Week 8: Fluids and Electrolytes Flashcards
Name the compartments of the body
Intracellular compartment
Extra-cellular compartment – the interstitial fluid compartment between cells
Extra cellular compartment - Circulating fluid in the cardiovascular system – the intra-vascular compartment.
Compare the composition of the intracellular compartment with the intravascular compartment.
The intra-cellular compartment has a high concentration of large organic molecules that support the metabolic processes of cells. The principal cation is Potassium (K+ at 140 mmol.l-1) with a much lower concentration of Sodium (Na+ 10 mmol.l-1). The main anions are chloride (Cl- at c 110 mmol.l-1), and Phosphate, with significant negative charge borne on large proteins.
The concentrations of electrolytes are the same in the interstitial and intravascular compartments. The principal cation is sodium (Na+ 140 mmol.l- 1), with potassium at a much lower concentration (K+ 4.5 mmol.l-1). The principal anions are Chloride (Cl-, 100 mmol.l-1), and hydrogen carbonate (HCO3-, 26 mmol.l-1).
The intravascular compartment has much higher concentrations of large organic molecules, principally plasma proteins, and circulating cells.
Describe the Exchange Between the intra and extra cellular compartments.
The intra-cellular and extra-cellular compartments are separated by cell membranes. Cell membranes are permeable to water, generally impermeable to large, hydrophilic organic molecules, and most electrolytes cross in a controlled way through channels or pumps.
The main pump is the Na+/K+ ATPase (the ‘sodium pump’) which moves Potassium into cells and Sodium out, in both cases against concentration gradients. At rest a very large fraction of metabolic energy is used by sodium pumps. A wide variety of other pumps and transporters allow cells to control their intra cellular environment by transport between the intra and extra cellular space.
Water will move into and out of cells if there is an osmotic gradient. Movement is normally driven by changes in extra-cellular osmolarity. If the osmolarity of extra cellular fluid exceeds intra cellular fluid cells will shrink as water moves out. If the osmolarity of extracellular fluid is less than that of intracellular fluid, then cells will swell as water moves in. These changes can be very damaging.
Sometimes cells defend their internal environment against changes in extra- cellular fluid composition by moving ions across the membrane. For example, if extra cellular pH changes K+ moves into or out of cells as they protect their internal pH.
Describe the Exchange Between the interstitial and vascular compartments
The interstitial and vascular compartments are separated by the walls of blood vessels. Nearly all exchange occurs through the walls of capillaries. Normal capillaries allow water, all electrolytes and small organic molecules to cross freely, but not large organic molecules. Movement is normally between cells rather than through them. Movements are driven either by osmotic gradients, in this case the osmotic pressure exerted by the one component that cannot move freely – plasma proteins (called the oncotic pressure), or by gradients of hydrostatic pressure.
Because of the pumping action of the heart the hydrostatic pressure in capillaries is above that in the surrounding interstitial fluid for nearly all of the systemic circulation. This will tend to drive water and electrolytes out into the surrounding tissue.
The oncotic pressure will tend to draw water and electrolytes and water back in again, so the net movement depends on the balance between the two forces (‘Starlings forces’).
Normally at the arteriolar end of capillaries there is net flow out of capillaries, as hydrostatic pressure exceeds oncotic, but at the venular end there is net flow back in again, as oncotic pressure exceeds hydrostatic. This allows exchange of material between the compartments with no net volume change in either. Any change in the balance between hydrostatic and oncotic pressure may lead to net movement between the compartments, such as happens in oedema.
Physiological control of the composition of compartments
The intracellular compartment
The Intracellular compartment is controlled by cells themselves moving electrolytes across their membranes. So long as they have energy available, their capacity to do this depends upon stable composition of the extracellular compartments. In clinical terms, therefore what matters is controlling the composition of the extra-cellular fluid.
Physiological control of the composition of compartments
The extracellular compartment
Many organs influence extra-cellular fluid composition. It will tend to be perturbed by organs that add or remove water and solutes in the course of their operation, such as the gut, lungs, sweat glands etc. It is normally controlled through selective excretion of water and solute via the kidney, or specific mechanisms to drive ingestion of water or particular electrolytes.
These control mechanisms operate to control three main properties of the extracellular fluid:
The osmolarity – this affects the movement of water into and out of cells. The volume – the volume in the intravascular space is of particular
significance as it affects the functioning of the cardiovascular system.
The pH – this affects plasma calcium concentrations, which alter the function of excitable tissues in the nervous system muscles.
The kidney plays a major role in each.
The osmolarity of the extracellular compartment is determined by the amount of water in it relative to the amount of electrolyte. Control mechanisms operate to change the excretion or ingestion of water so that osmolarity is maintained at a constant value around 300 milliosmoles.l-1. This avoids cells swelling or shrinking.
These powerful mechanisms operate though osmo-receptors in the hypothalamus of the brain. They detect changes in osmolarity and alter the secretion of anti-diuretic hormone (ADH) which acts on the collecting duct of the nephron to reduce the excretion of water by producing more concentrated urine. At the same time, the sense of thirst is altered to increase or reduce ingestion of water.
How is the volume of the extra-cellular fluid compartment regulated?
The volume of the extra-cellular fluid is principally affected by the total amount of electrolyte in it, principally sodium. This is because the mechanisms above operate to control osmolarity, so that the total amount of water in the extracellular space is altered to match the solute present and keep osmolarity at 300 milliosmoles.l-1.
Control mechanisms operate to change the amount of sodium in the extra- cellular fluid and keep the circulating volume, in particular, constant. The total circulating volume determines the average arterial blood pressure in the long term, so indirectly sodium control mechanisms control blood pressure.
These mechanisms operate through the renin-angiotensin system. Changes in circulating volume are detected in the juxta-glomerular apparatus of the kidney, leading to production of the hormone Renin, which cause the release of Angiotensin I, which is converted into Angiotensin II in the lungs, before stimulating the release of aldosterone from the adrenal cortex, which acts back on the distal convoluted tubule of the nephron to cause retention of sodium. If circulating volume is too high atrial natiuretic peptide (ANP) is release from the venous side of the circulation which leads to extra excretion of sodium.
How is the pH of the extra-cellular fluid compartment regulated?
pH is controlled through an interaction between the lungs, which control pCO2 and the kidney which controls hydrogen carbonate concentration.
Any disturbance of fluid or electrolyte balance will ultimately be corrected by these mechanisms in a healthy subject, but it is often necessary to intervene to supplement their actions in a patient who is very unwell.
Loss of circulating volume
This is most commonly because of haemorrhage. In this case all components of the circulating fluid are lost in their normal proportionsDescribe the principle immediate effect.
The principal immediate effect is on the circulation. Reduced venous pressure leads to lower cardiac output and arterial blood pressure. Mechanisms operating to increase blood pressure reduce perfusion to some organs, which may lead to shock.
The reduced pressure in capillaries draws fluid back into the circulating compartment, which generates an auto-transfusion helping to restore circulating volume at the expense of interstitial volume.
If the patient survives the cardiovascular consequences, the control mechanisms operate to restore all components of the circulating compartment in their normal proportions. Sodium is retained, water retained and ingested and both plasma proteins and cells restored.
If a patient is unable or unwilling to ingest water, or excretes too much water what will happen to the volume of the extracellular compartment and how will the body correct this?
If a patient is unable or unwilling to ingest water, or excretes too much water, then the volume of the extracellular compartment will decrease and its osmolarity will increase as the concentration of electrolytes, principally sodium rises. If this dehydration is severe the volume change may compromise circulating volume and therefore arterial blood pressure. The change in osmolarity will trigger the release of ADH which should lead to retention of water, and thirst which should restore osmolarity and volume.
In many conditions water and electrolytes are lost in different proportions to those normally present in extra-cellular fluid. This is usually due to abnormal loss of some secretions, such as from the gut or sweat glands. In these cases, there is an interaction between the control mechanisms as each of volume and electrolyte concentrations are separately restored.
Describe this with the example of severe vomitting.
Here the loss is of gastric secretions. Some water is lost, but in addition there is loss of stomach acid and electrolytes such as sodium. The loss of stomach acid is significant, as that has been secreted by a process that leads to increased hydrogen carbonate in blood. Normally that increase is reversed as the stomach empties into the duodenum and the pancreas secretes hydrogen carbonate back into the gut. With vomiting that does not happen, leaving increased hydrogen carbonate in the blood – a metabolic alkalosis. This causes potassium to move into cells, lowering blood potassium levels – hypokalaemia with potentially severe consequence for the heart.
The associated dehydration stimulates the kidney to recover water and salts, which compromises its capacity to excrete the excess hydrogen carbonate. The metabolic alkalosis and associated hypokalaemia therefore only resolves once the dehydration is corrected.
Symptoms and signs of hypervolaemia (too much fluid)
Hypervolaemia: breathlessness, orthopnoea, paroxysmal nocturnal dyspnoea, ankle swelling, weight gain, peripheral and sacral oedema, ascites, hepatomegaly, hypertension, raised jugular venous pressure, displaced apex beat, third heart sound, crepitations, and wheeze.
Symptoms and signs of hypovolaemia (too little fluid)
Hypovolaemia: thirst, weight loss, dizziness, confusion, sleepy, reduced skin turgor, dry mucous membranes, sunken eyes, reduced capillary refill, tachycardia, postural hypotension, and oliguria.
In many conditions water and electrolytes are lost in different proportions to those normally present in extra-cellular fluid. This is usually due to abnormal loss of some secretions, such as from the gut or sweat glands. In these cases, there is an interaction between the control mechanisms as each of volume and electrolyte concentrations are separately restored.
Describe this with the example of severe diarrhoea.
The fluid that is lost in diarrhoea is principally water and electrolytes that have been secreted into the intestines and not re-absorbed as usual. There is, therefore a large net loss of both water and electrolytes. The principal electrolytes lost are sodium, potassium and hydrogen carbonate.
The dehydration will compromise circulating volume. Generally, more water is lost than electrolytes, so osmolarity increases, but hydrogen carbonate loss may lead to metabolic acidosis.
In many conditions water and electrolytes are lost in different proportions to those normally present in extra-cellular fluid. This is usually due to abnormal loss of some secretions, such as from the gut or sweat glands. In these cases, there is an interaction between the control mechanisms as each of volume and electrolyte concentrations are separately restored.
Describe this with the example of severe sweating.
Sweat is largely water, but as the rate of sweating increases the secretion has progressively higher concentrations of sodium. Severe sweating without copious ingestion of water therefore reduces water, so reducing volume, but also leads to loss of total extra-cellular sodium (even if the concentration of sodium increases because of the dehydration).
Fluid and electrolyte changes as a result of illness or injury
Normal fluid and electrolyte balance can also be altered by disease and injury by non-specific metabolic responses to stress, inflammation, malnutrition, medical treatment, and organ dysfunction. During the catabolic phase of the stress response potassium is lost, sodium and water are retained, and oliguria ensues. After surgery, it is therefore important to differentiate oliguria caused by the stress response (harmless) from that caused by acute kidney injury. Inflammatory conditions (for example, sepsis or after trauma or surgery) reduce the endothelial barrier function. Fluid may leak from the intravascular space into the interstitial fluid compartment producing interstitial oedema. Malnutrition can lead to sodium and water overload and depletion of potassium. Many drugs can upset fluid and electrolyte balance e.g. loop diuretics (hypovolaemia and hypokalaemia) and corticosteroids (fluid retention). Heart failure and cirrhosis may lead to an expanded extracellular fluid compartment, peripheral oedema, ascites, and circulatory overload with intravenous fluids.
Describe Hartmann’s solution
Hartmann’s solution is most physiological (similar to plasma) which means it is good for replacing plasma loss. It is however not good for normal maintenance as it will give too much sodium and not enough potassium.
what is ‘normal saline’?
Sodium Chloride (0.9%) – so called ‘normal saline’ is high in sodium and chloride. Too much sodium causes a high sodium load on the kidney and too much chloride could lead to hyper-chloraemic acidosis.
An infusion of 0.9% sodium chloride (a common IV fluid) increases the volume within the blood vessels which increases the hydrostatic pressure and reduces the oncotic pressure within the blood vessels. This results in fluid moving from the plasma into the interstitial fluid. Sodium will diffuse into the interstitial space, but not into the intracellular space because of the Na+/K+-ATPase pump actively secreting sodium out of the cell space. This leads to the extracellular fluid being hypertonic to the intracellular space due to the increased sodium load which results in water movement from the intracellular space to the extracellular space.
when would 5% Glucose IV be used?
5% Glucose is used for maintenance to give water when needed. It has no place in replacing plasma/ blood loss – it is not physiological. Giving too much too quickly can lead to hyponatraemia.
Administering an infusion of glucose 5% (another common IV fluid) will result in expansion of the vascular compartment with fluid which will become hypotonic towards plasma (the glucose is metabolised). Fluid will therefore distribute itself into the interstitial space and into the cell compartment.
Very little glucose 5% remains in the vascular compartment, which is why it is not the fluid of choice for resuscitation.
Sodium chloride infusion therefore results in the expansion of the vascular compartment more effectively compared to Glucose 5%.
when would Glucose-saline IV be used?
Glucose-saline contains some glucose and some sodium chloride. It is a good fluid for maintenance – it contains approximately the correct sodium (when given at the correct rate for weight).
What are the advantages and disadvantages of Colloid solutions ?
Colloid solutions contain large molecules which stay in the vascular compartment. A Cochrane review showed no evidence from randomised controlled trials that resuscitation with colloids reduces the risk of death, compared to resuscitation with crystalloids, in patients with trauma, burns or following surgery. Colloids may cause anaphylaxis
Using IV fluids
When thinking about IV fluids use the 5 Rs. Describe these.
(Resuscitation, Routine maintenance, Replacement, Redistribution and Reassessment)
Consider prescribing less fluid (20–25 ml.kg-1.day-1 fluid) for patients who are older, frail, have renal impairment or cardiac failure.
For routine maintenance alone consider using 25–30 ml.kg-1.day-1 sodium chloride 0.18% in 4% glucose with potassium. This is an initial prescription and further prescriptions should be guided by monitoring.
When thinking about IV fluids and the 5 Rs, describe Resuscitation
IV fluids may need to be given to restore circulatory volume in patients with hypovolaemia. Hypovolaemia may result from bleeding, plasma loss (e.g. burns), fluid and electrolyte loss (e.g. diarrhoea & vomiting). If patients need IV fluid resuscitation, use crystalloids that contain sodium in the range 130– 154 mmol.l-1, with a bolus of 500 ml over less than 15 minutes.
When thinking about IV fluids and the 5 Rs, describe Routine Maintenance
Patients who are unable to take fluids orally will need their routine fluid requirements met by prescribing IV fluids. This fluid is in addition to deficits or continuing abnormal loses.
IV fluids for routine maintenance alone can be calculated using
Water: 25–30 ml.kg-1.day-1
Potassium, sodium and chloride: approximately 1 mmol.kg-1.day-1
Glucose: approximately 50–100 g.day-1 of glucose (limits starvation
ketosis).
For obese patients adjust the IV fluid prescription to their ideal body weight (patients rarely need more than a total of 3 litres of fluid per day).
Consider prescribing less fluid (20–25 ml.kg-1.day-1 fluid) for patients who are older, frail, have renal impairment or cardiac failure.
For routine maintenance alone consider using 25–30 ml.kg-1.day-1 sodium chloride 0.18% in 4% glucose with potassium. This is an initial prescription and further prescriptions should be guided by monitoring.
When thinking about IV fluids and the 5 Rs, describe Replacement
Many of your patients will not need urgent fluid resuscitation but will need IV fluids in addition to their routine maintenance needs. The additional fluid is to correct existing deficits or to meet ongoing losses.
When thinking about IV fluids and the 5 Rs, describe Redistribution
Some patients with sepsis, major surgery, liver, kidney or renal disease may have abnormal fluid handling. Many develop oedema from sodium and water excess. Fluid may accumulate in the GI tract or thoracic and peritoneal cavities.
Describe the content of the different Fluid compartments
We are about 60% water, divided across compartments of the body
• Intracellular compartment
• About 40% of body water – c28l
• Extracellular compartment
• About 20% of body water – c14l
• Divided into interstitial (15%) and intravascular (5%) components
• Plus a small number of specialised spaces (eg CSF, joint fluid, pleural fluid etc)
How does water move through the compartments?
Water can move freely between all compartments
• Movement driven by osmotic forces
• Generated by changes in the concentration of
solute in compartments
• Solutes may or may not cross between compartments
• Movement is often controlled
• Movement may be against concentration gradients (active) or with them (passive)
What determines the volume of a compartment?
• Determined by the amount of water in them
• The volume of the intracellular compartment determined by movement of water to and from the extracellular compartment
• Mostly determined by the solute concentrations in the extracellular compartment
• Shrinkage or swelling of cells is very harmful
• The volume of the extracellular compartment is determined by exchange of water and solute with the environment
Distribution of volume between the interstitial and intravascular compartments determined by exchange of water and solute with interstitial fluid at capillaries
