Respiratory pathophysiology


The effect of disease on respiratory function is best demonstrated by changes in the measurements of air flow, lung volumes and gas transfer. Diagnosis, management and monitoring of respiratory problems rely on non-invasive measurements of the movement of air breathed in and out of the lungs, and the diffusion of gas between inspired air and the blood. Ventilation is assessed by both expiratory flow rate and lung volume measurements. Normal values will vary with gender, age, height, weight and ethnicity. Thus measured values are usually expressed as a percentage of average normal predicted values. Abnormal flow rates reflect changes in airway resistance indicating obstructive pathology; whereas abnormal lung volumes reflect changes in compliance indicating restrictive pathology.




Obstructive respiratory deficits


Increased airway resistance obstructs air flow and thus impairs exhalation. If airways are narrowed it is still possible to fill the lungs to capacity, but resistance will increase on exhalation, so air will come out more slowly. Possible pathological causes include chronic obstructive pulmonary disease (COPD), irreversible increases in airway resistance involving a combination of chronic bronchitis (recurrent inflammation with mucus secretion) and emphysema (permanent destructive enlargement of alveoli)), and asthma (reversible increases in airway resistance involving a combination of bronchoconstriction and inflammation).



Restrictive respiratory deficits


Decreased compliance restricts lung expansion and thus impairs inhalation. If lungs are difficult to fill to capacity because they are stiff, have weak muscles, or there is a problem with the chest wall, they will be less full before exhalation. Lung volume will be reduced, but air will come out at a normal rate. Possible physiological causes include obesity, pregnancy, or restrictive clothing. Possible pathological causes are parenchymal (e.g. pulmonary fibrosis, pulmonary oedema), chest wall (e.g. kyphoscoliosis, pneumothorax), or neuromuscular (e.g. myasthenia gravis). Mixed respiratory deficits are also possible (e.g. tumours or cystic fibrosis) or with a combination of obstructive and restrictive conditions (e.g. if both obese and asthmatic).



Spirometry


Spirometry records the volumes breathed in and out over time. Peak expiratory flow rate (PEFR) depends on the patient inhaling as deeply as possible and then exhaling as quickly as possible. Forced expiratory volume in the first second (FEV1) and forced vital capacity (FVC) are measured by the same manoeuvre as PEFR, but the patient continues to try to breathe out until no more air can be exhaled (Figure 28.1). The FEV1 is expressed as a percentage of FVC. PEFR is the simplest technique to perform, but it is less reliable for assessing conditions where airways are not affected uniformly. With variable airway damage, a very brief momentary high PEFR may still be achieved, whereas measuring over a whole second with FEV1 would be a more reliable. With obstruction, FEV1 will be less than 70% of predicted, but FVC will be more than 70% of normal. With restriction, FVC will be less than 70% of predicted, but FEV1 will be more than 70% of FVC. A flow-volume loop is measured by the same manoeuvre as FEV1 and FVC, but once no more air can be breathed out then the patient inhales as rapidly as possible until they can breathe no more air in (Figure 28.2). A flow-volume loop has the advantage of assessing inhalation as well as exhalation, along with producing shapes characteristic of certain conditions (Figure 28.2).

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Apr 22, 2017 | Posted by in GENERAL & FAMILY MEDICINE | Comments Off on Respiratory pathophysiology

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