ANESTHETIC IMPLICATIONS OF MATERNAL AND FETAL PHYSIOLOGY
A 19 y.o. primigravida is admitted to L&D for induction of labor at 34 weeks gestation, secondary to preeclampsia with severe features. The anesthesia service is consulted in anticipation of the need for a labor epidural. Upon examination, the patient appears morbidly obese (height 157 cm/weight 130 kg). Her blood pressure is 147/89, heart rate 92, respiratory rate 24, O2 saturation 97%, and she has significant edema of her hands and ankles. CBC shows hemoglobin of 9.4 g/dl and platelets of 112,000. The coagulation profile is within normal range. Examination of her airway shows poor visualization of the uvula as well as a thick and short neck. Examination of the lumbar area reveals poor external landmarks.
Physiologic changes of pregnancy have a profound impact on anesthetic care; therefore a thorough understanding is necessary to provide safe and effective anesthesia to the patient. In addition, the dual-patient nature of pregnancy, involving both mother and fetus, must be considered at all times and may pose unique and sometimes conflicting concerns.
Pregnancy is associated with a 25% decrease in systemic vascular resistance and an increase in cardiac output by up to 40%, resulting in an overall mild reduction in blood pressure. Labor causes a further increase in cardiac output, which reaches maximal levels in the immediate postpartum period.1 For high-risk patients, these changes may be cause for special consideration.
In singleton pregnancies >20 weeks gestation, aorto-caval compression by the gravid uterus can reduce venous return to the heart, thereby reducing cardiac output and uterine blood flow. Left uterine displacement (LUD) is necessary whenever the fundus extends above the level of the umbilicus.2 Given that neuraxial anesthesia produces a regional sympathectomy resulting in hypotension, and that aorto-caval compression can exaggerate this, effective LUD should be performed.3
Weight gain and edematous changes, particularly in the airway, can make mask ventilation and intubation difficult. Decreased functional residual capacity (FRC) and increased oxygen consumption also make the patient more susceptible to hypoxemia.1
Plasma volume increases by 40% to 50% and creates a compensatory mechanism during hemorrhage.1 However, this compensation may impede early recognition of potentially significant bleeding. Since red cell mass increases by only 20%, pregnancy is associated with relative anemia, which puts the patient at a further disadvantage during hemorrhage. In contrast, the concentration of certain clotting factors and the fibrinogen level increases, which predisposes the patient to venous thrombosis and possible embolism.
It is believed that the patient has decreased gastric emptying time and decreased esophageal sphincter tone.1 These observations, in addition to the increased intra-abdominal pressure caused by the gravid uterus as well as physiologic changes, can lead ...