Why do some people pass out while on a dialysis machine?

Low Blood Pressure. Unfortunately, dialysis is an exchange of blood between the patient and the hemodialysis machine. At certain times, too much blood can be taken from a patient that leads to an acute drop in blood pressure. This decrease in blood pressure likely leads to some people passing out while on the hemo-dilaysis machine.
Low blood pressure. The most common cause is probably low blood pressure during treatment. Another cause could be low blood sugar, especially in diabetics.