Post cardiac surgery it is not required to use aPTT monitoring for heparin infusion as it is used as a prophylaxis if we have stent or weak pulsation in lower limbs. It is used then for short term and monitored clinically until regain pulsation.

We use aPTT when it is used for proved thrombus or DVT then we control the dose according to nonogram in Dr.Hossam’s answer

If you want any more clarification donot hesitate to ask.

Thanks