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

Abstract

Introduction 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]. Methods 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 (DELTAAcPO2), high V/Q as alveolar to lung capillary PCO2 difference (DELTAAcPCO2) 2]. Nonaeration, poor aeration, and normal aeration plus hyperinflation were calculated from Hounsfield units. Aeration and V/Q were compared (Pearson, rho). Results PEEP improved V/Q in four patients, shunt reducing 7 to 42% with no/small increase in DELTAAcPCO2. Two deteriorated, with large DELTAAcPCO2 or shunt increase. No systematic changes in DELTAAcPO2 were seen. Figure 1 shows response to PEEP in two patients. Changes in nonaerated regions and shunt were correlated (rho = 0.94, P = 0.002). No correlations were found between poorly aerated regions and DELTAAcPO2 (rho = -0.09, P = 0.84) or hyperinflated regions and DELTAAcPCO2 (rho = 0.07, P = 0.88). Conclusion 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.