During an attack of bronchial asthma, a generalized obstruction of the bronchi is observed due to a spasm of their smooth muscles, vasodilation , swelling of the mucous membrane and accumulation of thick viscous mucus in the bronchi. As a result, the airway resistance increases and the forced expiratory volume and the expiratory volume rate decrease . The lungs swell and expand the chest, increase breathing, and assist respiratory muscles. Due to focal hypoventilation and the mismatch between ventilation and perfusion , hypoxemia develops . In severe cases, ECG shows signs of overload and hypertrophy of the right ventricle. In a patient seeking emergency medical care, FVC is usually reduced by 50% or more, FEV1 is no more than 30% of the norm, and SOS25-75 is no more than 20% of the norm.
An important role in the pathogenesis of an attack is played by an increase in the residual volume due to the effect of an air trap. During an attack, the residual volume often reaches 400% of the norm, and the FOE doubles. When the residual volume decreases to 200%, and FEV1 rises to 50% of the norm, the patient feels the end of the attack.
An attack of bronchial asthma is always accompanied by hypoxia . Acute respiratory failure is quite rare, in about 10-15% of cases. In most patients, hyperventilation , hypocapnia, and respiratory alkalosis are observed during the attack . With severe bronchial obstruction, hypoventilation develops , therefore, normal levels of PaCO2 and especially hypercapnia and respiratory acidosis should be considered as signs of acute respiratory failure and indications for intensive care.
Assessing the severity of gas exchange disorders , one cannot rely on clinical data, since cyanosis (a sign of hypoxia ) appears very late, and symptoms of hypercapnia ( sweating , tachycardia , high pulse pressure , tachypnea ) in emotionally labile patients are even with mild attacks.
The severity of ventilation disorders during an attack also does not always correspond to the severity of the patient’s condition, therefore, when suspected of hypoventilation , SAC must be measured .