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Negative Expiratory Flow
Negative Expiratory Flow Document©
Author: Oisin Mcmanigan
Physiology
Applying a negative pressure at the mouth during tidal expiration and comparing the flow volume curve with that of the proceeding tidal expiration. The current range for the negative in cmH20 is up to 5.
Application of a negative pressure should increase the expiratory driving pressure (the pressure gradient between the alveoli and the airway opening) so increasing the flow if the patient is not flow limited. All individuals when a negative pressure is applied will show an initial peak on the flow volume curve as a result of air in the upper airways being displaced.
Where expiratory flow limitation is present the gradient will not increase resulting in the flow volume curve overlaying the proceeding tidal expiration, (the extent of this overlay is expressed as a percentage and will define the extent of flow limitation in conjunction with the modified scale).
Where the two curves do not overlay then we assume that expiratory flow limitation is not present.
Patient with no flow limitation
Patient exhibiting flow limitation in seated and supine position
(Ref: Valta et al. Am J Resp Crit Care Med 1994)
Past studies have compared maximal and tidal flow volume curves but this is problematic/erroneous for two main reasons:
1. volume dependent changes in airways resistance and lung recoil prior to the FVC manoeuvre.
2. time dependent inequalities between the viscoelastic properties of pulmonary tissue and lung emptying times
The NEP technique is effort independent and does not require the patient to perform the FVC manoeuvre.
NEP must be used with a mouthpiece and noseclip or a face mask and the subject should be tested in the seated and the supine position. Expiratory flow limitation is assessed using a modified dyspnea scale as shown from the ATS Questionnaire/ Medical Research Questionnaire.
Dyspnea Scale
| Category | Degree | Result |
| 0 | None | Not troubled by dyspnea |
| 1 | Slight | Troubled by shortness of breath when hurrying on the level or walking up a slight hill |
| 2 | Moderate | Walks slower than people of the same age on the level or up a slight hill |
| 3 | Moderately Severe | Has to stop when walking at on pace on the level |
| 4 | Severe | Stops for breath after walking about 100yds or after a few minutes on the level |
| 5 | Very Severe | Too breathless to leave the house or breathless when dressing or undressing. |
Classification of FL according to % control tidal volume encompassed by FL and Body position
| Supine | Seated | Category | Degree |
| No FL | No FL | 0 | None |
| FL<50% VT | No FL | 1 | Mild |
| FL>50% VT | No FL | 2 | Moderate |
| FL | FL<50% VT | 3 | Severe |
| FL | FL>50% VT | 4 | Very Severe |
Uses of NEP
To assess the degree of airflow limitation in relation to dyspnea:
Many patients appear very dyspneic whilst those with the same lung function results do not and are minimally symptomatic - NEP is much more sensitive in assessing expiratory flow limitation than FEV1 or FVC
Loubna Etyara et al 1996 Am.J. Resp crit. Care
Studies have shown that the intensity of dyspnea during exercise in patients with COPD is closely related to dynamic hyperinflation (when breathing takes place from Lung volumes higher than the relaxation volume of the respiratory system)
To assess extent of airflow limitation before and after lung transplantation
Individuals with chronic COPD may exhibit flow limitation even at rest-due to many patients breathing tidally along their maximal expiratory curve (MEFV)
Airflow limitation promotes hyperinflation and inspiratory positive end-expiratory pressure (iPEEP) resulting in increased respiratory work impairment or respiratory muscle function and adverse effects on hemodynamics.
Lung transplantation is common in severe COPD individuals and NEP can be used to assess it's severity before and after.
Murciano et al Am J. Resp Crit care med 1997.
Mechanical Ventilation
NEP can be used to easily assess ventilated patients airflow limitation.
Individuals who are flow limited during mechanical ventilation often go unrecognised because:
- FVL comparisons are not practical in I.T.U wards
- Bronchodilators are not always administered to the correct patients
- PEEP (Positive end expiratory pressure) has been suggested to offset inspiratory PEEP in an effort to reduce the workload of breathing but MUST ONLY be applied to individuals with evidence of Expiratory Flow Limitation (EFL).
Restrictive Lung Diseases
Some individuals who suffer from severe chest wall abnormalities e.g. Scoliosis, Kyphosis have been shown to exhibit EFL and this will increase with age.
Also useful for assessing EFL in individuals with neuromuscular disorders.
Clinical Papers Index
NEP/Expiratory Flow
The American Physiological Society 1998; Application of negative expiratory pressure during expiration and activity of genioglossus in humans; C Tantucci,
S Mehiri, A Duguet, T Similowski, I Arnulf, M Zelter, J Derenne, J Milic-Emili
Chest, 112/4 , October 1997, Expiratory Flow Limitation during Spontaneous breathing* Comparison of Patients with Restrictive and Obstructive Respiratory Disorders. A Baydur, J Milic-Emili
Am J Respir, Crit Care Med, Vol 156, Pp 752-757, 1997, Expiratory Flow Limitation in Stable Asthmatic Patients During Resting Breathing, J Boczkowski
D Murciano, M Pichot, A Ferretti, R Pariente, J Milic-Emili
Am J Respir, Crit Care Med, Vol 155P, p1036-1041, 1997 Expiratory Flow Limitation is COPD Patients after Single Lung Transplantation,M Pichot, D Murciano, J Boczkowski, C Sleiman, R Pariente, J Milic-Emili
The American Physiological Society 1997, Detection of expiratory flow limitation during exercise in COPD patients, I Dimopoulou, N Koulouris, P Valta, R Finkelstein, M Cosio, J Milic-Emili
Am J Respir, Crit Care Med, Vol 154, Pp1726-1734, 1996, Relationship between Chronic Dyspnea and Expiratory Flow Limitation in Patients with Chronic Obstructive Pulmonary Disease,M Rigsby Becklake, Alberto Volta, J Milic-Emili, L Eltayara
Am J Respir, Crit Care Med, Vol 150, Pp 1311-1317, Detection of Expiratory Flow Limitation during mechanical Ventilation, P Valta, C Corbeil, A Lavoie, R Campodonico, N Koulouris, M Chassé, J Braidy, J Milic-Emili
Am J Respir, Crit Care Med, Vol 150, Pp 1581-1586, 1994, Dependence of Maximal Flow-Volume Curves on Time Course of Preceding Inspiration in Patients with Chronic Obstruction Pulmonary Disease,E D'Angelo, E Prandi
L Marazzini, J Milic-Emili
Monaldi Arch, Chest Dis, 1993: 48: 1, 80-82, Respiratory mechanics in chest wall disease: implications for expiratory flow limitation during resting breathing
J Milic-Emili
Bull. Physio-path Maximum, Resp 1971, 7, 113-123 Expiratory flow-volume curves in children and adolescents, A Bouhuys