Why would a dr want to do a laying, sitting and then standing bp after an keg

Orthostatic..: If you experienced lightheadedness or dizziness... you could have low BP due to dehydration or other factors. BP responds to change of position for sitting, standing, lying down by " how fast " blood flow reaches your brain. major difference in these BP taken as we call " orthostatic" can help to establish diagnosis. Make sure to be always well hydrated, avoid much of stimulants and consult doc.