The Study On The Mechanism Of The Influence Of Mechanical Ventilation On Cardiac Function

The effects of mechanical ventilation on hemodynamics are important to make the ventilation strategy and avoid hemodynamic complications, but the mechanism is still unclear. The new theory of influence of respiration on cardiac function sheds light on it.Objective 1) To discuss and find out a new theory on the mechanism of theeffects of mechanical ventilation on cardiac function by analyzing thehemodynamic change during spontaneous respiration and mechanicalventilation with echocardiography.2) To provide evidence for the new theory by analyzing the variaty ofpreload and afterload and their impacting factors during mechanicalventilation before and after acute lung injury3)To prove the new theory further by observing the changes of cardiac volumeand hemodynamics during Valsava maneuver and Mueller man, which onlyincrease or decrease intrathoracic pressure without and change of lungvolume.Methods 1) The velocity variety of four cardiac valves during therespiration cycle of spontaneous breathing and mechanical ventilation was obtained with pulse Doppler and the difference of the velocity variety between two breathing mode was analyzed.2)The pressure variety of right atria, right ventricle and pulmonary artery during respiration cycle of mechanical ventilation was record before and after acute lung injury model was made. The change of cardiac cavity volume and heart function was observed one by one beat with echocardiography during the breathing cycle.3) Valsava maneuver and Mueller maneuver were performed by 10 health volunteer while the intrathoracic pressure was controlled up and down to 5mmHg, 10mmHg and 15mmHg. The change of the position of ventricular septum and ventricular diameter were obtained. The velocity variety of mitral valves and tricuspid valves during the maneuver were measured.Results 1) The velocities of blood flow across tricuspid valve and pulmonaryvalve varied with the phase of spontaneous breathing cycle, increasing while inspiration and decreasing while expiration. But the velocities across mitral valve and aortic valve didn't vary with the respiratory cycle, and no consistent pattern of velocity variety with breathing cycle could be identified. The maximum velocity didn't appear in a constant position in the respiratory cycle. Under mechanical ventilation, the velocity across the four cardiac valves changed regularly. The velocity of blood flow through tricuspid valve and pulmonary valve decreased during positive pressure inspiration, increased during expiration, while the velocity of mitral valve and aortic valve changed reversely.2) The pressure of right atria, right ventricle, pulmonary artery varied with the change of airway pressure during mechanical ventilation before and after acute lung injury and the magnitude of increase of the systolic pressure of pulmonary artery was larger than that of the diastolic pressure during mechanical inspiration. Before acute lung injury, the varying magnitude of the systolic and diastolic pressure of right ventricle were different individually. But after acute lung injury, the systolic pulmonary artery pressure increased more than the diastolic pressure during inspiration.Before and after lung injury, RVEF and RVSV decreased in the phase of inspiration, but at the same time LVEF, LVSV, LAA and LVEDVI increased comparing with that in the phase of expiration and there was no difference in LVESVI between inspiration and expiration. Before lung injury, RAA decreased during inspiration and increased during expiration and RVESVI in the phase of inspiration was greater than that in the phase of expiration, but RVEDVI changed reversely. After acute lung injury, RAA did not change significantly and RVEDVI and RVESVI of the individual object changed differently.3) As the 10 volunteers performed the Mueller manoeuvre increasing ITP level to 10mmHg and 15mmHg, the ventricular septum moved leftward. Correspondingly, the diameter of left ventricle decreased and that of right ventricle increased (P<0.05) . When they performed the Valsava maneuver decreasing ITP level to 10mmHg and 15mmHg, the ventricular septum moved rightward and the diameter of both ventricles changed reversely. When ITP increased or decreased to 5mmHg, 10mmHg and 15mmHg, there was no difference between the flow velocity across MV and TV after the manoeuvre than that before the manoeuvre.