Healthy Lifestyle & Kidney Health

The Reflex Autotomy — Protective Isolation

Manual Bypass appearing on the display is an absolutely critical clinical presentation for your new technicians to master.

In our medical analogy, if the machine enters Bypass Mode, it is performing a protective Reflex Autotomy—similar to how a biological organism cuts off blood supply to a damaged limb to protect its core organs.

The Mechanism: When the sensory nervous system (the conductivity cells or temperature sensors) detects that the dialysate fluid composition is toxic, the brain (LLC processor) instantly isolates the dialyzer to prevent these toxins from crossing into the patient's bloodstream.

1. Anatomy of the Bypass Loop (The Physical "Shunt")

Flow Path

Image Placeholder: Bypass Valve Block — V26, V27, V28 Assembly

Insert photo: Valve block showing V26 (bypass), V27 (inlet), and V28 (outlet) with connecting fluid lines.

To understand bypass, your staff must look at the physical flow-diversion valves situated right behind the dialyzer quick-connect hands (the blue and red couplings on the front of the machine).

[Normal Flow Mode] [Bypass Active Mode]
──[V27 (In)]──> [To Patient] ──[V27 (In)]──X (Closed)
│ │
[Dialyzer] ▼
│ [V26 (Bypass)] (Open)
<──[V28 (Out)]── [From Patient] <──[V28 (Out)]──X (Closed)

The system uses three interacting electromagnetic valves working in perfect synchronization:

Valve 27 (Inlet)

Fresh Dialysate Entry
Guards the fresh dialysate entry line to the dialyzer.

Valve 28 (Outlet)

Waste Dialysate Exit
Guards the waste dialysate exit line from the dialyzer.

Valve 26 (Bypass)

Direct Short-Circuit Bridge
Connects inlet line directly to outlet line, completely skipping the dialyzer.

2. Pathophysiology (Why the Machine Drops into Bypass)

Etiology

There are two distinct types of bypass presentations: Automated Safety Bypass and True Manual Bypass.

State A: Automated Safety Bypass (The Machine's Self-Preservation)
- The Mechanism: The sensory nervous system (CD_TOTAL or the TSD/TSE temperature thermistors) reports variables that fall outside safe physiological boundaries (e.g., Temperature >40°C or Conductivity <13.0 mS/cm or >15.5 mS/cm).
- The Reflex: The LLC instantly shuts V27 and V28, and throws V26 wide open. The dialysate fluid bypasses the patient completely and circles internally to protect the patient from hemolysis (caused by incorrect temperature/hypotonicity) or severe electrolyte shifts.
State B: True Manual Bypass (The Technician Overrides the Loop)
- The Mechanism: The technician or nurse physically presses the Bypass Button on the TFT touchscreen monitor.
- The Pathophysiology: This overrides the automated mixing loop logic for servicing or priming. In this state, even if the conductivity and temperature are mathematically perfect, the machine forces V26 open and locks the fluid out of the dialyzer.

Critical Clinical Warning — Bypass is a Safety Feature, Not a Malfunction:

New staff must understand that Bypass Mode is the machine protecting the patient. Never override or ignore a bypass alarm without first identifying and correcting the root cause (temperature, conductivity, or blood leak).

3. Signs & Symptoms (What Happens on the Machine)

Clinical Picture

When the machine drops into a manual or forced bypass state:

Symptom 1 (The Dialysate Flow Halts): Fluid stops moving through the dialyzer. The lines attached to the dialyzer go static, and the physical transmembrane pressure (TMP) drops to zero.
Symptom 2 (The Screen Presentation): The fluid lines on the display screen turn from Green to Orange or Red, and the text "Bypass" flashes.
Symptom 3 (The UF Pump Halts): The ultrafiltration pump stops stroking because pulling fluid during a bypass state could cause a severe collapse of the dialyzer membranes.

⚠️ BYPASS ⚠️
Fluid lines: ● ● ● | TMP: 0 mmHg | UF Pump: STOPPED

4. Differential Diagnosis (Ruling out Mimics)

Rule Out

If a machine gets "stuck" in a bypass state and refuses to return to a green, healthy flow state even after pressing the button, your team must diagnose which underlying organ is keeping it locked out:

The Sensor Board: Check TSM Menu 1.02. Is the temperature drifting, or are the conductivity cells dirty and under-reporting? (The machine will keep the bypass locked as long as a sensor reports a violation).
Optical Blood Leak Detector (BLD): If the BLD detects blood cells (or heavy dirt/scale) in the spent dialysate line, the machine locks into bypass to prevent further blood loss.
Mechanical Valve Failure (Stuck Valves): The electronic board might be telling V27 and V28 to open, but if their internal rubber plungers are physically swollen or jammed with scale, they remain physically closed, keeping the machine trapped in a bypass condition.

                
                Clinical Reasoning: The Listening Test (below) is the fastest way to differentiate between a sensor-driven bypass and a mechanical valve failure. Listen for the click-clack!
            

5. Management (Clinical Engineering Intervention)

Treatment Plan

Diagnostic Measures — Testing the Valves

If your staff suspects the physical valves are failing to respond to the bypass commands:

[TSM Mode → Menu 1.09] ──> Inputs/Outputs → Valves Step-by-Step:
1. Flip Switch S1 to Position 2 and enter Technical Service Mode (TSM).
2. Navigate to TSM Menu 1: Inputs/Outputs → Submenu 1.09 (Valves).
3. Locate V26, V27, and V28 in the logic list.
4. Toggle them manually one by one. Listen closely to the side panel. You must hear a sharp, metallic click-clack for each valve.
5. The Voltage Check: If a valve is silent, use a digital multimeter across its 24V DC terminal plug while toggling. If 24V is present but the valve remains silent, the coil winding is open-circuit. ✅ Hear sharp click-clack + 24V present → Valves are Electrically Healthy ❌ Silent valve + 24V present → Coil is burnt out (Replace valve) ❌ Silent valve + 0V present → Board/Logic Failure (Check LLC output)

Image Placeholder: Valve Toggle Test — Multimeter at 24V Terminal

Insert photo: Multimeter probes on valve terminal plug during TSM toggling test.

Technical Management (The "Treatment Plan")

The Citric Flush (Chemical Decalcification) If the valves are sticking due to chemical crystallization, perform a heavy hot citric descaling cycle. This breaks down the scale holding the bypass seals closed.
This resolves ~60% of "stuck bypass" cases without any disassembly.

Plunger Rebuilding (The Minor Surgical Fix)

Unbolt the metal top plate of the failing valve block (V26, V27, or V28).
Pull out the spring and chemical-resistant rubber plunger.
Clean out any debris or biofilm trapped in the housing.
Replace the rubber seal tip if it shows signs of swelling or pitting.

Verification & Clearance (Mandatory)

Exit TSM.
Return Switch S1 to Position 0.
Boot into therapy mode.
Ensure that when conductivity stabilizes, the machine successfully clears the "Bypass" status and transitions the screen icons to a solid, healthy green flow line.

                
                Post-Intervention Verification:
                Run a full conductivity and temperature stabilization test to ensure the machine exits bypass automatically.
Manually press the Bypass Button and confirm V26 opens (listen for click), then press again to confirm V27/V28 reopen.
Document the valve test results in the machine's service log.