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Anesthesia for Cardiac Patients During Labor and Delivery
Pregnant women with preexisting cardiac disease provide a unique challenge to anesthesiologists in the peripartum period. Pregnancy-related death has been increasing over the past three decades, and cardiovascular disease is the number one cause of pregnancy-related death in the United States. Caring for this challenging patient population depends on an understanding of the cardiovascular changes associated with pregnancy, effective communication between all members of the care team, adequate and accurate physiologic monitoring during labor and delivery, and an understanding of how peripartum anesthesia and analgesia may affect existing cardiac lesions. In formulating a plan of care, the anesthesiologist must consider the mode of delivery, the nature and severity of cardiac disease, the anticipated location of delivery and recovery, the anticipated physiologic consequences of each method of anesthesia, and the necessary monitors. This chapter reviews the main considerations for anesthetic care of patients with cardiac disease during labor and delivery.
Cardiac Changes During Labor and Delivery
The cardiovascular changes associated with pregnancy, labor, and delivery are dramatic. Compared with the majority of pregnant women, those with heart disease may not tolerate these changes as well. During the course of pregnancy, cardiac output increases by 50% through increases in both heart rate and stroke volume. Increased plasma volume and stroke volume results in increased left ventricular (LV) end-diastolic dilation. Systemic vascular resistance (SVR) is decreased, and central filling pressures remain unchanged. These physiologic changes are exacerbated during labor and immediately postpartum, with increases in cardiac output by an additional 50% to 100% during labor. This is largely caused by autotransfusion from uterine contractions. Labor pain complicates cardiovascular physiology further by elevating heart rate and mean arterial pressure (MAP). Immediately after delivery, autotransfusion from the involuting uterus rapidly increases circulatory volume by an additional 30%, further increasing preload and cardiac output. Collectively, these remarkable hemodynamic changes can strain a previously weakened cardiovascular system. With appropriate planning, pain control, and physiologic monitoring, the difficulty that these changes pose can be anticipated and mitigated.
The Basics of Neuraxial Anesthesia and Analgesia
Although women have various choices for labor analgesia, neuraxial analgesia is a common and effective method of labor pain relief, especially for those with cardiac disease. Neuraxial anesthesia and analgesia involve delivery of local anesthetics, analgesics, or both to the epidural or subarachnoid space. Typically, this is achieved in one of three ways: spinal (intrathecal) injection, epidural catheter placement, or combination spinal and epidural (CSE). Each technique works by blocking preganglionic afferent and efferent nerve fibers, including pain and temperature, motor, and sympathetic fibers. The blockade is typically achieved with a local anesthetic, such as bupivacaine or ropivacaine, which acts as a sodium channel blocker, and/or an opiate receptor agonist, such as fentanyl. Blockade of sensory fibers provides effective pain relief during labor, which mitigates the hypertension and tachycardia that can accompany uterine contractions. However, blockade of sympathetic fibers can cause hypotension and decreased venous return. These effects can be anticipated and treated by the anesthesiologist.
Certain women may not be eligible for neuraxial techniques. Contraindications to spinal or epidural placement include patient refusal, certain infections, bleeding disorders, active maternal use of anticoagulants, or certain spine disorders. The main risks to the patient include infection, bleeding, postdural puncture headache, and nonfunctional or one-sided epidural catheter resulting in ineffective analgesia.
Analgesia for Vaginal Delivery
Pudendal and Paracervical Blocks
Infiltration of the pudendal or paracervical nerve bundles with local anesthetic can be used for pelvic floor or cervical analgesia, respectively. The benefit of these procedures is the absence of sympatholysis and hypotension that may occur with neuraxial analgesia. Blockade of L2 to L3 nerve fibers at the paracervical ganglion, if successful, can help reduce pain associated with cervical dilation in the first stage of labor. However, even if successful, a paracervical block does not affect pain associated with distension of the vagina, vulva, and perineum. A pudendal nerve block can provide pain relief to the aforementioned structures and may provide some relief during the second stage of labor. However, pudendal nerve blocks have not demonstrated reduction in pain associated with the first stage of labor and are not as effective as neuraxial analgesia at reducing labor pain. Although there are data that compare paracervical blockade and epidural analgesia with regard to maternal blood pressure, the 44 patients included were described as healthy, making the data difficult to generalize to patients with existing cardiac disease.
Patient-Controlled Analgesia
For parturients who cannot receive neuraxial or regional analgesia, intravenous (IV) patient-controlled analgesia (PCA) presents an alternative that may be effective for some women. Short-acting opioid analgesics, such as remifentanil and fentanyl, are typically used for labor analgesia. The available data show that pain relief from PCA is acceptable to many patients but inferior to epidural analgesia. It also has documented fetal effects because these medications cross the placenta. In nulliparous patients without access to alternative pain control, 74% of those who used a fentanyl-containing PCA reported adequate pain relief in the first stage of labor compared with control participants without any form of analgesia. Apgar scores did not differ at time of delivery. However, for patients who used fentanyl PCA throughout both stages of labor, 44% of neonates had an Apgar score less than or equal to 6. This physiologic depression remained statistically significant at 5 minutes of life compared with the infants who had a 1-minute Apgar score greater than or equal to 7. An isolated study that compared remifentanil with fentanyl PCA use demonstrated that the only difference in pain control efficacy exists during the first hour of use, when remifentanil was more successful. Compared with epidural analgesia, patient satisfaction with a remifentanil PCA was comparable, although pain relief was inferior. Compared with epidural analgesia, women using remifentanil PCA required more oxygen supplementation and experienced lower rates of hypotension. Most studies that evaluate PCA use in parturients exclude high-risk deliveries or women with significant comorbidities. Given the paucity of data in women with cardiac disease, use of opioid PCA for labor analgesia needs to carefully weigh the risks of hypoxia against the risk of blood pressure and heart rate lability if other forms of analgesia are contraindicated or refused.
Nitrous Oxide
Nitrous oxide is an inhaled hypnotic agent commonly used in labor and delivery units throughout the world. Ease of administration and rapid onset of effect make nitrous oxide an appealing option. Although it does not achieve analgesic equivalency with epidural analgesia, it does provide greater overall satisfaction with the birthing experience compared with other forms of analgesia. The cardiovascular effects of inhalational nitrous oxide are secondary to an associated catecholamine release that can result in mild elevations in heart rate and blood pressure. These effects are exacerbated during uterine contractions, resulting in measurable increases in heart rate, stroke volume, MAP, and cardiac output. Nitrous oxide depresses myocardial contractility in vitro and has the potential to unmask existing myocardial depression in vivo. There are no studies that have evaluated the use of nitrous oxide for labor and delivery in women with known cardiovascular disease; however, a large randomized trial in general surgery patients showed no increase in major cardiovascular events at 1 year in patients with preexisting cardiac disease undergoing general anesthesia with nitrous oxide. Because of its noted vasodilatory effects on the pulmonary vasculature, nitrous oxide should not be used in patients with elevated pulmonary artery pressures and intracardiac shunts to avoid ventilation/perfusion mismatch and pulmonary to systemic flow ratio (Qp:Qs) imbalance, respectively. Patients receiving nitrous oxide are also at increased risk of nausea and vomiting. The use of nitrous oxide for parturients with cardiovascular disease should be considered on a case-by-case basis in consultation with the entire care team.
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