Short daily home hemodialysis: The low dialysate volume approach

INTRODUCTION — The use of daily home dialysis has increased over the past decade. This increase is driven by increased awareness of better outcomes and quality of life provided by home dialysis and by short daily hemodialysis compared with conventional, in-center, three times per week hemodialysis. (See "Short daily hemodialysis" and "Choosing home hemodialysis for end-stage kidney disease".)

This trend has prompted the development of effective hemodialysis equipment that may be used outside of a dialysis center. Although conventional hemodialysis machines have been used for home dialysis, such machines generally employ a high dialysis flow rate. High flow rates potentially increase water and electrical costs, often mandating home electrical and/or plumbing modifications that render the use of such machine impractical for some patients [1]. This topic reviews the requirements for a home hemodialysis machine and the approach used by NxStage to develop an effective dialysis machine for home use.

Issues relating to patient survival and the technical aspects of home hemodialysis, short daily hemodialysis, and nocturnal hemodialysis are discussed elsewhere. (See "Outcomes associated with nocturnal hemodialysis" and "Home hemodialysis (HHD): Establishment of a program" and "Technical aspects of nocturnal hemodialysis" and "Short daily hemodialysis" and "Choosing home hemodialysis for end-stage kidney disease".)

REQUIREMENTS OF HOME HEMODIALYSIS MACHINE — An ideal home hemodialysis machine should have the following characteristics:

●Easy to use

●Requires minimal storage space

●Requires no electrical or plumbing modification to the home

●Is energy and water efficient (ie, not significantly increase water and electricity costs)

●Is portable for travel

●Can provide adequate dialysis working around lifestyle preferences

Many home machines that meet some or all of these requirements have been developed or are under development. (See "Home hemodialysis (HHD): Establishment of a program".)

FREQUENT LOW-VOLUME DIALYSATE — In order to provide effective dialysis with a low-dialysate volume, the dialysate must be nearly saturated with uremic solutes such as urea. One home hemodialysis platform that utilizes such an approach is the NxStage home hemodialysis device [2]. Near saturation of dialysate is achieved by maintaining a very low ratio of dialysate flow rate (Qd) to blood flow rate (Qb); this ratio (Qd/Qb) is termed the flow fraction by NxStage. The degree of dialysate saturation is the ratio of dialysate concentration to plasma concentration (D/P). Clearance is the product of saturation (D/P) times the dialysate flow rate Qd.

Utilizing a low flow fraction is in marked contrast to standard hemodialysis platforms that utilize a nearly unlimited supply of dialysate, which is generally not as saturated with uremic solutes upon leaving the dialyzer. In conventional hemodialysis, urea clearance increases as either blood flow or dialysate flow rate increases. However, the dialysate is minimally saturated, resulting in inefficient dialysate use (figure 1).

By contrast, when a low Qd/Qb (flow fraction) is utilized, there is much more efficient use of dialysate (urea saturation can theoretically approach 100 percent with a flow fraction of 25 to 33 percent) (figure 2), and, therefore, less dialysate can be used. Under these conditions, urea clearance is approximately equal to the dialysate flow rate. However, we typically use a flow fraction of 40 to 50 percent to make efficient use of the dialysate and achieve the target Kt/V without substantially increasing the time on dialysis [3]. (See 'Target Kt/V' below.)

A decrease in dialysate flow rate (ie, a lower flow fraction) while maintaining the same volume of dialysate per treatment results in an increased time on therapy. Given blood flow rates of 400 to 450 mL/min, typical dialysate flow rates are in the range of 100 to 300 mL/min.

One consequence of maintaining a fixed, low ratio of dialysate to blood flow rate (flow fraction) is that an increase in the prescribed dialysate volume will result in an increased time on dialysis. (See 'Ultrafiltration' below.)

Similarly, a decrease in blood flow rate dictates a proportionate decrease in dialysate flow rate, or else dialysate urea saturation and Kt/V will decrease.

More importantly, a fixed flow fraction maintains dialysate saturation and delivered Kt/V regardless of any intradialytic changes in blood flow.

Ultrafiltration — As with conventional hemodialysis or peritoneal dialysis, ultrafiltration is required in order to maintain euvolemia. The ultrafiltration volume generally does not affect the dialysis time of patients who are dialyzed utilizing standard technology (providing the volume remains within hourly ultrafiltration parameters). The ultrafiltration volume increases the dialysis time of patients using the NxStage System One dialysis device since these machines fix the ratio of Qd (fluid leaving dialyzer)/Qb. The amount of total ultrafiltrate volume per treatment is added to the total dialysate volume per treatment, which increases the total fluid volume leaving the dialyzer (prescribed dialysate volume plus ultrafiltration volume). Since the flow fraction is fixed (as it is with NxStage), the increased volume requires an increase in the treatment time. In some patients, this provides an incentive to limit fluid intake. For example, to ultrafiltrate an extra liter, it might require another 6 to 10 minutes of treatment time, depending on the flow fraction.

Target Kt/V — Urea kinetic modeling has not been validated as a measure of adequate dialysis for more frequent, low-volume hemodialysis modalities. Despite this, among patients on short daily hemodialysis, we use Kt/V to assess dialysis dose and target a weekly standard Kt/V of 2 to 2.1, similar to the target among patients receiving in-center hemodialysis. The definition, calculation, and limitations of Kt/V are discussed at length elsewhere. (See "Prescribing and assessing adequate hemodialysis" and "Prescribing and assessing adequate hemodialysis", section on 'Definition and calculation' and "Prescribing and assessing adequate hemodialysis", section on 'Limitations' and "Prescribing and assessing adequate hemodialysis", section on 'The optimal amount of dialysis'.)

The best measure of adequacy of more frequent dialysis therapies is unknown, though several have been proposed [4]. Dialysis Outcomes Quality Initiative (DOQI) guidelines and recommendations for target standardized Kt/V per week are discussed elsewhere. (See "Short daily hemodialysis", section on 'Dialysis prescription and dose monitoring' and "Short daily hemodialysis", section on 'Summary and recommendations'.)

Volume of dialysate — Prescribing the appropriate volume of dialysate is conceptually similar to prescribing appropriate dialysate volume in peritoneal dialysis patients. (See "Prescribing peritoneal dialysis", section on 'Calculation'.)

The volume of distribution of urea (V) is estimated to be equal to body water. Larger patients have larger volumes of distribution of urea (V) and therefore require a higher product of clearance X time (Kt) to achieve the target Kt/V. The clearance (K) per dialysis session is theoretically adjusted by changing either the volume of dialysate per session or the saturation of urea in the dialysate fluid. If the saturation is >90 percent, then the only effective recourse is to increase dialysate volume per treatment.

Among patients undergoing low-volume frequent hemodialysis five days per week, most clinicians target a standardized weekly Kt/V urea of 2 to 2.1 or a per-treatment Kt/V urea of approximately 0.6. As an example, a 70 kg male who has a urea volume of distribution of 35 liters (assuming V is 50 percent of body weight in kilograms) would need to use 21 liters of 100 percent saturated dialysate (21 L/35 L = 0.6), provided that no or only minimal ultrafiltration was required. If Qd/Qb is in the range of 40 percent, then saturation is approximately 90 percent (figure 1). Thus, a 70 kg male would need 23.3 liters of fluid to meet the daily Kt/V goal of 0.6/treatment if there was no ultrafiltration to add to diffusive urea clearance (23.3 L x 0.9 / 35 L = 0.6). In hemodialysis, if dialysate saturation is maximized with a low ratio of dialysate to blood flow rate (low flow fraction), then the only way to increase Kt/V is to increase the volume of dialysate used. If the patient is undergoing frequent low-volume hemodialysis using a system with a fixed ratio, such as NxStage, the dialysate volume is limited. The average dialysate volume used with NxStage therapy is approximately 20 to 30 liters for an adult performing five or six treatments per week. The same reasoning as just described can be applied to a patient dialyzing six times per week. The typical reason for six treatments per week is usually to attain euvolemia, but occasionally, the indication may be solute control. For a six times per week schedule, a per treatment Kt/V of 0.5 is sought. Though initial dialysis prescription is estimated by above calculations, standardized weekly Kt/V is calculated from measured single-pool Kt/V. Approximate initial volume of dialysate determined by weight and range of initial flow fraction is demonstrated in the table (table 1).

If the standard Kt/V does not meet the target, then the prescription needs to be adjusted to increase Kt. This can be achieved by increasing the volume of dialysate or the percent saturation of dialysate fluid or both. Increasing the amount of dialysate volume or increasing the percent saturation (by decreasing flow fraction) will increase dialysis time, which may negatively impact the patient-perceived burden of therapy. However, if the dialysate volume is increased (eg, 5-liter increase), then the flow fraction could be simultaneously increased, resulting in a higher Kt, despite lower saturation, and less time on therapy. Depending upon the model used, the NxStage home dialysis devices allow for maximum dialysate flow rates of 200 mL/min or 300 mL/min. The NxStage machine with a maximum dialysate flow rate of 300 mL/min allows for less time on therapy for patients who require large, per treatment dialysate volumes. Performing short daily dialysis five days per week requires a significant commitment by the patient and partner. Avoiding unnecessary, long treatments might lessen the burden. However, the cumulative weekly time on short daily hemodialysis should be at least as long as the cumulative weekly time on three times weekly, in-center hemodialysis. The rationale for this is that larger, slow-moving uremic toxins require time on dialysis for adequate depuration. The distinct advantage of short, frequent treatments is the avoidance of the long interdialytic interval and the more frequent removal of interdialytic accumulated fluid.

DIALYSATE COMPOSITION — Depending on the machine used to deliver the frequent low-volume dialysis treatments, the dialysate fluid is made differently. For the NxStage platform, the dialysate is either generated by a self-contained dialysate generating system (PureFlow SL) or is available as prepackaged "travel fluids" in 5-liter sterile bags (similar to peritoneal dialysis fluids).

There are limited options for altering the electrolyte content of the dialysate if using the NxStage. The dialysate buffer used is lactate, which generates bicarbonate when metabolized in the liver.

The composition of dialysate is:

●Lactate – 40 or 45 mEq/L

●Potassium – 1 or 2 mEq/L

●Sodium – 140 mEq/L

●Calcium – 3 mEq/L

●Magnesium – 1 mEq/L

●Chloride – 105 mEq/L

●Glucose – 100 mg/dL

PureFlow SL generates dialysate in 40-, 50-, or 60-liter batches from prepacked concentrate that must be used within 96 hours of initiation of production. Different options for electrolyte content are not available for every size batch, and compositions change periodically. We contact the dialysis provider or manufacturer for updates on options.

If using a standard, conventional hemodialysis platform to perform frequent, low-volume hemodialysis, dialysate electrolyte options are similar to those typically available for in-center hemodialysis treatments.

Marked changes in serum electrolytes do not occur during dialysis among patients using frequent, low-volume hemodialysis, because of the relatively low volume of dialysate used.

ANTICOAGULATION — If using standard center hemodialysis platforms for frequent, low-volume hemodialysis, heparin pumps can be used. NxStage System One does not have a heparin pump. Instead, a single bolus of heparin is used at the initiation of dialysis. A typical dose of heparin for a two- to three-hour treatment is 2000 to 3000 units.

DIALYZER — Standard center hemodialysis platforms use typical blood lines and dialyzers. NxStage System One utilizes a prepackaged cartridge system that includes dialysate, blood, and saline infusion lines. The cartridge is available with or without a preattached, gamma-sterilized, polyethersulfone dialyzer (membrane area 1.6 m²).

SUMMARY

●Requirements of home dialysis machine – The increase in home dialysis has led to the development of effective hemodialysis equipment that may be used outside of a dialysis center. Ideally, a home hemodialysis machine should be portable, easy to use, have minimal storage space requirement, require no electrical or plumbing modification to the home, and be energy and water efficient. (See 'Introduction' above and 'Requirements of home hemodialysis machine' above.)

●Frequent low-volume dialysate – Low-volume, high-efficiency dialysis platforms, such as the NxStage System One, provide effective dialysis with very low volumes of dialysate by maintaining a very low flow fraction (the dialysate flow rate divided by the blood flow rate). This allows for the efficient use of dialysate and minimizes the requirement for storage supply space and the use of water. (See 'Frequent low-volume dialysate' above.)

•Low flow fraction – To be most efficient with dialysate saturation, and therefore use the least amount of dialysate per session, the blood flow rate should be approximately three times greater than dialysate flow rate, resulting in flow fractions of approximately 33 percent. However, a flow fraction of 40 to 50 percent is typically used because a lower flow fraction requires increased dialysis time, which may negatively impact the patient-perceived burden of therapy. (See 'Frequent low-volume dialysate' above.)

•Ultrafiltration – In systems that use a fixed ratio of dialysate effluent flow to blood flow, increases in the dialysate volume (such as ultrafiltration) mandate increased time on dialysis. A decrease in blood flow rate also requires an increase in treatment time. (See 'Ultrafiltration' above.)

•Volume of dialysate – Delivering the appropriate amount of dialysis to meet Kt/V targets requires adjustment of the volume of dialysate. The average dialysate volume used with NxStage therapy is approximately 20 to 30 liters for an adult. (See 'Volume of dialysate' above.)