Typically, subjects are attached to a mouthpiece connected in series to a pneumotachograph and a T-tube. One side of the T-tube is open to the atmosphere, while the other side is equipped with a pneumatic valve, which allows for the subject to be rapidly switched to a negative-pressure source (eg, a vacuum). The negative-pressure source typically has been set at —3 to —10 cm H2O in various populations. NEP is generally applied at the initiation of expiration and is maintained throughout most of expiration for comparison to non-NEP control breaths. Figure 3 shows an example of the NEP technique applied in a subject who is not flow limited (ie, who has an increase in flow during NEP compared with a control breath) during spontaneous breathing and in a patient who is flow limited (ie, who has no increase in flow with NEP compared with a control breath). Commercial systems are becoming available for interface with existing metabolic carts.
The application of NEP during expiration increases the pressure gradient between the alveoli and the airway opening. The induction (initial onset) of NEP results in an artificial increase in expiratory flow in both flow-limited and non-flow-limited patients. This is thought to be due to a reduction in volume from the more compliant oral and neck structures as well as from the larger intrathoracic airways., Generally, use of NEP also will result in an augmented expiratory volume, particularly if flow limitation does not exist. In flow-limited subjects, the use of NEP will augment dynamic compression, however, this is thought to occur downstream from the flow-limiting segments and, thus, typically should not influence the available maximal expiratory flow in these patients. birth control pills online

Clinical Application
The NEP method has been applied to assess whether expiratory flow limitation exists during spontaneous breathing in patients with COPD, before and after single lung transplantation, in patients with stable asthma, and in patients with restrictive respiratory disorders. It has also been successfully applied during mechanical ventilation and in infants with cystic fibrosis, as well as during exercise in COPD patients. The degree of expiratory flow limitation obtained with NEP also has been shown to be more highly correlated with dyspnea than other indexes of pulmonary function (eg, FVC and FEV1) in COPD patients.