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

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 H ₂O, in steps of 2 cm H ₂O) and blood gases, Vcap and EIT were measured.

Results

Tidal impedance variation of the non-dependent region was highest at 6 cm H ₂O PEEP, and decreased significantly at 14 cm H ₂O PEEP indicating decrease in the fraction of Vt in this region. At 12 cm H ₂O PEEP, homogenous ventilation was seen between both lung regions. Bohr and Enghoff dead space calculations decreased from a PEEP of 10 cm H ₂O. Alveolar dead space divided by alveolar Vt decreased at PEEP levels ≤6 cm H ₂O. The normalized slope of phase III significantly changed at PEEP levels ≤4 cm H ₂O. 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.