A 44-year-old obese woman undergoes an open cholecystectomy for complicated acute cholecystitis. She is receiving intravenous hydration, hydromorphone, and antibiotics. On the second postoperative day, her temperature is 36.7° C (98.2° F), blood pressure is 110/80 mm Hg, pulse is 92/min, and respirations are 12/min. Oxygen saturation is 89% on room air. Her arterial blood gas results are as follows: What is the most likely cause of hypoxemia in this patient?

Question

A 44-year-old obese woman undergoes an open cholecystectomy for complicated acute cholecystitis.  She is receiving intravenous hydration, hydromorphone, and antibiotics.  On the second postoperative day, her temperature is 36.7° C (98.2° F), blood pressure is 110/80 mm Hg, pulse is 92/min, and respirations are 12/min.  Oxygen saturation is 89% on room air.  Her arterial blood gas results are as follows:  What is the most likely cause of hypoxemia in this patient?

Quizplus · Accepted Answer

Explanation: This patient has respiratory acidosis evidenced by arterial blood gas values that show a decreased pH (<7.35) and a primary increase in PaCO₂ (>40 mm Hg). High PaCO₂ and low PaO₂ levels are suggestive of alveolar hypoventilation, although an elevated PaCO₂ alone, in the range of 50-80 mm Hg, is sufficient to make the diagnosis. Causes of alveolar hypoventilation and respiratory acidosis include the following: Pulmonary/thoracic diseases: Chronic obstructive pulmonary disease, obstructive sleep apnea, obesity hypoventilation, scoliosis Neuromuscular diseases: Myasthenia gravis, Lambert-Eaton syndrome, Guillain-Barré syndrome Drug-induced hypoventilation: Anesthetics, narcotics, sedatives Primary central nervous system dysfunction: Brainstem lesion, infection, stroke In addition, the A-a gradient (P_AO₂ - P_aO₂) can help determine the specific cause of hypoxemia. The A-a gradient is a measure of oxygen transfer from the alveoli to the blood. The alveolar oxygen tension can be calculated using the following equation: PAO₂ = (FiO₂ x [Patm - PH₂O]) - (PaCO₂/R)= (0.21 x [760-47]) - (59/0.8) = 76 Then, calculate the A-a gradient: A-a gradient = P_AO₂ - P_aO₂ = 76 - 62 = 14 A normal A-a gradient is <15. Values increase with age, but an A-a gradient >30 is considered elevated regardless of age. The A-a gradient is normal in patients with reduced inspired oxygen tension and hypoventilation. Recent subdiaphragmatic surgery, narcotic pain medications, and obesity make this patient at risk for alveolar hypoventilation. The resultant respiratory acidosis confirms the hypoventilation (CO₂ retention). (Choices A, C, D, and E) Pulmonary embolism, atelectasis, pleural effusion, and pulmonary edema cause V/Q mismatch. In V/Q mismatch, the A-a gradient is elevated. In all these conditions, there is a decrease in PaCO₂ (respiratory alkalosis) due to compensatory tachypnea. Educational objective: Hypoxemia can be caused by reduced inspired oxygen tension, hypoventilation, diffusion limitation, shunt, and V/Q mismatch. Hypoventilation is associated with a normal A-a gradient and respiratory acidosis.

A 44-Year-Old Obese Woman Undergoes an Open Cholecystectomy for Complicated