ELI5: If you can't defibrillate a stopped heart, how are hearts restarted after bypass surgeries?

Submitted by Tos-ka t3_yhxfik in explainlikeimfive

So, I know that defibrillators only work to restore a heart rhythm that's already present, and you can't defibrillate a stopped heart. When heart bypass surgeries or other similar surgeries are done, the heart is apparently disconnected from blood supply and completely still (machine acts as the heart) so that surgeons can work on it. If you can't restart a heart once it's stopped, how on earth does this work? Thanks

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Fred2718 t1_iug6d2x wrote

Basically, the heart is stopped by pushing a slug of cold saline into the coronary arteries, and restarted by flushing that out with warm oxygenated blood.

Source: I survived this process

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[deleted] t1_iug6kmt wrote

[deleted]

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Fred2718 t1_iug6x3t wrote

Nah, I survived because my surgeon, a tough Taiwanese lady, threatened me with dire consequences if I messed up her win streak.

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Sualocin t1_iugcb9p wrote

That's oddly comforting, she's not going to screw up so it's up to you to survive or not.

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breckenridgeback t1_iug9es1 wrote

> pushing a slug of cold saline into the coronary arteries

Is it weird that I kind of want to know what that would feel like?

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Fred2718 t1_iug9n5k wrote

Can't help you there, bro. I was kind of out of it At the time.

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frakc t1_iuh6kpr wrote

I assume nowone ever can answer that question, because without anesthesia, they will be too occupied with their chest rip open.

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Notorious_Rug t1_iugbrma wrote

It actually usually restarts on it's own. Sometimes a mild electric shock is used to restore normal rhythm, or the surgeon may massage the heart to get it going.

Defibrillation uses an electric shock to convert abnormal heart rhythms, called fibrillations, into a normal heartbeat (called a sinus rhythm). Defibrillation causes much of the heart muscle to depolarize (a change in electric charge within a cell). This depolarization causes the sinoatrial node (your heart's natural pacemaker) to reestablish sinus (normal) rhythm.

With cardiopulmonary bypass, you're actually weaned off the machine. Your body is re-warmed (they cool you down, which actually reduces risks of brain damage and other issues that arise from being put on the bypass), and your arterial blood gas will be measured. Arterial blood gas is a measurment of how much oxygen is in your bloodstream. It is measured via a blood sample from the arteries. Then, the anesthesiologist turns back on silenced alarms (they don't want to hear the flatline (no heartbeat) alarm throughout the surgery, so the monitor is put on "silence"), adjusts monitor and ventilator settings, and adjust oxygen flow rate, as necessary.

After the aorta is de-clamped, the heart usually starts to beat without help, but sometimes drugs (called inotropes- they are drugs that "tell" your heart muscles to contract; some inotropic drugs "tell" heart muscles to contract with more force; other inotropes "tell" heart muscles to contract less forcefully) are needed.

Cardiac (the medical term for anything to do with the heart) massage may also be needed to get the heart restarted. The medical personnel will use their hand and directly massage the heart itself. It's basically direct compressions, like in CPR, but instead of using the force from your arms and hands over the sternum (breastbone) to "massage" the heart back to beating, it's directly to the heart muscle itself.

It does sometimes beat irregularly at first, and an electric shock directly applied to the heart muscle may be needed to restore sinus rhythm, much like in automatic external defibrillation. Sometimes, the heart will fail to maintain a normal rhythm, and an internal pacemaker (a device that shocks the heart into sinus rhythm) may be implanted.

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Fluffy-Jackfruit-930 t1_iuhae2o wrote

The heart will automatically try and start, provided that it has oxygen, energy, correct pH and the correct electrolytes.

In surgery, the heart has its blood supply replaced with ice cold potassium chloride solution. The potassium is completely the wrong electrolyte, and the temperature is low, so the heart stops.

To restart it, the blood supply is reconnected and once warm, fresh blood with oxygen and the correct electrolytes reaches the heart it will restart. It may restart erratically on ventricular fibrillation, so it may need a shock to get back into normal rhythm.

The reason during resuscitation you can't usually restart a stopped heart is because you have to reverse the cause of the heart stopping first. Often it is severe illness, with major biochemical abnormalities or multiple organ failure, which can't easily be reversed.

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