ABSTRACT
The positive and negative swings in intrathoracic pressures associated with all forms of respiration directionally influence intraluminal pulmonary vascular pressures. During spontaneous breathing, intraluminal pressure is lower in inspiration than in expiration. However, during mechanical ventilation, pressures on inspiration exceed those of expiration. Intrathoracic pressures are closest to zero during end-expiration regardless of whether the patient is breathing spontaneously or being mechanically ventilated. Thus, all pressures should be measured at endexpiration to minimize the influence of intrathoracic pressure swings. Carefully measured end-expiratory pulmonary diastolic pressures can be substituted for mean PAWPs whenever wedge tracings are unobtainable, as end-expiratory pressures are often most accurate. In the absence of elevated pulmonary vascular resistance, pulmonary artery diastolic pressure usually approximates the wedge pressure (less than 2-4 mmHg difference). This relationship is essentially fixed through the physiologic range of pressure. For example, if the initial PAWP is 12 mmHg and the pulmonary artery diastolic pressure is 15 mmHg, when left ventricular failure causes the PAWP to increase to 20 mmHg, the pulmonary artery diastolic pressure would then be approximately 23 mmHg. This gradient will remain fixed unless lung disease becomes more severe or an acute process occurs during the pressure monitoring. The wedge pressure should not be greater than the pulmonary artery diastolic pressure. In the presence of pulmonary hypertension as with pulmonary embolic disease, pulmonary fibrosis, ARDS, or reactive pulmonary hypertension, pulmonary artery diastolic pressure (PADP) may markedly exceed mean PAWP and is an unreliable index of left ventricular function.
