Most women experience moderate to severe pain during labor and delivery, often requiring some form of pharmacologic analgesia.1 The lack of proper psychological preparation combined with fear and anxiety can greatly enhance the patient’s sensitivity to pain and further add to the discomfort during labor and delivery. However, skillfully conducted obstetric analgesia, in addition to relieving pain and anxiety, may benefit the mother in many other ways. This chapter focuses on the management of obstetric patients with a primary focus on regional anesthesia techniques.
Physiologic Changes of Pregnancy
Pregnancy results in significant changes affecting most maternal organ systems (Table 41–1). These changes are initiated by hormones secreted by the corpus luteum and the placenta. Such changes have important implications for the anesthesiologist caring for the pregnant patient. This chapter reviews the most relevant physiologic changes of pregnancy and discusses the approach to obstetric management using regional anesthesia.
Table 41–1.Summary of physiologic changes of pregnancy at term. ||Download (.pdf) Table 41–1. Summary of physiologic changes of pregnancy at term.
|Variable ||Change ||Amount |
|Total blood volume ||Increase ||25%–40% |
|Plasma volume ||Increase ||40%–50% |
|Fibrinogen ||Increase ||50% |
|Serum cholinesterase activity ||Decrease ||20%–30% |
|Cardiac output ||Increase ||30%–50% |
|Minute ventilation ||Increase ||50% |
|Alveolar ventilation ||Increase ||70% |
|Functional residual capacity ||Decrease ||20% |
|Oxygen consumption ||Increase ||20% |
|Arterial carbon dioxide tension ||Decrease ||10 mm Hg |
|Arterial oxygen tension ||Increase ||10 mm Hg |
|Minimum alveolar concentration ||Decrease ||32%–40% |
Changes in the Cardiovascular System
Oxygen consumption increases during pregnancy, as the maternal cardiovascular system is required to meet the increasing metabolic demands of a growing fetus. The end result of these changes is an increase in heart rate (15%–25%) and cardiac output (up to 50%) compared with values before pregnancy. In addition, lower vascular resistance is found in the uterine, renal, and other vascular beds. These changes result in a lower arterial blood pressure because of a decrease in peripheral resistance, which exceeds the increase in cardiac output. Decreased vascular resistance is mostly due to the secretion of estrogens, progesterone, and prostacyclin.2 Particularly significant increases in cardiac output occur during labor and in the immediate postpartum period owing to added blood volume from the contracted uterus.
Cardiovascular changes and pitfalls in advanced pregnancy include the following:
Increase in heart rate (15%–25%) and cardiac output (up to 50%).
Decrease in vascular resistance in the uterine, renal, and other vascular beds.
Compression of the lower aorta in the supine position may further decrease uteroplacental perfusion and result in fetal asphyxia.
Significant hypotension is more likely to occur in pregnant versus nonpregnant women undergoing regional anesthesia, necessitating uterine displacement or lateral pelvic tilt maneuvers, intravascular preloading, and vasopressors.
From the second trimester onward, aortocaval compression by the enlarged uterus becomes progressively more important, reaching its ...