Negative expiratory pressure: a new tool.

The term expiratory flow limitation is used to indicate that maximal expiratory flow is achieved during tidal breathing and is characteristic of intra-thoracic airflow obstruction. Despite the severe consequences of expiratory flow limitation, the prevalence and clinical significance of this phenomenon have not been adequately studied in chronic obstructive pulmonary disease (COPD), asthma, and patients with other pulmonary and non-pulmonary disease. This is due to the fact that the conventional method to detect expiratory flow limitation, i.e. the one proposed by Hyatt based on comparison of maximal to tidal expiratory flow-volume curve, has several methodological and theoretical deficiencies. Therefore, its use is no longer recommended. In order to overcome these difficulties, a better technique, namely the negative expiratory pressure (NEP) method has been introduced. It essentially consists in applying negative pressure (-3 to -5 cmH2O) at the mouth during tidal expiration. The NEP technique has been applied and validated in mechanically ventilated intensive care unit (ICU) patients by concomitant determination of iso-volume flow-pressure relationships. With this method the volume and time history of the control and test tidal expiration are the same. The NEP method is based on the principle that in the absence of pre-existing flow limitation, the increase in pressure gradient between the alveoli and the airway opening caused by NEP should result in increased expiratory flow. By contrast, in flow-limited subjects application of NEP should not change the expiratory flow. Application of NEP is not associated with any unpleasant sensation, cough, or other side effects. This method does not require patient collaboration, performance of forced vital capacity (FVC) manoeuvres or use of a body plethysmograph. It can be used, apart from in spontaneously breathing subjects in any body position, during exercise, and in the ICU setting. This new tool may provide new insights into the physiology and pathophysiology of several diseases and the symptom of dyspnea.