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Changes in computed tomography and ventilation/perfusion mismatch with positive end-expiratory pressure


The purpose was to compare effects of PEEP on computed tomography (CT) and estimated ventilation/perfusion (V/Q) mismatch. Previously, an oxygenation-based method was shown more related to the CT-measured effect of PEEP than lung mechanics [1], indicating lung aeration is better quantified using V/Q mismatch. Pulmonary shunt and low and high V/Q mismatch can be estimated from varying FIO2 and measuring ventilation and blood gas contents [2].


Preliminary results in six ARDS patients. CT scans were taken in static conditions at PEEP 5, 45 and 15 to 20 cmH2O. V/Q was estimated at 5 and 15 to 20 cmH2O as: shunt, low V/Q as alveolar to lung capillary PO2 difference (ΔAcPO2), high V/Q as alveolar to lung capillary PCO2 difference (ΔAcPCO2) [2]. Nonaeration, poor aeration, and normal aeration plus hyperinflation were calculated from Hounsfield units. Aeration and V/Q were compared (Pearson, ρ).


PEEP improved V/Q in four patients, shunt reducing 7 to 42% with no/small increase in ΔAcPCO2. Two deteriorated, with large ΔAcPCO2 or shunt increase. No systematic changes in ΔAcPO2 were seen. Figure 1 shows response to PEEP in two patients. Changes in nonaerated regions and shunt were correlated (Δ = 0.94, P = 0.002). No correlations were found between poorly aerated regions and ΔAcPO2 (Δ = -0.09, P = 0.84) or hyperinflated regions and ΔAcPCO2 (Δ = 0.07, P = 0.88).

Figure 1
figure 1

Gas exchange and CT for patient improving (A) or worsening (B) with PEEP change.


In these preliminary cases, changes in shunt and nonaerated tissue correlated well. However, results indicate poor agreement between changes in low and high V/Q and lung morphology.


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  2. Karbing DS, et al.: Med Eng Phys. 2011, 33: 240-248. 10.1016/j.medengphy.2010.10.007

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Karbing, D., Panigada, M., Bottino, N. et al. Changes in computed tomography and ventilation/perfusion mismatch with positive end-expiratory pressure. Crit Care 18 (Suppl 1), P276 (2014).

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