EFFECTS OF PEEP ON (V)OVER-CIRCLE-A (Q)OVER-CIRCLE MISMATCHING IN VENTILATED PATIENTS WITH CHRONIC AIR-FLOW OBSTRUCTION/

Recent work in patients with acute respiratory failure (ARF) due to ex acerbation of chronic airflow obstruction (CAO) suggests that applicat ion of low degrees of positive end-expiratory pressure (PEEP) can impr ove rather than impair respiratory mechanics, because PEEP replaces in trinsic PEEP (PEEPi). However, the impact of PEEP on pulmonary gas exc hange has not been fully investigated. We designed this study to exami ne the effects of PEEP and those of PEEPi on ventilation/perfusion (VA /Q) mismatching in mechanically ventilated patients with CAO. Eight pa tients were studied under four conditions: (1) during controlled mecha nical ventilation with the ventilatory setting established by the atte nding physicians (PEEPi-100%), according to standard criteria; (2) aft er application of PEEP amounting to 50% (PEEP-50%), and then (3) to 10 0% (PEEP-100%) of the original PEEPi; and finally, (4) after reduction of PEEPi to 50% of the initial value (PEEPi-50%), obtained by increas ing expiratory time and decreasing respiratory rate and tidal volume. Respiratory mechanics, hemodynamics, respiratory blood gases, and VA/Q distributions were measured during each ventilatory mode. At low valu es of PEEP (PEEP-50%) no changes in respiratory mechanics nor in hemod ynamics were observed, but Pa-O2 moderately increased (from 103 +/- 25 .2 to 112 +/- 29.6 mm Hg) and Pa-CO2 slightly decreased (from 42 +/- 3 .7 to 40 +/- 3.3 mm Hg) essentially because of an increase in the mean VA/Q ratio (first moment) of both food flow (Q, from 0.65 +/- 0.28 to 0.78 +/- 0.29) and ventilation (V, from 4.02 +/- 1.55 to 4.93 +/- 2.0 0) distributions (p < 0.05, each). With PEEP equaling PEEPi (PEEP-100% ), airway pressures slightly increased (by +3 cm H2O, p < 0.05) withou t further improvement in pulmonary gas exchange. Reducing minute venti lation and changing the ventilatory pattern to decrease PEEPi (PEEPi-5 0%), Pa-O2 fell (to 83 +/- 14.3 mm Hg) and Pac(O2) moderately rose (to 54 +/- 5.1 mm Hg) (p < 0.05, each). The fall in Pa-O2 was partially o ffset because P-VO2 increased (from 38 +/- 3.3 to 45 +/- 4.9 mm Hg) du e to increased cardiac output (from 3.3 +/- 0.8 to 5.2 +/- 1.1 L/min) which, in turn, increased systemic O-2 delivery (from 0.57 +/- 0.06 to 0.89 +/- 0.11 L/min, p < 0.05, each). We conclude that the applicatio n of PEEP equivalent to 50% of the initial PEEPi (PEEP-50%) improves p ulmonary gas exchange, without adverse effects on respiratory mechanic s nor on hemodynamics. Hypoventilation associated with reduction of PE EPi (PEEPi-50%) significantly reduces alveolar pressure, while increas ing cardiac output and systemic O-2 delivery Our data support the use of ''controlled hypoventilation'' associated with low values of PEEP i n CAO patients with acute respiratory failure requiring mechanical ven tilation.