Ventilator Work Ratio: A Guide to the Adequacy of Weaning
During weaning periods, work of breathing may be divided into the following two components: spontaneous effort of the patient and assisted ventilation given by a ventilator. Provided that the minute ventilation remains constant, improved patient’s effort may be evidenced by a decreased ventilator assist. Since work of breathing provided by a ventilator is easily obtained by a computer technique, we tried to evaluate a progress of weaning as a function of the work of a ventilator. Analog output signals from Servo 900C ventilator were converted to digital signals and fed to HP340C+ computer. Sampling frequency of the A-D converter was every 20msec. The product of airway pressure and airway flow rate gives the power. Integration of the power at the every instant gives the work of breathing. We obtained the work of breathing provided by a Servo 900C by calculating the work only during the inspiration. The work is referred to VW here. We normalized VW in terms of total minute ventilation. This study, we investigated how VWR changed in different modes of assisted ventilation throughout the course of weaning.