Modern anesthesia practice has an excellent record of safety for the parturient. The anesthesia-related maternal mortality rate in the United States is estimated at 1 per 1 million live births (Hawkins, 2011). Indeed, in the latest report from the Centers for Disease Control and Prevention, Creanga and colleagues (2015) cited anesthesia as the cause of pregnancy-related death in only 0.7 percent of maternal deaths in the United States from 2006 to 2010. Also, the 2010 to 2012 triennial report from the United Kingdom and Ireland described a direct anesthetic mortality rate of 0.17 per 100,000 maternities (Knight, 2014). Finally, the Serious Complication Repository (SCORE) Project of the Society for Obstetric Anesthesia and Perinatology (SOAP) captured data from 257,000 parturients receiving an anesthetic between 2009 and 2014 (D'Angelo, 2014). No deaths were reported, and serious anesthesia-related complications occurred in 1 of 3000 patients. The most frequent was high neuraxial block.
For safe anesthesia administration, the obstetric anesthesiologist should understand the unique characteristics of the gravida. These include alterations in maternal physiology, maintenance of uterine perfusion, and fetal response to anesthetic interventions. So too must the obstetrician be familiar with the effects of anesthesia on these parameters during surgery.
Changes in maternal physiology affect several aspects of anesthetic management (Gaiser, 2014). Cardiovascular changes include increases in cardiac output and blood volume that begin in the first trimester. By 28 weeks' gestation, these measure 30 to 40 percent above baseline (Table 19-1). Dilutional anemia caused by plasma expansion reduces the hematocrit.
TABLE 19-1.Cardiovascular Implications of Physiologic Changes of Normal Pregnancy ||Download (.pdf) TABLE 19-1. Cardiovascular Implications of Physiologic Changes of Normal Pregnancy
|Variable ||Change ||Clinical Implications |
|Blood volume ||↑40% ||Hypervolemia; can tolerate 1000 mL blood loss well |
|Plasma volume ||↑50% ||Greater plasma than red cell expansion causes dilutional anemia |
|Heart rate ||↑15 bpm ||Mild baseline tachycardia |
|Cardiac output ||↑40% ||More cardiac work to accommodate the increased blood volume |
|Systemic vascular resistance ||↓20% ||Blood pressure remains normal despite ↑ cardiac output and blood volume |
|Aortocaval compression ||Varies ||Reduces cardiac preload in supine position |
Despite the increase in blood volume and cardiac output, the parturient is susceptible to hypotension from aortocaval compression in the supine position. This is especially true after loss of sympathetic tone associated with regional anesthesia. If the uterus occludes the vena cava in the supine position, preload to the heart is obstructed (Lee, 2012b). Only about 10 percent of pregnant patients at term develop symptoms of shock in the supine position. However, fetal compromise from lowered uterine perfusion can develop even in an asymptomatic mother. For this reason, uterine displacement is an encouraged practice after midpregnancy.
The most important respiratory change during pregnancy is the decrease in functional residual capacity (FRC) (Table 19-2). ...