The Science Journal of the American Association for Respiratory Care
Background: Rise time (RT), pressure slope or flow acceleration percent (FAP) is an adjunct designed to improve delivery of a pressure-targeted breath in mechanical ventilators. We compared the RT's of 4 different mechanical ventilators. Method: A 2-chambered test lung was used with a set compliance of 20 ml/cm H2O and a normal resistance. Pressure signals at the ?Y? of the test lung were digitized by a computer data-acquisition system. Two of each of the test ventilators (NPB 760, Bear 1000, NPB 840 and NPB 7200) was connected one at a time to the test lung. Each test ventilator was set in the assist control mode with a set pressure control level of 20 cm H2O, an I:E ratio of 1:1 and 1:2.5, a respiratory rate of 12, room air and no PEEP. The sensitivity was set to 1 L/min in all ventilators except the 7200, set to 0.5 cm H2O. A FAP setting of 5, 50 and 100 was used for the 760 and 840 ventilators. A pressure slope on the Bear 1000 was set to match the RT of the 840 ventilator when the I:E ratio was 1:1 and the FAP was 50% as a reference for comparison. The 7200 does not have an adjustable RT.
Results: At a ratio of 1:1, a FAP of 5, 50 and 100% on the 840 and 760 corresponded to a pressure slope of -8, -6 and +8 respectively on the Bear 1000 (p > 0.05). An increase in the I:E ratio to 1:2.5 expectedly decreased the RT of the NPB ventilators. The RT of the 7200 was always significantly higher than the other ventilators (p < .005). The 760 and 840 RT's were similar to each other. There was always a very transient pressure overshoot in all ventilators when FAP or pressure slope was set at 100% or +8, respectively (exception was the 7200). The following example shows the mean RT's of 4 ventilators at an I:E ratio of 1:2.5 and an inspiratory time of 1.43 seconds.
|Ventilator||FAP% or PS set||RT (seconds)|
|* p < 0.05 when compared with other ventilators|
Conclusions: It is useful to know how rise time settings relate to each other if a patient on pressure control or pressure support needs to be switched to another type of ventilator. Accounting for the differences in RT's, therefore will help improve consistency of a pressure-targeted breath.