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

Since the feature n_p_ecg_p_04 represents the presence or absence of Type 1 Second-degree AV block on the ECG at the time of admission to the hospital, there are only two possible values: "no" and "yes". 

In the given task, we are trying to determine if the patient has chronic heart failure based on the myocardial infarction complications. However, the feature n_p_ecg_p_04 itself does not directly indicate whether the patient has chronic heart failure or not. It only provides information about the presence or absence of Type 1 Second-degree AV block on the ECG.

Therefore, both possible values of the n_p_ecg_p_04 feature ("no" and "yes") are relevant to the task, as they could be present in patients with or without chronic heart failure.