Most women experience moderate to severe pain during labor and delivery, often requiring some form of pharmacologic analgesia. 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 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 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.
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. Particularly significant increases in cardiac output occur during labor and in the immediate post-partum 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 maximum effect at 36–38 weeks, after which it may be relieved some as the fetal head descends into the pelvis. Cardiac output may decrease when patients are in the supine position but not in the lateral decubitus position. Venous occlusion by the growing fetus causes supine hypotensive syndrome in 10% of pregnant women and manifests as maternal tachycardia, arterial hypotension, faintness, and pallor.
Compression of the lower aorta in this position may further decrease uteroplacental perfusion and result in fetal asphyxia. Uterine displacement or lateral pelvic tilt should be applied routinely during the anesthetic management of the pregnant patient. Uterine displacement is best achieved by placing the patient in the left lateral decubitus position. In this position, cardiac vagal activity will be augmented as compared to the supine position. Placing a wedge under the bony pelvis has been used to achieve uterine tilt. However, it has recently been demonstrated that uterine tilt is more effective when the mother is placed in the full left lateral decubitus position and then is turned supine onto the pelvic wedge.
Changes in the electrocardiogram are common in late pregnancy. The QRS axis may initially shift to the right during the first trimester, rotating to left axis by the third trimester as a result of the expanding uterus. A shortening of the PR and QT intervals and an increase in heart rate are also present. The QT interval shortening may have implications for women with long QT syndrome. Indeed, Seth et al. found a reduced risk (risk ratio [RR] = 0.38) of cardiac events during pregnancy in woman with prolonged QT syndrome. However, an increased risk of postpartum cardiac events in the first nine months after delivery was also found, which suggests that the QT interval becomes prolonged again in the early post-delivery period. There is also a tendency toward premature atrial contractions, sinus tachycardia, and paroxysmal supraventricular tachycardia.postoperative wound healing.
Journal of Perioperative Medicine