A 64-year-old man comes to the emergency department due to 2 days of fever, chills, productive cough, and left-sided pleuritic chest pain. The patient has a history of hypertension, hyperlipidemia, and type 2 diabetes mellitus. He is a former smoker and drinks alcohol occasionally. Temperature is 38.9 C (102 F), blood pressure is 110/60 mm Hg, pulse is 110/min, and respirations are 22/min. Oxygen saturation is 94% on room air when he lies on his right side but drops to 89% when he lies on his left. Dullness to percussion and bronchial breath sounds are present on the left side. Which of the following pathophysiologic mechanisms in the left lung is most responsible for the drop in oxygen saturation after the change in position?

Question

A 64-year-old man comes to the emergency department due to 2 days of fever, chills, productive cough, and left-sided pleuritic chest pain.  The patient has a history of hypertension, hyperlipidemia, and type 2 diabetes mellitus.  He is a former smoker and drinks alcohol occasionally.  Temperature is 38.9 C (102 F), blood pressure is 110/60 mm Hg, pulse is 110/min, and respirations are 22/min.  Oxygen saturation is 94% on room air when he lies on his right side but drops to 89% when he lies on his left.  Dullness to percussion and bronchial breath sounds are present on the left side.  Which of the following pathophysiologic mechanisms in the left lung is most responsible for the drop in oxygen saturation after the change in position?

Quizplus · Accepted Answer

11ec25c8_d49d_2cb8_bc81_cf4c01dbf2b3_MD0003_00 This patient's fever, pleuritic chest pain, hypoxemia, and dullness to percussion with bronchial breath sounds in the left lung field are consistent with alveolar consolidation due to pneumonia. Pneumonia causes hypoxemia due to ventilation-perfusion (V/Q) mismatch from localized intrapulmonary shunting (ie, pneumonic consolidations are not ventilated [V = 0] but continue to receive blood flow [Q]). ventilation-perfusion (V/Q) mismatch When this patient with a left-sided pneumonia is in the left decubitus position (pneumonia side down), gravity increases the fraction of blood going to the consolidated lung. This leads to a more profound V/Q mismatch: Ventilation in the consolidated lobe remains very low, but blood flow increases, further lowering the overall V/Q ratio and worsening the hypoxemia. The opposite occurs when this patient is in the right decubitus position (pneumonia side up) because blood is redistributed away from the consolidation, leading to more favorable V/Q matching and improved hypoxemia. For this reason, patients with unilateral lung airspace disease (eg, pneumonia) often prefer to lie on the contralateral side (ie, trepopnea). (Choice A) Dead space ventilation refers to the wasted ventilation of alveoli not perfused with blood (eg, pulmonary embolism). Pneumonia does not cause significant alterations in dead space ventilation. (Choices B and C) Decreased minute ventilation occurs when either the respiratory rate or the tidal volume is decreased. This patient's pleuritic chest pain may have a positional component leading to some effort-dependent hypoventilation (shallow breathing resulting in decreased tidal volume), but his respiratory rate is elevated, and a significant decrease in minute ventilation is unlikely. (Choice D) Hypoxic pulmonary vasoconstriction (HPV) is the physiologic mechanism by which the lungs reduce perfusion to poorly ventilated alveoli (eg, consolidated by pneumonia) to preserve V/Q matching. The degree of HPV is determined by the alveolar oxygen tension and is not affected by changes in position. (Choice F) A low partial pressure of inspired oxygen (PiO₂) occurs at high altitude and leads to hypoxemia. However, positional changes at the same altitude result in no measurable decrease in PiO₂. Educational objective: Alveolar consolidation in pneumonia causes hypoxemia due to ventilation-perfusion (V/Q) mismatch from localized intrapulmonary shunting. Positional changes that promote gravity-dependent blood distribution to the consolidated areas create a more profound V/Q mismatch and worsen the hypoxemia. References: Pulmonary pathophysiology of pneumococcal pneumonia. (http://www.ncbi.nlm.nih.gov/pubmed/10501309)

A 64-Year-Old Man Comes to the Emergency Department Due to 2