Shivering is defined as an involuntary , spontaneous oscillatory mechanical activity of skeletal muscle.It is a fasciculation of the face,  jaw , head and muscle hyperactivity lasting longer than 15 seconds. It is a frequent complication following anaesthesia with an incidence of between 40 to 60% and 56.7% for general anaesthesia and neuraxial anaesthesia respectively. Shivering can cause due to multiple aetiology, it may be due to

1.physiological regulatory response to central hypothermia.

2. cytokine release due to surgical stress.

3. Uninhibited spinal reflexes, post-operative pain.

4. Hyper active sympathetic activities.

It is worsened by vasodilatation from spinal anaesthesia that redistribute core body heat. Ondansetron is the first 5HT3 antagonist most widely used drug for prophylaxis of shivering due to its lower cost. It act both centrally and peripheral. It’s time of onset of action is 30 minutes and its elimination half life is 3 to 4 hours with pre-dominantly hepatic  metabolism(2). It’s mechanism of action as an anti shivering is not clear but purposed to act centrally by inhibiting serotonin reuptake at the level of pre optic anterior hypothalamus.It’s most common minor adverse effects are headache and diarrhoea.

Palonosetron is a second- generation  5HT3 receptor  antagonist  with a prolonged duration of action and higher receptor binding affinity than first generation agents (ondansetron). The mean terminal elimination half-life, following single iv dose, is approximately 40 hours. Palonosetron is eliminated from body through both renal excretion and metabolic pathways (CYP2D6). widely use to prevent postoperative and pregnancy induce nausea, vomiting. It is now being used as an post spinal shivering prevention and management as well. Its anti-shivering effect may be related to serotonin reuptake inhibition in pre-optic anterior hypothalamic region. Its common side effects are headache and constipation, rare effects include bradycardia and QTc prolongation <1% 

Since, chemically, ondansetron and palonosetron belong to the same class of drugs, both are expected to exert similar effects. However, most studies have reported ondansetron administration to be efficacious for prophylaxis of post spinal shivering, but studies on palonosetron have yielded conflicting results. Thus, this study will be carried out to compare the efficacy of ondansetron and palonosetron of post spinal shivering in lower limb surgeries.