Hemodialysis Therapy and Vascular Access
Core concepts
Hemodialysis removes solute by diffusion down a concentration gradient and water by ultrafiltration. Dialysate is formulated to approximate plasma so that only wastes move: potassium is deliberately low (typically 1–4 mEq/L), calcium 2.5–3.5 mEq/L, bicarbonate 32–39 mEq/L. Sodium modeling reduces intradialytic hypotension and cramping.
Adequacy is measured, not estimated: single-pool Kt/V ≥1.2 per treatment three times weekly, or urea reduction ratio ≥65%. The post-dialysis BUN must be drawn correctly - reduce blood flow to about 100 mL/min for roughly 15 seconds before sampling - or clearance is overestimated.
Access follows "fistula first": AV fistula, then AV graft, then central venous catheter. A mature fistula meets the rule of 6s - flow ≥600 mL/min, diameter ≥0.6 cm, depth ≤0.6 cm - and is usually usable by about 6 weeks. Assess by palpating a thrill and auscultating a bruit every treatment, and teach the patient to check daily. Never take a blood pressure, draw blood, or start an IV in the access arm.
Trap: a clotted access or sluggish catheter is not something to fix by forceful irrigation. Never force saline into an access; report it.
Worked example: hypotension at hour two with a 3 L ultrafiltration goal. Stop or reduce ultrafiltration first, place the patient supine or in Trendelenburg, give a 100–250 mL normal saline bolus, then reassess. Do not redefine the dry weight from a single episode - review the trend with the team.