Expiratory flow limitation

This is a test to assess the degree of airway limitation. It is easy to perform and relatively inexpensive. The patient takes a full inspiration to total lung capacity and then blows out forcefully into the peak flow meter. The best of three recordings are normally taken. The peak flow measures the expiratory flow rate in the first 2 ms of expiration and can overestimate the extent of lung function in patients with moderate to severe airway limitation. Other tests such as... 

Peak flow measurement of peak expiratory flow rate (PEFR) on waking, before bed, before and after bronchodilator medication is useful to assess the extent of airflow limitation and the characteristic of the disease in terms of reversibility. There is some evidence of diurnal variability. PEFR is also useful in assessing the disease progression longer term and the response to therapy. Patients are advised to keep an asthma diary and record regularly the peak flows to ascertain their diurnal pattern. 

Q5 Measurement of peak expiratory flow this is a simple measure of expiratory function. The peak flow meter measures the velocity of expired airflow and is suitable for both adults and children. The patient breathes out a short blast of air, as fast as possible, into the device. Normal individuals can achieve airflow velocity of 450-6501 min-1. The peak flow meter is a cheap device which is used by patients at home to monitor their asthma. If a patient s peak flow diminishes below a certain level which has been set by their nurse practitioner or family doctor, they can adjust their own treatment, within specified limits, and control their condition better. 

A second way of looking at forced expiration is with a maximum expiratory flow-volume (MEFV) curve, which describes maximum flow as a function of lung volume during a forced expiration (Fig. 12). In healthy human subjects, flow rates or flow-volume curves reach a maximum and will not increase with additional effort after the lungs have emptied 20-30% of their volume (Fry and Hyatt, 1960). This phenomenon of flow limitation is due to airway compression over most of the lung volume. Thus, flow rate is independent of effort and is determined by the elastic recoil force of the lung and the resistance of the airways upstream of the collapse point. In obstructive diseases of the lung this curve is shifted to the left, whereas restrictive diseases shift the curve in the opposite direction (also shown in Fig. 12). 

Dawson SV, Elliott EA. Wave-speed limitation on expiratory flow—a unifying concept. J Appl Physiol Respir Environ Exerc Physiol 1977 43 498-515. 

The data on dmg delivery to children via the Authohaler are limited. Rug-gins and colleagues (71) compared the effect of giving salbutamol via the Rota-haler (400 (tg) and the Autohaler (200 pg) in 51 children aged from 4-13. They found no significant difference in peak expiratory flow rate between the two devices. Only 11 children less than 6 years were studied, of whom one 4-year-old... 

Dawson, S. V., and Elliott E. A., Wave-speed limitation on expiratory flow A unifying concept, J. Appl. Physiol., 43 498-515,1977. 

A randomized, placebo-controlled trial examined the effects of menthol (10 mg nebulized twice daily for 4 weeks) on airway hyperresponsiveness in 23 patients with mild to moderate asthma. The diurnal variation in the peak expiratory flow rate (a value reflecting airway hyperexcitability) was decreased but the forced expiratory volume was not significantly altered. This indicated an improvement of airway hyperresponsiveness without affecting airflow limitation (Tamaoki et al., 1995). Later in vivo research examined the effect of menthol on airway resistance cansed by capsaicin- and neurokinin-induced bronchoconstriction there was a significant decrease in both cases by inhalation of menthol at 7.5 pg/L air concentration. The in vitro effect of menthol on bronchial rings was also studied. It was concluded that menthol attenuated bronchoconstriction by a direct action on bronchial smooth muscle (Wright et al., 1997). 

Alterations in cardiopulmonary exercise tests (CPET) have been noted in 28% to 47% of patients with sarcoidosis (6,63,65,66). Typical findings include ventilatory limitation or increased dead space/tidal volume ratio (Vd/Vx) or widened alveolar-arterial O2 (A-a O2) gradient with exercise (63,65). CPET may be abnormal when static PFTs are normal (63,67). Exercise-induced desaturation correlated with reductions in DLco (63,68-70), whereas lung volumes and expiratory flow rates did not (70). 

The airflow limitation will be mirrored by a reduction in the FEVi/FVC ratio, FEVi and expiratory flow. In patients with symptomatic COPD, and especially with emphysema, the diffusion capacity (TL,CO, Kco) is reduced. Reduction in Kco is associated with the severity of the emphysema assessed by CT scanning (Gould et al. 1991). Static lung volumes, TEC and RV will be normal or increased. The severity of the airflow limitation in COPD is graded arbitrarily by the reduction in FEVp Mild FEVj a80% of predicted... 

Three anatomic patterns of TBA exist, each with distinct pulmonary function test characteristics (i) proximal trachea, ii) mainstem bronchi, and (Hi) distal airways. Proximal disease limits expiratory airflows, producing flow-volume loop changes consistent with extrathoracic upper airway obstruction. Mainstem bronchial disease affects large airways flow, decreasing FEVl/FWC ratio. In contrast, distal airway involvement results in decreased small airway or FEF 25 to 75 flows (56). Bronchoscopically, TBA appears as submucosal plaques or diffuse infiltration in 44% cases, nodular disease in 28%, and circumferential lesions in 28% (58). 

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