Tidal volume for mechanical ventilation of healthy lungs

Intermittent Positive Pressure Ventilation (IPPV) at our days is the gold standard treatment for patients requiring mechanical ventilation during general anesthesia and intensive care.1 The principal goals of IPPV are: improve tissue oxygenation, reduce hypercapnia and the related respiratory acidosis and reduce the work of breathing of the patient.1 Despite its important therapeutic functions, IPPV represents an un-physiological condition for the respiratory system and the thoracic cavity. During IPPV the pressure developed in the respiratory system during a respiratory cycle is overall positive while during spontaneous ventilation the pressure is mainly negative.1 Most of our understanding in physiology and pathophysiology of mechanical ventilation came from studies regarding the ventilation support in human patients affected by Acute Lung Injury (ALI) and Acute Respiratory Distress Syndrome (ARDS), pathological condition where IPPV represents the most effective treatment. The term ‘Acute Lung Injury’ (ALI) refers to an acute change in lung function, typically in gas exchange, in the prevention of abnormal water or solute accumulation in the air spaces. This lung injury, identifi ed histologically as diffuse alveolar damage, is the result of structural changes in the alveolocapillary unit. Injury to the alveolocapillary membrane inevitably disrupts the endothelial barrier, leading to the development of noncardiogenic pulmonary edema through increased vascular permeability. As the air spaces fi ll with fl uid, the gas exchange and mechanical properties of the lung deteriorate. When the injury is severe, the clinical manifestations are those of Acute Respiratory Distress Syndrome (ARDS).1 The European-American Consensus Conference on ARDS developed the following working defi nition of ALI/ARDS: Acute onset of respiratory distress (< 72h) Presence of bilateral infi ltrates at the chest radiographs Dott. Francesco Staffi eri, DVM, PhD: Clinical Anaesthetist, Sezione di Chirurgia Veterinaria, Università degli Studi di Bari, Facoltà di Medicina Veterinaria, Bari, Italy.