Welcome back to the tasty morsels of critical care podcast.

Today we’re covering the ambitious topic of CRRT in the ICU. Something that occupies a central part of the daily job, but also occupies Oh Chapter 48, Irwin and Rippe chapter 201 and a few other review papers thrown in for good measure. We’re only going to get so far as the modes today so let’s not get too carried away.

The obvious initial distinction in RRT modes is between IHD and CRRT with IHD being intermittent as the name suggests and CRRT being continuous. These are obvious temporal discriminators to do with how long the machine is attached to the patient but under the hood there are more fundamental differences between how the two modes work.

In broad terms we can compare dialysis (the movement of small molecules across a membrane along an osmotic gradient) with ultrafiltration (the squeezing of plasma through a big sieve that retains the big bits of the plasma and lets the other bits leak out). The best analogy I’ve seen for this comes from one of my colleagues in his yearly introduction talk to RRT. Dialysis is the tea bag as ultrafiltration is to the espresso machine.

Alas such simply categorisations fall by the way side when we encounter the actual workings of one of the big green machines in the unit as it often presents several modes to us. We can run a continuous heamofiltration, a continuous haemodialysis or a combo mode of continuous haemodialfiltration. These rejoice in the acronyms CVVH, CVVHD and CVVHDF respectively.

Lets start with CVVH, continous venoveno heamofiltration. In this set up blood is drained from the venous side and entered a circuit initially under negative pressure then post pump becomes positive pressure. This positive pressure is used to force blood through a haemofilter containing many hollow fibre microtubules making up around a metre squared squeezed into that tiny plastic cylinder. Hydrostatic pressure drives the water into the filter compartment from the blood compartment with “solute drag” bringing along small and middle molecules with it. The principle here is convection with both the transmembrane pressure and the semipermeable barrier characteristics both contributing to how much filtrate is generated. The filtrate produced looks just like urine and collects in a big bag at the bottom of the machine. The yellow stuff contains things we want out of the body like potassium and urea. It is very easy to remove water from the body in this manner and in general if left to filter without replacement fluid then your patient will become very negative very quickly, hence the large 5L bags of replacement crystalloid fluid that run simultaneously as the yellow stuff is being produced.  At its simplest the yellow ultrafiltrate has all the same concentrations in it as the plasma minus the large molecules like albumin.

In pure CVVHD (continuous venovenous haemodialysis) the patient’s blood is on one side of a membrane with a dialysate fluid running in the opposite direction on the other side of the membrane. In this scenario solutes (such as Na and K and urea) leave the blood compartment to the dialysate compartment down a concentration gradient. In this scenario the water follows the solute which is in distinction to haemofiltration. When running CVVHD the dialysate flow rates are usually very modest at maybe 30ml/min in distinction to IHD dialysate flow rates of 500-800ml/min

CVVHDF is a combination of the two with a little bit from column A and little bit from column B so to speak, with plasma being squeezed through the haemofilter and a modest counter current dialysate flow happening at the same time. The yellow stuff produced in this mode is a combination of ultrafiltrate and the spent dialysate that has passed through the filter.