Five days after a laparoscopic left colectomy for colon cancer, a 55-year-old female presents 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 has a temperature of 37.9 °C, respiratory rate of 26 cycles/min, heart rate of 110 beats/min, and blood pressure of 130/85 mmHg. Lungs are clear to auscultation without wheezing or rales. On cardiac exam, the patient has no murmurs or rubs. Her abdomen is soft and nontender. The wound appears to be clean without erythema or 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 studies reveal a leukocytosis of 10,600/mm3, hemoglobin of 12 g/dL (12–15.2 g/dL), and hematocrit of 36% (37–46%). Chest X-ray is normal. Electrocardiogram (ECG) demonstrates sinus tachycardia but is otherwise unremarkable.