Tuesday, February 17, 2015

Study group discussion: Mechanism of pulsus paradoxus in severe acute bronchial asthma

Which one of the following does not cause pulsus paradoxus?

a. Severe aortic regurgitation
b. Cardiac tamponade
c. Constrictive pericarditis
d. Acute severe bronchial asthma

In cardiac tamponade and constrictive pericarditis: As during inspiration more blood enters in to right heart there is myocardial contsraint and decrease myocardial expansion in above two scenario so ventricular septum bulges towards left side causing less ventricular filling i.e. end diastolic volume less stroke volume and exaggerated fall in BP. Read more here.

In obstructive lung diseases, lungs hyperinflated, more blood in pulmonary vasculature, less returns to heart, so it can also manifest with pulsus paradoxus.
Pulsus paradoxus is also taken consideration in asthma severity assessment.

Could you expand on the mechanisms behind how and why it occurs in acute severe bronchial asthma?

You know that pulsus paradoxus is caused by negative intrathoracic pressure, right?
So if you have an increased resistance to your airways, like in severe asthma, you'll be developing a more negative intrathoracic pressure to overcome the resistance and breathe.
This will exaggerate the normal physiological response.

About pulsus paradoxusus, in asthma due to hyperinflated lungs there is increased afterload for the RV and hence it press on LV, apart from that there is decreased preload to the LV, which ultimately causes an inspiratory fall in systolic BP more than 10 mm Hg.

Pulsus paradoxus is one of the ominous signs in acute exacerbation of bronchial asthma. This is the most common extra-cardiac cause of this physical sign. The main mechanism operational in respiratory disease is the unusually great fluctuations in intrathoracic pressures that are transmitted to the aorta. The following theories have been proposed:

1. During increased airway resistance, there is an exaggeration in the inspiratory-expiratory difference in stroke volume mediated primarily by the effects of intrathoracic pressure on ventricular preload. Shim et al found that patients of asthma with pulsus paradoxus had greater airflow obstruction than patients without. Also, it was often present in mild obstruction and absent in severe obstruction. In acute exacerbation of childhood asthma, pulsus paradoxus often correlates with both the severity and response to bronchodilators.

2. Hyperinflation of the chest due to air trapping is also plays a role in pulsus paradoxus. Factors other than hyperinflation also contribute to the fall in systolic pressure that occurs at full inflation of the lungs. This is also observed in patients with chronic stable obstructive airway disease.

Never heard of it in aortic regurgitation and can't even think any mechanism in which it will lead to this! My choice will be aortic regurgitation. 

Interesting fact: I was trying to relate aortic regurgitation with pulsus paradoxus and found out that it's one of the conditions where, if accompanied by cardiac tamponade, will not give pulsus paradoxus.

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