Ventilator circuits mechanics and ventilator compensation

Introduction Invasive mechanical ventilation usually uses a double limb circuit. However, those circuits are not standardized and usually come from different manufacturers with different lengths, diameters, compliances and resistances. Those differences can affect the delivered tidal volumes and pressures delivered to the patient. We aimed to test three different circuits to measure their compliance and resistance and measure their effects on delivered tidal volume and airway pressures after being calibrated by the ventilator. Methods Descriptive study, three different double limb circuits: separate inspiratory and expiratory limb (Vyaire 72 inch length, 22 mm diameter), separate inspiratory with heated wire and expiratory limb (Fisher and Paykel 71 inch, 22 mm diameter), and separate inspiratory and expiratory limb in one tube (Vyaire length 75 inch and 22 mm diameter). Bellavista 1000e ventilator (Zoll, MA, USA) was used to conduct the experiment with the volume controlled mode, tidal volume 500 ml, inspiratory flow of 30 L/min, PEEP of 5 cmH2O, respiratory rate of 20 bpm. Leak, resistance and compliance obtained from the ventilator display during circuit calibration. A passive single lung model with compliance of 80 ml/cmH2O and resistance of 10 cmH2O/L/s constructed using lung simulator (ASL 5000) used to measure the delivered the tidal volume, flow, airway pressures (Peak, plateau, PEEP) delivered to calculate the total compliance and resistance and the effect of the circuits. Results The three circuits had different compliances and resistances. Separate limbs: compliance of 0.84 ml/cmH2O, Inspiratory resistance 3.37 cmH2O/L/s, expiratory resistance 5.54 cmH2O/L/s resulting in total compliance 84.06 ml/cmH2O, inspiratory resistance 4.8 cmH2O/L/s. Separate limbs with heated wire in inspiratory limb: compliance 1.88 ml/cmH2O, Inspiratory resistance 3.22 and expiratory resistance 4.67 cmH2O/L/s, resulting in total compliance 82.74 ml/cmH2O, inspiratory resistance 4.81 cmH2O/L/s. Double limb in one circuit: compliance 2.25 ml/cmH2O, Inspiratory resistance 4.07 cmH2O/L/s, expiratory resistance 4.38 cmH2O/L/s, resulting in total compliance 82.91 ml/cmH2O, inspiratory resistance 4.81 cmH2O/L/s. Conclusion The measured compliances and resistances of the circuits differed slightly, however the ventilator compensated well for the differences with minimal difference in the combined total compliances and resistances. Further investigation across a broader range of circuit designs and ventilator models could help establish more standardized guidelines and recommendations for circuit selection in clinical practice. Keywords: Ventilator circuits, circuit compliance and resistance, ventilator compensation