Continuous monitoring of central venous pressure is now considered standard practice in the management of the critically ill patients. Some of these patients require support by mechanical ventilation, often with the concomitant use of PEEP. To determine the existence and amgnitude of systemic differences during mechanical ventilation and after disconnection of ventilation, we studied 120 central venous pressure measurements serially. The results show that mechanical ventilation reading were 11.0 CmH2O (+/-2.53) and ventilation disconnection readings were 8.0CmH2O(+/-2.39) with a mean difference of 3.0 CmH2O. both mechanical ventilation and ventilation disconnection readings were sensitive to hemodynamic changes when taken continuously. We feel that mechanical ventilation readings will more accurately reflect the patient's cariopulmonary status during ventilation support.