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Chapter 12: Respiratory Emergencies During Pregnancy

Alfredo F. Gei; Victor R. Suarez

INTRODUCTION

Respiratory complications during pregnancy are not unusual and can be life threatening. A careful interview and physical examination, a chest x-ray, and an arterial blood analysis are the most useful interventions in the evaluation of these conditions.

Understanding of the cardiorespiratory changes during pregnancy is essential for the diagnosis and treatment of emergencies in normal pregnant women and in women with underlying cardiopulmonary diseases.

BASIC SCIENCE

Oxygen is the basis of every aerobic reaction in our organism. The procurement and delivery of oxygen is a vital process that the pregnant woman has to perform for herself and her unborn child. Nature has ensured adequate mechanisms of adaptation in order to exchange oxygen with air and deliver it to her unborn child (and adapting body) through complex anatomic (Table 12-1) and physiologic changes (Table 12-2).

TABLE 12-1.Anatomic and Physiologic Respiratory Adaptations to Pregnancy
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TABLE 12-1. Anatomic and Physiologic Respiratory Adaptations to Pregnancy
Upper airways

  • Mucosal edema and friability

  • Capillary engorgement

    (A smaller-sized endotracheal tube may be required for intubation because of swelling of the arytenoid region of the vocal cords)
Chest wall

  • Increases in chest wall circumference (6 cm)

  • Elevation of the diaphragm (5 cm)

  • Widening of the costal angles (from 70° to 104°)

  • Increase in diaphragmatic excursion (1.5 cm)

    (All these changes occur before significant increases in uterine size, maternal body weight, or intra-abdominal pressure)
Respiratory musculature

  • Respiratory muscle function is unchanged

  • Diaphragm and intercostals accessory muscles contribute equally to tidal volume during pregnancy

  • Maximum inspiratory and expiratory pressures are unchanged
TABLE 12-2.Changes in Respiratory Variables During Pregnancy
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TABLE 12-2. Changes in Respiratory Variables During Pregnancy
Parameter Definition Change in pregnancy
Respiratory rate Number of breaths per minute • No change
Tidal volume Volume of air inspired and expired at each breath • Increase up to 40% since early breath pregnancy; remains essentially constant for the remainder of gestation (100-200 mL)
Minute ventilation (RR × Vt)

Total amount of air (gas) inspired and each minute

Sum of the volume of air (gas) participating in gas exchange plus the one filling the airway’s dead space (ie, not participating in gas exchange)

 • Increase up to 40% since early pregnancy and remains essentially constant for the remainder of gestation (100-200 mL)
Vital capacity Maximum volume of air that can be forcibly inspired after a maximum expiration • Unchanged
Residual volume Volume of air remaining in the lungs after a maximum expiration • Decreases by ~20% due to elevation of the diaphragm
Functional residual capacity (FRC) Volume of air in lungs at resting expiratory level • Decreases by ~20% due to elevation of the diaphragm
Inspiratory capacity Maximum volume of air that can be inspired from resting expiratory level • Increases 100-300 mL ...

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