During severe asthma, paradoxic pulse may result from increased impedance to left ventricular ejection, mechanical impairment of left ventricular filling by ventricular interdependence or decreased pulmonary venous return augmented by hypovolemia. We studied the effect of reversible blood volume expansion by MAST inflation during severe attacks of asthma. Ten patients with clinically detectable paradoxic pulse of more than 20 mm Hg were studied. All had a history of reversible bronchial asthma with evidence of respiratory and circulatory failure. Standard therapy for asthma was started. We observed no difference in respiratory and heart rates during MAST inflation. Paradoxic pulse was consistently decreased during MAST inflation; paradoxic pulse returned to baseline values after MAST deflation. The decrease in paradoxic pulse was produced by an increased inspiratory systolic arterial pressure. We conclude that a reduction in pulmonary venous return is more important than ventricular interdependence in producing paradoxic pulse during severe asthma.