PEEP is used to keep the lung aerated and prevent lung collapse at the end of expiration. However, PEEP may over-distend the normally aerated lung and impair lung perfusion. Therefore, any change in PEEP may affect the overall ventilation/perfusion ratio in an unpredictable way.
Volumetric capnography measures the volume of CO2 exhaled breath-by-breath (VeCO2). After a change in PEEP, assuming that the cardiovascular function and tidal volume are stable, an increase in VeCO2 means that the overall ventilation/perfusion ratio has improved. Conversely, a decrease in VeCO2 means that the ventilation/perfusion ratio is worsening. VeCO2 changes rapidly and returns to the baseline after a few minutes.
The limitation to this method is that the clinician monitors rapid changes to the ventilation/perfusion ratio, such as those due to lung over-distension and lung-perfusion impairment or improvement.
After a change in PEEP, recruitment or derecruitment may take a longer time to occur and cannot be assessed by this method.
Watch the video to see a demonstration during ventilation with a Hamilton Medical ventilator.
- Detection of optimal PEEP for equal distribution of tidal volume by volumetric capnography and electrical impedance tomography during decreasing levels of PEEP in post cardiac-surgery patients. P. Blankman, A. Shono, B. J. M. Hermans, T. Wesselius, D. Hasan, D. Gommers. British Journal of Anaesthesia, 116 (6): 862–9 (2016)
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