From the Guidelines
In a pregnant woman at 32 weeks gestation with wheezing and dyspnea, the threshold for respiratory failure is typically a PCO2 level above 40-45 mmHg. During normal pregnancy, women experience a compensated respiratory alkalosis with PCO2 levels typically ranging from 28-32 mmHg, significantly lower than non-pregnant values 1. This physiologic hyperventilation occurs due to progesterone-mediated increased respiratory drive and anatomical changes of pregnancy. Therefore, what would be considered a "normal" PCO2 of 35-40 mmHg in non-pregnant patients actually represents early respiratory failure in a pregnant woman. This is particularly concerning in a patient with wheezing and dyspnea for three days, which suggests asthma exacerbation or bronchospasm. Some key points to consider in the management of such a patient include:
- The importance of careful monitoring for hypercapnic respiratory failure with respiratory acidosis, which may develop in the course of a hospital admission even if the initial blood gases were satisfactory 1.
- The need to avoid excessive oxygen use in patients with COPD, as the risk of respiratory acidosis in patients with hypercapnic respiratory failure is increased if the PaO2 is above 10.0 kPa due to previous excessive oxygen use 1.
- The potential benefits and risks of prenatal physical activity in women with mild–moderate or severe respiratory disorders, and the need for future experimental research to examine the safety of different modalities of exercise in these populations 1. Immediate intervention should be initiated when PCO2 approaches 40 mmHg, as this indicates the patient is tiring and may soon require mechanical ventilation. Treatment should focus on bronchodilators, systemic corticosteroids, and supplemental oxygen while continuously monitoring both maternal and fetal status. In terms of oxygen therapy, the target oxygen saturation should be 94–98% unless there is a history of previous hypercapnic respiratory failure requiring NIV or intermittent positive pressure ventilation or if the patient’s usual oxygen saturation when clinically stable is below 94% 1. Blood gases should be repeated at 30–60 min to check for rising PCO2 or falling pH, and the patient should be monitored closely for signs of clinical deterioration 1.
From the Research
Threshold for Respiratory Failure in Pregnancy
The threshold for respiratory failure in a pregnant woman, in terms of partial pressure of carbon dioxide (PCO2) levels, is not explicitly stated in the provided studies. However, some studies provide information on PCO2 levels and respiratory failure in pregnancy:
- A study on transcutaneous PCO2 monitoring during laparoscopic cholecystectomy in pregnancy found that the estimated maximum PaCO2 during insufflation was 39-40 mmHg 2.
- Another study discussed the management of acute respiratory failure in pregnancy, but did not provide a specific threshold for PCO2 levels 3.
- A narrative review of literature on acute respiratory failure and mechanical ventilation in pregnant patients also did not provide a specific threshold for PCO2 levels 4.
- A study on prediction of respiratory failure in late-preterm infants with respiratory distress at birth found that a threshold of alveolar-arterial oxygen tension difference (A-aDO2) >200 mmHg was effective in predicting respiratory failure, but this study was focused on neonates, not pregnant women 5.
- A review of respiratory failure and mechanical ventilation in the pregnant patient discussed several obstetric and nonobstetric diseases that lead to respiratory failure, but did not provide a specific threshold for PCO2 levels 6.
PCO2 Levels in Pregnancy
In general, the normal range for PCO2 in pregnancy is not well established, and the provided studies do not offer a clear threshold for respiratory failure in terms of PCO2 levels. However, it is known that pregnancy can affect maternal respiratory physiology and increase the risk of respiratory failure 3, 4.