General Anesthesia for Cesarean Delivery
Outline
Introduction
Advantages
of General Anesthesia
Disadvantages
of General Anesthesia
Neonatal
Effects of General Anesthesia
Preparation
Intravenous
Induction Agents
Neuromuscular
Blocking Agents
Maintenance
of Anesthesia
Awareness
Summary
References
Introduction
In
the
Advantages of General Anesthesia
General anesthesia for cesarean delivery has a long history of maternal and fetal safety. It produces excellent operating conditions in the least possible time and for an unlimited duration, maintains relatively stable sympathetic tone and provides a controlled airway.
Disadvantages of General Anesthesia
The most significant disadvantage of general anesthesia is the loss of protective airway reflexes. Maternal death related to airway-related problems including failure to ventilate the patient and/or intubate the trachea and/or aspiration of gastric contents has remained relatively constant despite advances in patient monitoring, airway techniques and provider education. Failure to intubate the trachea in the obstetric patient is 8 times more likely than in the surgical patient secondary to such factors as generalized airway edema. The obstetric patient has decreased oxygen reserve because of a decrease in functional residual lung capacity and increased oxygen consumption. Hypoxia occurs more rapidly in the apneic obstetric patient,1 leaving little room for delayed ventilation. And because of the emergent nature of obstetrics, as well as hormonal and mechanical changes that occur because of pregnancy, these patients are at increased risk of aspiration of gastric contents.
Other disadvantages of general anesthesia include the loss of maternal(and paternal) participation in the delivery; increased stress response to laryngoscopy, tracheal intubation and extubation; the potential for undesirable awareness, and possibly an increase in blood loss. Because regional anesthesia is not performed, neuraxial opiates are not administered for postoperative pain control.
Neonatal Effects of General
Anesthesia
Although anesthetic technique does not correlate with neonatal mortality, controversy exists when comparing general verses regional anesthesia and neonatal outcome after cesarean delivery. In a large study of elective and nonelective cesarean deliveries, Ong, et al.2 showed that compared to neonates whose mothers received regional anesthesia, those born under general anesthesia had lower 5 minute Apgar scores following cesarean delivery for fetal distress but not after elective cesarean delivery. In contrast, Mueller and colleagues3 in a large study of patients undergoing elective cesarean deliveries, demonstrated lower Apgar scores with regional anesthesia. Perhaps preexisting fetal status is more important to neonatal outcome after cesarean delivery than anesthetic technique.
Note in the picture
patient preparation prior to induction of general anesthesia.
left
uterine displacement is done with a wedge under the right hip and the patient
is
being
preoxygenated.
PREPARATION:
All operating rooms where cesarean deliveries are conducted should be checked daily and between cases to make sure all necessary drugs and equipment are immediately available should the need to induce general anesthesia occur. A difficult airway cart for unanticipated airway emergencies should also be stocked and readily available.
A thorough history and physical examination should be performed if possible, including a careful airway assessment and fetal heart rate (FHR) evaluation. If the cesarean delivery is urgent, a focused history and physical examination concentrating on the airway, allergies, medical problems and previous anesthetics is performed as preparation is made to anesthetize the patient.
Airway Assessment
Although airway assessment is important, there are
limitations in the preinduction diagnosis of patients
at risk of a difficult intubation. Regardless of these limitations, 4 factors
correlate with a difficult intubation. They are the size of the patient’s tongue (Mallampati Class III or IV), protruding maxillary incisors,
a short neck and a decreased thyromental distance
(receding jaw).4
A combination of these factors increases the probability of
experiencing a difficult intubation. Because of the nature of the urgent cesarean
delivery, airway assessment can be difficult.
Preparation, education and practice drills are therefore critical. If possible, early identification of the
patient with a potentially difficult airway and consultation with her
obstetrician is essential. In this case,
maternal safety concerns may take precedence over fetal indications for an
emergent delivery and may necessitate either an early epidural or awake intubation of the trachea.
Acid-Aspiration Prophylaxis: Note in
the picture to the left sodium citrate (0.3%) containers each
containing 30
ml. At
The obstetric patient is at risk of
aspiration of gastric contents secondary to recent food ingestion, delayed
gastric emptying, increased stomach acid secretion, increased gastric pressure
and decreased lower esophageal sphincter (LES) tone. Historically, risk of aspiration pneumonitis is present when gastric volume is greater than
25ml and gastric pH is less than 2.5 (although a lower pH is worse than a
higher volume). Of uncertain usefulness,
the practice of increasing gastric pH and decreasing gastric volume is
considered the standard of care. This includes
the administration of non-particulate antacids, and possibly the administration
of H2-receptor blockers, metoclopramide
and proton pump inhibitors.
Thirty milliliters of 0.3M sodium citrate should always be given prior to inducing
general anesthesia (Fig. 1). If time
permits, 50mg of ranitidine or 40mg of omeprazole,
intravenously, can be administered 30 minutes prior to surgery and is effective
in decreasing gastric acidity and volume.
Metoclopramide, 10mg, can be administered
intravenously; it is an effective antiemetic and
increases LES tone.
The best way to prevent aspiration is either to
maintain the patient’s protective airway reflexes by using regional anesthesia
or if general anesthesia is necessary, to induce anesthesia using a rapid-sequence
induction with cricoid pressure and and awake extubation of the
patient’s trachea.
Positioning
As with regional anesthesia, left uterine displacement with a wedge under the patient’s right hip to decrease the incidence of aortocaval compression is also necessary when general anesthesia is used.
The patient’s head should be elevated slightly using a small pillow to reproduce the “sniffing” position to facilitate tracheal intubation. In the obese patient, it is often helpful to place a rolled blanket under the patient’s shoulders to elevate the thorax, improving tracheal intubating conditions.
Although usually of little clinical importance, the Trendelenburg position should be avoided if possible
because of the incidence of small venous air emboli.
Monitoring and Equipment
The patient should be monitored using a non-invasive blood pressure monitor with a “stat” mode, a 5-lead EKG, a thermometer and a precordial or esophageal stethoscope. Pulse oximetry and capnography are manditory. Capnography is used to confirm a tracheal intubation and helps avoid hyperventilation that decreases uterine blood flow. FHR monitoring should continue as long as possible since this can influence the speed with which the surgery proceeds.
Preoxygenation
Because pregnancy increases oxygen consumption and decreases oxygen reserve, the pregnant patient cannot tolerate long periods of apnea. It is therefore imperative to preoxygenate prior to the induction of general anesthesia. Preoxygenation also “buys time” in a difficult airway situation. If time permits, the administration of 100% oxygen at high flow rates using a tight fitting mask for at least 3 minutes is best. However, 4 vital capacity breaths should be used when delivery is urgent.
Pretreatment/Defasciculation
There is no need to pretreat with a non-depolarizing muscle relaxant prior to succinylcholine administration to prevent muscle fasciculations since they are usually mild in intensity or absent.
Intravenous Induction Agents
Because of hormonal and other physiologic changes
during pregnancy, the anesthetic requirements of the pregnant patient and the
dosages of the intravenous induction agents used are decreased by 30-40%. These drugs are all highly lipid soluble and
readily cross the placenta, however, fetal metabolism and redistribution
decreases their influence on the neonate.
Because long uterine incision-to-delivery intervals may affect neonatal outcome, general anesthesia is induced only after the patient’s abdomen is prepped and sterile drapes applied (Fig 2.).
The dosage of medications given as
mg/kg are based on the patient’s pregnant body weight.
Thiopental (4mg/kg to a maximum of 500mg) is the
most common drug used to induce general anesthesia in the patient for cesarean
delivery. A single bolus of thiopental
(4-7mg/kg) does not affect Apgar scores. Because thiopental decreases myocardial
contractility and produces vasodilation, it may be
unwise to use this drug for patients with cardiac disease or hypovolemia.
Ketamine (1mg/kg) is a potent
analgesic and amnesic drug that causes the release of endogenous catecholamines. It
may be useful when circulatory support is needed as in the hypovolemic
patient but dangerous in the hypertensive
patient. Ketamine
is a potent bronchodilator and is useful in severe
asthmatic patients. The incidence for postoperative hallucinations following
the use of ketamine is low in these patients and
reduced further if benzodiazapines are used as part
of the anesthetic.
Etomidate (0.3mg/kg), although used infrequently for cesarean delivery because of pain on injection, maternal myoclonus, and possible neonatal effects, can be useful in the pregnant cardiac patient because it produces minimal cardiovascular depression.
Propofol (2mg/kg) offers no advantages over thiopental. In addition, propofol produces pain on injection, a more rapid decrease in maternal blood pressure and possibly more negative neonatal effects than thiopental.
Neuromuscular Blocking agents
All commonly used neuromuscular blocking agents are highly ionized at physiologic pH and therefore do not cross the placenta. In addition, if the patient is receiving magnesium sulfate, expect a greater duration of action of both depolarizing and nondepolarinzing muscle relaxants.
Succinylcholine (1.7mg/kg to a maximum of 200mg) is the most commonly used neuromuscular blocking agent to facilitate tracheal intubation for a rapid-sequence induction. Although frequently unnecessary, a succinylcholine infusion (0.2%) can be used to maintain relaxation.
Nondepolarizing neuromuscular relaxants can be substituted when succinylcholine is contraindicated. Rocuronium (1.2mg/kg) produces good to excellent tracheal intubating conditions for a rapid-sequence induction in approximately 1 minute. However, the duration of paralysis is longer than succinylcholine which may be important if a difficult tracheal intubation is anticipated.
Maintenance of Anesthesia
Because of questionable neonatal benefits of administering inspired oxygen concentrations above 50 to 60%,5 nitrous oxide(50-40%) is commonly administered along with a volatile agent immediately following induction of general anesthesia and increased to 70% following delivery.
Volatile agents used to maintain anesthesia before delivery have minimal effects on mother and fetus. 0.75% isoflurane in 50% nitrous oxide readily crosses the placenta, however, fetal uptake is not clinically significant unless induction to delivery time is excessive; and uterine relaxation is minimal. After delivery, the volatile agent is reduced or discontinued. Oxytocin (15units/250ml) is administered after the delivery of the placenta to contract the uterine smooth muscle and minimize postpartum blood loss. Intravenous narcotics (fentanyl, 4-7:g/kg or morphine, 0.3-0.4mg/kg), which are usually avoided prior to delivery are given after delivery. Midazolam (0.03-0.04mg/kg) is administered to minimize awareness.
Awareness
Because the doses of anesthetics used for general anesthesia are decreased in order to limit fetal exposure, maternal awareness and recall are possible. The use of a volatile agent prior to delivery and midazolam after delivery can reduce the incidence of maternal awareness and recall.
Summary
In the presence of acute fetal distress, a well
conducted general anesthetic for cesarean delivery is vital to preserve
maternal and fetal well-being. However,
because of the uncommon and often unexpected need for general anesthesia in the
parturient, it is essential that anesthesia providers who care for these
patients be prepared in advance to deal calmly and
efficiently with the challenges that can accompany this anesthetic.
References
1. Archer GW, Marx GF:
Arterial oxygen tension during apnea in parturient women. Br J Anaesth
46:358-360, 1974
2. Ong BY, Cohen MM, Palahniuk RJ: Anesthesia for cesarean section: Effects on neonates. Anesth Analg 68:270-275, 1989
3. Mueller MD, Bruhwiler H, Schupfer GK, Luscher KP: Higher rate of fetal acidemia
after regional anesthesia for elective cesarean delivery. Obstet Gynecol 90:131-134, 1997
4. Rock DA, Murray WB, Rout CC, Gouws
E: Relative risk analysis of factors
associated with difficult intubation in obstetric
anesthesia. Anesthesiology 77:67-73,
1992
5. Marx GF, Mateo CV:
Effects of different oxygen concentrations during general anaesthesia during caesarean section. Can Anaesth Soc J
18:587-593, 1971
Figure
Legends
1. Pulmonary aspiration prophylaxis with sodium citrate
2. Preparation of the parturient for cesarean delivery with
general anesthesia