5.4 Dialysate Proportioning, Conductivity & pH

Dialysate Proportioning, Conductivity & pH

Quick Answer: Dialysis machines use volumetric 3-stream proportioning (water, acid, bicarbonate) to create dialysate. Safety is ensured through continuous conductivity (13.5-14.5 mS/cm) and pH (6.9-7.6) monitoring backed by an automated bypass failsafe valve.

Once the water has been successfully purified by the water treatment system and distributed to the patient floor, it enters the individual hemodialysis machines. Inside the machine, this highly purified water is mixed with concentrated chemical solutions to create dialysate. Dialysate is the fluid that flows on the opposite side of the dialyzer membrane from the patient's blood, drawing out toxins and excess electrolytes while replacing essential buffers. The process of precisely mixing the purified water with the chemical concentrates is known as proportioning. Modern dialysis machines use a volumetric proportioning system to ensure the mixture is perfectly accurate and safe for patient exposure.

Three-Stream Proportioning: Acid & Bicarbonate Concentrates

Standard hemodialysis utilizes a three-stream proportioning system. The three streams are purified water, acid concentrate, and bicarbonate concentrate. The acid concentrate contains precise amounts of essential electrolytes, including sodium, potassium, calcium, magnesium, and chloride. It also contains a small amount of glucose (to prevent hypoglycemia during treatment) and acetic acid or citric acid. The acid concentrate is responsible for removing the metabolic waste products from the patient's blood through diffusion. The bicarbonate concentrate contains sodium bicarbonate. The primary purpose of the bicarbonate is to serve as a buffer; it crosses the dialyzer membrane into the patient's bloodstream to neutralize the metabolic acidosis that naturally occurs in patients with end-stage renal disease (ESRD). The bicarbonate is kept in a separate jug from the acid concentrate because if calcium and magnesium (from the acid) were stored in the same highly concentrated container as the bicarbonate, they would react and precipitate out of solution, forming solid calcium carbonate (limestone). By keeping them separate until the moment they are diluted with purified water inside the machine, this precipitation is avoided.

Proportioning Ratios, Osmolality & Red Cell Hazards

The dialysis machine mixes these three components at a very specific ratio, known as the proportioning ratio. A common proportioning ratio is 45x, which means the final dialysate is made of 1 part acid, 1.72 parts bicarbonate, and 42.28 parts purified water (totaling 45 parts). Because the patient's blood will be in direct contact with this fluid (separated only by the thin, semi-permeable membrane), the exact concentration of the final dialysate is of critical importance. If the dialysate is too concentrated (hypertonic), water will be rapidly pulled from the patient's blood cells into the dialysate, causing the red blood cells to shrink and shrivel—a condition known as crenation. If the dialysate is too dilute (hypotonic), water will rush from the dialysate into the patient's red blood cells, causing them to swell and burst—a fatal condition known as hemolysis.

Dialysate Conductivity Monitoring & Independent Verification

To guarantee that the proportioning system is functioning correctly, the dialysis machine continuously monitors the final dialysate using two critical parameters: conductivity and pH. Conductivity is a measure of the solution's ability to conduct an electrical current. Because the electrolytes in the dialysate (like sodium, potassium, and chloride) carry electrical charges, the overall conductivity of the fluid provides a highly accurate measurement of the total ion concentration. The acceptable, safe range for dialysate conductivity is generally between 13.5 and 14.5 mS/cm (milliSiemens per centimeter). However, the specific target conductivity depends on the physician's prescription. The machine must read within +/- 0.5 mS/cm of the expected theoretical conductivity. Before every single treatment, the technician or nurse must independently verify the machine's conductivity reading using a calibrated external meter (such as a Phoenix meter or similar device) to ensure the machine's internal sensors are accurate.

Dialysate pH Parameters & Acid-Base Balance

The second critical monitoring parameter is pH, which measures the acidity or alkalinity of the dialysate. The pH scale ranges from 0 (highly acidic) to 14 (highly alkaline), with 7.0 being perfectly neutral. Human blood has a very narrow, slightly alkaline pH range of 7.35 to 7.45. To safely buffer the patient's blood without causing acid-base imbalances, the ideal pH of the final dialysate must be maintained between 6.9 and 7.6. Just like conductivity, the pH of the dialysate must be independently verified with an external meter or pH test strips prior to initiating the treatment. If the pH is too low, the patient will suffer from severe metabolic acidosis; if it is too high, they will suffer from metabolic alkalosis.

Automated Bypass Failsafe Mechanisms

Because safety is the paramount concern, the dialysis machine features a robust failsafe mechanism known as the bypass valve. If the machine's internal sensors detect that either the conductivity or the pH has fallen outside of the safe, established alarm limits, the machine will immediately trigger an audible and visual alarm. Simultaneously, the machine engages the bypass valve. The bypass valve instantly shunts the flow of dialysate directly to the drain, completely bypassing the dialyzer. This mechanical action ensures that inappropriately mixed, potentially lethal dialysate never comes into contact with the dialyzer membrane, thereby completely protecting the patient. Furthermore, the machine also strictly monitors the temperature of the dialysate, maintaining it close to body temperature (typically 35°C to 37°C). If the temperature falls outside safe limits (usually > 40°C), it can cause hemolysis, and the machine will again engage the bypass valve. Through this continuous, automated vigilance, the proportioning system safely delivers life-sustaining therapy.