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

Abstract

Ulf Ludwigs. Inverse ratio ventilation: Experimental studies on pulmonary integrity and gasexchange. Medical Intensive Care Unit, Department of Medicine, S dersjukhuset, KarolinskaInstitutet, Stockholm, Sweden.In this thesis, pulmonary and circulatory effects of inverse ratio ventilation (IRV) have been studiedexperimentally. Comparisons have been made between volume and pressure controlled IRV andconventional ventilation under conditions of equal minute ventilation and end-expiratory alveolarpressure levels. The ventilatory modes have been analysed with regard to effects on airway pressuresand 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 ofventilator 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 airwaypressure whereas end-inspiratory alveolar pressure is unchanged. Mean lung volumeincreases markedly with IRV. It is not evident that these pressure and volume changes isadvantageous in the management of patients with acute lung injury or the acute respiratorydistress syndrome.Global and regional lung aeration do not improve with IRV, nor does recruitment of lungtissue. The proposed theoretical advantages of IRV, in regard of a more homogenousdistribution 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 patientswith acute respiratory failure. Its use should be restricted to controlled clinical trials. The complexityof the IRV concept necessitates not only a high degree of awareness of possible complications suchas circulatory compromise and undesirably high levels of intrinsic PEEP, but also the means ofdetecting such adverse effects, i. e. frequent measurements of static airway pressures and invasivehemodynamic monitoring.Keywords: respiration, artificial; gas exchange; lung function; acute respiratory distress syndrome;acute lung injury; inverse ratio ventilation; ventilator induced lung injury; intrinsic PEEP; lungclearance