Mitral and aortic blood flows during spontaneous respiration in dogs

Left sided hemodynamic events during respiration remain a controversial subject. Left ventricular (LV) hemodynamic events were evaluated during obstructed and partially obstructed inspiration in anesthetized dogs acutely instrumented with mitral (Q̇(m)) and ascending aortic (Q̇(a)) flow probes. This allows classification of the inspiratory decrease in LV stroke volume as either a diastolic event (e.g., ventricular interdependence) in which case the LV inflow volume (∫ Q̇(m)) should decrease before the LV outflow volume (∫ Q̇(a)), or a systolic event (e.g., afterload or contractility) in which case outflow (∫ Q̇(a)) should decrease before inflow (∫ Q̇(m)). During either unobstructed (n = 8) or partially obstructed (n = 5) spontaneous ventilation, ∫ Q̇(m) reached both its inspiratory minimum and expiratory maximum prior to the associated minimum and maximum values for ∫ Q̇(a) in 80% or more of the respiratory cycles. Thus, a diastolic event dominates both in reducing the subsequent LV outflow during the inspiratory fall in intrathoracic pressure and in increasing LV outflow during the expiratory increase in intrathoracic pressure. However, because a diastolic event did not occur first at all times, a systolic event must also be present. If a rapid change in intrathoracic pressure occurred during diastole, ∫ Q̇(m) invariably immediately increased. If a rapid change in intrathoracic pressure occurred during systole, ∫ Q̇(a) could change independently of the preceding ∫ Q̇(m). Both systolic and diastolic mechanisms contribute to the inspiratory fall in LV output. These mechanisms will not be clearly delineated without evaluating the effects of intrathoracic pressure within a single cardiac cycle.