A 25-year-old woman comes to the hospital due to worsening dyspnea. The patient was previously healthy; 2 months ago, she delivered a healthy baby boy at 38 weeks gestation. Her arterial blood gas analysis shows evidence of hypoventilation. During further evaluation, she is asked to take several deep breaths, and the following physiologic information is obtained: Which of the following locations is the most likely site of this patient's disease?

Question

A 25-year-old woman comes to the hospital due to worsening dyspnea.  The patient was previously healthy; 2 months ago, she delivered a healthy baby boy at 38 weeks gestation.  Her arterial blood gas analysis shows evidence of hypoventilation.  During further evaluation, she is asked to take several deep breaths, and the following physiologic information is obtained:  Which of the following locations is the most likely site of this patient's disease?

Quizplus · Accepted Answer

Intrapleural pressure (ie, between the lung and chest wall pleura) is slightly negative at rest due to the inward elastic recoil of the lungs and outward recoil of the chest wall. During inspiration, diaphragmatic contraction is primarily responsible for creating a more negative intrapleural pressure that causes expansion of the thoracic cavity, drawing air into the lungs. Passive expiration occurs with respiratory muscle relaxation and relies on the natural inward elastic recoil of the lungs to expel air as the intrapleural pressure returns to baseline. The graph shows progressive attenuation of the intrapleural pressure change that occurs during deep, repetitive inspiration. Because there is no apparent decrement in phrenic nerve activity, this patient's progressively weaker diaphragmatic contractions are likely due to excessive skeletal muscle fatigue (eg, severe obstructive or restrictive lung disease) or impaired neuromuscular transmission. In myasthenia gravis, autoantibodies against postsynaptic acetylcholine receptors cause fatigable skeletal muscle weakness as acetylcholine stores within the presynaptic nerve terminal become progressively depleted with repetitive stimulation. Women in the postpartum period (like this patient) are at particularly high risk of developing myasthenia gravis. (Choice A) Impairment of the respiratory control centers in the brain stem can result in decreased frequency and/or amplitude of involuntary respirations (Ondine curse). Voluntary breathing would be unaffected. (Choice B) Nasopharyngeal lesions are unlikely to cause inspiratory airflow obstruction severe enough to cause rapid diaphragmatic fatigue as the oropharynx remains unobstructed (allowing the patient to breath from the mouth). (Choices D and E) Metastatic disease to the pericardium could compress the phrenic nerve due to its location along the pericardium. In addition, the phrenic nerve can be affected by a peripheral neuropathy. However, both diseases cause uniform weak inspirations during maximal voluntary ventilation rather than progressively weaker diaphragmatic contractions. Educational objective: Progressively weakening diaphragmatic contractions during maximal voluntary ventilation with intact phrenic nerve stimulation indicate neuromuscular junction pathology (eg, myasthenia gravis) and/or abnormally rapid diaphragmatic muscle fatigue (eg, restrictive lung or chest wall disease).

A 25-Year-Old Woman Comes to the Hospital Due to Worsening