Lung recruitment and PEEP response in ARDS-related H1N1 virus patients

Introduction

Starting in April 2009 the swine-origin influenza (H1N1) virus is considered a worldwide pandemic and is associated with a large number of patients admitted to intensive care. In a proportion of these patients, acute respiratory distress syndrome (ARDS) can develop. We described the case of two patients that presented ARDS related to H1N1 virus confirmed by a nasal swab tested with real-time reverse transcription-polymerase chain reaction. Using a lung CT scan we measured the potential of lung recruitment and the response to positive end-expiratory pressure (PEEP) in terms of gas exchange and respiratory mechanics.

Methods

Patients underwent a lung CT scan at an airway pressure of 5 and 45 cmH₂O; lung profiles were outlined and quantitative analysis was performed with custom-made software. The potential of lung recruitment was computed as the amount of nonaerated lung tissue at 5 cmH₂O in which aeration can be restored at an airway pressure of 45 cmH₂O. The PEEP test was performed at two PEEP levels (5 and 15 cmH₂O), maintaining constant the minute ventilation and the inspiratory oxygen fraction; gas exchange and respiratory mechanics were recorded 20 minutes after each PEEP change.

Results

Two sedated and paralyzed subjects with acute lung injury (ALI)/ARDS (age 46 ± 5 years, BMI 38.6 ± 5.5 kg/m², PaO₂/FiO₂ 187 ± 30 mmHg, PEEP 17 ± 1 cmH₂O; both patients were obese and one of them suffered from diabetes) were enrolled. One patient required extracorporeal membrane oxygenation support for 5 days. Lung CT scans were performed 5 ± 3 days after ICU admission. The total lung weight at 5 cmH₂O was 1,598 ± 142 g. The percentage of potentially recruitable lung was 24 ± 8% of the total lung weight, corresponding to an absolute weight of 371 ± 109 g recruitable lung tissue. A 15 cmH₂O compared with 5 cmH₂O of PEEP improved the compliance of the respiratory system from 42.2 ± 7.0 to 46.6 ± 2.1 ml/cmH₂O and the gas exchange from PaO₂ 62.7 ± 0.1 to 123.2 ± 61.8 mmHg and PaCO₂ 47.1 ± 0.9 to 45.9 ± 3.2 mmHg, respectively.

Conclusions

The ARDS-related H1N1 virus presented, in the early phase, a high potentially recruitable lung in which elevated PEEP levels can be beneficial.