Comparison of the pressure time product during synchronous intermittent mandatory ventilation and continuous positive airway pressure

Objective-To compare the effect of continuous positive airway pressure (CPA P) with synchronous intermittent mandatory ventilation (SIMV) during weanin g, by measurement of the pressure time product (PTP). The PTP is an estimat e of the metabolic work or oxygen consumption of the respiratory muscles. Patients-Forty children, median age 14 months (range 1 month to 17 years) w ere studied immediately prior to extubation. Methods-The PTP, derived from airway pressure changes, was measured while t he child received SIMV (with or without pressure support) and then CPAP. A pneumotachograph was placed between the endotracheal tube and ventilator ci rcuit. Its flow signal was used to indicate the beginning and end of inspir ation. Airway pressure was measured from the pneumotachograph and the area subtended by the pressure curve (PTP) calculated using a modified Labview p rogramme. Results-The median PTP was significantly lower on SIMV (17.1 cm H2O/sec/min , range 0.4 to 60.5) compared to on CPAP (32.6 cm H2O/sec/min, range 0.1 to 116.7), p < 0.001. Conclusion-These results suggest SIMV, rather than CPAP, may be the more ef ficacious weaning mode in children, and that the time spent on CPAP during weaning should be minimised.