High frequency jet ventilation and gas trapping

We have compared three types of high frequency jet ventilation (HFJV) with conventional positive pressure ventilation in patients recovering from elec tive coronary artery bypass surgery. Twelve patients were allocated randoml y to receive HFJV at ventilatory frequencies of 60, 100, 150 and 200 bpm fr om a standard jet ventilator at either the proximal or distal airway (HFJV. p and HFJV.d), or from a valveless high frequency jet ventilator acting as a pneumatic piston (VPP). Trapped gas volume (Vtr), cardiac index (CI) and right ventricular ejection fraction (RVEF) were measured. Vtr was related t o the type of HFJV used (P<0.05) and ventilatory frequency (P<0.05). CI dec reased with increasing rate of HFJV (P<0.05) and there were significant dif ferences between the three types of HFJV (P<0.05). RVEF showed a linear rel ationship with ventilatory frequency (P<0.05) decreasing most with the VPP. The decrease in RVEF was associated with an increase in right ventricular end-systolic volume (P<0.05) suggesting that an increase in right ventricul ar afterload was the cause. The same three types of HFJV were compared usin g a lung model with variable values of compliance and resistance, to assess the impact of lung mechanics on gas trapping (Vtr, mi). Lung model complia nce (C) was set at 50 or 25 mi cm H2O-1 and resistance (R) at 5 or 20 cm H2 O litre(-1) s, where values of 50 and 5, respectively, are normal. Vtr incr eased with ventilatory frequency for all types of jet ventilation (P<0.05), varying with the type of jet ventilation used (P<0.05).