Airway resistance

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— Effects of autonomic activity upon airway smooth muscle
— Physiological basis of the clinical airway effects of asthma
— Peak expiratory flow rate: its dependence upon initial lung volume
— Appreciation of normal PEFR values for healthy adult males and females
— FEV(1)/ FVC as a composite measure of function [obstructive vs restrictive disease
— Normal value of FEV(1)/FVC in a healthy adult [/box]

Diagram

Flow through the airways is described by Darcy’s law.
Flow = (P1 P2)/R, where P1 is the alveolar pressure, P2 is the mouth pressure and R is the resistance to air flow.
Poiseuille’s law; Airway resistance is determined by the airway radius and whether the flow is laminar or turbulent.

Airway resistance is increased by bronchoconstrictors (muscarinic – parasympathetic activation from irritant mediators released by inflammatory cells (e.g. histamine, prostaglandins, leukotrienes), e.g. in asthma. Increased mucus production also narrows the lumen.

Sympathetic stimulation, adrenaline (epinephrine) and salbutamol (asthma therapy) cause bronchodilatation via β2-adrenoceptors on the smooth muscle.

Effect of transmural pressure.

Expiration is normally passive. Forced expiration increases the intrapleural and alveolar pressure and therefore theoretically leading to increased flow. However, although expiration from fully inflated lungs is indeed effort dependent, towards the end of the breath, increasing force does not increase flow, i.e. it is effort independent. Midway, the pressure in the airway falls below the intrapleural pressure, causing the airway to collapse (dynamic compression). In diseases in which the airways are already narrowed (e.g. asthma, COPD), this leads to expiratory wheezing and air trapping.

Lung function tests

Lung volumes can be measured using a simple spirometer.
The easiest and quickest measurement is the peak expiratory flow rate (PEFR).
PEFR is decreased if the airway resistance is increased (obstructive disease) (used as follow up tool in management, e.g. asthma). It is, however, dependent on the initial lung volume.
Subjects breathe out from total lung capacity to residual volume as fast as possible; this is the forced vital capacity (FVC).
Forced expiratory volume in 1 second (FEV1) reflects airway resistance; it is normally expressed as a ratio to FVC (FEV1/FVC) to correct for lung volume, and is usually 0.75–0.90. It can be used to distinguish between obstructive (increased airway resistance) and restrictive (decreased lung compliance) diseases.
In asthma, for example, FEV1/FVC is typically <0.7. In restrictive disease (e.g. lung fibrosis), FEV1 and FVC are low, but FEV1/FVC is normal or even increased due to greater elastic recoil.

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