Fluid Balance
See Also: Fluid Administration
Questions Received:
Responses:
Could you explain what is meant by CVVH?
26th May 1999
CVVH stands for continuous veno-venous haemofiltration. It is a procedure that is practised in specialist areas such as Intensive Care Units and Renal Units and requires skilful management.
Hydrostatic ultrafiltration is a method by which "water is forced across a semi-permeable membrane by hydrostatic pressure exerted across the membrane" (Adam and Osborne, 1998). Physiological hydrostatic ultrafiltration of the blood - haemofiltration - takes place within the kidneys, and is the means by which the body is able to dispose of certain waste products and maintain a suitable fluid balance. The pressure of the blood circulating through the capillary network in each glomerulus causes water and some small-to-medium-sized solutes to pass into the renal tubules as an ultrafiltrate. The solutes include inorganic salts, glucose, amino acids, and creatinine - all substances with a molecular weight of less than 70,000 Daltons. Larger materials, for example plasma proteins and blood cells, do not pass through the membrane and remain in the plasma.
Artificial haemofiltration uses similar principles and is a very efficient method for removing fluid and solute from the body, particularly when cardiac impairment causes arterial hypotension (Woodrow, 1993). Unlike the kidney, however, the removal of solute by haemofitration is a non-selective, convective, process. Biochemical monitoring and isotonic solute replacement is therefore essential during the procedure. Continuous veno-venous haemofiltration involves an extracorporeal circuit and a pump. The pump provides the hydrostatic pressure to push the water and solutes in the plasma across the semi-permeable membrane to become the filtrate. CVVH can create fluid 'space' in the intravascular compartment to allow other fluids to be administered, for example infusion and transfusion fluids and parenteral nutrition.
Method
A double lumen cannula is inserted by a doctor into the jugular, subclavian or femoral vein. The extracorporeal circuit is from the vein to the haemofilter and back to the vein, with an ultrafiltration line from the haemofilter to a collection bag.
Objective Monitoring
Meticulus monitoring is essential and involves:
Fluid Balance - The volume of filtrate removed can be over 2 litres per hour (Adams and Osborne, 1998). Fluid input has to be balanced against output with the achievement of either equilibrium or a positive or negative balance; determined by the patient's clinical condition
Haemodynamic - Continuous ECG, blood pressure, pulse, and temperature measurements are required. Hypovolaemia can occur as result of the extracorporeal circuit and requires corrective treatment. Also because blood is circulating outside the body it cools and contributes to heat loss. Added to this the infusion of large volumes of fluid will contribute to a fall in peripheral temperature unless the fluid is warmed prior to infusion
Air Detection - The circuit must be kept free of air and the "Air Detect" device must be constantly used. If a femoral vein is used for venous access the dependent limb must be observed for signs of extravasation - swelling or bruising.
Visual checks of the circuit need to be made regularly to avoid the serious consequences that would arise if a leak occurred. Also the sites for venous access must be observed for signs of inflammation that might be due to infection.
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Advantages and disadvantages of CVVH (Woodrow, 1993) |
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| Advantages | Disadvantages | |
| Major | Patient Comfort Cardiovascular Stability Good Middle Molecule Clearance Reduced Morbidity |
Patient Immobility Fluid Balance Error Poor Small Molecule Clearance Heat Loss |
| Minor | Creates Space for Feeding Less Risk of Disequilibrium |
Increased Nursing Workload Expense |
References
Adams, S.K., and Osborne, S. (1998) Renal problems. In: Critical care nursing: science & practice, edited by M. Singer. Oxford: Oxford University Press (pp. 220-226).
Woodrow, P. (1993) Resource package: haemofiltration. Intensive and Critical Care Nursing, 9, 95-107.
Our grateful thanks to Mrs.S.Hudson, Senior Sister, Intensive Care Unit, Royal Devon & Exeter Healthcare N.H.S. Trust, Exeter, Devon, England for her help in compiling this response.
Please can you explain the differences between CVVH and haemodialysis, and explain their physiological effects?
28th July 2000
Haemodialysis is used to restore physiological homeostasis in acute or chronic renal failure, while haemofiltration is used to either support the aims of dialysis or to remove fluid in cases of fluid overload and to create space if fluid needs to be administered intravenously.
Dialysis (Renal Replacement Therapy)
This is a procedure designed to sustain life in someone who has developed acute or chronic renal failure. One of two procedures may be employed, either haemodialysis or peritoneal dialysis.
Dialysis requires a semipermeable membrane as an interface between the blood and a specially prepared physiological solution known as the dialysate. In haemodialysis the specially-designed fine tubes of the artificial kidney act as the semipermeable membrane. In peritoneal dialysis the peritoneal membrane covering the abdominal viscera and lining the inside of the abdominal cavity provides the semipermeable membrane.
The semipermeable membrane allows water and small molecules to pass through by a process of diffusion (from a region of high concentration to region of low concentration) whilst preventing the transfer of larger molecules such as plasma proteins. This selective process forms the basis of dialysis. In the case of haemodialysis, substances in high concentration in the blood of a patient with renal disease, for example urea, creatinine, sodium, and potassium, will move across the membrane into the dialysate. The dialysate can be prepared with substances such as bicarbonate that will diffuse in the opposite direction through the semipermeable membrane and into the patient's blood to correct metabolic imbalances such as acidosis.
The patient's serum electrolytes and biochemistry determine the exact composition of the dialysis fluid. Diffusion is achieved by providing a concentration of salts within the dialysate that is lower than that which exists in the patient's blood. The dialysate can also be made more concentrated - hyperosmolar - in order to remove water from the blood by osmosis. Water transfer can also be achieved by increasing pressure in the blood compartment or reducing pressure in the dialysis compartment, a process known as ultrafiltration. Thus the three processes, diffusion, osmosis and ultrafiltration form the basis of dialysis. During dialysis the patient's blood flows through the artificial kidney in the opposite direction to the flow of the dialysate to produce a countercurrent effect which enhances transfers between the two flows.
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Example of dialysis fluid composition* |
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| Component | Concentration |
| Sodium | 130-135 |
| Potassium | 1.5 - 3.0 |
| Calcium | 1.6 - 1.8 |
| Magnesium | 0.25 |
| Chloride | 95 - 100 |
| Lactate/Acetate or Bicarbonate | 40 |
| Glucose | 0 - 10 |
| *the exact composition will be adapted to the needs of the individual patient | |
Haemofiltration
Dialysis has been in existence for about forty years, but haemofiltration is a relatively new technique, being introduced in the UK in the mid 1970's. Like dialysis, haemofiltration uses a semipermeable membrane to remove fluid and solutes from the blood. Two processes are involved: ultrafiltration and convection. As with dialysis, ultrafiltration requires the application of a pressure differential. This is achieved through arterial pressure in continuous arteriovenous haemofiltration (CAVH) and the use of a pump on the blood delivery side of the circuit in continuous venovenous haemofiltration (CVVH). As fluid moves across the semipermeable membrane it carries solutes such as sodium, potassium, urea, and creatinine by the process of convection. The rate of convection depends upon the size of solute molecules, hydrostatic pressure, membrane permeability and the ultrafiltration rate (Challinor et al, 1998).
Haemofiltration can be used in place of haemodialysis. However, its use is mainly as an adjunct to haemodialysis and whenever there is a need to create fluid space, as for example before administering intravenous fluids in large quantities as in the case of total parenteral nutrition (TPN). The procedure can also be used to remove fluid from someone who has developed a fluid overload.
According to Coraim and Wolner (1995), '[w]hen used early in the management of moderate to severe heart failure, haemofiltration can increase mean arterial pressure, reduce preload and afterload, reverse cardiac dysfunction, help restore renal function, and eliminate cardiopulmonary toxic metabolites from the plasma, all of which improve survival'.
As both haemodialysis and haemofiltration constitute artificial means for restoring homeostasis, careful monitoring of the patient's fluid balance, weight, blood pressure, pulse, temperature and blood clotting status is required. Arteriovenous shunt sites, cannula sites, and any central lines also need close observation by nursing staff.
References
Challinor, P., and Sedgewick, J. (1998) Principles and practice of renal nursing. Cheltenham: K. Stanley Thornes, Publishers (Chapter 5, p 83).
Coraim F.E., and Wolner, E. (1995) Continuous hemofiltration for the failing heart. New Horizons, 3(4), 725-731.
Evans, D.B., and Henderson, R.G. (1985) Lecture notes on nephrology. Worcester: Blackwell Scientific Publications (Chapter 16, p.161).
Could you please explain the principles underpinning fluid and electrolyte balance/imbalance, and what the nurse should be looking for if there is a problem?
26th January 2001
We hope the following notes will help as a first step towards an understanding of the mechanisms involved in fluid and electrolyte balance and how these principles can be applied to nursing care.
Fluid Balance
Approximately 60 to 70% of the total body weight of an average adult is composed of water. In an infant the proportion is between 75 to 80%. If we exclude the period of growth between infancy and adulthood and ignore the factors that lead to obvious weight loss or weight gain, we can say that body weight - at least on a day-to-day basis - remains relatively constant. As part of this balance, water intake must be balanced with water loss.
We gain water mainly by eating and drinking, and a smaller contribution comes from metabolic processes occurring within the body. We lose water from the lungs during expiration, via the skin, particularly when sweating, from the gastrointestinal tract during defaecation, and from the urinary system as urine. Disorders in any of these systems can severely affect fluid balance.
The average adult takes in approximately 2.5 to 3.0 litres of fluid from food and drinks over a 24-hour period. We do not measure the exact amount of fluid each time we drink but consume what we feel we need and leave the body to make the necessary adjustments.
The kidneys produce between 1.5 and 2.0 litres of urine per day, and this is regarded as a measurable loss since it can be readily quantified. The remaining water is lost via the lungs, skin and alimentary tract - insensible losses since they are harder to measure.
If the intake of fluid over a 24-hour period significantly exceeds the amount lost, a state of over hydration will exist. On the other hand if the amount of loss exceeds the amount gained a state of dehydration will exist. Nurses are commonly involved in providing care that is not only designed to monitor and adjust fluid imbalance, but also to prevent such deviations from occurring.
Electrolyte Balance
Apart from water, carbohydrate, protein, fat, vitamins and trace elements, the diet also contains inorganic ions such as sodium (Na+), potassium (K+), chloride (Cl-), and hydrogen (H+). Again, as with water consumption we do not consciously measure out our daily requirements for each of these substances but ingest varying quantities and leave our body to make the necessary adjustments. In the body these chemicals are in solution in the body fluids, and it is the concentration of each that is important. The survival and normal functioning of cells in the body can only occur if the concentration of electrolytes is precisely maintained. In other words the maintenance of a constant internal environment is essential for the constituent cells.
As with water balance, the kidneys play a key role in the maintenance of sodium, potassium, chloride and hydrogen ion balance. The amount of plasma filtered by the kidneys each day and released as urine is largely regulated by antidiuretic hormone (ADH) which is released from the posterior lobe of the pituitary gland. ADH acts directly on the collecting ducts within the kidney. The production of ADH is influenced by the state of hydration - as fluid levels increase the release of ADH falls. As a result the kidney produces more urine until fluid balance has been restored. By contrast, a water deficit triggers the release of more ADH with the result that the kidneys conserve water.
The hormone aldosterone, which is produced by the adrenal cortex, regulates the concentration of sodium and potassium in the extra cellular fluid. Aldosterone achieves this by its action on the collecting tubules of the kidney. The mechanism is similar to ADH, release being triggered by the amount of sodium and potassium in the extra cellular fluid. In this way the kidney conserves these substances when the amount in circulation is low and releases them when they are in excess.
Hydrogen ions are also excreted by the kidney. The amount excreted depends on how much acidity they are creating in the plasma. If the blood becomes too acidic, excess hydrogen ions are excreted until the pH of the blood returns to the optimum value of 7.4, which is slightly alkaline. The other system that plays a key role in maintaining the pH of the blood is the respiratory system - by breathing more rapidly the release of carbon dioxide can be increased and blood acidity reduced.
The kidneys are therefore very important in maintaining fluid and electrolyte balance, and we need to have an understanding of the control systems such as the hormones ADH and aldosterone that regulate their functioning.
The concentration of electrolytes in body fluids is maintained within narrow limits. By collecting a venous sample of blood and sending it to the laboratory for analysis the doctor is able to obtain information about a person's state of hydration and renal function. Leaving aside the cells in the venous specimen of blood, examination of the plasma will provide information about the concentration of the substances so far discussed. Normal values for these are:
| Sodium | 135-145 mmol/l |
| Potassium | 3.5-5.2 mmol/l |
| Chloride | 96-107 mmol/l |
These values can change as a result of either over or under hydration, increased or diminished intake of inorganic salts, failure in one of the excretory pathways such as the kidneys, or in response to drugs, for example certain diuretics.
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Related Terminology |
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| Hyponatraemia | Plasma Sodium <135 mmol/l |
| Hypernatraemia | Plasma Sodium >145 mmol/l |
| Hypokalaemia | Plasma Potassium <3.5 mmol/l |
| Hyperkalaemia | Plasma Potassium >5.2 mmol/l |
Nursing Care
Nurses often care for people across the whole age spectrum with fluid and electrolyte disturbances. Providing such care requires careful monitoring.
| Objective Monitoring | Recording fluid intake and output and determining whether positive or negative balance is being achieved |
| Recording weight, possibly on a daily basis | |
| Taking and recording the pulse (important if there is a potassium imbalance) and blood pressure | |
| Maintaining fluid replacement regimes, (either enteral or parenteral), at the correct rate | |
| In extreme cases, it may be necessary to monitor the central venous pressure (CVP). |
| Subjective Observations | Observing the person's skin (the skin of someone who is dehydrated becomes inelastic) |
| Observing the tongue and mucus membranes for dryness, and observing whether the degree of dryness makes speaking difficult | |
| Observing the urinary output (the urine of someone dehydrated is concentrated, the urine of someone over hydrated by contrast is dilute) | |
| Observing for signs of weakness (electrolyte disturbances can produce weakness, particularly hypokalaemia) | |
| Observing the person's mental state as disorientation and impaired consciousness may develop. |
What can you tell me about Alaris intravenous infusion (IVI) pumps and their use in fluid replacement? I would appreciate instructions on priming an IVI and use of the pump. Please supply any references.
13th May 2001
Alaris Medical Systems have arisen out of IMED and IVAC, two companies that have long been involved with the design and manufacture of electronic infusion devices.
Electronic infusion pumps are designed to ensure accurate administration of intravenous fluid volume (which may contain drugs) over a given period of time. Such devices, which include syringe drivers, are known as volumetric pumps as they control and monitor volume, rather than drops per minute.
The IVAC Model 598 is an example of a volumetric pump that is in common use. Here the total volume to be infused, together with the number of millilitres to be delivered per hour, according to the period of time over which the infusion has been prescribed, is keyed into the pump's memory. When the pump is set to ‘run' the fluid passes down the giving set into the patient until the pre-set volume has been delivered
Example
Prescribed: 1000ml sodium chloride to run over 8 hours
Infusion pump is first set at 1000 (volume to be infused - VTBI)*
Infusion pump is next set to deliver 125ml per hour
*if a flow sensor is used and the volume in the container is exactly the amount to be infused it is not necessary to set the VTBI.
Most pumps operate by transmitting a series of peristaltic waves along the tubing of the giving set as it passes through the pump's clamped chamber. A sensing device within the pump detects air in the line and when present sounds an alarm.
Principles of Administration
Before the tubing of the specified giving set is attached to the pump it is primed with the prescribed intravenous fluid, care being taken to ensure that all the air has been expelled and that asepsis is being maintained. A special section within the giving set is then placed through chamber of the infusion pump according to the manufacturer's instructions and clamped into position. The giving set is then attached to an intravenous cannula and the pump is set to run.
Regular checks are conducted on the infusion line and the pump throughout the intravenous regime. The patient's condition is monitored throughout according to local practice.
N.B. Nothing stated here is meant to override or replace either the manufacturer's instructions or local hospital or community policies on intravenous fluid administration and the use of volumetric pumps.
We should like to thank Mr. Carlos MUNOZ, Product Manager at Alaris Medical Systems for his help with this response.