Massumi, in 1973, presented three distinct clinical conditions in which he described a reversal of the expected variations of blood pressure relative to respiratory activity. He termed this phenomenon reverse pulsus paradoxus. One of his cases was a patient with congestive heart failure receiving intermitten positive pressure ventilation (IPPV). The same phenomenon has been documented in many of our intensive care patients who were receiving IPPV, with or without PEEP. A variety of underlying clinical conditions were included. We have called this response of blood pressure respirator paradox. A theoretic mechanism related to changes in afterload is offered as one possible explanation of these findings. Occurrence of respirator paradox after open heart surgery may complicate the problem of early diagnosis of cardiac tamponade.