asthma
9:20 p.m. - 2006-05-27

Airway drying and cooling provokes bronchoconstriciton on exercise, Contraction of smooth muscle in bronchial walls, mucosal oedema and secretions within airway lumen all contribute to decrease in airway diameter, increasing airway resistance. This causes obstruction of intrathoracic airways, compromising airflow on expiration. Arterial blood gases show low O2 and low CO2(resp alkalosis). Mechanism for hypoxemia is ventilation-perfusion mismatch. The increased airways resistance in asthma is not evenly distributed, some airways more affected than others. Therefore inspired air is not distributed evenly but tends to go to less diseases areas> But blood flow remains preserved in regions that are ventilated poorly. The regions of low V/Q ratio result in decreased PO2 of arterial blood that cannot be made up by increase in V/Q ratio from other regions of lungs. Despite the abnormality in PO2, the patients are able to hyperventilate and PCO2 is usually low. When PCO2 increases to normal/elevated, means worsening airflow obstruction. During acute asthma attack, possible that activation of irritant receptors stimulates ventilation or other reflxes originated in airways, lung, chest wall may stimulate ventilation.

Clinical features
symptoms: cough, wheeze, short of breath, chest tightness. On examination, tachypnea(fast breathing). Wheezing shows airflow through narrowed airways

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