Lung model and testing protocol
Spontaneous breathing was simulated by a lung model using a mechanical ventilator (LTV 1200, CareFusion, San Diego, CA) and two Training & Test Lungs (TTLs) (Dual Adult Lung Simulator 5600i, Michigan Instruments, Inc.; Grand Rapids, MI). In the first TTL, paired bellows called the driving bellow and ventilation bellow were linked together by a rigid metal strip. In the second TTL, one bellow called the expiratory gas modification bellow was linked to the first TTL by a rigid metal strip to wash out previous gas in the anatomic dead space in the airway trainer. During simulation, the driving bellow was connected to LTV1200 ventilator and the ventilation bellow was connected to a human-like anatomy model (Laerdal Airway Management Trainer 25 00 00) with oxygen therapy applied via a nasal cannula (VADI Medical Technology, Taoyuan, Taiwan). The carina position of the human-like anatomy model was connected to the oxygen analyser (MiniOX I MSA Medical,Gurnee, Ill.). Between the TTLs, we incorporated four one-way valves (valves 1 to 4) to prevent the mixing of inspired and expired gases (Fig. 1). When inspiration was simulated by the LTV 1200, the ventilator delivered a TV to the driving bellow, causing the bellow to expand and force the metal strap to pull on the ventilation bellow, which expanded passively. This action was detected as an “inspiratory” effort, which in turn triggered spontaneous breathing and drew in gas inhaled only from the nose and mouth into the “ventilation bellows” of the TTL through the in-line one-way valve 1 (directed to the ventilation bellows). Thus, the expiration washing out ventilation bellow was also inflated simultaneously and aerated through valve 4. During expiration, the air was exhaled from the “ventilation bellow” through the other one-way valve 2 (directed to the outside), and the expiration washing out ventilation bellow passively deflated. Consequently, the air was exhaled to the anatomic dead space in the airway trainer through the in-line one-way valve 3.
This experiment included three levels of resistance(R) and compliance(C) of TTLs to represent the lung mechanics of normal, obstructive and restrictive lung diseases (Fig. 2), as suggested by the manufacturer and previous studies.27-30 In the normal lung model was R5/C60, R20/C80 and R5/C40 were represented to obstructive and restrictive lung diseases, respectively. The protocols were performed with a wide-ranging change in the ventilatory pattern of the LTV1200 ventilator [VT:300, 500 and 700mL; f:10, 20 and 30breaths/min]; Oxygen flow rates of 1, 3 and 5L/min were set for the nasal cannula.