Ventilation perfusion relations

Hypoxia. Reduction of oxygen supply to tissue below physiological levels despite adequate perfusion of the tissue by blood. Hypoventilation. Underventilation reduced alveolar ventilation in relation to the oxygen consumption. 

Although the reversibility of airways obstruction is an important characteristic of asthma, those asthmatics with chronic symptoms do not reverse completely. The chronic airways obstruction is probably related to mucus plugging of bronchi and hypertrophy of the bronchial smooth muscle. These changes are not reversible by bronchodilators, and it is not clear that they can be reversed with any specific therapy. Both types of obstruction contribute to an imbalance of the ventilation to perfusion ratio in the lung and can result in hypoxemia and hypocarbia. The decreased carbon dioxide content of the blood is the result of the asthmatic s hyperventilation and will persist until the respiratory muscles fatigue and hypoventilation becomes prominent. Asthmatics can die quite quickly when this occurs. 

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