Respiratory Mechanics During Labor
During labor, the baseline hyperventilation of pregnancy intensifies further, increasing minute ventilation beyond the elevated oxygen demands of labor and delivery, which normally prevents hypoxia or hypercapnia in healthy women. 1
Physiological Changes in Respiratory Mechanics
Ventilatory Pattern Changes
Resting hyperventilation that exists throughout pregnancy becomes even more pronounced during active labor, with minute ventilation increasing substantially above baseline pregnancy levels 1
In healthy women, this augmented hyperventilation exceeds the increased metabolic oxygen demands of labor and delivery, maintaining adequate oxygenation and preventing carbon dioxide retention under normal circumstances 1
End-tidal CO2 tension typically remains low during labor, usually below 30 mmHg, reflecting the persistent hyperventilation pattern 2
Impact of Pain and Anxiety
Pain and anxiety during labor can trigger rapid, shallow breathing patterns that paradoxically decrease alveolar gas exchange efficiency, despite increased respiratory rate 1
Oxygen desaturation below 90% occurs in approximately two-thirds of laboring women (10 of 15 in monitored studies), often associated with periods of apnea or shallow respirations between contractions 2
Hyperventilation-induced hypocarbia combined with intermittent apneic episodes between contractions represents the primary mechanism for oxygen desaturation during labor 2
Clinical Implications for Women with Respiratory Disease
Increased Vulnerability
Women with more severe underlying pulmonary disease develop hypoxia, hypercapnia, and respiratory acidosis much more readily during labor compared to healthy parturients 1
The mechanical work of breathing is already compromised in pregnancy due to the gravid uterus limiting diaphragmatic excursion, and this becomes further challenged during the physical exertion of labor 1
Management Priorities
Adequate pain relief during labor is a high priority as it reduces anxiety, maternal stress, and prevents the rapid shallow breathing pattern that impairs gas exchange 1
Early epidural analgesia with local anesthetics (with or without opioids) is the preferred pain management method, as it provides effective analgesia while avoiding the ventilatory suppression associated with systemic opioids 1
Systemic opioids should be used cautiously as they suppress cough, suppress ventilation, and may worsen respiratory mechanics, particularly problematic in women with chronic suppurative lung diseases 1
Monitoring and Support Requirements
Continuous pulse oximetry monitoring during delivery is recommended to detect oxygen desaturation early 1
Supplemental oxygen should be provided to maintain normal oxygen saturations as per standard care, particularly in women experiencing pain, shortness of breath, or documented desaturation 1
Bronchodilator therapy and assistance with sputum clearance may be required during labor in women with chronic airways disease 1
For women with established bronchiectasis, positive end-expiratory pressure may assist by splinting open smaller airways to prevent dynamic airway closure and improve secretion mobilization 1
Common Pitfalls
Avoid assuming that increased respiratory rate during labor indicates adequate ventilation—rapid shallow breathing actually decreases alveolar gas exchange 1
Do not rely solely on clinical assessment for oxygenation status—objective pulse oximetry monitoring is essential as desaturation is common even in apparently healthy laboring women 2
Recognize that narcotic administration increases the risk of oxygen desaturation (7 of 10 women with desaturation had received narcotics in one study), making epidural analgesia preferable 2