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. 2016 Jun;116(6):862-9.
doi: 10.1093/bja/aew116.

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

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Free PMC article

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 et al. Br J Anaesth. .
Free PMC article

Abstract

Background: Homogeneous ventilation is important for prevention of ventilator-induced lung injury. Electrical impedance tomography (EIT) has been used to identify optimal PEEP by detection of homogenous ventilation in non-dependent and dependent lung regions. We aimed to compare the ability of volumetric capnography and EIT in detecting homogenous ventilation between these lung regions.

Methods: Fifteen mechanically-ventilated patients after cardiac surgery were studied. Ventilator settings were adjusted to volume-controlled mode with a fixed tidal volume (Vt) of 6-8 ml kg(-1) predicted body weight. Different PEEP levels were applied (14 to 0 cm H2O, in steps of 2 cm H2O) and blood gases, Vcap and EIT were measured.

Results: Tidal impedance variation of the non-dependent region was highest at 6 cm H2O PEEP, and decreased significantly at 14 cm H2O PEEP indicating decrease in the fraction of Vt in this region. At 12 cm H2O PEEP, homogenous ventilation was seen between both lung regions. Bohr and Enghoff dead space calculations decreased from a PEEP of 10 cm H2O. Alveolar dead space divided by alveolar Vt decreased at PEEP levels ≤6 cm H2O. The normalized slope of phase III significantly changed at PEEP levels ≤4 cm H2O. Airway dead space was higher at higher PEEP levels and decreased at the lower PEEP levels.

Conclusions: In postoperative cardiac patients, calculated dead space agreed well with EIT to detect the optimal PEEP for an equal distribution of inspired volume, amongst non-dependent and dependent lung regions. Airway dead space reduces at decreasing PEEP levels.

Keywords: capnography; mechanical ventilation; peep; ventilator induced lung injury.

Figures

Fig 1
Fig 1
Schematic model of three phases of the expiration. The Vcap curve is divided in three phases. Phase I represents exhaled CO2 from the airways, whereas phase III represents exhaled CO2 from the alveoli. Phase II is a mixed phase with CO2 from both the airways and alveoli. Point A is the inflection point of phase II, which is the theoretical point where the airway compartment is separated from the alveolar compartment. VDaw, airway dead space, Vtalv, alveolar tidal volume, Vt, tidal volume.
Fig 2
Fig 2
Tidal Impedance Variation at different PEEP levels. Data are shown as mean (sd). In the dependent lung region the ventilation distribution was decreased when PEEP was lowered, whereas the non-dependent region received more ventilation as compared with PEEP 14 cm H2O. * Indicates a significant reduction in TIV of the non-dependent region according to 6 cm H2O. † Indicates a significant reduction in TIV of the dependent region as compared with 12 cm H2O. Dashed lines represents the interpolation lines; open circles=non-dependent region; solid circles=dependent region. P<0.05 was considered significant.
Fig 3
Fig 3
Mean intratidal gas distribution (ITV) curve of all the patients at each PEEP step. Data are shown as mean (sd). ITV curve represents the mean percentile contribution (%) of ventilation distribution in non-dependent and dependent lung regions during the entire inspiration. At PEEP 12 and 10 cm H2O, intratidal gas distribution curves stayed close to each other, showing equal distribution to both regions. Dashed lines represents the interpolation lines; open circles=non-dependent region; solid circles=dependent region.
Fig 4
Fig 4
Effect of increased PEEP on the Vcap curve. Figure 4 demonstrates the effect of two PEEP levels on the Vcap curve. As a result of the PEEP application the Vcap curve shifts to the right, indicating an increase in airway dead space. PeCO2 = Expiratory carbondioxide pressure; Vcap = volumetric capnography.

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