The rule of 7 is an important concept in dialysis treatment. It refers to the recommended limits for fluid removal during a dialysis session in order to avoid complications. Dialysis is a treatment for kidney failure that removes excess fluid and waste products from the blood by filtering it through a dialysis machine. Fluid overload is a common issue in kidney failure, so removing fluid is a key goal of dialysis. However, removing too much fluid too quickly can cause low blood pressure, cramping, nausea, and other problems. This is where the rule of 7 comes in.
The Basics of the Rule of 7
The rule of 7 states that fluid removal should not exceed 7% of the patient’s weight. For example, if a patient weighs 70kg, then no more than 4.9L (70kg x 0.07) of fluid should be removed during that dialysis session. This limit helps prevent the blood pressure from dropping too low during treatment. The 7% limit is a general guideline and the exact fluid removal goal should be individualized for each patient based on factors like their blood pressure, heart function, and fluid status.
Where Does the Rule of 7 Come From?
The rule of 7 originates from multiple research studies in the 1960s and 70s that looked at the impacts of fluid removal rate in dialysis. Researchers found higher rates of intradialytic complications like muscle cramping, nausea, vomiting, and hypotension (low BP) when fluid was removed too aggressively. Over time, nephrologists found that aiming to keep fluid removal under 7% of body weight helped minimize these intra-dialytic side effects for most patients. This limit continues to be recommended in dialysis clinical practice guidelines today.
How is the Rule of 7 Used?
Here is a step-by-step overview of how the rule of 7 is applied in dialysis:
- The patient’s post-dialysis dry weight is estimated based on clinical evaluation of fluid status.
- The patient is weighed before their dialysis treatment session.
- The pre-dialysis weight is compared to the estimated dry weight.
- The difference between pre-dialysis weight and dry weight determines the amount of excess fluid present.
- The fluid removal goal for that session is calculated as no more than 7% of post-dialysis dry weight.
- During the treatment, the patient’s blood pressure and symptoms are monitored closely to assess tolerance of fluid removal.
- If blood pressure becomes unstable or the patient experiences significant symptoms, the fluid removal rate may be slowed or stopped before reaching the 7% target.
The rule of 7 serves as a maximum limit rather than a fixed target. Patient tolerance is always the priority. Some patients may only tolerate removing 3-5% of their weight as fluid in a single session.
Why is the Rule of 7 Important?
The rule of 7 sets an important safety limit for fluid removal during dialysis. Removing fluid too quickly can cause:
- Severe drops in blood pressure
- Ischemia – inadequate blood flow to the heart, brain, and other tissues
- Muscle cramping
- Nausea and vomiting
- Headaches and dizziness
- Heart arrhythmias
These complications can be minimized by respecting the maximum 7% fluid removal rule. Slower, more gradual fluid removal is gentler on the body and avoids large fluid shifts between compartments. The rule of 7 provides a useful guideline for dialysis nurses and technicians to gauge safe fluid removal rates for their patients.
Are There Any Exceptions to the Rule of 7?
There can be exceptions where removing more than 7% of body weight as fluid may be appropriate. Some examples include:
- Patients with significant fluid overload and edema that needs urgent management
- Hypertension emergencies requiring rapid fluid removal
- Patients on dialysis for many years who have adapted to more aggressive fluid removal
- Younger, healthier patients who tolerate fluid removal well
However, these situations should be carefully evaluated by the nephrologist. Fluid removal above 7% increases the risks of complications. If faster fluid removal is desired, the patient’s blood pressure, heart rate, oxygen levels and symptoms should be very closely monitored.
How is Dry Weight Determined?
Estimating the patient’s ideal “dry weight” is key for applying the rule of 7. Dry weight refers to the patient’s weight without fluid overload. Determining accurate dry weight involves assessing the patient’s fluid status through:
- Physical exam of fluid retention in legs, lungs and abdomen
- Blood pressure trends
- Fluid removal patterns and blood pressure response during prior dialysis sessions
- Chest x-rays to evaluate heart size and pulmonary edema
- Inferior vena cava size on ultrasound
- Bioimpedance devices that measure body water composition
Dry weight is not constant. It may need to be adjusted based on changes in the patient’s underlying condition and fluid status. Ongoing evaluation helps determine the optimal dry weight target.
How Does Blood Pressure Change During Dialysis?
Blood pressure behavior during dialysis provides important clues about the patient’s fluid status and dry weight. Typical blood pressure patterns include:
- Low BP at start – May indicate hypovolemia and dry weight is set too low
- BP drop during treatment – Fluid removal rate may need to slow down
- BP rise at end of treatment – Suggests dry weight target is appropriate
- High BP throughout – Dry weight may be set too high
The nurse monitors for significant BP drops during dialysis and intervenes to avoid complication. The nephrologist uses BP trends to adjust dry weight up or down.
What Happens if Fluid is Removed Too Fast?
Removing fluid too fast can lead to serious complications:
- Severe hypotension – BP drops dangerously low during session. Lightheadedness, confusion, loss of consciousness.
- Ischemic events – Inadequate blood flow can damage the heart, brain, intestines, etc.
- Muscle cramping – Painful cramping occurs as fluid shifts out of cells.
- Nausea/Vomiting – Associated with large fluid shifts.
- Disequilibrium syndrome – Rare complication from rapid osmolar shifts in the brain. Causes neurological symptoms.
Removing fluid rapidly can also cause chronic damage over time to the heart, blood vessels and other organs from repeated ischemia. It is extremely important to avoid exceeding the 7% fluid removal limit.
Tips for Preventing Adverse Effects
Here are some tips for safely managing fluid removal and preventing complications:
- Set realistic fluid removal goals based on clinical evaluation, not just pre- to post-dialysis weight differential.
- Start fluid removal slowly and ramp up gradually. Avoid abrupt high-rate fluid removal early in treatment.
- Use saline infusions to maintain BP if needed rather than decreasing fluid removal rate.
- Monitor blood pressure, pulse, and oxygen saturation frequently for stability.
- Ask patients to report any lightheadedness, cramping, nausea, etc early.
- Consider using dialyzate cooling to help stabilize blood pressure.
- Adjust dry weight gradually. Avoid large jumps up or down.
How is the Rate of Fluid Removal Calculated?
The dialysis machine controls the rate of fluid removal, measured in milliliters/hour. The machines can remove fluid at variable rates through the treatment. Some ways the prescribed rate is determined include:
- Constant rate: Example – removing 700mL/hr throughout treatment.
- Gradual increase: Start at 400 mL/hr and increase 100mL/hr every 30 minutes.
- Higher initial rate then slowed: Remove 800 mL/hr initially, then back down to 300 mL/hr.
- Variable by BP: Set wide range like 200-700 mL/hr targeting BP 120-140/80-90.
The nephrologist prescribes the protocol, and the nurse can also make adjustments within prescribed parameters based on the patient’s response and symptoms during dialysis.
Conclusion
The rule of 7 provides an important safety guideline to avoid removing excessive fluid too rapidly during dialysis. It states that fluid removal should not exceed 7% of the patient’s post-dialysis estimated dry weight. This helps prevent symptomatic hypotension and other complications from intra-dialytic fluid shifts. However, the rule of 7 is not intended as a rigid target. Removal rates should be individualized and reduced if a patient experiences any concerning symptoms. Ongoing clinical assessment of fluid status and hemodynamic response during dialysis is key for applying the rule of 7 safely and effectively.