A bedside index assessing the reliability of pulmonary artery occlusion pressure measurements during mechanical ventilation with positive end-expiratory pressure

Abstract

Pulmonary artery occlusion pressure (Ppao) represents the pressure in a pulmonary vein (Ppv) and reflects left atrial pressure (P la) provided alveolar pressure (Palv) does not exceed the pressure existing at the venous end of the collapsible alveolar vessels. Three major factors may influence the relationships between Palv and Ppv: mechanical ventilation with positive end-expiratory pressure (PEEP), vertical location of the pulmonary artery catheter (PAC) tip relative to the left atrium, and volemia. In an experimental and clinical study, we assessed a simple means to ensure meaningful measurements of Ppao during mechanical ventilation with PEEP, based on the following reasoning: if Ppao reflects Palv and not Ppv, it will increase during positive pressure inspiration by an amount (ΔPpao) similar to Palv; thus, ΔPpao will widely exceed the increase in pulmonary arterial pressure (ΔPpa), which approximates the increase in pleural pressure. Conversely, when Ppao actually reflects Ppv, ΔPpao will be close to ΔPpa. A preliminary experimental study in an animal with a left atrial catheter and PAC in place showed that Ppao widely exceeded P la when ΔPpao/ΔPpa widely exceeded 1 during mechanical ventilation with PEEP. After fluid infusion, P la and Ppao were almost identical and ΔPpao/ΔPpa was now close to 1. In 12 patients ventilated for adult respiratory distress syndrome and with a left ventricular catheter and PAC in place, we verified at four levels of PEEP (0, 10, 15, and 20 cm H 2O) that a Ppao close to left ventricular end-diastolic pressure was always associated with a ΔPpao/ΔPpa ratio close to 1 (38 measurements). In 34 additional patients ventilated for noncardiogenic pulmonary edema, we also tested the ΔPpao/ΔPpa ratio during the same incremental PEEP protocol and found that ΔPpao/ΔPpa was close to 1 at each level of PEEP in 30 of the patients (108 measurements). In five measurements in four hypovolemic patients, ΔPpao/ΔPpa widely exceeded 1; only one of these four patients was found to have a PAC tip located above the left atrium level on a lateral chest x-ray film. We conclude that the bedside determination of the ΔPpao/ΔPpa ratio should identify the cases in which Ppao measurements are not valid more reliably than lateral chest x-ray film, thus avoiding misleading interpretations of Ppao during PEEP ventilation.