Dialysis Procedures and Equipment
The extracorporeal circuit
Blood leaves the access through the arterial needle, reaches the arterial pressure monitor (which reads the negative pressure created by the blood pump), passes through the blood pump, enters the dialyzer, then moves past the venous pressure monitor, through the air detector and clamp, and returns via the venous needle. Dialysate flows countercurrent to blood-opposite direction-to maximize the concentration gradient. Ultrafiltration is the volume of plasma water removed as prescribed; on volumetric machines the UF goal, not transmembrane pressure alone, determines removal, although TMP is the pressure that drives it.
Alarms and checks
Separate alarms by location. A pre-pump arterial alarm often signals an access problem (infiltration, stenosis, needle against the vessel wall, kinked line). A high venous pressure alarm suggests a clot, a kink, or a poorly positioned venous needle. The air detector protects against air embolism and will clamp the line and stop the pump; never bypass it. Blood leak detectors on the dialysate side guard against membrane rupture. A conductivity alarm means the proportioning system is producing dialysate outside the safe range-do not reconnect the patient until it is corrected and verified.
Traps and a rule of thumb
Trap questions mix up TMP with the UF goal, or ask which alarm a kinked arterial line triggers. Another common trap is "troubleshooting" by silencing an alarm without assessing the patient. Rule of thumb: for any alarm, look at the patient first, then the circuit, then the machine-and never return blood after an air detector alarm until the cause has been found and corrected.