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The importance of the duration of a Hemodialysis Session

nephroplus | April 21, 2019

Hemodialysis does two main things – remove excess fluids and remove the built-up toxins in the body. Since the kidneys are not functioning and removal of these toxins and excess fluids is one of the most important tasks of the kidney, dialysis is used to sustain life by removing these two. Uncontrolled build-up of toxins and fluids can kill a person whose kidneys are not functioning.

Most people on dialysis die earlier than the normal life-span of humans because of problems with the heart. The main cause of heart failure and other issues of the heart among dialysis patients is the presence of excess fluid in the body.

An average human adult has about five litres of blood in the body. When the kidneys are functioning, they remove any excess fluid in the form of urine. So at any given time, the body has only the amount  of fluid it really needs. However, when the kidneys don’t function, the fluid needs to be removed by dialysis. If there is about 2 litres if excess fluid in the body, there is about 40% excess volume of blood for the heart to pump. This puts enormous strain on the heart. This can be tolerated only for some time. Beyond a point the heart cannot continue to pump this excess volume. It fails after that.

The fluid in the human body can be thought to be present in three ‘compartments’ as Dr. John Agar, a reputed nephrologist in Australia refers to. The there compartments are

  • blood
  • cells in the body (intracellular fluid)
  • space surrounding the cells (extracellular fluid)

Under normal circumstances, in someone with healthy kidneys, any change in the fluid status of any of these compartments results in fluid moving into or out of the other compartments to ensure that the fluid is well-balanced across all the three compartments. However, there is a limit on the rate at which this movement can happen. This rate is around 400 ml per hour. Any movement into or out of these compartments can happen only at around 400 ml per hour or less.
During dialysis, the access is only to the fluid in the blood. Since blood is being drawn out of the body and passed through the artificial kidney (or the dialyser), fluid can be removed only from this compartment. However, excess fluid that builds up in a dialysis patient is there in all three compartments.
Let us assume that a dialysis patient came in for dialysis one day with a weight gain of 3 kgs. This 3 kg is equivalent to about 3000 ml of water. This 3000 ml is present across all three compartments – the blood, the cells and the space surrounding the cells. For simplicity let us assume that this water is distributed equally across all three compartments. So it is roughly 1000 ml per compartment. Now when dialysis begins, water starts getting removed from the blood (since the dialyser only has access to the blood).

Until the 1000 ml gets removed, there is no problem. Since this is anyway excess fluid in the blood. However, the ultrafiltration is set for 3000 ml which means fluid removal goes on beyond 1000 ml. At this point, water is being removed from the blood which is actually a natural part of the blood. So the water starts moving from the other compartments into the blood.

The fluid is being removed at the rate of 1000 ml/hour. However, the movement of the excess water from the other two compartments into the blood can happen only at around 400 ml per hour. This cause the blood to become deficient in water at the rate of 600 ml per hour. This causes a fall in the Blood Pressure as blood becomes thicker and moves slowly in the body. Similar complications like cramps also happen due to the rapid removal of fluid from the blood.

That is why the ultrafiltration rate should ideally not be more than 400 ml/hour. If you are doing a four hour session, the ultrafiltration should not be more than 1.6 kgs. If you need to remove 3000 ml, you need to set your dialysis duration to 7.5 hours to have a safe, complication-free dialysis session.

Some claims have surfaced recently that you can do three-hour dialysis sessions and achieve the same results. They claim to use high flux dialysers and get equivalent clearances. This claim is true only partially. Only toxins can be removed more efficiently using high flux diayzers. Fluid removal,  unfortunately, is a constraint of the body and not the dialysis machine or the dialyser. The 400 ml/hour limit is something that the body imposes and using any kind of diayser or machine cannot change this. There is a possibility of lesser complications happening during a session by some techniques. However, in the long run, removing fluid at a rate more rapid than 400 ml per hour can result in serious cardiac issues and affect longevity of the patient.

So, please ensure that you try to stay close to the 400 ml/hour limit set by your body. The four hour dialysis session is an arbitrary number that is chosen and should be the bare minimum duration of your dialysis session. In reality, that much is usually not enough for optimal removal of fluids and toxins. So, never even think of reducing that duration. If your centre is dialysing you for less than three hours by making ludicrous claims of doing more efficient dialysis, please insist that you get the full four hours (if not more).

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