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Uterus is a pear-shaped muscle made of 3 layers; endometrium (the inner lining that is shed during menses), myometrium (middle muscle layer) and perimetrium (outer layer providing extra support to the whole structure). A combination of factors initiate labor, oxytocin and prostaglandins being the most important biochemical factors (hormones) in stimulating uterine contractions.
Labor is the progressive dilatation of the uterine cervix in association with repetitive contractions. During labor, subjective changes that occur include regular uterine contractions that become stronger, longer and at closer intervals, while objective changes include the cervical changes i.e. descent of the presenting part. Consequentially, there is effacement and dilatation of the cervix and delivery of the fetus and placenta.
When the uterus begins to contract, labor pain or contractions commence. The experience of labor for every woman is dissimilar. Labor contractions are of two types, true and false contractions.
True labor contractions occur at regular intervals and the intensity and discomfort increase steadily. There is a gradual decrease in the interval between the contractions. Usually there is discomfort in the lower back/ abdominal region and the vaginal mucous or bloody discharge increases. The contractions ultimately lead to cervical dilatation. A feature of these contractions is that they do not cease with rest, hydration or medications.
False labor contractions, also known as Braxton-Hicks contractions, are those which occur at irregular intervals and they do not gradually increase in either intensity or duration. There are long intervals in between two contractions. Pain is mainly described as a feeling of tightening in the lower abdominal region. There is no accompanying vaginal discharge and/or cervical dilatation. The discomfort due to contractions can be completely relieved with the aid of rest and medications.
Pathophysiology of Labor Pain
Labor pain consists of visceral and somatic pain. Visceral pain is the first stage (mediated by T10 – L1) in which there is distension and stretching of lower uterine segment. Somatic pain is the second stage (mediated by S2-S4) in which there is distension of the pelvic and perineal structures and compression of the lumbosacral plexus.
Clinical Signs and Symptoms
In general, contractions are usually felt in the lower middle abdomen. Occasionally, they are felt as unrelenting low back pain as well. They often feel like pre-menstrual cramps and are very common as the pregnancy comes to an end.
- Painful contractions or tightening that may be irregular in strength and frequency
- Broken waters i.e. rupture of membranes with spurting of amniotic fluid and a brownish or blood-tinged mucus discharge
Nerve Supply of the Uterus
Motor nerve supply is from the sympathetic divisions T11, T12 and L1 while the parasympathetic supply is from S2, S3 and S4.
Sensory nerve supply for the uterine body is derived from the pelvic, superior hypogastric and aortico-renal plexus of the D10, D11, D12 and L1 segments of the spinal cord. Sensory nerve supply of the cervix is through the pelvic plexus to S2, S3 and S4 segments of spinal cord.
For the upper vagina, the sensory supply is derived from S2, S3 and S4 while for the lower vagina it is through the pudendal nerve.
For the perineum, both motor as well as sensory nerve supply is derived from S2, S3 and S4 through the pudendal nerve.
There are three hormones that regulate labor contractions. They include oxytocin, progesterone and adrenaline.
Methods to Manage Obstetric Pain
Management of obstetric pain is essential as it causes discernible stimulation of respiration and circulation in mother, activation of sympathetic nervous system and mental disturbances such as postpartum depression and post traumatic stress disorder.
Basically, there are pharmacologic and non-pharmacologic methods to manage the obstetrical pain.
- Mind–body interventions (psycho-prophylaxis)
- Bioelectromagnetic methods
- Physical methods: massage, heating pads, warm bath
- Alternative medications: acupuncture, hypnosis
- Systemic analgesia: IV, inhalational
- Regional techniques
- General anesthesia
- Patient controlled analgesia (PCA)
- Transcutaneous electrical nerve stimulation (TENS)
Psycho prophylaxis implies the antenatal psychological preparations of the parents, chiefly the mother, aimed to minimize, if not eliminate labor pain. Mother-crafting is a key feature of psycho-prophylaxis wherein the mother is educated about the process of pregnancy and labor progression.
Also, the partner is co-educated which acts as an immense emotional assistance to the mother. The mother is also taught relaxation exercises and an encouraging environment is promoted throughout pregnancy.
This method endows benefits such as enhanced bearing-down efforts, minimal pain during child birth, reduced need of pain management by pharmacotherapy and early ambulation post-partum. It also includes hypnosis, biofeedback, energy yoga and music therapy.
Various acupuncture techniques are widely used in China both for surgery as well as for pain relief during and after labor.
Intradermal saline injections
In this technique, a 25G needle is used to administer 0.1-0.15 ml injections of sterile water by intracutaneous route. It can be administered at either point 1, the Posterior Superior Iliac Spine or point 2 that is 1cm medial and 1-2 cm inferior. There is sharp burning pain for 20-30 sec followed by pain relief after 2 min which last for 45 min to 3 hours. There are no side effects.
Sedatives or analgesics
Selection of an appropriate analgesic agent is crucial to prevent neonatal asphyxia. Progression of labor has been arbitrarily divided into early and late phases in order to standardise the choice of analgesic agent.
Early phase is when the cervical dilatation is up to 8 cm in primigravida and up to 6 cm in multipara mothers while late phase is when the cervical dilatation goes beyond the early phase and up to delivery. While sedatives and analgesics are used in the early phase, inhalation agents are preferred in the late phase of labor.
The commonly used sedative and analgesic agents are:
- Opioid narcotics – Pethidine, Fentanyl and Promethazine
- Benzodiazepines – Diazepam
- Combinations therapy – Opioids + Promethazine/Metoclopramide/Ondansetron
There are various factors that moderate the dose of sedatives like fetal maturity, parity of mother, level of pain threshold for the mother. At times, there can be certain complications related to this therapy. Nausea and vomiting, delayed emptying of stomach, respiratory depression of foetus and neonatal hypotonia are the known complications. However, all of these can be effectively managed with the use of Naloxone (Opioid antagonist), Flumazenil (Benzodiazepine antagonist), Ranitidine (Antacid for gastric symptoms) or use of combination therapy.
Entonox i.e. 50 % nitrous oxide and 50 % oxygen is the commonly used inhalation agent. It can be self-administered by the mother by slow and deep inhalation which is advised before contractions. Inhalation is stopped as soon as contractions are induced.
However, there are some associated complications which include hyperventilation, altered consciousness, hypocapnia, dizziness. Intra-inhalation monitoring in form of arterial blood gas analysis and pulse oximetry is necessary to start early interventions in case of complications.
Patient controlled analgesia (PCA)
Novel continuous infusion pumps have been developed with the capability to receive patient input and deliver medication on demand. It is a technique wherein the patient herself can administer the drug and can also control the dose.
The agents used here are meperidine, pethidine and fentanyl. The mother herself can administer the drugs intravenously using a pump. There can be continuous or intermittent infusion. This method is beneficial as it helps in achieving higher maternal satisfaction and at the same time there is no need of administration of higher doses of narcotics.
There are less unwanted effects such as motor block , hypotension, reduction in the demands on staff on the labor floor and finally it gives many parturients a feeling of empowerment.
Transcutaneous electrical nerve stimulation (TENS)
In this method, electrodes are placed over the region of spinal cord supplying the uterus i.e. T10 to L1 and S2 to S4. The flow and amount of current is adjusted according to the severity of pain and threshold. The current that flows helps in limiting as well as ceasing the neuro-transmitter release via synapses. This is a non-invasive procedure which helps in reduction of dependence on opioid analgesics. Also, there are no associated systemic side effects.
This method includes epidular analgesia, para-cervical nerve block, pudendal nerve block and spinal anaesthesia.
- Lumbar epidular analgesia
It is the gold standard technique for pain control in obstetrics. Low doses of local anesthetic or opioid combinations are administered to provide a continuous T10-L1 sensory block during the first stage of labor. It is administered only after the onset of labor and not anytime before that. Agents used in this method are local anaesthetics; 0.5 % bupivacaine or 1 % lignocaine.
It is administered with the help of a needle that is inserted to puncture and reach the epidural space between L2 and L3. In order to achieve complete analgesia, it is inserted between T10 to S5, while for caesarean it is inserted between T4 to S1.
When this method is used, the mother is conscious and able to experience child birth. There is easier control of post-partum pain and reduced risk of aspiration-related complications. Simultaneously, there is significant improvement of newborn APGAR score.
However, reduced blood pressure due to blocking of sympathetic plexus is a known complication. This can be avoided with appropriate hydration of mother, either with isotonic saline or, Hartmann’s fluid. There can also be back pain due to persistent catheter insertion, accidental injection into subarachnoid space and complete spinal blocking, headache due to leakage of cerebro-spinal fluid from puncture site and nerve and vessel injury.
Contra-indications for this method include postural hypotension, coagulation disorders, kypho-scoliosis, hypovolemia and local infection.
- Para-cervical nerve block
This method can be used at the end of first stage of labor. It minimizes the urge to bear-down before onset of second stage. Para-cervical block reduces pain of only uterine contractions and has no role in relieving perineal pain.
1 % lignocaine with adrenaline is the agent used in this method. Bupivacaine is avoided due to its cardiotoxic properties. 15 cm or longer needle is used to administer the agent in lateral fornix of cervix at 3 o’clock and 9 o’clock position. Similar procedure is repeated on the other site. This method should be avoided in cases of established placental insufficiency. Fetal bradycardia is a common incidence.
- Pudendal nerve block
In this technique, there is no effect on pain from uterine contractions but perineal pain is relieved. Co-administration is done at vulva and perineum. It can be administered through trans-vaginal and trans-perineal routes at the level of the ischial spines. There can be complications such as injection into vessels, hematoma or local infection.
- Spinal anaesthesia
This is a preferred method of pain relief during delivery or third stage of labor. Hyper-baric bupivacaine, lignocaine and fentanyl are commonly used agents for early onset of action and morphine is used for pain control.
It is administered in the subarachnoid space between L3 and L4 vertebrae. For normal or forceps delivery, it is given between T10 to S1 while for caesarean delivery it is extended up to T4. In this technique, there is relatively blood-less field as well as reduced incidence of fetal hypoxia. Most complications resemble that of epidular analgesia. Some spinal analgesia specific complications are respiratory depression, urinary retention, chemical meningitis and paralysis.
This is a commonly employed technique for emergency or elective caesarean section delivery. When this technique is to be used, pre-operative preparation with sedatives or opioid narcotics is to be avoided as these agents may precipitate fetal respiratory depression. Different agents are used for induction of anesthesia, maintenance, relaxation and post-partum.
The agent used for induction is intravenous 2.5 % thiopentone sodium, for maintenance is 50 % nitrous oxide, 50 % oxygen and 0.5 % Halothane, for relaxation is vecuronium or atracurium and for post-partum use is nitrous oxide concentration increased to 70 % and intravenous narcotics. Mendelson’s syndrome is a complication in which there is aspiration of contents of stomach. There can also be chemical pneumonitis and delayed gastric emptying.