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Dialysis Issues


Blood Pressure and Ultrafiltration in Dialysis





Fluid taken in, stays in


For most haemodialysis (HD) patients, fluid in the mouth means fluid in the body, with no way out except by dialysis. You see…

·       Most HD patients pass insufficient urine to keep up with a comfortable fluid intake

·       Fluid restriction is worst restriction every HD patient must bear

·       Fluid restriction is even more difficult for diabetic patients where high blood sugars aggravate an already raging thirst


Drink too much fluid and risk drowning


As fluid taken in stays in until removed by the next HD, ‘weight gain’ between HD treatments is not true body weight gain but simply reflects the amount of fluid gained in excess of loss since the last dialysis. This fluid gain:

·       Swells the blood volume

·       Distends the blood vessels

·       Raises the blood pressure

·       Wets the lungs (sometimes to the point of near-drowning)

·       Strains the heart and makes it weaker, like a repeatedly over-stretched rubber band


The sad facts of a typical dialysis day for some …


The patient struggles in for HD on a Monday morning …

·       Short of breath, heart thumping, 4 kg heavier than at the end of Friday’s dialysis

·       4 kg weight gain equates to 4 litres of retained fluid

·       The plan? … proceed to remove 4 litres over 4 hours HD at 1 litres/hour

·       The result? … cramps, nausea and a ‘flat’ - that awful moment as the blood pressure drops and the eyes roll back in a dead faint

·       The consequence? … ‘revival’ with IV fluids when the whole object of the last 4 hours has been to remove fluid

·       The outcome? … the patient goes home, washed out, exhausted and thirsty as hell, writes-off the rest of the day to recover while drinking (when knowing its wrong) to slake a raging thirst

·       The inevitable … gain 4 kg (4 litres of fluid) and start all over again


With NHHD, it isn’t like that at all


·       Imagine dialyzing for 8 out of every 24 hrs

o     As NHHD is for 8 and not 4 hours, there is twice as long to remove excess fluid

·       Imagine dialyzing every 24 hrs, not just every 48-72 hrs

o     As NHHD is every night, fluid can be removed twice as often

·       Do the mathematics ... it’s simple

o     Twice as long + twice as often = a rate of fluid removal four times as gentle


Ultrafiltration, what is it?


·       Ultrafiltration is the process by which an artificial kidney (dialyser) removes fluid

·       The rate at which the fluid is removed is called the ultrafiltration rate (or UFR)

·       The more fluid weight gained from one dialysis to the next – the higher the UFR needed to remove that fluid

·       The higher the UFR, the greater the risk of cramps, nausea, vomiting and ‘going flat’


Here is a complex concept … so, take this bit slowly…


The fluid contained in the body is effectively divided into 3 main ‘compartments’

·       There is fluid contained within the body’s cells - the cellular fluid

·       There is fluid in the tissues surrounding the cells - the extracellular fluid

·       There is fluid contained within the blood vessels - the blood plasma

·       The fluid (or body water) contained in these three compartments can move from one to the other – especially if one compartment suddenly loses some of its fluid volume. This is shown in the following diagram…



 extracell. fluid



·       Fluid can move between each of the three compartments, though the rate at which it can move does have a ‘speed limit.’

·       If fluid is removed from any one compartment (the cells, the extracellular fluid or the blood), replacement fluid will seep from the other two compartments to replenish it and to restore the balance of all three

·       Cellular fluid will then replenish the extracellular fluid – re-creating a balance

·       But … the rate at which these ‘fluid shifts’ can occur has a ‘speed limit’. This speed limit is about 0.4 litres/hr


… Exceed the speed limit and pay the price!


·       If fluid is removed from the blood stream by ultrafiltration at a rate greater than 0.4 litres/hr, the extracellular fluid cannot replace it fast enough to maintain a balance and, further, the cellular fluid falls behind in replenishing the extracellular fluid

·       The balance is upset and the blood volume must begin to fall

·       The more the UFR exceeds 0.4 litres/hr, the faster the blood volume must fall and, ultimately, the blood pressure will destabilize and fall


Back to our fluid-soaked patient


Remember our typical HD patient?

·       4 kg (litres) overloaded with fluid and 4 hours to remove it

·       Mathematics calculates a UFR of 1 litre/hr from the blood compartment

·       This UFR exceeds the capacity of the extracellular fluid to replace it by 0.6 litres/hr

·       The blood volume thus drops by 0.6 litres/hr

·       Over 4 continuous hours, the blood volume will therefore fall by 2.4 litres

·       This is like a massive ‘bleed-out’ every 2nd day


… Now you understand why HD ‘knocks people around’ so much!


The incredible gentleness of NHHD


NHHD is dialysis for twice as long and twice as often

·       Fluid gain is ½ as much (½ the time to drink)

·       UFR on NHHD is ¼ as severe - twice as long, twice as often

·       Typically, the UFR of NHHD is approximately 0.2 to 0.25 litres/hr - well within the rate at which the extracellular fluid can replenish the blood volume


… and so


·       Blood volume cannot fall

·       Blood pressure cannot collapse

·       The dreaded ‘flats’ of conventional HD cannot occur


Dialysis is safe at night because:


·       The fluid removal rate is slow

·       The blood volume does not contract

·       The blood pressure doesn’t fall

·       ‘Flats’ do not happen


Conclusions …


·       With NHHD, no-one need be there to ‘revive’ you from a ‘flat’, because ‘revival’ is no longer an issue.

·       This makes dialysis safe to perform whilst asleep and makes NHHD safe for single people at home

·       This makes NHHD a revolution like no other in dialysis history as long-held theory is turned on its head.



This makes NHHD a revolution like no other in dialysis history

as long-held theory is turned on its head.


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Authored by Prof John Agar. Copyright © 2012
Nocturnal Haemodialysis Program, Barwon Health.
All rights reserved. Revised: July 1st 2012