Mean airway pressure: physiologic determinants and clinical importance--Part 1: Physiologic determinants and measurements.

PURPOSES: To discuss the theoretical relationship of mean alveolar pressure to its most easily measured analog, the mean airway pressure, and to describe the key determinants, measurement considerations, and clinical implications of this index. DATA SOURCES: Relevant articles from the medical and physiologic literature, as well as mathematical arguments developed in this article from first principles. STUDY SELECTION: Theoretical, experimental, and clinical information that elucidates the physiologic importance, measurement, or adverse consequences of mean airway pressure. DATA EXTRACTION: Mathematical models were used in conjunction with data from the published literature to develop a unified description of the physiological and clinical relevance of mean airway pressure. SYNTHESIS: Geometrical and mathematical analyses demonstrate that shared elements comprise mean airway pressure and mean alveolar pressure, two variables that are related by the formula: mean alveolar pressure = mean airway pressure + (VE/60) x (RE-RI), where VE, RE, and RI are minute ventilation and expiratory and inspiratory resistances, respectively. Clear guidelines can be developed for selecting the site of mean airway pressure determination, for specifying technical requirements for mean airway pressure measurement, and for delineating clinical options to adjust the level of mean airway pressure. Problems in viewing mean airway pressure as a reflection of mean alveolar pressure can be interpreted against the theoretical basis of their interrelationship. In certain settings, mean airway pressure closely relates to levels of ventilation, arterial oxygenation, cardiovascular function, and barotrauma. Because mean airway pressure is associated with both beneficial and adverse effects, a thorough understanding of its theoretical and practical basis is integral to formulating an effective pressure-targeted strategy of ventilatory support.
CONCLUSIONS: Mean airway pressure closely reflects mean alveolar pressure, except when flow-resistive pressure losses differ greatly for the inspiratory and expiratory phases of the ventilatory cycle. Under conditions of passive inflation, mean airway pressure correlates with alveolar ventilation, arterial oxygenation, hemodynamic performance, and barotrauma. We encourage wider use of this index, appropriately measured and interpreted, as well as its incorporation into rational strategies for the ventilatory management of critical illness.