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

Based on the feature description, the possible values for the feature n_r_ecg_p_06 are 'no' and 'yes'. For the target class of chronic heart failure, the feature value of 'no' indicates that there is no persistent form of atrial fibrillation on ECG at the time of admission to hospital, while the feature value of 'yes' indicates that there is a persistent form of atrial fibrillation on ECG at the time of admission to hospital. So, the only possible values for this feature in relation to the task are 'no' and 'yes'.