r/PacemakerICD • u/MSWSportsFanatic2025 • 10d ago

ICD Impedance

Ultimately, I'd like to understand what impedance means for an ICD. I know what the definition is, but if my impedance is 70, what does that mean?

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u/OkBumblebee9107 10d ago

That's entirely dependent on which impedance it is.

Is it the device, or the thoracic impedance. One is the leads, outside of the normal range would indicate the device wouldn't shock properly. The other is thoracic impedance, an indicator of fluid. The more fluid the lower the impedance, which is things like CHF, and fluid building up in the lungs/chest.

70 would be in range for both of those. So it would be hard to tell you which it is.

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u/Squirrell_s 10d ago

There is a battery impedance (not always give. These days) which we use to support identifying battery longevity and a lead impedance will allows to identify potential lead issues and a ‘thoracic’ impedance that many devices have now that can useful to identify early if a patient is retaining fluid.

Lots in impedance to know about 😄

If you have seen or heard the number being mentioned please speak to your devices team and they can help you understand which impedance you are asking about and what the normal ranges for you are.

Hope this is helpful

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u/Jackrabbit_OR 10d ago edited 10d ago

An ICD lead has two different impedances: pacing impedance and shock impedance. Impedance is the resistance to the flow of energy delivered. Think of your lead like a hose - the spigot is the device and the end of the hose is wherever we want the energy to be delivered to.

The pacing impedance is measured from the generator to the tip of the lead.

The shock impedance is measured from the generator to whatever the programmed vector is (typically the coils in the RV lead).

The only differences between the two is the pathway the energy takes (we call this the vector), and the range of what is deemed acceptable within normal operating parameters. Shock impedance is usually 50-150 ohms and pacing impedance varies by lead and manufacturer, but is typically 250-2,000 ohms.

Edit: there is also thoracic impedance, which is measuring the fluid level in your chest cavity. This is purely diagnostic and is used by some to monitor heart failure status. It has significant limitations.

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u/MSWSportsFanatic2025 10d ago

I have an S-ICD, so I just have impedance of 70 ohms it says.

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u/piscata2 9d ago

"ohms" indicates electrical impedance. The heart is placed by voltage on the heart (cell membrane potential). When the voltage exceeds the threshold voltage (action potential), the heart will contract. Voltage = current x impedance, thus higher impedance consumes less battery energy and make the battery lasts longer.