Role of general anesthesia in obstetric practice: preparation, induction, intubation, gaseous agents used and complications.

Nowadays the usage of general anesthesia is decreasing as  regional analgesia is becoming popular. General anesthesia  is having relatively higher risk. Though incidence of case fatality is low, it  is more with general anesthesia for cesarean delivery than regional anesthesia.

Compared with the non pregnant population a 10-fold higher rate of failed intubation noted in pregnant women and it occurs in approximately 1 of every 250 general anesthetics administered.

According to these findings it can be concluded that concluded that regional analgesia is the preferred method of pain control in pregnant and should be used unless contraindicated.

Compared with regional analgesia trained personnel and specialized equipment

 including fiberoptic intubation are mandatory for the safe use of general anesthesia.

Preparation of the  patient for general anesthesia:

To minimize the risk of complications for the mother and fetus several steps should be taken prior to anesthesia induction, like  the use of antacids, lateral uterine displacement, and preoxygenation.

Giving antacids:

Giving antacids before induction is more important in general anesthesia than any other method to decrease mortality from chance of aspiration of gastric contents.

Normally preferred is 30 mL of sodium citrate with citric acid  called Bicitra, given about 45 minutes before surgery, neutralizes gastric contents in nearly 90 percent of women undergoing cesarean delivery. It can also be given within a few minutes of the anticipated time of anesthesia induction, either by general or major regional block.

If more than 1 hour has passed between when the first dose was given and when anesthesia is induced, then a second dose to be given.

Intravenous administration of ramiprazole or pantaprazole or omeprazole can also be used.

Uterine Displacement:

 Generally the  uterus may compress the inferior vena cava and aorta when the mother is in the supine position. With lateral uterine displacement, the duration of general anesthesia has less effect on neonatal condition than when the woman remains supine.

Preoxygenation:

As gravid uterus pushes the diaphragm above the functional reserve capacity of the lungs is reduced in pregnant women . So, they  become hypoxemic more rapidly during periods of apnea than do non pregnant patients.

It  is important first to replace nitrogen in the lungs with oxygen  to minimize hypoxia between the time of muscle relaxant injection and intubation. It  is done by administering 100 percent oxygen via face mask for 2 to 3 minutes prior to anesthesia induction. In case of emergency, similar benefit can be obtained by  four vital capacity breaths of 100-percent oxygen via a tight breathing circuit.

Induction of general anesthesia:

Thiopental:

 Thiopental belongs to thiobarbiturate group which is given intravenously is widely used and having  the advantages of ease and extreme rapidity of induction as well as prompt recovery with minimal risk of vomiting.

But thiopental and similar compounds are poor analgesic agents. So, large amount of drug is needed  to maintain anesthesia by it alone. And it may cause appreciable newborn

depression. Because of this thiopental is not used as the sole anesthetic agent and  is administered in a dose that induces sleep.

Ketamine:

 This agent may also be used to make the patient unconscious. Ketamine may be used to produce analgesia and sedation just prior to vaginal delivery  intravenously in low doses of 0.2 to 0.3 mg/kg. Whereas in doses of 1 mg/kg used to induce general anesthesia.

 Unlike thiopental ketamine is not associated with hypotension. So, it is useful in women with acute hemorrhage. But  it usually causes a rise in blood pressure, and thus to be avoided in women who are already hypertensive. Unpleasant delirium and hallucinations are commonly induced by this agent.

Intubation:

Immediately after the patient becomes unconscious, a muscle relaxant to be given to facilitate intubation.

The commonly used agent is succinylcholine, which is havind  a rapid-onset and short duration of action.

 To occlude the esophagus from induction until intubation is completed, cricoid pressure is used in sellick maneuver. Before the operation begins, proper placement of the endotracheal tube must be confirmed by  auscultating the bilateral breath sounds and by end-tidal carbon dioxide analysis.

Failed or difficult intubation:

Though rare,  failed intubation is a major cause of anesthesia-related maternal mortality.

Among the maternal deaths associated with general anesthesia,  22 percent of deaths can be attributed to induction or intubation problems.

Difficult intubation can be predicted to some extent by verifying the history of previous difficulties with intubation and by careful assessment of anatomical features of the neck, maxillofacial, pharyngeal, and laryngeal structures.Sometimes intrapartum edema may develop though initial assessment of the airway was uneventful, and can create problems in intubation. Morbid obesity is also a major risk factor for failed or difficult intubation.

To prevent or to face these problems appropriate preoperative preparation with immediate

availability of specialized equipment, including a variety of different shaped laryngoscopes, laryngeal mask airways, a fiberoptic bronchoscope, a transtracheal ventilation set, as well as liberal use of awake oral intubation techniques are important.

Management of failed intubation:

 The operative procedure should be started only after it has been ascertained that tracheal

intubation has been successful and that adequate ventilation can be accomplished. This should be followed  even in case of an abnormal fetal heart rate pattern.

In case of failed intubation:

The  woman must be allowed to awaken and a different technique used, such as an awake intubation or regional analgesia.

Or  the woman is ventilated by mask and cricoid pressure is applied to reduce the chance of aspiration and the surgery may proceed with mask ventilation or the woman may be allowed to awaken.

In some cases where the woman has been paralyzed, and  ventilation cannot be reestablished by insertion of an oral airway, laryngeal mask airway, or use of a fiberoptic laryngoscope to intubate the trachea, then a life-threatening emergency exists. In this case to restore ventilation, percutaneous or even open cricothyrotomy is performed, and jet ventilation to be given.

Gas anesthetics used in general anesthesia:

After  securing the endotracheal tube, a 50:50 mixture of nitrous oxide and oxygen is

administered to provide analgesia. To  provide amnesia and additional analgesia  a volatile halogenated agent can be added.

Volatile anesthetics:

Isoflurane  is the most commonly used volatile anesthetic. Isoflurane  and halothane both are potent, nonexplosive agents and produce remarkable uterine relaxation when given in high, inhaled concentrations. High concentrations are restricted to uncommon situations where uterine relaxation is a requisite rather than a hazard, like for internal podalic version of the second twin , breech decomposition and replacement of the acutely inverted uterus. As soon as the manoeuvre has been completed, anesthetic administration should be stopped and immediate efforts begun to promote myometrial contraction to

minimize hemorrhage.

Side effects:

 Halothane and isoflurane may intensify the adverse effects of maternal hypovolemia by their cardiodepressant and hypotensive effects.

Occasionally  these  agents have been associated with hepatitis and massive hepatic necrosis.

Complications of general anesthesia:

Anesthesia gas exposure and pregnancy outcome:

 All  anesthetic agents that depress the maternal central nervous system will cross the placenta and depress the fetal central nervous system. So, the newborn immediately following delivery with a general anesthetic should get respiratory support. Induction to delivery time should be minimized when general anesthesia is used.

 Fetal exposure of more than 8 minutes is shown to be associated with increased neonatal depression.

But some studies showed no significant differences in short-term measures of neonatal outcome, including Apgar scores, umbilical artery blood gas determinations, or length of stay in between general or regional anesthesia.

Extubation:

The extubation can become a problem sometimes as patient may not recover from general anesthesia in case of poor general condition or because of prolonged anesthesia period.

When  the woman is conscious to a degree that enables her to follow commands and is capable of maintaining oxygen saturation with spontaneous respiration then the tracheal tube can be safely removed.

 The  stomach should be emptied via a nasogastric tube prior to extubation.

Aspiration :

As  Mendelson, who is an obstetrician first described massive gastric acidic inhalation causing pulmonary insufficiency from aspiration pneumonitis, the syndrome bears his name.  

It was the most common cause of anesthetic deaths in the past in obstetrics. In a survey of

maternal deaths between 1979 and 1990, inhalation of gastric contents was associated with 23 percent  anesthesia related deaths.

Important  procedures in  effective prophylaxis are use of antacids, skilful intubation accompanied by cricoid pressure, emptying of the stomach with a nasogastric tube and use of regional analgesia whenever possible.

Fasting time required to prevent aspiration:

In  uncomplicated laboring women clear liquids such as water, clear tea, black coffee, carbonated beverages and fruit juices without pulp may be allowed but obvious solid foods should be avoided.

A fasting period of 8 hours or more is preferable for uncomplicated parturients undergoing elective cesarean delivery. Even after taking these precautions, it should be assumed that any woman in labor is having both gastric particulate matter as well as acidic contents.

Pathophysiology of aspiration pneumonitis:

 If  the pH of aspirated fluid was below 2.5 chances of developing severe chemical pneumonitis are more.

The  right lower lobe is most often involved as the right main stem bronchus usually offers the simplest pathway for aspirated material to reach the lung parenchyma. In severe

cases, there may be bilateral widespread involvement.

Depending  on the material aspirated and the severity of the process the woman may develop evidence of respiratory distress immediately or as long as several hours after

aspiration,. Aspiration of a large amount of solid material causes obvious signs of airway obstruction. Smaller particles without acidic liquid may lead to patchy atelectasis and

later to bronchopneumonia.

In case of  inspiration of highly acidic liquid , decreased oxygen saturation along with tachypnea, bronchospasm, rhonchi, rales, atelectasis, cyanosis, tachycardia, and hypotension are likely to develop. At the sites of injury, pulmonary capillary leakage

results in protein-rich fluid containing numerous erythrocytes exuding from capillaries into the lung interstitium and alveoli to cause decreased pulmonary compliance, shunting of blood, and severe hypoxemia.

In X ray, radiographic changes may not appear immediately and they may be quite variable, although the right lobe most often is affected. Therefore, chest radiographs alone can not exclude aspiration.

Treatment of aspiration pnemonitis:

 In case of having suspicion of aspiration of gastric contents  very close monitoring is needed to note the evidence of any pulmonary damage. By  pulse oximetry, respiratory rate and oxygen saturation are measured. These  are the most sensitive and earliest indicators of injury.

If chance of aspiration is predicted, immediately as much of the inhaled fluid as possible should be wiped out of the mouth and removed from the pharynx and trachea by suction. Saline lavage is not recommended as it may further disseminate the acid throughout the lung. Bronchoscopy may be indicated to relieve airway obstruction if large particulate matter is inspired.

There is no convincing clinical or experimental evidence that corticosteroid therapy or prophylactic antimicrobial administration is beneficial .

But clinical evidence of infection develops, however vigorous treatment with above agents to be given. In case of development of acute respiratory distress syndrome, mechanical ventilation with positive end-expiratory pressure to  given to save the life.