The Science Journal of the American Association for Respiratory Care

2001 OPEN FORUM Abstracts

OptimizingMAP in HFOV by Maximizing Amplitude

JasonHiggins, BS RRT, Dean Holland, RRT, Bob Estetter, RRT Ken Hawkins, RRT, Grant O?Keefe, MD,MPH Departments of Respiratory Care andSurgery Parkland Health & Hospital System,Dallas, Texas

Introduction:The purpose of this work is to establish an objective method to optimize meanairway pressure (MAP) during HFOV. Current practice involves setting MAP 2-8cm H2O higher than the MAP on conventional ventilation, in order to optimizeoxygenation. However, little attention is directed towards amplitude, sub-tidalvolume and ventilation. We sought to determine whether setting MAP at differentpoints on the static pressure-volume curve resulted in changes in amplitude,potentially providing the opportunity to optimize ventilation. We hypothesizedthat optimal MAP is achieved at the midpoint between the lower inflection point(LIP) and upper inflection point (UIP) of a static pressure-volume (P-V) tracingand is confirmed by maximal amplitude (delta P).

Methods: A staticP-V tracing was obtained using a balloon model that was submerged and stabilizedin a 50-gallon tank. The LIP and UIP were identified and used as markers fordata collection. HFOV was initiated through a #8.0 ETT at the following settings:Hz 4, I-time % 50, and Bias Flow of 30 L/m. Four MAPs were chosen along theP-V curve as follows: (1) below the LIP, (2) midpoint between LIP and UIP, (3)shoulder of the UIP, and (4) above the UIP. In each experimental condition,MAP was held constant and power changes were made in increments of 0.5 for arange of 5.0 to 10.0. Amplitudes were then documented at each study condition.

Results: Thelargest amplitudes were achieved when the MAP setting was positioned at themidpoint between the LIP and UIP. As the MAP was increased and was positionednear the shoulder of the UIP, amplitudes decreased but still remained higherthan when the lung model was above the UIP and below the LIP.

Discussion: Ourdata suggests that at a power of 10, the maximum amplitude is achieved whenMAP is set at the steepest slope of the static P-V tracing. These results indicatethat position on the static P-V curve can be determined based on the ratio ofpower to displayed amplitude. Prior studies and management strategy indicatethat MAP should be set near the shoulder of the UIP to improve oxygenation byrecruiting collapsed alveoli. The findings represented in this study indicatethat MAP can be adjusted until amplitude starts to decrease for a given power.As the lung recruits into overdistension a further reduction in amplitude willoccur. At this point, MAP should be reduced to increase amplitude to near previouslevels. Power should not be increased for a drop in amplitude as this will onlyincrease damage in the condition of overdistension. Given the difficulties oftenencountered with ventilation during HFOV this data establishes that MAP shouldbe adjusted to optimize position on the static P-V curve.


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