Wednesday, March 16, 2016

Mechanism of pulsus paradoxus in pericardial diseases like constrictive pericarditis and pericardial tamponade

Okay, so let's begin!

Normally, during inspiration, a decrease in intrathoracic pressure is transmitted to the right heart, augmenting venous return.

What happens to the left heart?

During inspiration, expansion of the lungs and pulmonary tissues causes pulmonary blood volume to increase, which transiently decreases the flow of blood from the lungs to the left atrium and therefore, left ventricle.

Understood this? Okay, cool.

Right ventricular volume increases, pushing the interventricular septum towards the left ventricle.

This leads to an increase in left ventricular filling pressure with an unchanged or lower left ventricular end diastolic volume. (Left ventricular filling pressure? Yeah, the pressure in the ventricle as it fills with blood.)

This is a diastolic interventricular interaction which is always present.

Okay, great.

Now it get's complex-y because we are going to talk about pericardial disease like constrictive pericarditis and pericardial tamponade.

Intrathoracic pressure changes are transmitted, normally, both to the pericardium and the cardiac chambers.

There's a gradient between the pulmonary veins and the left ventricle (The effective filling gradient) that allows filling of blood from the pulmonary veins into the left ventricle. This remains constant during inspiration and expiration because pericardial and pleural pressure follow the intrathoracic pressures.

Understood so far? Now, let's see what happens in constrictive pericarditis:

Anatomy fact: The pulmonary veins and a part of the left atrium are NOT covered by the pericardium.

Intrathoracic pressures are transmitted to the pulmonary veins and the portion of the left atrium not encased by the pericardium. But the pressure of the rigid pericardial bound left ventricle doesn't vary with the intrathoracic pressure.

So, the effective filling gradient is decreased. The left ventricular volume will be reduced even more because of this.

The right ventricular diastolic pressure and volume increases like it usually does in inspiration.
This causes the interventricular septum to shift to the left even more than it used to in normal inspiration.

So basically, the inspiratory decrease in left ventricular systolic pressure results in increase in right ventricular systolic pressure.

This is known as ventricular interdependence or discordance during the respirophasic cycle. It also occurs in preicardial tamponade where fluid is present in the pericardium.

It is also why pulsus paradoxus occurs. The decreased left side effective filling gradient and the decreased left ventricular stroke volume causes the greater than normal (>10 mm Hg) decrease in aortic blood pressure when inspiration occurs.

Hope that helped clear things a little!

-IkaN

2 comments:

  1. This explanation has been awesome & has cleared up so much things for me.

    ReplyDelete

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