Dialysis Equipment, Dialyzers, and Extracorporeal Circuit
The circuit
Blood leaves the access, passes the pre-pump arterial pressure monitor, the blood pump, a heparin infusion point, the dialyzer, the venous drip chamber with its pressure monitor, the air/foam detector, and returns to the patient. Pre-pump arterial pressure is normally negative; a very negative reading means the access is not delivering - kink, infiltration, hypotension, or a needle against the vessel wall.
The dialyzer
A hollow-fibre dialyzer runs blood inside the fibres and dialysate outside, in countercurrent flow to maximize the concentration gradient. Small-solute clearance depends on surface area, blood flow, dialysate flow, and membrane permeability (KoA); middle-molecule clearance requires high-flux membranes. Kuf, the ultrafiltration coefficient, is the mL/h of water removed per mmHg of TMP, and TMP is the pressure difference across the membrane.
Alarms as diagnostics
- High venous pressure: clot in the drip chamber, kinked line, infiltrated needle, patient lying on the line.
- Low venous pressure or blood in the venous line: disconnection - clamp and stop the pump.
- Air detector: check drip-chamber level; never bypass.
- Blood leak detector: pink dialysate signals membrane rupture.
Rule of thumb: UF rate ≈ Kuf × TMP. A dialyzer with Kuf 40 mL/h/mmHg at TMP 250 mmHg can remove about 10 L/h of ultrafiltrate if flows permit - which is why modern machines program the UF goal, not the TMP.
Traps
- Assuming blood flow is limited only by needle gauge, ignoring access and pre-pump pressure.
- Forgetting that raising blood flow raises TMP and therefore UF at a fixed setting.
- Treating pre- and post-dilution HDF as equivalent.