Inverse ration ventilation : experimental studies on pulmonary integrity and gas exchange

Abstract: In this thesis, pulmonary and circulatory effects of inverse ratio ventilation (IRV) have been studied experimentally. Comparisons have been made between volume and pressure controlled IRV and conventional ventilation under conditions of equal minute ventilation and end-expiratory alveolar pressure levels. The ventilatory modes have been analysed with regard to effects on airway pressures and volumes, global and regional lung aeration as measured by computed tomography, gas exchange, hemodynamics and oxygen transport. In addition, pulmonary epithelial permeability, as a marker of ventilator induced lung injury, has been studied by [99mTc]DTPA lung clearance. It is concluded that: I. Inverse ratio ventilation leads to decreased peak airway pressure and increased mean airway pressure whereas end-inspiratory alveolar pressure is unchanged. Mean lung volume increases markedly with IRV. It is not evident that these pressure and volume changes is advantageous in the management of patients with acute lung injury or the acute respiratory distress syndrome. Global and regional lung aeration do not improve with IRV, nor does recruitment of lung tissue. The proposed theoretical advantages of IRV, in regard of a more homogenous distribution of volume and pressure within the lungs, are therefore contested; II. Blood oxygenation does not increase as a result of IRV with increased mean airway pressure. The level of end-expiratory alveolar pressure appears to be more important with regard to oxygenation. Under certain circumstances, IRV results in improved alveolar ventilation. Pressure controlled IRV causes hemodynamic compromise in the form of decreased cardiac output, blood pressure or oxygen transport. This effects is most probably a consequence of an increased mean alveolar pressure; III. Pressure controlled IRV leads to increased pulmonary epithelial permeability as measured by [99mTc]DTPA clearance. This probably reflects a functional influence on pulmonary epithelial integrity. The conclusions reached in this thesis make it appropriate to be cautious when using IRV in patients with acute respiratory failure. Its use should be restricted to controlled clinical trials. The complexity of the IRV concept necessitates not only a high degree of awareness of possible complications such as circulatory compromise and undesirably high levels of intrinsic PEEP, but also the means of detecting such adverse effects, i.e. frequent measurements of static airway pressures and invasive hemodynamic monitoring.