```json
{
    "no": ["no"],
    "yes": ["yes"]
}
```

Based on the feature description of `n_p_ecg_p_12`, it represents the presence or absence of Complete RBBB (Right Bundle Branch Block) on the patient's Electrocardiogram (ECG) at the time of admission to the hospital. In this specific task, we are trying to determine if the patient with myocardial infarction complications also has chronic heart failure.

Since the feature only has two categories ('no' and 'yes'), we can conclude that the target class of patients without chronic heart failure (no) will have 'no' as the value for this feature. Similarly, the target class of patients with chronic heart failure (yes) will have 'yes' as the value for this feature. Therefore, the possible values of `n_p_ecg_p_12` for the target class 'no' will be ['no'] and for the target class 'yes' will be ['yes'].