Intermittent Hemodialysis (IHD)

This is the gold standard. Patients who are hemodynamically stable are suitable for this mode. The set up is a double lumen catheter, a pump which forces blood into a filter (semi permeable membrane) a dialysate (usually deionized water) which flows in and out, and a return line to the patient. The blood flow rate is 200-400ml/minute, the dialysate flow is approx 500ml/minute, the filtration rate is between 300 and 2000ml/hour, with urea clearance of 150-250 ml/min. With this high flow and clearance rate patients, depending on the extent of their catabolism, only require 3-4 hours of dialysis, two or three times a week. If you consider that the kidney works 24 hours a day 7 days a week, there must be a trade off: there is. Patients switch from being metabolically “clean” immediately post dialysis, to being uremic before the next session. There are huge swings in fluid between the intravascular and extravascular compartments, causing transient hypotension and disequilibrium. Many ICU patients cannot tolerate this.

Typically patients undergo dialysis for 3 to 4 hours daily or on alternate days depending on their catabolic state, although there is some evidence that daily dialysis may improve outcomes. Vascular access for short-term hemodialysis or hemofiltration is usually achieved using a double-lumen catheter inserted into the internal jugular.

Anticoagulation with heparin is the standard method for preventing thrombosis of the extracorporeal circuit during acute intermittent dialysis

The major complications of acute intermittent hemodialysis relate to rapid shifts in plasma volume and solute composition, vascular access, the necessity for anticoagulation and dialysis membrane incompatibility.

What is important about the dialysis membrane?

The membrane is the surface through which dialysis or ultrafiltration occurs: it is the core component of the hemofilter. Different membranes are used in renal replacement therapy: they may be cellulose based or synthetic. The cellulose membranes are "low-flux" - they are very thin, have a low permeability co-efficient and are strongly hydrophilic: they are known to activate the inflammatory cascades, particularly complement, and are thus unsuitable (bioincompatible) in critical illness. Synthetic membranes should be used in this setting for both intermittent and continuous hemodialysis. These membranes tend to be slightly thicker than cellulose ones, and have very high sieving coefficients at a wide range of molecular weights: these properties make synthetic membranes, particularly effective at convective clearance. Thus regardless of the technique involved, renal replacement therapy with synthetic filters will always include significant ultrafiltration.

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