A 56-year-old man comes to the office due to chronic cough and fatigue. The patient has smoked 2 packs of cigarettes daily since age 18. Physical examination shows cyanosis and expiratory wheezes scattered throughout the lungs. During evaluation for long-term oxygen therapy, his respiratory rate decreases shortly after he begins nasal cannular oxygen supplementation. This patient's reduced respiratory rate is most likely caused by a sudden decrease in stimulation of which of the following sensory receptors?

Question

A 56-year-old man comes to the office due to chronic cough and fatigue.  The patient has smoked 2 packs of cigarettes daily since age 18.  Physical examination shows cyanosis and expiratory wheezes scattered throughout the lungs.  During evaluation for long-term oxygen therapy, his respiratory rate decreases shortly after he begins nasal cannular oxygen supplementation.  This patient's reduced respiratory rate is most likely caused by a sudden decrease in stimulation of which of the following sensory receptors?

Quizplus · Accepted Answer

11ec160b_3cdc_13d3_9249_abe6ef4b08ed_MD0001_00 This patient with chronic obstructive pulmonary disease (COPD) developed a decrease in respiratory rate as a result of oxygen supplementation. Oxygen has a minimal effect on respiratory drive unless the arterial partial pressure of oxygen (PaO₂) drops below 60-70 mm Hg; therefore, the arterial partial pressure of carbon dioxide (PaCO₂) is the major stimulator of respiration in healthy individuals. However, patients with long-standing COPD have decreased sensitivity to PaCO₂ (due to chronic CO₂ retention) and may also have profound hypoxemia (PaO₂ <60 mm Hg); therefore, PaO₂ levels can become a significant contributor to respiratory drive. The depth and rate of respirations are controlled by the medullary respiratory center based on input from central and peripheral chemoreceptors and airway mechanoreceptors. Peripheral chemoreceptors found in the carotid and aortic bodies are the primary sites for sensing PaO₂ and are stimulated by hypoxemia. When supplemental oxygen is administered, the rapid increase in PaO₂ can reduce peripheral chemoreceptor stimulation and decrease the respiratory rate. Reduced respiratory rate is a minor contributor to the oxygen-induced hypercapnia that can occur in patients with COPD; increased ventilation-perfusion mismatch triggered by alleviation of pulmonary vasoconstriction in poorly ventilated areas is the major mechanism. (Choice B) Central chemoreceptors, located in the medulla, are more involved in the respiratory response to hypercapnia than to hypoxemia. CO₂ readily diffuses through the blood-brain barrier and forms hydrogen ions in the cerebrospinal fluid; the resulting decrease in pH is detected by medullary neurons, triggering an increase in respiration. Because the blood-brain barrier is relatively impermeable to hydrogen ions, blood pH itself has little effect on central chemoreceptors. (Choice C) The juxtaglomerular apparatus is located in the kidneys and consists of juxtaglomerular cells and the macula densa. It helps regulate renal blood flow and the glomerular filtration rate via renin secretion. (Choices D and E) Pulmonary stretch receptors include myelinated and unmyelinated C fibers in the lungs and airways. These receptors regulate the duration of inspiration depending on the degree of lung distension (Hering-Breuer reflex). Educational objective: Arterial partial pressure of carbon dioxide (PaCO₂) is the major stimulator of respiration in healthy individuals; even a slight increase in PaCO₂ stimulates central chemoreceptors and triggers increased ventilation. In patients with chronic obstructive pulmonary disease, the response to PaCO₂ is blunted and hypoxemia can contribute to respiratory drive. Peripheral chemoreceptors are primarily responsible for sensing arterial partial pressure of oxygen (PaO₂) and can be suppressed with oxygen administration.

A 56-Year-Old Man Comes to the Office Due to Chronic