Fluid and electrolyte balance Flashcards
What are the features of Fluid Compartments ?
The fluid compartments are fairly tightly controlled in the body both in volume and composition
Major divisions of them are;
- Intracellular
- Extracellular
Extracellular can be further divided into;
- Plasma
- Interstitial
- (Synovial, intra-ocular, CSF, etc)
Composition is determined by the movement across the plasma membrane - ions must move through channels !
How do fluid compartments look in Men and Women, and what can they be broken down into?
Male;
- 60% fluids
- 40% solids
Female;
- 55% fluids
- 45% solids
Fluids can be broken down into;
- 2/3 Intracellular fluid (ICF)
- 1/3 Extracellular fluid (ECF)
Extracellular fluids can then be broken down into;
- 80% interstitial fluids
- 20% plasma
How does pressure change across a capillary and what does this cause?
Arterial end;
- Hydrostatic pressure > Osmotic pressure
- Get net filtration (moving out vessel into cells)
Venous end;
- Osmotic pressure > Hydrostatic pressure
- Get net absorption (moving into vessels from cells)
17L reabsorbed a day
3L leaks into lymphatic systems and is returned to venous circulation
How do the plasma, interstitial and intracellular compartments of the cell vary in concentration?
Plasma and interstitial are Extracellular and very similar, main difference is Plasma has slightly more protein because its harder for proteins to leak out of the plasma, than interstitial (interstitial has a little more ions)
There is a marked contrast between Extracellular and Intracellular;
- Intracellular has barely any sodium, calcium, chlorine and bicarb (all ions BUT potassium)
- Intracellular has a much higher potassium and protein levels
How much fluid do we Lose and Gain each day?
Lose and Gain 2200ml fluid a day, not net gain for loss (balanced)
Fluid loss;
- Urine 1300ml (59%)
- Skin 600ml (27%)
- Respiration 200ml (9%)
- Faeces 100ml (5%)
Fluid gain;
- Drink 1200ml (55%)
- Food 700ml (32%)
- Metabolic 300ml (14%)
What is the difference between Sweat and Insensible water loss?
Insensible water loss you are unaware that you are loosing fluid !
Sweat;
- From specialised skin appendages called sweat glands
- Lose a variable amount of solute (sweat tastes salty!)
- Regulates body temperature
Insensible water loss;
- From skin (transepithelial) and respiratory tracts
- Doesn’t secrete solutes
- Cannot be prevented
- Evaporation of insensible fluid is a major source of heat loss from the body each day but is not under regulatory control
What are the ways we can control Water Balance?
Control of Water Balance;
- Sensors/Central controller/Effectors
- Changes to gains/losses results in a change in osmolarity and/or volume (ADH)
How do sensors work in controlling Water Balance?
Sensors;
- Osmoreceptors in hypothalamus
- Low pressure baroreceptors in right atria and great veins
- High pressure sensors (carotid sinus/aorta)
NB - pressure is used as a surrogate marker of blood volume
How does the control of Antidiuretic hormone affect water balance ?
Increase blood osmolality or a decrease in blood volume;
- An increase in blood osmolality or a decrease in blood volume stimulates neurons in the hypothalamus, resulting in an increase in ADH release from the posterior pituitary
- ADH increases water reabsorption in the kidney, resulting in retention of a greater volume of water in the blood, a reduced urine volume, and a decreased blood osmolality. There is also an increase in blood volume
Decrease in blood osmolality or a increase in blood volume;
- A decrease in blood osmolality or a increase in blood volume inhibits neurons in the hypothalamus resulting in a decrease in ADH release from the posterior pituitary
- Reduced ADH decreases water absorption in the kidney, resulting in reduction of the volume of water in the blood, increased urine volume and increased blood osmolality. There is also a decrease in blood volume
What are the 4 things to consider with any Electrolyte Abnormality?
- What are the physiological mechanisms to control the concentration of ‘x’?
- What are the consequences for the patient if the levels are to high/low?
- Is it a change of intake/loss of ‘x’ or a change of location for dilutional (E.g same amount of Ribena, add more water = more dilute)
- Does it need to be corrected and if so how quickly and with what ?
E.g if sodium level falls doesn’t mean less sodium, can just be more diluted
What is the key driver of total volume and how can it affect volume?
Key driver of total volume is total sodium;
- If total sodium drops and osmolality (tightly regulated) stays the same, the total volume falls (including plasma volume)
- If total sodium rises and osmolality stays the same, the total volume will rise
- Compensatory mechanisms are really linked to low volume (low GFR, stimulation of Juxtaglomerular apparatus (JGA) r high volume (increased GFR and release of ANP)
(Think sodium drags water with it!)
How do we gain and loose sodium ?
- Gains: Food and drink (recommended max 2.4g Na+)
- Losses: Sweat (0.25g), faeces (0.25g), urine (2-10g - main loss is in urine)
- If you eat too much/too little salt, the only controllable route of loss is via urine: hormonal control
- May increase losses via the other routes in a non-hormonally ways (e.g exercise/heat etc causing increased sweating Na+ 50mml/l; diarrhoea - 130mmol/l, vomit - 50mml/l
- Intake is normally in excess of need, e.g processed food
What are the features of Hypovolaemia, Euvolaemia and Hypervolaemia ?
Pathophysiology - More water relative to sodium needs assessment of “-volaemia”
Hypovolaemia;
- Deficient in water an Na, Na greater than water due to excess sodium loss
Hypovolaemia Examples;
- Renal: Diuretics, Adrenocortical failure
- GI: Vomiting/diarrhoea (more water lost than sodium)
- Skin: Burns
Euvolaemia;
- Normal Na, excess water
Examples of Euvolaemia;
- Polydipsia
- Iatrogenic - IV fluids
- SIADH
Hypervolaemia;
- Water and Na retention
Examples of Hypervolaemia;
- Heart failure
- Cirrhosis
- CKD
What are the causes of Hypernatraemia ?
Hypernatraemia - High Sodium Level
Free water loss;
- Renal: Osmotic diuretic, e.g Manitol
- GI: Diarrhoea or vomiting
- Skin: Sweating
- Diabetes insipidus (cranial or nephrogenic)
Inadequate intake;
- Limited access to water
- Blunted thirst
Excess Na (rare);
- Excessive salt (IV/oral)
- Note - “Normal” saline = 154 mol/l of Na+
- Usually always caused by too much water - Too much Ribena not enough water
What are the Gains and Losses of Potassium?
Remember K+ is mainly intracellular;
- Gains: via food/drink
- Losses: Predominantly via urine. Little is lost in sweat or faeces in normal conditions
- Control is therefore at the kidney
- K+ is freely filtered through the glomerulus
- Predominantly absorbed again in the PCT with controlled section at the DCT (swaps sodium with potassium here)
- Secretion is linked to Na+ reabsorption (sodium pump)
What are the causes of Hyperkalaemia and some examples ?
Hyperkalaemia;
Excessive intake of Potassium;
- Dietary potassium (but usually only a problem if reduced excretion) E.g - Eating lots of fruit juices, powdered coffee - only issue if renal impairment
- IV fluids (with additional potassium in it)
Inadequate excretion;
- Reduced GFR - AKI/CKD
- Reduced aldosterone - Addison’s, ACE-I, ARB, spironolactone
- Renin inhibition - NSAIDS/Beta-blockers
- Tubule-intestinal disease
Redistribution;
- Acidosis (Potassium level high as acidosis causes K leak out)
- Insulin deficiency
- Rhabdomyolysis
- Tumour lysis syndrome (people on chemo, cell breakdown)
What are the causes of Hypokalaemia and some examples ?
Hypokalaemia;
Excessive loss;
- Diuretics - loop/thiazides
- Urine loss
- Conns syndrome (aldosterone excess)
- Cushing’s syndrome (high cortisol)
- Genetic causes
GI loss;
- Upper: Vomiting, Nasogastric aspiration (draining contents of stomach)
- Lower: Diarrhoea, laxative
Inadequate intake;
- Dietary deficiency
- IV fluids (without potassium replaced)
Redistribution;
- Alkalosis
- Insulin
- Beta agonists (salbutamol)
What are the risks associated with IV fluids and when can we use them ?
Risks with IV fluids;
- Peripheral Vascular Catheter (or other device) required
- Easy to give too much fluid (especially in sick people)
- Errors in prescribing (e.g water, sodium, potassium load)
You must be able to justify the use of IV fluids, Oral fluids are safer!
E.g If given normal saline or dextrose saline can cause hypokalaemia
What information is important to gain in the History of a patient with fluid loss or gain ?
1). Limited intake ?
2). Abnormal losses?
- How much?
- What kind of fluid ?
- Ongoing ? Can you treat the cause?
3). Co-morbidities ?
4). Current illness? (likely to get better or worse?)
5). Symptomatic ?
6). Fluid balance charts ? (Hard to keep them all up to date, so do in patients who really need it)
What should you assess when examining a patient you expect to be fluid challenged?
Vital signs:
- Systolic BP
- Heart Rate
- Capillary Refill
- Respiratory Rate
- Urine output/colour
Hypovolemia;
- Systolic BP < 100mmHg
- Heart Rate > 90 BPM
- Capillary Refill > 2 secs
- Respiratory Rate > 20 breaths/min
- Urine output/colour < 0.5mls/kg/hr
- Dry mucous membranes
- Decreased skin turgor
- Responsive to passive leg raising
Remember: Weight gain or weight loss ?
Postural hypotension - sensitive marker of hypovolaemia
What is Postural hypotension and its significance ?
Postural hypotension - sensitive marker of hypovolaemia
Postural/Orthostatic hypotension is a form of low blood pressure that happens when standing after sitting or lying down. Orthostatic hypotension can cause dizziness or lightheadedness and possibly fainting. Orthostatic hypotension can be mild. Episodes might be brief.
What are the signs that a patient may be fluid overloaded ?
Fluid overload;
- History of cardiac or renal problems
- Raised JVP
- Peripheral oedema - but beware of other causes
- Inspiratory crackles at lung bases
- Hypertension
What are the causes of Oedema?
The causes of Oedema aren’t always fluid overload !
Increased total extracellular fluid;
- Congestive heart failure
- Renal failure
- Liver disease
High local venous pressure;
- Deep venous thrombosis or venous insufficiency
- Pelvic tumour
Lymphatic obstruction;
- Infection (filariasis, lymphogranuolma venereum)
- Malignancy
- Radiation injury
- Congenital abnormality
Low plasma oncotic pressure/serum albumin;
- Nephrotic syndrome
- Liver failure
- Malnutrition/malabsorption
Increased capillary permeability;
- Leakage of proteins into interstitial
- Infection/inflammation
- Sevre sepsis
- Calcium channel blockers
What other investigations can be helpful in the assessment of volume status ?
Other investigations helpful in the assessment of volume status;
- Full blood count
- Urea & electrolytes
- Chest x-ray
- Lactate
- Urine biochemistry
Kidneys trying to conc urine and hang onto sodium - prerenal
Established - kidneys given up, cannot hold onto sodium and cant excrete urea
What are the body’s Water and Electrolyte requirements ?
- Sodium 1mmol/kg/24 hours
- Potassium 1mmol/kg/24 hours
- Water 20-30ml/kg/24 hours
- Calories: min of 400kcal/24 hours
Note you are never going to give adequate nutrition with IV fluids, need to think other ways of nutrition
E.g 1L of 5% dextrose = 170kcal, but daily need 2000 - 2500kcal
Keep an eye on magnesium, calcium & phosphate & replace as required
What are some common IV line problems ?
Infiltraiton;
- When the catheter unintentionally enters the tissue surrounding the blood vessel and the IV fluid goes into the tissues
Phlebitis;
- Inflammation of a blood vessel
Hypothermia;
- When large amounts of cold fluids are infused rapidly
Local infection (Abscess)
- A microscopic organism may use the tiny hole in the skin created by the IV catheter to find its way into the body and cause an infection
What are the different Crystalloid IV fluids we can give?
Crystalloid IV fluids;
- 82% of Crystalloids go into the Interstitial and lymphatic compartment (part of extracellular) and 180% into the Plasma Volume (Capillary membrane)
- Crystalloids have small molecules, are cheap, easy to use, and provide immediate fluid resuscitation, but may increase oedema.
5% dextrose (glucose);
- Initially distributes through ESF and plasma; glucose metabolised so effectively adding just water
- Further distributes into cells as well as ISF and plasma
0.18% NaCI 4%dextrose (31mmoI/I Na+)
0.9% NaCl (154 mmol/l Na+)
Plasmalyte-148 (140 mEq sodium, 5 mEq potassium, 3 mEq
magnesium, 98 mEq chloride, 27 mEq acetate, and 23 mEq
gluconate)
- Distributes through ISF and plasma; does not enter cells
Fluid and Electrolyte Balance
What are the different Colloid IV fluids we can give?
Colloid IV fluids;
- 100% of Colloids fill up the Plasma Volume compartment (capillary membrane)
- Colloids have larger molecules, cost more, and may provide swifter volume expansion in the intravascular space, but may induce allergic reactions, blood clotting disorders, and kidney failure.
4.5% albumin (ie 4.5g per 100ml)
- Supplied in 0.9% NaCl
- Tends to stay in plasma; does not enter cells
- Blood product
Hydrolysed gelatin (derived from animal collagen)
- Supplied in 0.9% NaCI
- Initially tends to stay in plasma; does not enter cells
- Protein metabolised over time so then equivalent to 0.9% NaCl
Blood
- Stays in the vasculature and increases blood volume
Fluid and Electrolyte Balance
How are different IV fluids distributed ?
Glucose 5%;
- 600ml Intracellular
- 328ml Interstitial and lymphatic (Part of extracellular)
- 72ml Plasma volume (Capillary membrane)
Sodium chloride 0.18% and glucose 4%;
- 480ml Intracellular
- 426ml Interstitial and lymphatic (Part of extracellular)
- 94ml Plasma volume (Capillary membrane)
Saline 0.9%;
- 820ml Interstitial and lymphatic (Part of extracellular)
- 180ml Plasma Volume (Capillary membrane)
Balanced Crystalloids;
- 820ml Interstitial and lymphatic (Part of extracellular)
- 180ml Plasma Volume (Capillary membrane)
Colloid;
- 1 litre Plasma Volume (Capillary membrane)
When would you consider a patient to be “Fluid Challenged”?
Fluid Challenge;
- Oliguria or hypotension and no signs of overload consider a fluid challenge
- Therapeutic & diagnostic
- 500mls balanced salt solutions
- QUICKLY ! (<15mins)
- Re-assess (What’s happening to BP, tissue perfusion)
- Can repeat up to 2000mls
What patients do you need to take caution in when thinking of administering fluids?
Caution;
- Obese patients (use ideal body weight)
- Elderly or frail
- Renal impairment
- Cardiac failure
- Malnourished or at risk of refeeding syndrome (leaky?)
When the going gets tough what things should you do ?
When the going gets tough;
- CVP line
- Measure Right Atrial Pressure
- Target 8-12mmHg
- Point of care USS or ECHO
Assess & reassess - Please don’t just re-prescribe what the previous person has prescribed and hope for the best !
