Understanding Anesthesiology - The Global Regional Anesthesia ...

SECTION 2
Obstetrical Anesthesia
In This Section
1. Physiologic Changes of
Pregnancy
2. Labour Analgesia
3. Anesthesia for Operative
Delivery
Physiologic Changes of Pregnancy
Physiologic and anatomic changes develop across
many organ systems during pregnancy and the
postpartum period. Metabolic, hormonal and
physical changes all impact on anesthetic management.
To the anesthesiologist, the most important
changes are those that affect the respiratory and
circulatory systems.
Respiratory System
There is an increased risk of difficult or failed intubation
in the parturient. This is primarily due mucosal
vascular engorgement which leads to airway
edema and friability. Laryngoscopy can be further
impeded by the presence of large breasts.
In addition, the parturient is at risk for aspiration
of stomach contents. During pregnancy, the stomach
is displaced cephalad and intragastric pressure
increases. Gastric motility is decreased and
gastric secretions increase. This, combined with a
decrease in the integrity of the gastroesophageal
junction predisposes to pulmonary aspiration of
gastric contents. In fact, airway complications (difficult
intubation, aspiration) are the most common
anesthetic cause of maternal mortality. The best
means of avoiding this outcome is to avoid general
anesthesia (by using a regional technique)
and thus maintain intact laryngeal reflexes. If a
general anesthetic is required, NPO status for
eight hours is preferred although not achievable
in an emergency situation. Pretreatment of all parturients
with a non-particulate antacid (30 cc sodium
citrate p.o.) as well as with a histamine
blocker (ranitidine 50 mg IV) is important. Finally,
a rapid sequence induction with cricoid pressure
is mandatory.
With the apnea that occurs at induction of anesthesia,
the parturient becomes hypoxic much more
rapidly than the non-pregnant patient. The reason
for this is two-fold. Firstly, oxygen requirement
has increased by 20% by term. Secondly, the functional
residual capacity (FRC), which serves as an
“oxygen reserve” during apnea, has decreased by
20% due to upward displacement of the diaphragm.
Adequate ventilation must be maintained during
anesthesia. By term, minute ventilation has increased
to 150% of baseline. This results in a de-
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