1- Lung Stress and Strain During Mechanical Ventilation of the Acute Respiratory Distress Syndrome.

Dipartimento di Anestesia, Rianimazione (Intensiva e Subintensiva) e Terapia del Dolore, Fondazione IRCCS - "Ospedale Maggiore Policlinico Mangiagalli Regina Elena", Milan, Italy.

RATIONALE: Lung injury due to ventilator results from un-physiological lung stress (transpulmonary pressure) and strain (inflated volume to functional residual capacity ratio). OBJECTIVE: To determine whether the plateau pressure and the tidal volume are adequate surrogates for stress and strain. To quantify the stress to strain relationship in patients and control subjects. METHODS: Nineteen post-surgical healthy patients (group 1), 11 patients with medical diseases (group 2), 26 patients with acute lung injury (group 3) and 24 patients with ARDS (group 4) underwent a PEEP trial (5 and 15 cmH2O) with 6, 8, 10 and 12 mL/kg tidal volume. MEASUREMENTS AND MAIN RESULTS: plateau airway pressure, lung and chest wall elastances, lung stress and strain significantly increased from group 1 to 4 and increasing PEEP and tidal volume. Within each group a given applied airway pressure produced largely variable stress due to the variability of the lung elastance to respiratory system elastance ratio (range 0.33-0.95). Analogously for the same applied tidal volume the strain variability within subgroups was remarkable, due to the functional residual capacity variability. Therefore low or high tidal volume as 6 and 12 mL/kg could produce similar stress and strain in a remarkable fraction of patients in each subgroup. In contrast, the stress to strain ratio, i.e. specific lung elastance, was similar throughout the subgroups (13.4+/-3.4, 12.6+/-3.0, 14.4+/-3.6 and 13.5+/-4.1 cmH2O, P=0.58) and did not change with PEEP and tidal volume. CONCLUSIONS: Plateau pressure and tidal volume are inadequate surrogates for lung stress and strain.

2- Acute lung injury/acute respiratory distress syndrome pathophysiology: what we have learned from computed tomography scanning.

Istituto di Anestesiologia e Rianimazione, Dipartimento di Anestesia, Rianimazione e Terapia del Dolore, Fondazione IRCCS - Ospedale Maggiore Policlinico, Mangiagalli, Regina Elena di Milano, Università degli Studi di Milano, Milano, Italy.

PURPOSE OF REVIEW: Although many years have passed since its first application in acute respiratory distress syndrome, computed tomography remains widely employed for research and clinical purposes. Here, we review recent findings derived from computed tomography scanning during acute respiratory distress syndrome, particularly concerning setting positive end-expiratory pressure and mechanisms of ventilator-induced lung injury. RECENT FINDINGS: Several studies have provided evidence for the validity of monitoring dynamic mechanics of the respiratory system to estimate the balance between beneficial (i.e. reduction of alveolar derecruitment) and harmful (i.e. lung hyperinflation) effects, consequent to positive end-expiratory pressure increase. The combination of different respiratory variables to estimate lung recruitment has become a more accepted approach. Computed tomography scanning has provided important evidence of lung hyperinflation even after the use of low tidal volume in a specific category of patients. Alternative techniques, such as electrical impedance tomography and lung ultrasound, appear as promising tools potentially available at the bedside. SUMMARY: As far as setting positive end-expiratory pressure is concerned, further randomized clinical studies are warranted to verify the pathophysiologic findings recently observed with computed tomography scanning. Similarly, the safety of the widespread use of low tidal volume should be brought into question, possibly pointing out a category of patients who may benefit from alternative techniques of respiratory support.