A control system for high-frequency jet ventilation

The design of a system for high-frequency jet ventilation is described. Jets of 10-40 psig humidified air were introduced into the trachea through an 8-gauge needle inserted through the cricothyroid membrane. Gas flow to the needle was interrupted by a solenoid valve operated by a controller circuit. The controller allowed the independent setting of duty cycle (ratio of inspiratory time to total cycle time) from 10-50% and respiratory frequency from 10-350/min. When the electrical control signal was set equal to the desired duty cycle, the actual mechanical duty cycle erred from the desired value by as much as 109%. These errors were caused by unequal time delays for solenoid opening and closing. Tests of a commercially available jet-ventilator also demonstrated frequency-dependent errors caused by unequal time delays and changes in wave shape. The controller described here compensates for differences in opening and closing delays and delivers volumes within 3.4% of those calculated from continuous flows.