The study of arterial blood gases.
Imbalance between ventilation and perfusion of the lungs leads to a decrease in paO2 , which can be observed even in the interictal period. However, even with normal paO2, p (Aa) O2 can be increased (more than 20 mmHg).
To determine p (Aa) O2, pAO2 is first calculated using the following formula:
pAO2 = pIO2 – paCO2 / R
where pIO2 = 150 mmHg. Art. (pO2 in inhaled air at sea level), R = 0.8.
It can be seen from the formula that with a decrease in paCO2, p (Aa) O2 can increase despite normal paO2.
paO2 during an attack of bronchial asthma decreases in proportion to the degree of bronchial obstruction . paCO2 first also decreases, and then rises. So with mild attacks, hyperventilation is usually observed, which leads to a decrease in paCO2. In severe attacks, hypoventilation often develops, an indicator of which is an increase in paCO2 of more than 42 mm Hg. Art. A normal or elevated level of paCO2 during an attack of bronchial asthma indicates a pronounced obstruction of the bronchi and the need for immediate treatment under the constant control of arterial blood gases.
With mild and moderate attacks of bronchial asthma, the pH is usually normal, less often respiratory alkalosis is observed . It can be compensated and uncompensated. Severe attacks of bronchial asthma , in which FEV1 is less than 15-20% of normal, are accompanied by a decrease in pH. This is due to the accumulation of lactate and metabolic acidosis ( BE less than -2 meq / l). Metabolic acidosis may be accompanied by respiratory acidosis , which develops as a result of hypoventilation. Metabolic acidosis results in:
– increased work of breathing due to severe bronchial obstruction and pulmonary distension;
– tissue hypoxia due to difficulty in venous outflow and hypoxemia.
The study of arterial blood gases is indicated for severe shortness of breath, a decrease in the effectiveness of bronchodilators, significant tachypnea and tachycardia, an increase in the size of the chest, the inclusion of auxiliary muscles in the act of breathing, retraction of the pliable areas of the chest, the appearance of a paradoxical pulse, and impaired consciousness. With a significant decrease in paO2 (less than 60 mmHg) or an increase in paCO2, oxygen inhalation is indicated. To control oxygenation of arterial blood, ventilation, and perfusion of tissues, arterial blood gases are regularly examined.