A 53-year-old homeless man comes to the emergency department due to several days of shortness of breath and productive cough. A month ago, the patient was evaluated for dysphagia and was found to have Candida esophagitis. He received nystatin but refused any further workup and left against medical advice. The patient uses cocaine and intravenous heroin. Temperature is 38.8 C (101.8 F), blood pressure is 121/72 mm Hg, pulse is 124/min and regular, and respirations are 22/min. Oxygen saturation is 89% on 2 L/min of oxygen by nasal cannula. He is awake, alert, and in mild respiratory distress. There are extensive white plaques over the oral mucosa. A 2/6 midsystolic murmur is heard at the left upper sternal border. Lung auscultation is remarkable for faint, bilateral crackles. There is no jugular venous distension or lower extremity edema. Which of the following is the most likely underlying mechanism responsible for this patient's hypoxemia?

Question

A 53-year-old homeless man comes to the emergency department due to several days of shortness of breath and productive cough.  A month ago, the patient was evaluated for dysphagia and was found to have Candida esophagitis.  He received nystatin but refused any further workup and left against medical advice.  The patient uses cocaine and intravenous heroin.  Temperature is 38.8 C (101.8 F), blood pressure is 121/72 mm Hg, pulse is 124/min and regular, and respirations are 22/min.  Oxygen saturation is 89% on 2 L/min of oxygen by nasal cannula.  He is awake, alert, and in mild respiratory distress.  There are extensive white plaques over the oral mucosa.  A 2/6 midsystolic murmur is heard at the left upper sternal border.  Lung auscultation is remarkable for faint, bilateral crackles.  There is no jugular venous distension or lower extremity edema.  Which of the following is the most likely underlying mechanism responsible for this patient's hypoxemia?

Quizplus · Accepted Answer

11ec25c8_d4ad_a794_bc81_ab4ecf3ff299_MD0003_00 This patient with extensive mucosal candidiasis (white plaques) that suggests an immunosuppressed state (eg, AIDS) now has cough, dyspnea, fever, and hypoxemia, which together are highly suspicious for pneumonia. Immunosuppressed patients are susceptible to pneumonia due to typical organisms (eg, Streptococcus pneumoniae) as well as opportunistic organisms (eg, Pneumocystis jiroveci). In pneumonia, the alveoli become filled with inflammatory exudate, leading to hypoxemia due to marked impairment of alveolar ventilation in the affected portion of the lungs. The result is right-to-left intrapulmonary shunting, an extreme form of ventilation/perfusion (V/Q) mismatch in which there is perfusion of lung tissue in the absence of alveolar ventilation (V ≈ 0). Depending on how much of the lungs are affected by intrapulmonary shunting, the resulting hypoxemia may or may not correct with an increase in the fraction of inspired oxygen (FiO₂). Severe Pneumocystis jiroveci pneumonia in particular may cause widespread intrapulmonary shunting with hypoxemia that is difficult to correct with supplemental oxygen. hypoxemia (Choice A) Diffuse alveolar hypoventilation refers to a uniform fall in ventilation in all regions of the lungs that results from a decrease in either tidal volume or respiratory rate. Potential causes include narcotic overdose and neuromuscular weakness. This patient's fever, tachypnea, and bilateral crackles make pneumonia more likely. (Choice B) Endocarditis can lead to valvular dysfunction, which can cause left-sided heart failure with increased left atrial pressure (pulmonary capillary wedge pressure) and bibasilar crackles. However, this patient has no other signs of heart failure (eg, jugular venous distension, lower extremity edema). Fever induces a hyperdynamic state, which most likely accounts for his murmur (flow murmur). (Choices C and D) Unlike right-to-left intracardiac shunts, left-to-right intracardiac shunts (eg, atrial septal defect) are typically asymptomatic and do not result in hypoxemia. However, large left-to-right intracardiac shunts may eventually lead to right-sided volume overload and pulmonary hypertension. Patients with pulmonary hypertension typically have shortness of breath and hypoxemia; however, symptoms usually develop gradually, and fever is not present. Educational objective: Pneumonia causes hypoxemia due to right-to-left intrapulmonary shunting, an extreme form of ventilation/perfusion mismatch. Depending on how much of the lungs are affected by intrapulmonary shunting, the resulting hypoxemia may be difficult to correct with an increase in the fraction of inspired oxygen. References: Pulmonary pathophysiology of pneumococcal pneumonia. (http://www.ncbi.nlm.nih.gov/pubmed/10501309)

A 53-Year-Old Homeless Man Comes to the Emergency Department Due