Based on my experience, I have seen two kinds of situations when someone cramps during dialysis:
- When the rate of fluid removal is too high - When too much fluid is removed
What's the difference between the two, you might wonder? There is. Let me explain.
My dry weight is 79 kgs. Dry weight for the uninitiated, is the body weight when there is no excess fluid in the body. Why might there be excess fluid in the body? For the horribly uninitiated, when the kidneys don't work, fluid builds up in the body because the kidneys do not remove it.
So, my dry weight is 79 kgs. Let's say I put on about 3 kgs as a result of drinking too much water. Now the goal of dialysis usually is to come back to your dry weight. So, ideally I must remove 3 liters of water (because a liter of water weighs about a kilogram) to come back to my dry weight.
If I dialyze for 4 hours, I must remove water from my body at the rate of 750 ml/hour which is too high. Studies have shown that when water is removed from the body at a rate greater than about 400 ml/hour, it is too fast. Cramps can happen.
Another possibility is that too much water is removed, even if the rate of removal is 400 ml/hour or less. For example, let's say I did a 9 hour session and tried to remove 3.5 liters of water during the 9 hour treatment. Even though the rate of fluid removal is less than 400 ml/hour, cramps can occur because I will be below my dry weight towards the end of treatment. Basically I am squeezing my muscles of the water needed for them to function. This results in cramps.
That is why proper monitoring of the dry weight is very important. Unfortunately there is no accurate method of determining the dry weight. We have to use indicators like general feeling of well being and the blood pressure to guess the dry weight.