Pulsus paradoxus

Paradoxical pulse refers to an inspiratory decline in systolic pressure greater than 10 mmHg. In normal circumstances, inspiration results in an increase in venous return as blood is ‘sucked into’ the thorax by the decline in intrathoracic pressure. This increases right ventricular stroke volume, but left ventricular stroke volume falls slightly (ventricular interdependence). When the heart is constrained in a ‘fixed box’ by a pericardial effusion (cardiac tamponade) or by thickened pericardium (pericardial constriction), the increased inspiratory right ventricular blood volume reduces left ventricular compliance, resulting in a more pronounced reduction in left ventricular filling stroke volume and systolic blood pressure during inspiration. ‘Pulsus paradoxus’ therefore represents an exaggeration of the normal inspiratory decline in systolic pressure and is not truly paradoxical. Pulsus paradoxus in acute severe asthma is thought to be due to negative pleural pressure increasing afterload and thereby impedance to left ventricular emptying. It is measured by inflating a blood pressure cuff until no sounds are heard. The pressure is then slowly decreased until systolic sounds are first heard during expiration but not during inspiration – note this reading. The pressure is slowly decreased further until sounds are heard throughout the respiratory cycle (inspiration and expiration) – note this second reading. If the pressure difference between the two readings is >10 mmHg, it can be classified as pulsus paradoxus. 

Ref- Hutchison's 24E