Fall in Arterial Oxygen Pressure
A fall in arterial oxygen pressure (PaO2) has been noted with isoproterenol administration during asthmatic bronchospasm, as ventilation improves and the exacerbation is relieved.
The same effect has subsequently been noted with newer beta agonists such as albuterol and salmeterol. The mechanism for this seems to be an increase in perfusion of poorly ventilated portions of the lung.
It is known that regional alveolar hypoxia produces regional pulmonary vasoconstriction in an effort to shunt perfusion to lung areas of higher oxygen tension.
Administration of inhaled beta agonists may reverse hypoxic pulmonary vasoconstriction by beta 2 stimulation, increasing perfusion to underventilated lung regions.
Preferential delivery of the inhaled aerosol to better ventilated lung regions increases the ventilation-perfusion mismatch. Oxygen tension falls most in subjects with the highest initial PaO2. Decreases in PaO2 rarely exceed 10 mmHg, and the PaO2 values tend to be on the flat portion of the oxyhemoglobin curve, so that drops in arterial oxygen saturation (SaO2) are minimized. Oxygen tensions usually return to baseline within 30 minutes.
Source: http://www.allbusiness.com/pharmaceuticals-biotechnology/pharmaceutical/14173501-1.html#ixzz1njbKDcNh
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