Pulmonary artery wedge pressure measurements in patients on assisted ventilation

Abstract

When vascular pressures are measured on ventilated patients, end-expiration is commonly assumed to be at the lowest position of the respiratory cycle. However, for patients on assisted ventilation, the presence of a large negative deflection in the pressure tracing, corresponding to their inspiratory effort, may invalidate this assumption. Consequently, the position of end-expiration may be improperly located on the graphic tracing, causing errors in the measurement of the pulmonary artery wedge pressure (PAWP). Similarly, incorrect values of the PAWP will be obtained from a digital monitor if the digital diastolic register is used. To determine the frequency of this waveform and the accuracy of values obtained from the different digital registers in approximating the graphic PAWP, we studied 32 critically ill patients grouped according to the mode of ventilation: spontaneous breathing, controlled ventilation, or assisted ventilation. We observed a large negative deflection (greater than 4 mm Hg) in 13 of the 15 patients on assisted ventilation. In this group, the value obtained from the digital mean register approximated the graphic PAWP better than the value obtained from the diastolic register. We conclude that for patients on assisted ventilation, appropriate adjustments should be made when reading the PAWP from either a graphic printout or from a digital monitor if a large deflection is present in the vascular pressure tracing.