Hemodynamics of Positive Pressure Ventilation

All right, now let's talk about the hemodynamics of positive pressure ventilation.

So you just spent the last 20 minutes adjusting and meticulously optimizing this patient's ventilator.

They were hypoxemic.

They were hypercarbic.

Their lung mechanics were bad.

And you have done an amazing job.

And you're now patting yourself on the back because the oxygenation looks great.

The ABG looks great.

You're walking out of the room when the nurse is like...

Hang on a second.

Not so fast.

You seem to have failed to notice that your blood pressure is now 70 over 40.

A lot of the time in vent management lectures, understandably, we like to talk about vent management.

But it turns out that the lungs do not exist in a vacuum.

And we, when we're talking about vent management, at some point cannot ignore the fact that positive pressure ventilation has very significant hemodynamic effects.

Because when you transition a patient to positive pressure ventilation, that's a huge physiologic shift.

Now, the hemodynamic consequences of that don't really matter that much in, I don't know, say your otherwise healthy patient who gets intubated for their knee replacement.

But if you have a critically ill patient and their hemodynamics are operating right on the edge anyways, this can become quite significant and is something that you need to take into account when you're thinking about vent management and management of BiPAP.

Now, the principle here is what increasing the intrathoracic pressure does in terms of altering normal physiologic pressure gradients.

So it's intuitive to us that waterfalls flow down, not up, right? That's obvious.

If you walked by a waterfall and it was flowing up, not down, you'd be like, that seems really abnormal.

But waterfalls, of course, flow down, not up.

And fundamentally, the direction of flow is determined by a gradient.

And our circulatory system works in the same way.

And a major contributor to the way that hemodynamic physiology functions is the magnitude of various pressure gradients around the circulatory system.