Determination of Ventilator Flow Patterns by Simple Spirography

Abstract

Numerous inspiratory-flow patterns are available for use during mechanical ventilation. These flow patterns may have various effects on cardiac output and ventilation/perfusion relationships. A Collins spirometer was modified to provide a simple and inexpensive method of obtaining spirographic representations of ventilator flow patterns, and flow patterns produced by six commonly used positive-pressure ventilators were determined by this method. The results provided easily interpretable tracings produced by sine-wave, square-wave, and decelerating-flow generators. Static-pressure periods were also easily determined. These data can be used with the knowledge that carbon dioxide elimination can vary with static-pressure periods, that decelerating-flow patterns in flow-sensitive generators may improve ventilation of restricted airways, and that cardiac output can be influenced by intrathoracic pressure, which depends on inspiratory time, static-pressure periods, and peak airway pressure. The subject of ventilator flow patterns is often misunderstood or ignored. Flow patterns delivered by mechanical ventilators differ widely and have varying effects on cardiac output and ventilation/perfusion relationships. This paper describes a simple, inexpensive method of obtaining spirographic representation of ventilator flow patterns and presents and discusses spirographs produced by six commonly used posi- tive-pressure ventilators.