Thread by @lindaschulteMD

Ok let’s keep going with #cardiopulmonarybypass. Several requests for circulatory arrest, but first let’s understand how to go on and off bypass and a few flavors of cannulation and cardioplegia

🧵primer pt 2: going on and off bypass

#CardioTwitter #surged #meded #CTsurgery

Again, don’t get overwhelmed by the surgeon shoving a large number of tubes into the heart. Think logically what each one does and why it is needed.

If you know your stuff and you can’t come up with an answer, then maybe you’ve identified an inefficiency in their process. 🙃🙃

But maybe wait until you’re a PGY-10 to tell them 🙃

Let’s start with patient vitals pre-bypass.

🫀The patient is in his own rhythm. 🫀Arterial line (red) is pulsatile as the heart is ejecting to the body.
🫀🦢(yellow) is pulsatile: blood is moving from the right heart to the left (and this pt has some pHTN)
🫀SpO2 is pulsatile

Once heparin is given, ACT>450, cannulas are in, the perfusionist slowly takes the patient to “full flow” (~CI 2.2) - the heart is emptied through the venous cannula and non-pulsatile flow is delivered to the body through the arterial cannula.

At this point, the heart is not ejecting because it has been emptied into the venous reservoir, so the arterial waveform is flat. However, no change has been made to the rhythm, ie cardioplegia has not been delivered.

What is the purpose of being on bypass with the heart beating?

It allows us to move the heart around since the blood pressure does not depend on the heart ejecting!

This is when any remaining dissection can be done for redos, LV vent can be placed (coming up later), etc

DON’T try this at home but… you might see the surgeon sometimes *try* to pick up the heart without being on bypass.

The anesthesiologist will immediately yell HEY!

A contorted heart = can’t eject = no BP = unhappy anesthesiologist

That’s when I say sorrsorsorsorsoruurusurryyyyyyyy

Some surgeries can be done entirely in this state: “on pump beating”

🫀CABG with an unclampable aorta (below) (not the same as “off pump CABG”)
🫀right sided surgery: tricuspid repair/replacement

But for most operations (CABG, aortic valve and mitral valve surgery) we need the heart to be stopped and empty. This is done by applying a cross clamp and delivering cold cardioplegia to the coronaries.

During “clamp”, we look for the “good arrest”- meaning the heart stops after several hundred mL of cardioplegia delivery. We also monitor the temperature of the heart muscle and keep it as cold as possible.

This is the only place in the hospital where you’ll see these vitals:

…without screaming.

During clamp we also cool the patient slightly to decrease metabolic demand.

Notice that the sat seems low but it’s not pulsatile flow so poor pleth waveform. However, we know that the well saturated blood is being circulated.

When most of the work is done we ask perfusion to start rewarming. This lets the room know the clamp is almost ready to come off.

Once the clamp is off warm blood flows into the coronaries and the heart restarts, but is still fully supported by bypass.

At this point the heart may have rhythm but is not yet ejecting. Once we are comfortable that no bleeding needs to be fixed (while the heart is empty), we ask perfusion to start “leaving some in”.

As blood crosses from the right heart to the left, bumps come back:

If volume is flowing through the heart, make sure lungs are on! Otherwise you are creating an obligate shunt.

Once the patient is warm, anesthesia has their proper medications in line, lungs are on, and the heart has rhythm (can be from temporary pacing wires)…

You’re ready to start weaning from bypass

🫀the surgeon monitors the the heart, making sure the RV doesn’t balloon out
🫀anesthesia is monitoring TEE, looking at LV volume/function, titrating pressors/inotropes
🫀perfusion comes down on flow while leaving volume in the heart

Occasionally the heart does not tolerate the wean: either the LV or the RV or both just needs more help. This could mean increasing inotropes, adding pulmonary vasodilators, going back to rest on bypass, or occasionally leaving the OR on VA ECMO.

But most of the time the wean is successful, we remove the cannulas from the heart, repair the holes with suture, and reverse the heparin with protamine.

Next time in pt 3 we’ll cover why there are so many cannulas to achieve the three basic goals from pt 1.