Mechanism of reverse pulsus paradoxus
Reverse pulsus paradoxus is an expiratory fall in pulse volume, the opposite of pulsus paradoxus. The classical causes of reverse pulsus paradoxus described are :
- Isorhythmic atrioventricular (AV) dissociation
- Hypertrophic obstructive cardiomyopathy
- Intermittent positive pressure ventilation (IPPV)
During IPPV the swings in intrathoracic pressure is opposite to that of normal breathing. Hence the inspiratory blood pressure is higher than the expiratory blood pressure . During the inspiratory phase, increased intrathoracic pressure of positive pressure ventilation is transmitted to the thoracic aorta. In addition there is an increase in left ventricular stroke volume . The occurrence of reverse pulsus paradoxus during IPPV has also been called as respirator paradox . It has been mentioned that respirator paradox after cardiac surgery may delay the early diagnosis of cardiac tamponade. Reverse pulsus paradoxus in a patient on IPPV is thought to be a sensitive indicator of hypovolemia . Left ventricular end systolic volume as measured by radionuclide imaging is reduced during inspiratory phase of IPPV without significant change in end diastolic volume. This would suggest a reduced left ventricular after load in inspiratory phase as a mechanism for reverse pulsus paradoxus in IPPV .
In hypertrophic obstructive cardiomyopathy, Massumi RA et al  have documented higher left ventricular pressures and lower arterial pressures in expiration with simultaneous left ventricular-brachial arterial pressures. This would suggest an enhanced left ventricular outflow obstruction in expiration.
In case of isorhythmic AV dissociation, inspiratory increase in sinus rate results in sinus rhythm dominating over AV dissociation. In sinus rhythm there is AV synchrony and hence a higher pulse volume, compare to AV dyssynchrony in expiration.
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