Changes in computed tomography and ventilation/perfusion mismatch with positive end-expiratory pressure
Critical Care volume 18, Article number: P276 (2014)
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 , 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 .
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) . 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).
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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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). https://doi.org/10.1186/cc13466