A 52-year-old man is found to be tachypneic on the second day after an uncomplicated upper abdominal ventral hernia repair. Until now, his postoperative course had been unremarkable, and he has been treated with low-dose morphine for pain control. The patient says he feels short of breath but has no chest pain. He has an occasional cough with a small amount of early-morning sputum production. Other medical conditions include hypertension and hyperlipidemia. The patient is an active smoker with a 15-pack-year history. Temperature is 36.7 C (98.1 F) , blood pressure is 123/79 mm Hg, pulse is 90/min, and respirations are 28/min. BMI is 32 kg/m². Physical examination shows decreased breath sounds at the right lung base without wheezes or prolonged expiration. Heart sounds are normal. There is abdominal distension and diffuse tenderness without rebound. The surgical incision appears clean and dry. Chest x-ray reveals a dense opacity at the right lung base.
Arterial blood gas analysis done on room air is most likely to show which of the following?

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Quizplus · Accepted Answer

This patient most likely has postoperative atelectasis. Thoracic or abdominal pain following surgery can restrict chest expansion and lead to low tidal volume (TV) respiration with basilar alveolar collapse. Pharyngeal secretions, airway tissue edema, and residual anesthetic effects can also contribute. Postoperative atelectasis most commonly manifests on day 2 following surgery and may be seen up to postoperative day 5. Atelectasis causes hypoxemia (ie, low PaO₂) due to localized intrapulmonary shunting, leading to ventilation-perfusion mismatch. This triggers an increased respiratory rate (RR), which more than compensates for the reduction in tidal volume (TV). Because minute ventilation = TV × RR, there is an increase in minute ventilation (ie, hyperventilation), which leads to decreased PaCO₂ and consequently increased pH (primary respiratory alkalosis). In patients without underlying chronic lung disease, any cause of significant ventilation-perfusion mismatch (eg, pulmonary embolism, pneumonia) usually presents similarly with hypoxemia and primary respiratory alkalosis. ventilation-perfusion mismatch (Choice A) Low PaO₂, low pH, and high PaCO₂ are consistent with hypoxemia and primary respiratory acidosis, as would be expected with the hypoventilation that occurs in narcotics overdose. Although this patient has received morphine, his elevated RR makes hypoventilation unlikely. (Choice B) Normal PaO₂, low pH, and low PaCO₂ are consistent with primary metabolic acidosis and compensation by secondary respiratory alkalosis. Such findings are expected in sepsis or acute renal failure. (Choice C) These arterial blood gas values are approximately normal for a patient breathing room air (pH 7.4, PaO₂ >75 mm Hg, PaCO₂ 40 mm Hg). (Choice E) Normal PaO₂, high pH, and high PaCO₂ are indicative of primary metabolic alkalosis with compensation by secondary respiratory acidosis. Such a scenario may be seen with the loss of hydrogen ions that occurs with severe vomiting. Educational objective: Postoperative atelectasis is common and typically manifests 2-5 days following surgery. Hypoxemia results from localized intrapulmonary shunting and ventilation-perfusion mismatch, and hyperventilation leads to primary respiratory alkalosis with low PaCO₂ and high pH. References: Types and mechanisms of pulmonary atelectasis. (http://www.ncbi.nlm.nih.gov/pubmed/8820021)

A 52-Year-Old Man Is Found to Be Tachypneic on the Second