Critical Care Medicine
The ventilator graphics show that the expiratory flow rate does not reach zero, which is indicative of air trapping or auto-PEEP (also called occult PEEP or intrinsic PEEP). In patients with obstructive lung disease, emptying is slowed and expiration is interrupted by the next inspiratory effort, causing air to become trapped. Eventually, a static equilibrium volume is reached at higher FRC, but air trapping is still present, which causes positive alveolar pressure at the end of expiration (ie, auto-PEEP). Auto-PEEP is caused by 1 or more factors: reduced elastic recoil, excessively high VT or RR, and increased airway resistance or expiratory flow limitation. Auto-PEEP is not apparent by simply observing the pressure at the end of expiration because the expiratory port is open and allows VT to escape. Therefore, the pressure will display zero at end expiration unless extrinsic PEEP has been added. Simple observation of ventilator graphics showing airflow failing to return to zero at end expiration is the key to recognize auto-PEEP. In the absence of graphics, auto-PEEP can be measured by performing an end-expiratory occlusion (hold) and inspecting for an increase in airway pressure as the trapped air tries to escape. To manage auto-PEEP, minute ventilation should be reduced by decreasing VT or RR, increasing the IFR to allow for an expiratory time long enough to achieve lung emptying, and applying extrinsic PEEP to alleviate the increased breathing effort that auto-PEEP imposes on respiratory muscles. Increasing VT or decreasing IFR will prolong the time for inspiration, thereby reducing expiratory time and exacerbating auto-PEEP.4
- Decrease VT, increase IFR, add PEEP. This patient has high peak airway and Pps.
4. Blanch L, Bernabe F, Lucangelo U. Measurement of air trapping, intrinsic positive end-expiratory pressure, and dynamic hyperinflation in mechanically ventilated patients. Respir Care 2005;50:110-3.
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