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

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 (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).

Results

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

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

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.