Jet ventilation, gas exchange
Last updated: 03/03/2015
Jet ventilation can be performed via either high or low frequency means. High frequency jet ventilation (HFJV) is accomplished with specialized ventilators capable of producing the high pressure, low volume breaths necessary. Low frequency jet ventilation (LFJV), however, is usually accomplished with a manually triggered hand-held device. HFJV is primarily utilized in an ICU setting while low frequency jet ventilation (LFJV) is primarily utilized during short procedures (rigid bronchoscopy), airway surgery or after a cricothyrotomy. During jet ventilation the tidal volumes utilized are smaller than the total dead space (anatomical + equipment), therefore gas exchange must occur via alternative means in addition to the standard bulk flow (convective) ventilation that occurs during a normal inspiratory-expiratory cycle where gas is delivered directly to the alveoli (this is the case in HFJV more so than LFJV). These other means of gas exchange include Laminar flow, the Pedulluft effect, Taylor dispersion and cardiogenic mixing.
Laminar flow (non-turbulent, low Reynolds number) in small airways produces a scenario where the high-pressure of the delivered breath flows down the middle of the airway and the margins of the airway contain gas moving in the opposite direction.
The Pedulluft effect describes the movement of inspired gas from those alveoli that fill the fastest to those that are slower to fill. This is based on the fact that there is variability between alveoli in both resistance and compliance.
Taylor dispersion (convective streaming) is the diffusion of the high velocity central gases to the margins of the airway. This phenomenon primarily occurs in the smaller airways and further enhances gas mixing and hence exchange.
Cardiogenic mixing also contributes to gas exchange during jet ventilation, albeit likely to a lesser extent than those mentioned previously. It occurs as a result of the rhythmic, pulsatile nature of the heart conferring a mixing of gases.
- J Jane Pillow High-frequency oscillatory ventilation: mechanisms of gas exchange and lung mechanics. Crit. Care Med.: 2005, 33(3 Suppl);S135-41 Link
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