The effects of positive end-expiratory pressure on heart-lung interactions

Abstract

Pulmonary artery occlusion pressure (Ppao) may not reflect left ventricular (LV) filling pressure during mechanical ventilation with positive end-expiratory pressure (PEEP) either because the catheter tip is not in zone III conditions or because pericardial pressure (Ppc) is also elevated. Ppc can be estimated from pleural pressure (Ppl) or, when right ventricular (RV) function is normal, from right atria[ pressure (Pra) (Am Heart J 108:603–605,1984). If the pericardium limits LV diastolic filling or RV diastolic compliance changes, then these estimates of Ppc may be inaccurate. We previously showed that nadir Ppao, immediately after abrupt PEEP removal, reflects on-PEEP LV filling pressure (Am J Physiol 249:H770–H776, 1985). We performed this study to determine if (1) the pericardium limits LV filling pressure during PEEP and (2) on-PEEP LV filling pressure can be estimated by on-PEEP Ppao minus Pra or nadir Ppao. We compared these relations when RV performance was normal (control), after induction of acute ventricular failure (AVF) with propranolol, and after subsequent induction of acute lung injury (AVF-OA) with oleic acid in an acute, anesthetized, closed-chest intact canine model (n = 7). PEEP was increased from 0 to 15 cm H2O in 5-cm H 2O increments. Nadir Ppao was measured on termination of 15 cm H 2O PEEP. Juxtacardiac Ppl and Ppc increased parallel to increasing PEEP in all conditions except at 0 and 5 cm H 2O PEEP in two dogs during AVF, when Ppc exceeded Ppl by as much as 4 mm Hg. Pre and Ppc covaried linearly in all conditions and at all levels of PEEP, although Pre was greater than Ppc (by 5.5 + 1.2 mm Hg [x + SE] in control, 7.1 + 2.1 mm Hg in AVF, and 6.6 + 1.5 in AVF-OA; P < .01). As PEEP increased, Ppao increased more than left atrial pressure (Pla) ( P < .01). Nadir Ppao reflected the difference between Pla and Ppc during PEEP ( P < .01). Error in Pra and Ppao relative to Ppc and Pla, respectively, confounded the estimate of LV filling pressure, such that Ppao-Pra showed no relation to Pla-Ppc at greater than 10 cm H 2O PEEP in control or at any level of PEEP in AVF and AVF-OA conditions. We conclude that the pericardium does not usually limit diastolic filling during PEEP. Also, Ppao-Pra may inaccurately estimate LV filling pressure when either RV performance or PEEP changes.