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Fluid Management in Haemodialysis. Personal and confidential communication. Only for employees of the B. Braun Group. Copies, including excerpts, prohibited. Introduction.

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fluid management in haemodialysis

Fluid Management in Haemodialysis

Personal and confidential communication. Only for employees of the B. Braun Group. Copies, including excerpts, prohibited.

introduction
Introduction

Accurately assessing fluid status in a renal patient can be difficult. However, it is important to have an accurate dry weight because the morbidity associated with over-hydration and dehydration can have detrimental effects on the patients’ quality of life, and also on the quality of their dialysis.

Accurate fluid removal during dialysis is directly related to the accuracy of the estimated dry weight. This presentation describes the symptoms associated with an incorrect dry weight.

what is dry weight
What is ‘Dry Weight’?

Dry weight is defined as “the weight at which the patient has no excess fluid and below which the patient develops postural hypotension”.

Conventionally, a patient at his/her dry weight requires no anti-hypertensive medication (unless hypertension is the primary renal disease). In Renal Units practising long slow haemodialysis this is usually achievable, with the extended treatment allowing enough time to remove fluid during dialysis without risking hypotensive episodes due to vascular dehydration. However, in conventional 4-5 hour dialysis it is more difficult to take off the volume of fluid to achieve the dry weight without causing UF related hypotension.

the 3 compartment model assuming a 75kg patient
The 3 Compartment Model(assuming a 75kg patient)

The majority of fluid that a patient gains between dialysis sessions is stored within the intracellular and interstitial fluid compartments. During dialysis we only have direct access to the intravascular space, and so care must be taken to ensure that fluid is removed quickly enough to return the patient to their dry weight during the dialysis treatment, but not so quickly as to cause vascular hypotension

Toxins

Fluid

In this case fluid is removed from the blood via UF quicker than it can be replaced from the other two fluid compartments.

Toxins

Toxins

Fluid

Fluid

Intracellular Space

Interstitial Space

Intravascular Space

Dialysate Flow

total body water
Total Body Water

Water comprises approximately 58% of body weight in males and 55% in females. Water is distributed freely in three compartments, separated by semi-permeable membranes. The three compartments are:

1 – Intracellular fluid (ICF)

2 – Interstitial fluid (IF)

3 – Intravascular fluid (IVF)

IF and IVF combined form the extracellular fluid. Note that the 3 Compartment Model in the previous slide depicts blood volume (about 5 litres) as extracellular. In reality, it is comprised of both intra- and extracellular fluid because of the fluid contained in the cellular components of blood. Plasma water volume is extracellular and is approximately 3 litres.

total body water1
Total Body Water

An important principle is that extracellular water volume is primarily determined by total body sodium. This is because sodium is largely excluded from cells by an energy dependent mechanism, and the body maintains tight control of the concentration of extracellular sodium and other small molecular weight solutes (osmolarity). Thus, water follows sodium and not vice versa.

This principle of alterations in the salt concentrations and how it affects the movement of water via osmosis between different body compartments, is central to the treatment concept employed in sodium profiling. We will cover this in another presentation.

age considerations and total body water
Age Considerations and Total Body Water

However, as usual in dialysis it is not always that simple. As a person ages the total body water as a representation of their weight falls. Because of this it can be difficult to accurately assess some patients for the dry weight resulting in hypotensive episodes during the dialysis treatments.

AVERAGE TOTAL BODY WATER Child ....... 80%Adult ....... 55-60%Elderly ....... 50%

why does blood volume matter

Normal

Moderate Dysfunction

Cardiac Output

Severe Dysfunction

Blood Volume

Why Does Blood Volume Matter?

Maintenance of blood volume is critical to haemodynamic stability in health and disease. A contracted blood volume will lead to hypotension and ultimately death. This is because an adequate cardiac output requires cardiac filling which in turn is dependent on blood volume.

As blood volume rises, cardiac output rises until blood volume becomes excessive and the heart fails, resulting in pulmonary and interstitial oedema. This relationship is described by the Frank Starling curve (see next below).

fluid removal and blood volume
Fluid Removal and Blood Volume

During fluid removal on HD, the rate of fall in blood volume is determined by the balance between the ultrafiltration (UF) rate and the plasma-refilling rate. In general, when the extracellular volume is expanded (fluid overloaded patient), refilling will be rapid, allowing large UF volumes. The opposite is true when the extracellular volume is contracted.

This description is an over-simplification because a number of osmotic and hydrostatic forces influence refilling, known collectively as Starling’s Forces. Some of these are discussed further below in the section entitled “Fluid assessment and management of fluid loss in haemodialysis”.

slide10

Normal

Moderate Dysfunction

Cardiac Output

Severe Dysfunction

Blood Volume

Normal cardiovascular reflexes can compensate for a moderate fall in blood volume by vasoconstriction of the peripheral vessels, thereby maintaining blood pressure. In dialysis patents, especially diabetics, these reflexes are often impaired and this predisposes such patients to intradialytic hypotension. Further scrutiny of the diagram below reveals the fact that cardiac function also influences the relationship between blood volume and cardiac output.

Again, cardiac dysfunction is common in HD patients and this is another factor that contributes to cardiovascular instability.

sodium and fluid intake
Sodium and Fluid Intake

A logical extension of the ‘ water follows sodium’ principle is that sodium restriction should be the cornerstone of limiting interdialytic weight gains. Fluid restriction is the usual focus of the advice given, but water taken in excess of sodium will distribute in total body water and will have little influence on blood pressure. In reality this means that there is little point in advising patients about fluid intake unless this is also associated with dietary advice about sodium intake.

assessing dry weight
Assessing Dry Weight

The importance of monitoring the patients weight pre and post dialysis must be emphasised here. Accuracy is very important and it is the nurses responsibility to check the weight against previous sessions to monitor any trends and changes.

Clinical monitoring of dry weight through assessment of blood pressure, oedema, jugular venous pressure and cardio-respiratory examination has limitations.

It remains important as a means of detecting gross fluid excess or depletion but does not have the discriminatory power to predict an accurate dry weight. For example, oedema formation does not occur until extracellular volume is expanded by >10%.

Similarly the presence of a large heart or pulmonary oedema on a chest X-ray may reflect gross fluid excess.

Furthermore, dry weight is a dynamic parameter, influenced by nutritional status and intercurrent illness.

assessment of fluid status
Assessment of Fluid Status

There are a number of ways in which a patients’ fluid status can be assessed, and some for the more common methods are listed to the right. However, this presentation will concentrate on the clinical assessment criteria

  • Clinical assessment
          • difficult because of other CRF and dialysis complications
  • X-ray
          • good for overhydration
  • Biochemistry
          • iATN - overhydration, but expensive
  • Ultrasound
          • relationship between diameter of vena cava and fluid status
  • Haemoatocrit monitoring
          • Very useful tool, easy to use
  • Bio-impedance
          • Accurate assessment, however needs new equipment and training
the dehydrated patient
The Dehydrated Patient

The patient who is dehydrated will have a number of symptoms, the common of which are listed to the right. The degree to which these symptoms present will be related to the level of dehydration, and the time of the assessment. However, a number of these symptoms can also be associated with other co-morbid conditions, and care needs to be taken during the assessment.

If a patient presents with these symptoms pre-dialysis then dehydration may well be the cause, but a full assessment should be undertaken to re-evaluate the patients dry weight.

If the patient complains of these symptoms post-dialysis again a full assessment should be undertaken. For example the patient could be eating more and gaining body mass.

  • thirst
  • fatigue
  • giddyness
  • tachycardia
  • hypotension
  • skin turgor
effects of ischaemia
Effects of ischaemia

Ultrafiltration during dialysis resulting in a patient becoming dehydrated during treatment often results in distressing symptoms for the patient. Symptoms will depend upon the level of hypotension experienced. However, haemodialysis associated hypotension has been shown to be an independent and significant risk factor for 2 year mortality. Evidence now suggests that long-term effects can occur such as damage to vital organs, i.e. the brain and heart (Shoji et al, 2004).

effects of ischaemia1
Effects of Ischaemia

Vision Changes: blurred vision, loss of peripheral vision

Neurological Effects: headache, dizzyness, seizures, stroke

Renal Effects: decreased renal function, decreased residual urine

Cardiac Effects: hypotension, chest pain, silent ischaemia, MI

GI Effects: ischaemia, infarct of the gut, nausea/vomiting/diarrhoea

Adequacy Effects: decrease in KtV/URR

Muscular effects: cramps

the patient with fluid overload
The Patient With Fluid Overload

The most obvious symptoms associated with fluid overload is oedema, hypertension and pulmonary oedema. However, these symptoms can also be caused by other mo-morbid conditions, and so care must be taken when assessing the patient before reducing the dry weight target.

If the patient is fluid overloaded the blood pressure may not reduce during the dialysis treatment as would be normally expected. Studies have shown that 80% of all hypertension in dialysis patients is due to fluid overload.

Other symptoms associated with fluid overload include a feeling of being full as a result of gastro-intestinal congestion. This can lead to poor nutritional intake as a result and further loss of body mass.

One subtle symptom associated with pulmonary oedema is the patient may complain of being unable to sleep well. He may be unable to lay flat without becoming breathless. Fluid overload will also lead to a reduction in exercise tolerance.

risks of fluid overload
Risks of Fluid Overload

All anuric haemodialysis patients will retain sodium and water during the interdialytic interval. Sodium/water excess is an important determinant of hypertension and left ventricular hypertrophy (LVH) in haemodialysis patients. Hypertension and LVH are powerful predictors of cardiovascular death. There is also evidence that prolonged volume expansion eventually leads to LV dilatation and heart failure.

However, volume is not the only determinant of blood pressure but it is an indicator related to increased morbidity and mortality in volume-overloaded patients with large interdialytic weight gains. (This may reflect better nutrition in patients with large fluid gains). Therefore, achieving an accurate dry weight is a desirable goal in haemodialysis, as is minimising the number of anti-hypertensive drugs.

consequences of fluid overload
Consequences of Fluid Overload

The following is a list of consequences of fluid overload for the patient. All of which can affect their quality of life to varying degrees.

Breathlessness on exercise

Tiredness from orthopnea

Tissue oedema

Dialysis consequences which may include the following

Prolonged dialysis

Difficulty in removing fluid

Hypotension on dialysis

Reduced efficiency of dialysis

reducing the dry weight
Reducing the Dry Weight

If your assessment has determined that the patients’ dry weight needs to be reduced due to fluid overload, then this needs to be done with care. Too large a reduction in the dry weight could cause hypotension during or after dialysis. Not only will this make the patient feel unwell, affect their dialysis session but could also put their vascular access at risk.

The general rule to reduce the dry weight by 0.5kg per dialysis session and monitor the patient closely during dialysis, followed by a full assessment when they have reached their new dry weight target. Larger weight reductions may be possible if the patient is very fluid overloaded, but must be done under medical supervision.

slide21

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