Dimensional analysis of the left ventricle during PEEP: relative septal and lateral wall displacements.

We studied the effects of ventilation with positive end-expiratory pressure (PEEP) on left ventricular (LV) shape and wall motion with emphasis on the relative positions of the septum and lateral LV wall. In seven dogs we implanted radiopaque markers in the LV wall and septum to represent the major and two minor axes of a nonprolate LV ellipsoid and measured their spatial positions using 90 degrees biplane cineradiography. The relative positions of the septum and lateral LV wall were determined with respect to the central longitudinal axis of the heart defined by the apex and base markers. Animals were studied in the supine position during positive-pressure ventilation without PEEP, during PEEP before and after dextran administration to elevate stroke volume, and again without PEEP while blood was withdrawn to lower stroke volume. We found that ventilation with PEEP reduced all three LV chamber end-diastolic dimensions and distorted its shape primarily by restricting the outward expansion of the lateral LV wall during diastole, a change that persisted after stroke volume had been restored. The relative position of the septum was not displaced into the LV by PEEP to any greater degree than were the anterior or posterior walls. The inordinate reduction of the end-diastolic septal to lateral wall dimension, and specifically the lateral wall component thereof, was associated with a proportional reduction in the respective stroke lengths. We conclude that deformation of the LV is more likely caused by lung compression than by right ventricular dilation. This deformation contributes to the reduction in LV filling and thereby contributes to the reduction in stroke volume.