Table 1– Main mechanisms contributing to the decrease in systolic blood pressure during inspiration

Mechanisms	Normal subjects	Cardiac tamponade	Acute asthma
Passive transmission of decreased intrathoracic pressure during inspiration	+	+	++
Blood pooling in the pulmonary veins during inspiration	+/−	+/−	+/−
Series biventricular interdependence (pulmonary transit time)	+	++	+
Parallel biventricular interdependence (septum and pericardium)	0	++	+++
Decreased left ventricular afterload during inspiration	0	+	+

0: mechanism is absent; +/−: mechanism may be present or absent; +: mechanism is present and has a moderate contribution; ++: mechanism is present and has an important contribution; +++: mechanism is present and has a very important contribution (a dominant mechanism).