Applying external heat—such as heating pads, electric blankets, or even sitting in a hot tub—increases blood flow to the skin and accelerates the absorption of lidocaine into your bloodstream. This can lead to:

The medical consensus and official FDA labeling for Lidoderm (5% lidocaine patch) strongly advise using a heating pad directly over the patch . Why Heat is Dangerous with Lidoderm

Because the patch numbs the skin, you may not feel if a heating pad is becoming hot enough to cause severe burns.

If you have used heat and feel dizzy, lightheaded, or have ringing in your ears, remove the patch immediately and seek medical help. Usage Tips & Alternatives How To Use Lidocaine - BuzzRx

Absorbing too much lidocaine can cause life-threatening side effects, including seizures, cardiac arrhythmias, and respiratory arrest.