The authors respond:

We thank Dr Esquinas et al for the interest in our study¹ and for the questions raised. They questioned our expectancy over the application of sequential higher PEEP in AIDS patients with hypoxemic respiratory failure. When applying a stepwise alveolar recruiting maneuver in ARDS patients, the mass of collapsed lung tissue and the oxygenation improve substantially higher than a single 40 cm H₂O/40 second recruiting maneuver.² Our explanation for this fact is the alveolar overdistention in some areas in the heterogeneously injured lungs, causing alveolar collapse through the different compliance alveolar interactions.^3–5 Furthermore, there is the possibility of time-dependent alveolar recruitment using low pressures.⁶ Moreover, we hypothesized that a stepwise elevation of noninvasive applied relative low PEEPs could be effective in improving hypoxemia.

Dr Esquinas et al also addressed the insufficient time to achieve the complete and effective alveolar recruitment. We agree with Dr Esquinas et al, and, furthermore, complete alveolar recruitment in patients presenting such severe acute hypoxemia will probably be reached after the application of higher pressures than those applied in our study. However, the goal of noninvasive ventilation in this setting is not to reach the complete alveolar recruitment, but the necessary alveolar patency to improve the severe hypoxemia and the patient's respiratory distress. In this way, our result allows the conclusion that improvement of the patient's hypoxemia seems to occur faster when applying a stepwise progressive elevation of PEEP. A given patient undoubtedly can improve the hypoxemia receiving a higher and static PEEP for more than 20 min.

Their next question concerned the absence of a real sequence effect of progressively higher PEEPs applied with a washout period of 20 min. The rationale for this step, in our study, was that, in spite of massive alveolar derecruitment that happens in ARDS patients when suddenly lowering high pressures in the presence of F_IO2 = 1.0,² when lowering PEEP from 14 to 5 cm H₂O using an F_IO2 of 0.6, even using low tidal volumes, the loss of aeration is minimal within an interval of 30 min in ARDS patients.⁷ We used extrinsic PEEP values ≤ 15 cm H₂O alternated with PEEP = 0 cm H₂O through the native airway (keeping the natural expiratory pressurization of the trachea). In this situation, the loss of aeration is not expected to be high in a 20-min interval.

Regarding the P_aCO2 rising during the PEEP elevation, they suggest that a more reasonable explanation of this fact is the Haldane effect and the changes in the ventilation/perfusion ratio secondary to the oxygenation improvement. We congratulate Dr Esquinas and co-authors for the idea, and totally agree that such physiological considerations are one of the factors contributing to the P_aCO2 rising in this situation.

Their final comment was on the hemodynamic impact of each level of PEEP. The available data about hemodynamics of our study were shown in our Table 2,¹ in which we can observe the stability of mean arterial pressure and the fall of the heart rate during the exposure to progressively higher PEEPs. Therefore, the double product was felt, showing the possibility of a reduction in myocardial consumption of oxygen. The hemodynamic safety of noninvasive support in our study seems similar to other physiological studies about noninvasive mechanical ventilation.^8,9

Our study was designed to explore relevant pulmonary physiological characteristics of AIDS patients with hypoxemic respiratory failure at bedside, creating the possibility of noninvasive respiratory support optimization.