What does this arterial line tracing show, and what is the differential diagnosis for the abnormal arterial waveform morphology? An 83-year-old man presented with a contained rupture of a thoracoabdominal aortic aneurysm and a type I endoleak of a previously repaired aneurysm. Computed tomographic angiography results demonstrated an aneurysm of his native thoracic aortic; transthoracic echocardiography results revealed dynamic compression of the posterior left ventricle by the aneurysm itself during systolic midejection. The patient’s arterial waveform was notable for a double systolic peak followed by a dicrotic notch and single diastolic waveform (Figure 1 and Figure 2), consistent with pulsus bisferiens. There was no change in waveform morphology with the patient’s heart rate, fluid challenges, or blood pressure alterations. Following deployment of an aortic stent graft, the arterial waveform normalized (Figure 1). The patient died on postoperative day 5 of surgical complications. Pulsus bisferiens is associated with subaortic stenosis, severe aortic regurgitation, hypertrophic cardiomyopathy, and aortic dissection.1-3 The loss of pulsus bisferiens morphology with graft deployment strongly suggests it was directly associated with the ruptured aneurysm or endoleak.