A majority of women experience subjective breathlessness during pregnancy. This complicates the diagnosis of true respiratory pathology. “Dyspnea of pregnancy” is likely related to normal physiologic alterations, summarized in Table 26-1, that serve to maintain the fetus and prepare the mother for labor and delivery.1 The dyspnea associated with pregnancy does not interfere with activities of daily living and is not related to exertion, coughing, or wheezing. Physiologic dyspnea usually improves as pregnancy progresses, particularly with “quickening,” which is defined as the maternal perception of initial fetal movement. In contrast, pathologic dyspnea from cardiac or pulmonary origins may have an abrupt onset, be progressive in its severity, occur even at rest, or be associated with cough, chest pain, fever, or hemoptysis. Dyspnea due to cardiac or respiratory pathology worsens as the pregnancy reaches the third trimester. Pregnant women with a respiratory rate greater than 20 breaths/min, increased work of breathing, or the presence of rales, wheezing, or murmurs deserve prompt evaluation for potential cardiopulmonary pathology.2
Table 26-1.Physiologic Changes During Pregnancy |Favorite Table|Download (.pdf) Table 26-1. Physiologic Changes During Pregnancy
|Organ System ||Change During Pregnancy |
|Respiratory || |
| Tidal volume ||Increased 40% |
| Minute ventilation ||Increased 40%-50% |
| Respiratory rate ||Increased 10% |
| Oxygen consumption ||Increased 40% |
| Expiratory reserve volume ||Decreased 20% |
| Functional residual capacity ||Decreased 12%-25% |
| Arterial PCO2 ||Decreased ∼10% |
| Chest wall compliance ||Decreased 45% |
|Cardiovascular || |
| Cardiac output ||Increased 20%-50% |
| Stroke volume ||Increased 30% |
| Systemic vascular resistance ||Decreased 30% |
| Heart rate ||Increased 20% |
|Hematologic || |
| Plasma volume ||Increased 55% |
| Blood volume ||Increased 45% |
| Hematocrit ||Decreased (physiologic anemia) |
|Gastrointestinal || |
| Gastrointestinal motility ||Decreased |
| Lower esophageal sphincter tone ||Decreased |
| Intragastric pressure ||Increased |
ACUTE RESPIRATORY FAILURE
Acute respiratory failure (ARF) is defined by the inability to maintain adequate oxygenation or ventilation. Although ARF is rare (occurring in less than 0.1% of pregnancies), it remains one of the most common indications for intensive care unit admission in pregnant women.3 The etiology of ARF in pregnancy is diverse and may or may not be directly related to pregnancy (Table 26-2). Acute respiratory distress syndrome (ARDS) has been defined by the American-European Consensus Conference with the following criteria: lung injury of an acute onset, bilateral infiltrates present on chest x-ray, PaO2-to-FiO2 ratio less than or equal to 200, and pulmonary artery wedge pressure less than 18 mm Hg or the absence of clinical evidence of left atrial hypertension.3 The estimated maternal mortality rate due to ARF is reported to be 30% to 35% and, in the setting of ARDS, regardless of the inciting etiology, the mortality rate can be as great as 70%.4 Fetal mortality is also high, most commonly reported at 20% to 30%, and it is usually due to complications from premature delivery or perinatal hypoxia.3
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