Five days after a laparoscopic left colectomy for colon cancer, a 55-year-old female represents to the emergency department with shortness of breath for the past 6 h. She feels that she is breathing more rapidly and does not seem to be able to catch her breath. She denies any chest pain. Per report, the colon cancer was limited to the sigmoid colon, and the surgery was uneventful. She has no prior cardiac or pulmonary history. On physical exam, the patient was found to have a temperature of 100.8 °F, respiratory rate 26, heart rate 110 beats/min, and blood pressure of 130/85 mmHg. Lungs are clear to auscultation without wheezing or rales. The heart examination reveals a regular rate and rhythm without murmurs or rubs. Her abdomen is soft and nontender. The wound appears to be clean without drainage. Her left leg appears to be swollen up to the knee with pitting edema. The left calf is not tender to palpation. The right leg is not swollen. Distal pulses are normal. O2 saturation on room air is 92 %. Arterial blood gas on room air reveals a pO2 of 70 mmHg, a pCO2 of 33 mmHg, a pH of 7.47, and an A-a gradient of 25. Laboratory values reveal a WBC of 9,600/mm3 and Hgb/Hct of 12 g/dL and 36 %. CXR is normal. ECG demonstrates sinus tachycardia but is otherwise unremarkable.