Decreasing the Inspiratory Percent as an Alternate to Increasing Hertz During Weaning

Background: This study looks at the delivered amplitude (amp) and tidal volume (VT) using an inspiratory percent (I%) of 30 at one hertz (Hz) vs the next greater Hz with an I% of 33 using a 3100A high frequency oscillatory ventilator (HFOV). Increasing HFOV Hz is a method of weaning the HFOV ventilated patient. Increasing the Hz decreases the inspiratory and the expiratory time. Despite the fact I:E remains the same, the changes increase the possibility of inadequate time for exhalation. The resulting consequence can be air trapping. The trapping can result in hyper-expansion of the lungs, which can impair gas exchange, increase shunting, and in some cases impair cardiac function if severe enough. This study looks at the delivered amps and VTs to understand if decreasing the I% is a choice in weaning the HFOV. Methods: A calibrated HFOV was connected to one of the three test lungs using a full-length endotracheal tube. The lung compliances in cm H2O and ETTs used were; 0.20cc with a 2.5 ETT, 0.39cc with a 3.0 ETT, and 0.59cc with a 3.5 ETT. The HFOV bias flow was maintained at 15 L/min. The MAP at 14 cm H2O (+ - 0.1). The amps were measured in the test lungs using a TSI Certifier. The VT monitor was a Dragger VN 500 BabyLog ventilator. Results: The HFOV frequencies tested; 6Hz thru 12Hz. The I% used; 30 and 33 for each frequency, except 12 Hz at 33 I%, only. The amps tested; 16, 18, 20, 22, 24, 26, 28, and 30. The data was collected and placed into a category consisting of the averaged measured values from all 8 amps and VTs from a single Hz at a given I%. The categories were then paired, so a Hz at 30 I% was matched with the category of the next higher Hz at 33 I%(see graphs 1 thru 6). The VT categories were then stratified further using the DLCO2 equation [(f * VT2)/kg] (see graphs 7, 8, and 9). With using in vitro testing the weight aspect of the DLCO2 equation was not completed. Conclusions: By decreasing the I% to 30 there is a demonstrated reduction in the delivered amps and VTs. The paired categories results are similar. The 30 I% values are slightly larger than the 33% measurements, except for a few examples. Where 30 I% values are larger you may expect that the change would be a lesser wean. However, when the VT categories are stratified using the DLCO2 equation the data would suggest the Hz at 30 I% would be a marginally greater wean than increasing the Hz at 33 I%. Only clinical application/trial will determine if these assumptions are correct.