Left ventricular geometry during intermittent positive pressure ventilation in dogs

We have analyzed how regional left ventricular (LV) performance may contribute to global LV performance during a single intermittent positive pressure ventilation (IPPV) respiratory cycle using chronically instrumented dogs with endocardial piezoelectric crystals in the three orthogonal axes, anterior-posterior (AP), septal-lateral freewall (SL), and long axis (LA) dimensions (D). Right ventricular (RV) SLD and aortic flow were also measured. Changes in LV geometry were evaluated during IPPV at two respiratory rates (10 and 20 breaths/min) and during inferior vena caval (IVC) occlusion since a change in systemic venous return is a major component of an IPPV inspiration. During an IPPV inspiration the RV SL end-diastolic D decreased, while the LV AP end-diastolic D increased (P < .01) and LV SL end diastolic D decreased (P < .001). LV end-diastolic and end-systolic volumes tended to increase during early inspiration and then diminished to an early expiratory minimum (P < .05) associated with early expiratory minimum stroke volumes and measured aortic flows (P < .02). In contrast, during IVC occlusion, the LV SL end-diastolic D initially increased while the AP end-diastolic D was falling. Thus, entirely different changes in LV end diastolic geometry result from IVC occlusion and lung inflation. These studies suggest a common factor of cardiac compression by the lung during inspiration altering diastolic intraventricular distribution of the LV preload and raise the question of whether respiratory-induced changes in LV geometry may independently modulate LV systolic pump function.