Accurate measurement of pulmonary vascular pressures in critically ill patients maintained on intermittent mandatory ventilation is investigated during various phases of respiration. Simultaneous airway pressure and pulmonary vascular recordings are recommended to choose correct pressure values at the end of expiration without influences of apnea or positive end expiratory pressure (PEEP). Concurrent evaluation of cardiac output, arterial blood gases, urine output and systemic arterial pressure may effect appropriate fluid management and administration of vasoactive or cardiotonic drugs.