Hemodialysis

ProcedureTop

Hemodialysis (HD) is an extracorporeal blood purification technique that relies on 2 physicochemical laws:

1) Differences in concentrations (Fick laws of diffusion): Waste products (uremic toxins) are removed from the body through the blood owing to the difference in concentrations between blood and the dialysate. The solutes diffuse out from circulation to the dialysate across the dialyzer’s semipermeable membrane.

2) Differences in pressures: The difference in pressures enables removal of excess water through the dialyzer’s membrane by ultrafiltration. This phenomenon plays a bigger role in other, related techniques, such as hemofiltration (HF) and hemodiafiltration (HDF).

Types of Hemodialysis and Related Techniques

1. HD:

1) Low-efficiency low-flux HD.

2) High-efficiency high-flux HD.

3) Single-needle HD.

4) Sequential HD.

5) HD with sodium profiling.

6) HD with biofeedback system.

7) Sustained low-efficiency dialysis (SLED).

2. HD with variations in schedules:

1) Short daily hemodialysis (SDHD).

2) Intermittent hemodialysis (IHD) (eg, thrice weekly).

3) Nocturnal HD, can be daily or intermittent.

3. Related high-efficiency techniques:

1) HF.

2) HDF.

3) High-cutoff hemodialysis (HCO-HD).

4) Expanded hemodialysis (HDx).

4. Continuous renal replacement therapy (CRRT)/slow continuous therapies:

1) Continuous venovenous hemodialysis (CVVHD).

2) Continuous venovenous hemofiltration (CVVH).

3) Continuous venovenous hemodiafiltration (CVVHDF).

4) Slow continuous ultrafiltration (SCUF).

Technique

The HD system consists of an HD machine ("artificial kidney”), tube system, a dialyzer, and dialysate. The HD machine is used to produce the final dialysate formula, move blood and dialysate through the dialyzer in a countercurrent manner, and monitor the procedure by measuring a number of parameters. High-purity (ultrapure) water is an essential element of the dialysate composition. It is obtained with the use of a filtering system and reverse osmosis.

A typical HD session lasts 3 to 4 hours and is usually performed thrice weekly, not on consecutive days. It requires anticoagulation to prevent extracorporeal clotting. Anticoagulation is usually achieved using heparin (unfractionated or low-molecular-weight) or, if contraindicated, introduced in extracorporeal circulation (eg, predialyzer citrate infusion to lower ionized calcium levels). If regional anticoagulation is contraindicated, normal saline flushes can be used.

HD is the most common extracorporeal modality used for intermittent therapy, but there is emerging evidence showing that high-dose HDF may provide survival advantage over HD.Evidence 1Moderate Quality of Evidence (moderate confidence that we know true effects of the intervention). Quality of Evidence lowered due to heterogeneity and indirectness. Vernooij RWM, Hockham C, Strippoli G, et al. Haemodiafiltration versus haemodialysis for kidney failure: an individual patient data meta-analysis of randomised controlled trials. Lancet. 2024;404(10464):1742-1749.doi:10.1016/S0140-6736(24)01859-2.

CRRT, also referred to as slow continuous therapy, is used for the treatment of critically ill patients with renal failure. It is very resource intensive. Potential advantages include less hemodynamic instability, better control of electrolyte and acid-base balance, and effective fluid removal. Despite the potential advantages, randomized controlled trials comparing the use of CRRT to IHD in AKI showed no survival advantage.

Most commonly used CRRT techniques:

1) CVVHD: Dialysate is passed continuously and at a slow rate, countercurrent to blood flow. Diffusion is the primary method of solute removal. The ultrafiltered fluid volume across the dialyzer membrane is low (3-6 L/d).

2) CVVH: No dialysate is used. A large volume (25-50 L/d) of replacement fluid is infused into the inflow (predilution) or outflow (postdilution) blood line. The ultrafiltered volume of fluid across the dialyzer membrane includes both replacement fluid and excess fluid.

3) CVVHDF: This is a combination of CVVHD and CVVHDF, dialysate is used along with replacement fluid. The daily volume of fluid ultrafiltered across the dialyzer membrane is high but not as high as with CVVHF.

Dialysis Access

HD requires an efficient vascular access that enables a dialyzer blood flow rate (BFR) of 250 to 500 mL/min.

1. Autologous (native) arteriovenous (AV) fistula is the best mode of vascular access for maintenance HD. It is an anastomosis (surgical connection) between an artery and a vein in the forearm, upper arm or, less frequently, in a different location (eg, thigh). Fistulas require time to mature, usually several (>4) weeks to months. Once mature, needles (arterial and venous) are used to cannulate the vein at the start of each HD session. Between dialysis sessions, a scab forms at the cannulation sites.

2. Prosthetic AV grafts are created by interposing a graft between an artery and a vein. The graft must enable multiple needle punctures. Prosthetic fistulas are created when the patient’s vessels do not allow for the development of an efficient autogenous fistula, usually due to atherosclerosis or thromboinflammatory lesions.

3. Tunneled permanent hemodialysis catheter is a dual-lumen catheter with a Dacron cuff. It is tunneled under the skin through the internal jugular vein, with its tip usually in the superior vena cava. This type of vascular access is being increasingly used in patients in whom an AV fistula is not feasible due to urgency of dialysis initiation or vascular factors. Other tunneled locations may include femoral, translumbar, or transhepatic, and are used when internal jugular access is not feasible due to anatomy, stenosis, or thrombosis.

4. Nontunneled temporary hemodialysis catheter (NTHC) is a dual-lumen catheter inserted via the internal jugular vein, subclavian vein, or femoral vein in patients with acute kidney injury (AKI) or poisoning or in those with chronic kidney disease (CKD) if urgent initiation of renal replacement therapy (RRT) is needed.

Dialysate

Dialysate is typically produced by the HD machine through mixing dialysate concentrate and duly processed (purified) water. This allows tailoring the dialysate composition to patient needs, particularly in terms of potassium, calcium, and glucose concentration. The use of sodium bicarbonate (bicarbonate dialysis) as an alkalizer allows for effective correction of metabolic acidosis and has a beneficial effect on cardiovascular system stability during the HD session. Blood and dialysate flow through the dialyzer in opposite directions to increase the efficiency of diffusion (countercurrent multiplication). The currently used HD machines offer settings that ensure dialysate composition adjustment (profiling) during the procedure.

Assessment of Adequacy of Hemodialysis

1. Clinical criteria:

1) Absence of uremic symptoms.

2) Euvolemia.

2. Adequate dialysis dose, expressed as:

1) Kt/V in a single session ≥1.2; where K stands for urea clearance (mL/min or L/h); t, length of the dialysis treatment (min or h); V, urea distribution volume (mL or L).

2) Urea reduction ratio (URR) >65%.

ContraindicationsTop

Age, the underlying disease, and comorbidities are not contraindications to HD provided that the HD-associated suffering does not prevail over benefits. Currently 2 clinical situations are generally considered contraindications to the initiation of RRT:

1) Disseminated cancer (metastases) with limited expected survival (weeks to months).

2) Severe dementia, usually with preexisting atherosclerosis.

ComplicationsTop

1. Frequent complications:

1) Hypotension (20%-30%) during or immediately after the HD session. There are many reasons for hypotension, with the most frequent ones being excessive or too rapid decrease in intravascular volume, absence of appropriate vasoconstriction, or absence of increased cardiac output in response to hypovolemia. Several factors coexist in the majority of cases.

2) Muscle cramps (20%) caused by rapid decrease in intra- and extravascular volume; may accompany hypotension.

3) Headache (frequently): The cause is unclear; in some cases it may be a sign of mild disequilibrium syndrome (see below).

4) Pruritus (75%) may appear or exacerbate during HD sessions. The etiology is likely multifactorial, with calcium and phosphate metabolism disturbances among the main causes.

2. Rare complications:

1) Disequilibrium syndrome: A group of manifestations (nausea, vomiting, headache, seizures and coma in severe cases) that appear during or immediately after initial HD sessions. Disequilibrium syndrome is caused by sudden changes in the uremic environment in patients with severe uremia, leading to cerebral edema.

2) First-use syndrome: A group of symptoms that appear during an HD session with a new dialyzer (on its first use). Type A reaction (anaphylaxis) is a severe and potentially fatal anaphylactic reaction that develops within the first few minutes of dialysis performed on a new dialyzer sterilized with ethylene oxide during manufacture. The majority of patients with type A reaction had IgE antibodies against ethylene oxide–modified proteins detected in blood.

3) Hemolysis can be caused by improper physical parameters (too high temperature) or chemical parameters (hypo-osmolarity), dialysate contamination with chemical compounds, and/or mechanical damage to red blood cells.

4) Air embolism is caused by noncompliance with procedures when starting, conducting, or ending HD sessions or by the use of a nonoperational HD machine.

OutcomesTop

HD may prolong survival and quality of life in many but not all patients (eg, those at an advanced age with poor baseline functional status and comorbidities). The number of patients enrolled in the maintenance HD program for 10+ years is growing.

Of note, as a method of RRT, HD helps correct basic homeostasis disturbances but does not fully eliminate the sequelae of renal failure. For this reason, each patient treated with HD should be assessed for their candidacy as a kidney transplant recipient.