Operational characteristics

The operational characteristics are summarized in Table !..


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Table 1 Operational characteristics of CRRT

Continuous arteriovenous hemofiltration (CAVH) ( CDFigue4)

Continuous arteriovenous hemofiltration (CAVH) ( CDFigue4)

CD Figure 4. CAVH requires an arterial and a venous catheter. The patient's blood pressure is the driving force for the blood flow and ultrafiltrate production. A replacement fluid is pumped into the postfilter (as shown in the figure) or in the prefilter line.

Convective clearance of small unbound solutes is equal to the filtration rate ( Q f). Determinants of Q f are the membrane surface area (A), the membrane hydraulic permeability (Kf), and the transmembrane pressure (TMP):

TMP is given by

TMP = Pb + Puf - p where Pb is the hydrostatic pressure on the blood side, Puf is the negative pressure in the filtrate compartment, and p is the oncotic pressure of the blood.

Arteriovenous techniques operate under conditions of low flow and low pressure. Within the filter there is a progressive decline in hydrostatic pressure accompanied by an increase in osmotic pressure, leading to a point of filtration pressure equilibrium where ultrafiltration ceases and the risk of filter clotting increases. This phenomenon is attenuated by predilution, which decreases oncotic pressure in the filter. Resistance in the filter also depends on blood viscosity which is inversely proportional to the hematocrit.

Satisfactory filtration rates can only be achieved with arteriovenous techniques if the systolic blood pressure is over 80 mmHg and resistance in the extracorporeal circuit (catheters, lines, connections) is kept to a minimum. Attempts to increase filtration rate by lowering the filtrate collector or applying suction to the filtrate line (suction-assisted hemofiltration) augment the risk of reaching filtration pressure equilibrium. Although predilution increases filtration rate, it does not lead to a proportional increase of clearance because of lower solute concentrations in blood and filtrate.

Continuous venovenous hemofiltration (CVVH) ( CDFigure 5)

CD Figure 5. CVVH uses a single dual-lumen venous catheter and an external blood pump to provide the filtration pressure. An air detector with an external automatic clamp is required to protect against systemic air embolism. Pump-driven circuits also require pressure monitoring (not shown in the figure) in order to prevent exposure to excessive pressures. Some hemofiltration machines use an additional pump on the filtrate line.

The limited efficiency of CAVH resulted in the development of venovenous pump-driven techniques where transmembrane pressure is higher and primarily determined by the pump performance. Some hemofiltration machines also use a pump on the filtrate line. The use of a pump substantially enhances the complexity of the procedure and requires a variety of control and safety devices (air traps, automatic clamps, pressure monitors, and alarms).

Continuous hemodialysis (CAVHD, CVVHD)

The use of diffusive solute transport is another method of compensating for the limited azotemia control achieved with CAVH. With continuous dialysis, small-solute clearance equals dialysate flow (Qd) provided that it remains considerably smaller than blood flow, enabling complete equilibration inside the dialyzer.

Combined diffusion and convection

Critically ill patients probably benefit from a combination of diffusion and convection that provides sufficient elimination of small and larger toxins. This combination may be achieved with the following.

1. Hemodiafiltration or continuous hemodialysis with high flux membranes, producing a considerable amount of ultrafiltrate that requires replacement. Diffusive and convective clearance of small solutes are additive (CD Figure6 and CD E.i.g.u.r.e...Z).

CAUCF nifimMrt Hms

CD Figure 6. Continuous arteriovenous hemodiafiltration (CAVDHF) requires an arterial and a venous catheter. The patient's blood pressure is the driving force for the blood flow and ultrafiltrate production. In order to increase small-solute clearance, dialysate is driven through the outer compartment of the filter, adding diffusion to the convective solute transport. The outflow from the filter contains both the filtrate and the dialysate. As in CAVH, a replacement fluid is pumped into the postfilter (as shown in the figure) or in the prefilter line.

CD Figure 7. Continuous venovenous hemodiafiltration (CVVHDF) combines pump-driven venovenous hemofiltration with continuous dialysis and therefore is the most complex CRRT.

2. High-flux dialysis or continuous hemodialysis with high-flux membranes and ultrafiltration control, leading to ultrafiltration in the first half of the filter and back-filtration of dialysate in the second part. This backfiltration can be seen as a substitute for the replacement solution.

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