A microprocessor based feedback controller for mechanical ventilation

A microcomputer feedback system has been developed which adjusts the inspired minute volume of a ventilator based on the patient's end-tidal CO₂ concentration. The feedback controlled ventilator was evaluated in 6 dogs (18–20 kg). Arterial PCO₂ was monitored continuously while end-tidal CO₂ concentration was controlled by the microcomputer system and the following perturbations introduced: [1] NaHCO₃ was infused IV, [2] a pulmonary artery was occluded, [3] one lumen of a double lumen endobronchial tube was occluded, and [4] an air embolism was given. The end-tidal PCO₂ controller kept PaCO₂ within 1.2 mm Hg of the desired value when CO₂ production increased by as much as 44%. Changing the ventilation/perfusion ratios caused differences as large as 22 mm Hg between the arterial and end-tidal PCO₂ and the controller was not effective in keeping PaCO₂ at the desired level. Closed loop control of ventilation based on end-tidal PCO₂ measurements successfully compensated for increases in CO₂ production keeping PaCO₂ constant. The controller did not, however, keep PaCO₂ at the desired level when significant changes occurred in the distribution of blood flow to ventilation. This work was supported by Siemens-Elema, Biochem Int. and Shriners Childrens Hospital.