Deductible medical expenses are expenses that are incurred in order to cure, prevent, or treat diseases and conditions that affect the health of the body. If you are buying an AED for your personal use, because you or a family member has been diagnosed with heart trouble, and your doctor has recommended one, then it would be an allowable medical expense. Any expense that is deductible is also allowable for HSA reimbursement.
However, if you were buying one for your place of business as a general precaution for your employees or customers, then it would be a deductible business expense but it is not a deductible medical expense and cannot be paid for from an HSA.