Role of anesthesia in obstetrics: principles, parenteral drugs and nitrous oxide.

Introduction: 
 Anesthesia role will be there in all surgical specializations in those Obstetrics is important one. But in obstetrical anesthesia there are some  controversial issues also.

Giving anesthesia in a obstetrical patient is not as easy as in general people because nobody knows when the labor begins, and  anesthesia may have to be given  within short time  of a full meal. Patient may vomit and gastric contents may be aspirated leading to maternal morbidity and sometimes mortality.

Along with normal physiological adaptations of pregnancy which will affect the type of analgesia and anesthesia, the associated risk factors like preeclampsia, placental abruption, and chorioamnionitis etc will also affect the choice.

Though obstetrical anesthesia is serving patients in relieving their pain but the complications should not be ignored as these are responsible for 1.6 percent of pregnancy-related maternal deaths.

Nowadays  anesthesia-related maternal mortality has declined significantly because of the measures using to increase the safety. Among them first to mention is increased use of regional analgesia, rather than general anesthesia. Second one is the increased availability of in-house anesthesia coverage.

But still inadequate anesthesia services have been are remaining as a leading and potentially preventable cause of maternal deaths in some developing countries.

General principles followed in obstetrical anesthesia services:

Obstetrical anesthesia can be given either on request of the woman or as a responsibility of the obstetrician to help her as much as possible in relieving the pain.

Actually there should be combined effort of the obstetrician and the anesthesiologist in managing the patient. Before planning to give anesthesia patient should be examined to identify any risk factors and to plan according to them. By this we can minimize the need for emergency anesthesia as it may become hazardous.

The risk factors are,

If patient is morbidly obese it will become difficult to keep her in position to give spinal or local anesthesia and also the dose of the drugs also changes.

In case of any anatomical abnormalities or edema of the face or neck intubation during general anesthesia will become a problem. If spinal abnormality is there giving spinal or epidural anesthesia will become a problem. Any prior surgeries or trauma can also lead to these abnormalities. So, verifying the previous medical and surgical history of the patient is important.

In case of general anesthesia patients with short neck or with arthritis of neck or having goiter will create problems whereas in woman with extreme short stature giving spinal will become a problem as identifying the inter vertebral space will become a problem.

In woman with medical conditions like pulmonary or cardiac or neurological problems or bleeding disorders or with severe pre-eclampsia , sudden untoward effects can occur during anesthesia better to take physician consultation and he should be informed about the posting of the case.

If the woman is having any prior history of anesthetic complications, through history to be taken regarding it and measures to be taken to prevent them.

If planning for normal delivery, the chances of converting into operative delivery to be kept in mind like in cases of malpositions, bigbaby, placenta previa, high-order multiple gestation etc.

If a hospital is said to be having good obstetrical anesthesia services, ideally  should have,  

- A 24 hours availability of a licensed obstetrical practitioner, who is also trained to administer an appropriate anesthetic and to maintain  of vital functions in  case of an obstetrical emergency. The obstetrician should be proficient in giving local and pudendal analgesia. In case of emergency, regional analgesia may be administered by the properly trained obstetrician but anesthesiologist is preferred,  so that the obstetrician can concentrate on laboring woman and her fetus. As per general anesthesia is concerned it should be administered only by those with special training.

- A 24 hours availability of anesthesia personnel, so that  cesarean delivery can be started within 30 minutes of the decision or if the patient to be shifted immediately to caesarean section from the labor board and he should take responsibility of all the drugs he has administered.

- A 24 hours availability of a qualified physician with obstetrical exposure  to take care of emergencies that can occur during vaginal or cesarean delivery or during administration of anesthesia.

- Availability of good equipment, facilities, and supporting team to the obstetrician and anesthesiologist in the surgical suite.

- Immediate availability of pediatrician personnel to look after the baby if  resuscitation is needed as depressed newborn.

Individual pain tolerability:

How the woman experiences the labor pain is entirely based individual’s pain tolerability and response to variable stimuli.  This is modified by emotional, motivational, cognitive, social and cultural circumstances. It also depends on the ability of the woman and her caregivers to anticipate her pain experience prior to labor. So, in deciding the method of pain control individualization is desirable.

Non pharmacological  methods used for control of pain:

 The pain during labor is aggravated by fear and imaginations by the woman. If before delivery the woman receives counseling regarding the events that take place during delivery and the measures available to relieve the pain and if she gains confidence on the obstetrical staff that cares for her, the intensity of the pain can be decreased. This is psycho prophylactic method.

Pain can be decreased by teaching pregnant women to practice relaxed breathing and to her  labor partners psychological support techniques. By these techniques the need for  potent analgesic, sedative, and amnestic drugs during labor and delivery has decreased.

A well motivated women with supportive spouse or other family member and with a considerate obstetrician who instils confidence can easily pass through the labor and delivery normally.

Analgesics  and sedatives used during labor:

Analgesia during labor can be started when uterine contractions and cervical dilatation starts causing discomfort. To give pain relief a narcotic can be used such as meperidine along with one tranquilizer drug such as promethazine.

Successful analgesia and sedation means which allow the mother to  rest quietly in between contractions. So, the discomfort usually is felt at the peak of an effective uterine contraction but the pain is generally not unbearable.

The other parenteral drugs used for labor pain are Fentanyl, Nalbuphine, Butorphanol, Morphine etc.

Parenteral  agents:

Meperidine and Promethazine:

 For intramuscular injection Meperidine is given in dose of  50 to 100 mg with promethazine in dose 25 mg at intervals of 2 to 4 hours. By intravenous injection of meperidine in doses of 25 to 50 mg every 1 to 2 hours more rapid effect can be achieved.

Analgesic effect  is maximum after 30 to 45 minutes of an intramuscular injection but

Immediate effect can be seen after intravenous administration.

Meperidine will cross the placenta and its half-life is around 13 hours, it is longer in the newborn.  It is having  depressant effect in the fetus which occurs after the peak maternal analgesic effect. Sometimes it needs treatment with naloxone in 3 percent of newborns.

Butorphanol (Stadol):

 This is a synthetic narcotic and given in 1- to 2-mg doses either IV or IM.  

The major side effects are somnolence, dizziness, and dysphoria. In some cases neonatal respiratory depression is reported but in low incidence than with meperidine. And a sinusoidal fetal heart rate pattern noted in some cases following butorphanol administration.

These two drugs should not be given are contiguously, as butorphanol antagonizes the narcotic effects of meperidine.

Fentanyl:

Fentanyl is a short-acting and very potent synthetic opioid. It can be given in doses of 50 to 100 µg intravenously every hour.

The main disadvantage is a short duration of action. So, it requires frequent dosing or patient-controlled intravenous pump.

Compared with fentanyl, butorphanol provides better initial analgesia.

As per the efficacy of the parenteral agents is concerned, meperidine is found to be the most common opioid used worldwide for obstetrical analgesia. The  parenteral opioids will not influence the length of labor or need for obstetrical intervention. Compared with them epidural analgesia provides superior pain relief.

And  intravenous and intramuscular sedatives are not without risks. Among 129 maternal anesthetic-related deaths 4 were estimated from this type of sedation, in that one is from aspiration, two are from inadequate ventilation and one from over dosage. And also respiratory depression in newborn can occur due to meperidine or other narcotics.

Narcotic Antagonists:

Naloxone: The antagonist normally used to reverse the respiratory depression induced by opioid narcotics is nalaxone.

It acts by displacing the narcotic from specific receptors in the central nervous system.

Naloxone, along with proper ventilation should be given to reverse respiratory depression in a newborn.

Naloxone is contraindicated in a newborn of a narcotic-addicted mother because of the fear that withdrawal symptoms may become precipitated.

Nitrous oxide:

Nitrous oxide is used in obstetrical analgesia along with oxygen in the form of a self-administered mixture of 50-percent nitrous oxide (N2O) and oxygen.

In it two type of preparations are there, one is both gases premixed in a single cylinder called Entonox, and in the other form a blender mixes the two gases from separate tanks called Nitronox.

A  breathing circuit is used to which the gases are connected through a valve that opens only when the patient inspires.

The suggestions for the use of nitrous oxide are:

The woman should take slow deep breaths and she has to  inhale the gas 30 seconds before the next anticipated contraction and to stop when the contraction starts to recede.

She has to breathe normally in between contractions by removing the mask. Either the patient or a person who is having  knowledge about the circuit should hold the mask.

The attendant should be in verbal contact with the patient to monitor her conscious levels.

The patient should be previously instructed that the pain will not become completely eliminated but some amount of relief can be obtained.

During the process  intravenous access to be secured.

Pulse  oximetry used to measure the level of oxygen saturation and scavenging of exhaled gases to be maintained.

If previously opioids are administered  additional caution to be taken as the combination of opioids and nitrous oxide can make the woman unconscious and unable to protect her airway.