Dr Jonathan Round, PICU Consultant

You may be reading this out of interest or more likely because you’re about to look after a patient on CVVH. This pair of articles (click here for part 2) will first go over what it is, then how to use it to manage a patient on CVVH. In Part 2, we will look at why you might want to use CVVH and how to get it going.

If this seems a little backwards that’s because this article is written as “just-in-time” piece for those situations where one suddenly needs to have a small degree of understanding and ability. This is not intended to be exhaustive account of CVVH. It’s also nothing to do with dialysis, which is a related but quite distinct topic.

How it works

So, Continuous Veno-Venous Haemofiltration: It is what it says on the tin. The aim is to continuously take blood from a great vein into a CVVH  machine, adjust the blood chemistry using filtration (and other tricks), and then put it back in to that same great vein.

At its heart there is a membrane that acts as a one-way sieve. All of the paraphernalia of the circuit and the machine simply makes the membrane function properly and adjusts its function. So let’s look at the membrane in detail.

The CVVH machine has a cylinder around 10 to 20 cm long and around three or 4 cm in diameter (figs 1 & 2), which I will call “the filter”.

Fig 1 – the filter on the front of a CVVH machine
Fig 2: diagram of filter structure.

Blood is pumped into this and comes out the other end. This cylinder has 2 other ports. One port is rarely used in CVVH (called “dialysate in”, fig 2) and the other is where the filtrate comes out of the filter – it’s usually a light yellow colour because of all the urea in it.

This cylinder (fig. 2) is actually a collection of small tubes made out up of the membrane through which the blood passes. The total area of the membrane can range from under 50 cm2 to over 1 m2. A typical membrane has holes around 2 nM in diameter. So red cells, white cells and platelets cannot pass through, nor can blood proteins, even the smallest: albumin. However all of the electrolytes, water, glucose and even some complex molecules such as cytokines can cross this filtration membrane.

Fluid crosses this membrane because the pressure is higher on one side than the other. With the fluid also passes any electrolytes or molecules that can fit through the holes in the membrane. This is termed ‘convective clearance’, where the solvent crosses the down a pressure gradient and ‘solvent drag’ carries electrolytes, urea and glucose at the same time. Everything goes in the same direction, following the pressure gradient.

This article is not about dialysis, but this is where dialysis, superficially a similar process, differs from filtration. In dialysis, water molecules and electrolytes go in whichever direction the concentration gradient favours, with diffusion as the process. There are other differences, such as the smaller size of the holes in the membrane, but as I said, this article is not about dialysis.

The pressure gradient across the membrane is created by the blood pump that pushes blood into the membrane, and the filtrate pump that sucks blood away from the membrane. This we get to when we talk about the circuit.

The reader may by now have noted have noted a fairly big problem with the process as described above. Essentially the filter itself does not correct blood electrolytes, all it does is remove a proportion of the aqueous part of the blood. To be clear, table 1 shows what will happen as blood passes across the membrane, if 10% of the blood volume is removed (a typical scenario). This refers to blood with a haematocrit of around 0.46 – where 54% of blood volume is plasma, and of which only 90% of the plasma is the non-protein aqueous phase.

Table 1 – changes at the filter

This does not sound much use. All this has done is to make the blood more concentrated without fixing any electrolyte issue or removing any waste products such as urea. Urea and creatinine levels even seem to have gone up – due to not crossing the membrane quite as well as water. This haemoconcentration brings problems too – the blood is now thicker and less potentially more coagulable. This could clog up the filter and stop the CVVH working altogether. While this is an inherent problem with CVVH, often requiring anticoagulation – and more of that below – this coagulation tendency need not be exacerbated by this haemoconcentration process.

To escape this issue and make CVVH useful, it does not happen without another important step – dilution, usually pre-dilution. This works by adding fluid to the blood, diluting the blood before it enters the filter itself, which means there is spare fluid to remove at the filter, bringing the blood cell and protein concentrations back to similar levels they were as the blood was taken out of the body. If this doesn’t sound much use, this dilutional fluid changes the blood electrolyte, urea and other solute levels before entering the filter. By using a dilutional fluid that is fairly ideal in its electrolytes, the chemistry of the blood returning to the patient will become closer to physiological as it passes many times through the CVVH system.

I’ll try to show this in table 2, with a 10% pre-dilution with a standard filtration fluid, using the same starting figures we had in table 1.

Table 2 Showing the effect of predilution to correct small solutes concentrations

This is the essence of CVVH, and one could see that with repeated cycles through the filter, blood concentrations would return to something similar to the replacement fluid.

The filter is the heart of the CVVH process, but making it work in practice requires a ‘circuit’. This and other bits of terminology make CVVH fairly confusing.

We’ll pause to let all this information… filter in (sorry).

Part 2: Click here

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