce it with gentle pressure is considered as moulding +++; ++ indicates overlapping of the bones that can be reduced by gentle digital pressure and + indicates meeting of the bones without any overlap. The position (e.g. left occipito-anterior (LOA) or occipito-transverse (LOT)) and station which is the leading bony part of the skull in relation to ischial spines should be identified. Ideally the station should be below spines with descent of the head with contraction and bearing down effort.
In three dimensional terms, the female pelvis accommodates the fetal head at term. Therefore when the head is 0/5'th palpable above the pelvic brim the leading part of the head should be below the ischial spines. In an obese mother and in those with occipito-posterior positions, palpation of the fifths above the brim may be difficult and may be deceptive. If (1/5)th or (0/5)th of the head was palpable above the brim but on vaginal examination the head was above spines then the small amount of head palpated may have been the fetal chin and the vertex may be in occipito-posterior position. IVD should not be attempted when the head is more than (1/5)th palpable and/or when the station is above spines.
The station and position will determine whether to proceed with IVD and the type of instrument to be used. The position is determined by palpating for the suture lines, posterior fontanelle and occiput. The inverted Y shaped suture lines or overlapping of parietal bones over the occipital bone help to identify the posterior fontanelle. The posterior fontanelle is small and the caput and moulding may make identification difficult. Anterior fontanelle is easily identified as a soft diamond-shaped depression recognized at the junction of the two parietal bones with the two frontal bones. If anterior fontanelle is felt easily around the centre of the pelvis it indicates the possibility of a deflexed head. In a well-flexed head the anterior fontanelle is likely to face the side wall of the pelvis. It is useful to confirm the position by palpating and flicking the fetal ear. The finger has to be moved from the direction of the occiput for the ear to flick. Palpation of the ear also indicates that the largest diameter of the head (i.e. the biparietal eminences) has descended below the midcavity. The sagittal suture should cut the pelvis into halves. If the sagittal suture is far posterior or anterior there is asynclitism and it suggests the reason for the delay and should warn the possible difficulties with IVD. The degree of descent and rotation of the head with contraction and bearing down effort should give some idea about the possibility of successful IVD.
IVD can be performed with the mother in the dorsal position and the legs flexed and abducted or in the left lateral position, but it is much easier when the mother is placed in lithotomy with the buttocks just beyond the edge of the bed. The adequacy of pain relief should be checked. The procedure should be done under antiseptic and aseptic conditions. The vulva and perineum should be cleansed and the bladder catheterized if necessary.
The pain relief needed may be judged by the station of the head. Regional anaesthesia in the form of epidural or spinal anaesthesia is preferred for mid cavity IVD, that is, when the head is engaged but the station is above +2 cm but below the ischial spines . If the vertex is beyond +2 cm but not reached the pelvic floor it would be termed low cavity IVD and regional or pudendal block anaesthesia with local infiltration of the perineum may be adequate. Outlet IVD is carried out when the head is on the perineum with the scalp visible without any separation of the labia. In this situation the vertex would have reached the pelvic floor and would be in the direct, right or left occipito-anterior position needing no rotation or slight rotation of less than 45°. Pudendal block anaesthesia and local infiltration of perineum is generally adequate although some may prefer regional anaesthesia.
It is preferable to deliver by CS if the head is above ischial spines. When vertex is below the spines, IVD is possible and different types of forceps and vacuum could be used depending on the position and station of the vertex. In order to make comparison of outcome, there are suggestions to use the terminology of the specific station and the position (i.e. right occipito-transverse (ROT) at +2 or left occipito-posterior (LOP) at +3) at the time of instrumentation, instead of the broad categories of mid, low and outlet IVD .
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