Nomenclature: Techniques

Hardware and Devices

Continuous renal replacement therapy (CRRT) “hardware” includes the machine and all dedicated disposables. Knowledge of the nomenclature and the functions of the machine and its main components is extremely important, not only for nurses or technicians but also for clinicians.

Fig. 177.1 depicts a standard CRRT machine equipped with current technology. Its main components include the following:

• 1.

  Screen: The monitor through which the user interacts with the machine

• 2.

  Alarm light and sound indicators: Visual and auditory alarms must be clear and comprehensive. The alarm settings should be categorized unequivocally according to a specific standard

• 3.

  Inflow pressure (P _IN ) sensor (upstream of blood pump): Monitors the negative pressure in the blood inflow line between the patient's vascular access and the blood pump

• 4.

  Blood pump: Pump that controls the blood flow rate through the extracorporeal circuit

• 5.

  Pre-blood pump: Pump that controls the flow rate of solutions, mainly regional anticoagulants (e.g., citrate), into the blood inflow line before the blood pump

• 6.

  Preblood pump pressure sensor: Monitors the pressure before the pre-blood pump

• 7.

  Prefilter pressure (P _PRE ) sensor (downstream of blood pump): Located in the blood flow line between the blood pump and filter, this sensor monitors the positive pressure and enables calculation of the transmembrane pressure (TMP) and pressure drop (P _DROP ) in the filter

• 8.

  Filter holder: Holds the filter or the entire filter-tubing kit on the machine

• 9.

  Outflow pressure sensor (P _OUT ): Monitors the positive pressure between the filter and the patient vascular access. This sensor is used to calculate the TMP and pressure drop in the filter

• 10.

  Bubble detector: Transducer that detects the presence of air in the blood outflow line

• 11.

  Safety outflow electroclamp: Mechanism that produces occlusion of the blood outflow line

• 12.

  Effluent/ultrafiltrate pump: Pump that controls the rate of total fluid removal from the filter

• 13.

  Effluent/ultrafiltrate pressure sensor (P _EFF ): Monitors the pressure in the effluent compartment of the filter. This sensor is placed before the effluent pump and allows calculation of the TMP

• 14.

  Blood leak detector (BLD): Placed along the effluent line, it identifies unwanted blood leaks from the blood compartment of the filter

• 15.

  Replacement/infusion pump: Pump that controls the rate of replacement fluid flow into the blood inflow line (predilution, usually between the blood pump and the filter) and/or into the blood outflow line (postdilution, usually in the blood outflow chamber, such as the deaeration or venous drip chamber)

• 16.

  Prereplacement pump pressure sensor: Monitors the negative pressure before the replacement pump

• 17.

  Dialysate pump: Pump that controls the rate of dialysate flow into the filter

• 18.

  Predialysate pump pressure sensor: Monitors the negative pressure before the dialysate pump

• 19.

  Postdialysate pump pressure sensor (P _Di ): Monitors the pressure in the dialysate line before the connection with the filter. Permits a better estimate of TMP

• 20.

  Fluid control system: Allows direct monitoring of the fluid balance related to fluids exchanged by the CRRT machine during the treatment. It can be gravimetric, volumetric, fluximetric, or a combination of these mechanisms (see later in this chapter)

• 21.

  Heater: Heats the dialysate/replacement fluids, or the blood flowing through the blood outflow line of the extracorporeal circuit

• 22.

  Anticoagulant/specific antagonist pumps: Infuses anticoagulants/specific antagonists into the blood circuit. Depending on the anticoagulation modality chosen, these pumps can be divided further into systemic anticoagulation pumps (e.g., heparin), regional anticoagulation pumps (e.g., citrate) and reversal anticoagulation pumps (e.g., calcium). If necessary, specific antagonist drugs (e.g., protamine) can be infused via a separate pump (i.e., not integrated into the CRRT machine) into the blood outflow line

Continuous Renal Replacement Machine: Procedures and Phases of Treatment

The different procedures performed by the machine include the following:

• Prescription phase: Decisions by the prescribing clinician about the required modality and related operational parameters and includes periodic reassessment and/or change of the prescription

• Preparation phase: Collection of necessary disposable material, identification and checking of the disposable set, set loading (cassette tubing), connection to the filter, positioning of the tubing, and hanging of bags

• Priming phase: Priming solution is infused into the extracorporeal circuit to remove air and impurities remaining after sterilization of the set. When heparin anticoagulation is used, it usually is added to the priming solution. During this phase, the machine makes a general check of all components and sensors

• Connection to the patient : Connection of the extracorporeal lines to the patient's vascular access

• Treatment phase : Net ultrafiltration and diffusive and/or convective solute transport are activated (all the pumps are working) and blood purification is performed. Patient vital signs and circuit pressures must be monitored throughout the treatment phase

• Special procedures : During treatment, special procedures can include replenishment of dialysate, replacement fluid, citrate bags (when citrate anticoagulation is used) and change of syringes (when using heparin anticoagulation), repositioning of the vascular access, temporary disconnection, recirculation, and replacement of filter and kit

• Blood return, disconnection, and unload : Blood return procedure returns the blood to the patient. This usually is done by connecting a saline solution bag to the inflow blood line and running the blood pump. When the circuit is flushed, the blood pump is stopped, the blood outflow line disconnected, and the tubing and filter unloaded

Continuous Renal Replacement Thrapy Disposables

Disposables (single-use components of the extracorporeal circuit) are specific for every machine and usually are designed for a specific treatment modality. The main disposables and color codes that should mark each tubing line are listed in Table 177.1 .

During CRRT, the filter is the key disposable through which blood or plasma is purified effectively by ultrafiltration, convection, and/or diffusion. Historically, the designation “filter” describes the entire purifying extracorporeal device system (i.e., membranes, housing). Among the different types of filters, hemofilters, hemodialyzers, and hemodiafilters should be used when exclusively convective, diffusive, or convective plus diffusive modalities, respectively, are applied. In this manuscript we use these terms distinctly, taking into account the different CRRT modalities. A plasmafilter is defined as a specific filter that allows the separation of plasma from cellular elements. Sorbents, cartridges, and adsorbers do not belong to the category of filters; in this case, adsorption is the only purifying modality. The only available type of CRRT filter that can perform diffusive and/or convective transport is shaped as a collection of parallel “hollow fibers.” The filters can be identified mainly by membrane geometrics and performance characteristics.

Volume Management and Fluid Balance

Fluid management during CRRT must take into account the volume and hemodynamic status of the patient. The machine fluid balance error (FBE) is the fluid management error caused by CRRT machine malfunction. Based on the inherent variability (“tolerances”) in the performance of the fluid pumps, scales, and other components of a CRRT machine's fluid management system, the manufacturer provides a specified limit (“specification”) beyond which a fluid imbalance is considered an error. Fluid imbalances can be due to hardware (scales, pumps, tubes) or software (control system and protective subsystem) errors.

Various systems have been proposed for fluid balancing in CRRT machines:

• Gravimetric fluid balancing, using one or more scales, is used most commonly in CRRT because it is the most reliable technique during long treatment intervals. A fundamental aspect of this type of system is the continuous weighing of the effluent along with replacement fluid and/or dialysate, with weight acting as a surrogate for fluid flow rate. The machine software analyzes these scale data on an ongoing basis, and any discrepancies between prescribed and actual values lead to adjustments in pump rates based on a servo-feedback mechanism. Disadvantages include limitations in scale capacity, user errors, and other disturbances of the operating environment.

• In volumetric fluid balancing, a system of balancing chambers and valves is used. During long treatments, volumetric balancing is less accurate than gravimetric balancing because of systematic, cumulative errors, because there is no continuous servo-feedback safeguard for this approach. The advantage of this system is that it eliminates the need to collect effluent and thus reduces the frequency of fluid-related interventions.

• Fluxometric fluid balancing requires the application of accurate but expensive flowmeters (electromagnetic, ultrasonic and Coriolis flowmeters).

All these methods can be applied individually or in combination.

Extracorporeal Therapies and Treatments

Extracorporeal therapies can be categorized according to session frequency and duration.

Continuous Therapies

CRRT is any extracorporeal technique that replaces kidney function and more generally provides blood purification for an extended period of time. CRRT is considered by many clinicians to be the most appropriate modality for the management of hemodynamically unstable patients with AKI, promoting better hemodynamic stability, reduced transcellular solute shifts, and better tolerance to fluid removal than intermittent extracorporeal therapies. The need for expertise, the necessity of continuous anticoagulation, the nursing workload, the continuous alarm vigilance, and the higher costs are some of the limitations of this approach. CRRT can be provided in various forms depending on resources, patient needs, and staff skills ( Fig. 177.2 ).

Prescription should be reviewed regularly.

CRRT treatments currently are performed using a double-lumen catheter as vascular access, a venovenous technique, whereby blood is driven from a vein and, after being purified, returns to the same vein. Arteriovenous circuits have been virtually abandoned.

Slow Continuous Ultrafiltration

Slow continuous ultrafiltration (SCUF), based only on slow removal of plasma water, is used for patients with refractory fluid overload, with or without renal dysfunction. Its primary aim is to achieve safe and effective correction of fluid overload.

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