[scientizscht (OP)]

Hello

With pacemakers, we put electrodes in contact with blood. Blood is a highly ionic solution which makes it great electricity conductor.

Why when a pacemaker fires electricity, this electric current does not electrocute the patient as it is conducted via blood to the hole body and cause damage?

[alancalverd]

The current flows through the path of least resistance, so the electrodes are planted where the current is needed to stimulate the heart muscle. Not quite the same as an external defibrillator, but it doesn't matter anyway - death from electrocution is usually caused by contraction of the heart muscle, which is what you are trying to achieve to get it moving again!

[scientizscht (OP)]

I would assume that blood has less resistance than heart tissue.

[alancalverd]

Which is why the electrodes are implanted in the muscle tissue, not floated in the bloodstream.

[scientizscht (OP)]

Still the electrodes are in contact with interstitial fluid and/or pericardial fluid which is connected to blood.

[Bored chemist]

No matter what your opinion  may be regarding what "should" happen, In fact the current goes through the heart

[alancalverd]

Still the electrodes are in contact with interstitial fluid and/or pericardial fluid which is connected to blood.

It's a good idea to study physics before embarking on medical physics or physiological measurement. Doing it the other way around is very difficult because there's a lot going on in a live body, and even a corpse is more complicated than a laboratory bench.

[scientizscht (OP)]

So if an electrode is floating in blood, will it cause problem with some level of voltage?

[evan_au]

How do pacemakers not electrocute patients?

I think you may be confusing a defibrillator with a pacemaker?

An External  Defibrillator is much loved on TV shows, with someone applying the paddles to the unconscious patient, a convulsive leap (or three), the patient starts breathing and everyone heaves a sigh of relief. In practice, they are much less successful than the TV dramas would have you believe.

They are used on patients with an abnormal heart rhythm which isn't pumping blood effectively. The massive electrical shock flows through the entire chest, including the skin, chest muscles and blood vessels.  A small fraction of the current flows through the heart, stopping it entirely; and then (sometimes) the heart will resume a normal rhythm. This shock would be extremely painful to a conscious person.

People who are prone to these abnormal rhythms can have an internal defibrillator wired directly into their heart. Because it is direct contact with the heart muscle, all the current flows through the heart, and much less power is required. These devices can be battery-powered. I understand that a design criterion for these devices was that when they detect an abnormal rhythm, they wait long enough for the patient to become unconscious before they apply the shock - otherwise the patient would go direct to their doctor and demand that it be removed!
See: https://en.wikipedia.org/wiki/Defibrillation

A pacemaker is not featured so often on TV shows because it is not nearly so dramatic.

These are devices which provide a small, carefully-timed current to the heart, to trigger it to beat at the right rate.
In severe cases, this trigger must continue 24 hours per day, while the patient is awake and asleep, but the current is so low that the patient does not notice it. In less severe cases, the pacemaker continuously monitors the natural heart rhythm, and just sends an occasional pulse if and when it is needed.

The amount of power is minimised if the leads are placed in the right place in the heart muscle. This lets the battery last longer.
See: https://en.wikipedia.org/wiki/Artificial_cardiac_pacemaker#Permanent_transvenous_pacing

Just to confuse things, some of the more expensive implantable devices can do both functions - but they try the gentler pacemaker function first.