Although most of the information focuses on understanding how the ventilator produces lung damage, the pulmonary factors that predispose to ventilator-induced lung injury (VILI) have been less studied. Acute respiratory distress syndrome (ARDS) can adopt different morphological phenotypes, with its own clinical and mechanical characteristics. This morphological phenotypes may favor the development of VILI for same ventilatory strategy
The lung in acute respiratory distress syndrome ARDS) is a heterogeneous viscoelastic system, in which areas with different time constants coexist, causing tidal volume to be distributed unevenly within an anatomically and functionally reduced lung. The administration of a disproportionately high tidal volume for this lung predisposes to the over-distension of the better ventilated alveoli and to the injury by tidal opening and closing of the alveoli more unstable. In this sense, using low tidal volume and homogenizing the lung by means of the prone position have proven beneficial in ARDS. Tidal volume, driving pressure, inspiratory flow and respiratory rate have been identified as responsible for mechanical ventilation-induced lung injury (VILI). These factors together represent the mechanical power, the insulting energy which is repeatedly applied to a vulnerable lung parenchyma. Although most of the information focuses on understanding how the ventilator produces lung damage and/or amplifies the existing one, the pulmonary factors that predispose to VILI have been less studied. Acute respiratory distress syndrome can adopt different morphological phenotypes, with its own clinical and mechanical characteristics. Understanding how each subgroup of ARDS responds to the protective ventilatory strategy could help to personalize treatment. Objectives: To compare the risk of VILI in two groups of ARDS with different morphological phenotypes (focal and non-focal), ventilated with the same protective strategy. Design: Patients with ARDS were ventilated under the same conditions of both tidal volume (TV) and plateau pressure (PPlat). Positive End Expiratory Pressure (PEEP) was adjusted to reach 30 cmH2O of PPlat. A CT was performed in inspiration and expiration. Transpulmonary pressures (TP) were measured and lung volumes calculated (Volume Analysis Software,Toshiba, Japan). Stress was defined as TP at the end of inspiration (TPinsp) and strain: tidal volume/End Expiratory Lung Volume Patients were classified into focal and non-focal according to the distribution of aeration loss in CT. Mann - Whitney U test was used to compare variables and Pearson correlation coefficient to compare its correlation. Significant: p \<0.05
Patients with ARDS were included. We excluded patients with emphysema, asthma, pneumothorax, or serious conditions of instability: oxygen saturation ≤ 88%; severe shock, ventricular arrhythmia, or myocardial ischemia. To allow comparison between groups, patients were ventilated in volume control under similar conditions of tidal volume (TV; 6 ml/kg-PBW), plateau pressure (PPlat 30 cmH2O), respiratory rate (18 bit/min) and constant flow. PEEP was adjusted to reach objective PPlat. Transpulmonary pressures (TP) were measured and a chest CT scan performed during an expiratory and inspiratory pause. Global and regional volumes of lungs were measured using specific software (Volume Analysis Software,Toshiba, Japan). Three regions were identified: basal (from the diaphragm to the carina), middle (from the carina to the aortic arch) and apical (above the aortic arch).
San Juan Bautista, Buenos Aires, Argentina