A transcutaneous oxygen sensor was used continuously during surgical management of a ruptured abdominal aortic aneurysm. Closed chest compression initiated for intraoperative cardiac arrest gave an inadequate cardiac output on the basis of falling PtcO2 despite transmitted femoral pulses and an excellent PaO2. This discordance provided a rationale for open cardiac massage, which increased the cardiac output and tissue perfusion (PtcO2) needed for successful resuscitation. The PtcO2 sensor provides immediate, non-invasive, and continuous information regarding tissue oxygenation. It reflects the PaO2 in hemodynamically stable patients as well as providing a sensitive indicator for inadequate cardiac output during shock. In patients undergoing cardiopulmonary resuscitation, a falling PtcO2 with an acceptable PaO2 indicates poor tissue perfusion and, in select circumstances, may warrant open cardiac massage.