Hi,
one small worst-case example: if you would directly connect the battery with the patient and you would measure the patient leakage current (here: a possible patient auxiliary current), i.e. plugging the MD between the poles of the battery, this would result in a leakage current of 5 V / 1000 Ohms = 5 mA (DC) in normal condition. This value is much higher than any allowed value in respect to a patient.
Now in respect to your device it depends on how the patient is being connected to the applied part (one, two or even more connections to the patient). You have to think how current can flow with your device and a connected patient. Consider also if there is any return path by possible earth connections of device and patient.
At the end of the day you have different possiblities to reduce the leakage current: you can either use an insulation barrier or use protective impedances to protect the attached patient from the "evil" battery.