A child who is a known case of bronchial asthma comes with respiratory rate 48/min, cannot speak 2 words, occasional wheeze and oxygen saturation of 95%. You give 3 doses of salbutamol nebulisation then he staed to speak a sentence but saturation falls to 85%. What is the cause?
Oxygen saturation (SaO2) levels may not reflect progressive alveolar hypoventilation and the SaO2 may initially fall during therapy because b2-agonists produce both bronchodilation and vasodilation and may initially increase intrapulmonary shunting. SABAs, which are first-line therapy for asthma symptoms and exacerbations, increase pulmonary blood flow through obstructed, unoxygenated areas of the lungs with increasing dosage and frequency. When airways obstruction is not resolved with SABA use, ventilation-perfusion mismatching can cause significant hypoxemia, which can perpetuate bronchoconstriction and fuher worsen the condition. The main catch point in this question is that salbutamol is given without oxygen.(Nebulisation without oxygen which is a common practice). It is always essential to give salbutamol nebulisation through high flow oxygen (Standard practice). In status asthmaticus most children will have some degree of mucus plugging, atelectasis, ventilation-perfusion mismatch and hypoxemia. In that lung, segments with atelectasis, compensatory hypoxic pulmonary vasoconstriction is often present. Treatment with inhaled beta 2 agonists may induce generalized pulmonary vasodilation and as a result, exacerbate ventilation-perfusion mismatch and worsen hypoxemia. Oxygen should be a pa of management for all children with status asthmaticus. Inference: Giving only salbutamol nebulisation without oxygen can aggravate hypoxemia because of increase in dead space ventilation and VP mismatch.
Bronchomalacia
Right to left shunt
VP mismatch with dead space ventilation
Faulty pulse oxy-meter
COPD and Asthma