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INTRODUCTION

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There are a significant amount of changes that occur during pregnancy to a woman’s body (Table 151-1). Many alterations are physiologic and due to hormonal changes. Other effects are due to the increased size of the uterus, particularly noticeable later in pregnancy. The physiologic and mechanical changes both impact the appropriate anesthetic management of the parturient and the fetus.

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TABLE 151-1Average Maximum Physiological Changes Associated with Pregnancy1
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RESPIRATORY AND AIRWAY

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Pregnant women develop a more barrel-shaped chest due to increase in both transverse and anteroposterior chest lengths. Additionally, there is upward displacement of the diaphragm due to the growing fetus.

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Other respiratory changes include an increase in tidal volume by 40% and an increase in respiratory rate. These alterations are due to progesterone and both increase minute ventilation by 50% and cause a respiratory alkalosis with metabolic compensation. A normal arterial blood gas (ABG) in a pregnant woman consists of normal pH with a CO2 of 30 and a HCO3 of 20. If a parturient is mechanically ventilated, tidal volume and respiratory rate should be adjusted to maintain a higher minute ventilation and a PaCO2 of 30 to maintain normal acid–base status. Residual volume and expiratory reserve volume decrease as well. In addition, it is common late in pregnancy for women to experience dyspnea on exertion due to the respiratory changes, as well as anemia.

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Other concerns include the decrease in functional residual capacity by 20% and increase in oxygen consumption by 20%. In combination, these alterations make it is more likely for women to rapidly desaturate during times of hypoventilation or apnea. Adequate pre-oxygenation is necessary prior to induction of general anesthesia or any other anticipated period of apnea or hypoventilation. Induction and emergence with inhalational agents are both faster due to the increase in minute ventilation and decrease in functional residual capacity. There is also a higher probability of difficult and failed bag mask ventilation and intubation ...

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