Q: What is the hazard of using sub-optimal dose of Atropine in Bradycardia?

Answer: 

Minimum dose of Atropine in Bradycardia is 0.5 mg (0.4mg to be precise). Doses less than 0.5 mg (0.2 mg in peds) may further decrease the rate. Though this pradoxical bradycardia has been questioned in some literature papers.

Various explanations have been proposed for that including stimulation of the vagus nerve causing bradycardia at low doses, as Atropine may crosses blood brain barrier. Another explanation given is at low doses, atropine may have affinity for presynaptic autoreceptors thus blocking the negative feedback loop of acetycholine and increasing presynaptic output of acetylcholine into the synaptic junction. This will activate m2 receptors on the heart and lead to bradycardia.  At higher doses the muscarinic blocking effects of Atropine causes tachycardia.