5. Fluid + Electrolyte Na+ Flashcards
What are electrolytes?
Ions capable of carrying electric charge
Anions -
Cations +
What are the main physiological electrolytes to consider?
Sodium Na+ Potassium K+ Calcium Ca2+ Magnesium Mg2+ Chloride Cl-
Acid base;
Hydrogen phosphate HPO42-
Hydrogen carbonate HCO3-
What do the electrical charge symbols on an ion indicate?
Indicate substance is ionic in nature + has unbalanced distribution of electrons = result of chemical dissociation in solution
What are the main functions of electrolytes in physiology?
Conduct energy;
- neurons = electrical tissue, activated by electrolyte activity b/n ECF + ICF via ion channels
Regulate fluid balance + pH;
- Na (main in ECF) + K (main in ICF) involved in fluid balance + BP
- osmotic gradients affect + regulate hydration + blood pH = critical for nerve + muscle function
Support muscle function;
- muscle tissue = electrical tissue, activated by electrolyte activity b/n ECF + ICF via ion channels
- muscle contraction dependent on presence of Ca2+, Na+, K+: deficiency = muscle weakness/severe contractions
What physiological processes are electrolytes essential for?
Volume + osmotic regulation (Na+, Cl-, K+);
- osmoregulation = physiological process maintaining fixed conc of cell-membrane impermeable mol + ions in the interstitial fluid
Myocardium rhythm + contractility (K+, Mg2+);
- contraction beings with characteristic flow of ions across cell membrane = AP
- triggers muscle contraction by increased Ca2+ in cytosol
Enzyme cofactors (Mg2+, Ca2+, Zn2+);
ATP production + ion pumps (Mg2+, PO4-);
- binding of divalent cation (almost always Mg2+) strongly affects interaction of ATP with various proteins
- ATP exists in cell mostly as complex with Mg2+ bonded to phosphate oxygen centres
Neuromuscular excitability (K+, Ca2+, Mg2+)
How are electrolytes balanced in the body + why?
Electrolyte homeostasis must be maintained to allow proper function
Depends on integration of respiratory, renal + behavioural systems
H2O + Na2+ regulation maintained against variation in volume + osmolality
What contributes to electrolyte intake?
Food
Fluids
Metabolic reactions
What contributes to electrolyte output?
Respiration (H+/HCO3-)
Excretion (H2O/electrolytes)
Behaviour (alcohol = increased urination)
Describe the overall fluid state of the human body
Average H20 content in body = 40-75% total body weight
- H2O = solvent for body processes
- women >H2O as body comp different
2/3 body H20 = ICF
1/3 body H20 = ECF
Plasma (ECF component) = 93% H2O + lipids + proteins
What are the different tonicities?
Hypertonic = increased osmotic pressure aka more solutes in a solution
Hypotonic = decreased osmotic pressure aka less solutes in a solution
Relative to another solution
Describe the fluid shift between ICF + ECF
ECF + ICF are dynamic;
- link b/n external/internal environments = plasma (only fluid circulating whole body)
- osmolalities of all body fluids equal: changes in solute concentration quickly follow by osmotic changes
Water movement; ECF hypertonic (more solute) = water moves ICF -> ECF ECF hypotonic (less solute) = water moves ECF -> ICF (cells)
Solute movement;
Ion fluxes restricted + move selectively by active TP
Opposing conc of Na+ and K+ in ECF + ICF;
- maintained by activity of cellular ATP-dependent Na+/K+ pumps
Nutrients/resp gases/wastes move unidirectionally
Define + describe the ECF
ECF (1/3 BF) = all body fluid outside cells of multi cell org
Components;
- interstitial fluid: main, bathes cells: provides nutrients/ removes waste
- blood plasma (both = 97% ECF)
- transcellular fluid: smallest, contained within epithelial lined spaces e.g. CSF, aq humour, serous fluid, peri/endo lymph, joint fluid (2.5% ECF)
Plasma + IF v similar as water, ions + small solutes constantly exchanged b/n across walls of capillaries through pores + capillary clefts
Define + describe the ICF
ICF (2/3 BF) = the cytosol, the fluid contained inside cells
The matrix in which cell organelles are suspended
Define osmolality + its use in physiology
Osmolality = physical property of a solution based on concentration of solutes (mOsm/kg) per kg of solvent
Controlled by release of arganine vasopressin hormone (AVH/ADH) from post pit - changes water output in urine
Used to assess water balance + whether body is regulating properly
Define osmolarity + its use in assessing fluid balance
Osmolarity = concentration of solutes (mOsm/L) per litre of solvent
Can be inaccurate as affected by temp, H20 content, pressure
Define the osmolar gap + its use in assessing fluid balance
Osmolar gap = difference b/n measured osmolality + calculated osmolarity
Due to differences in the way blood solutes are measured + their calculation method
Normally osmolality contributing particles in serum = sodium, potassium, salts, glucose, urea
Osmolar gap calculates difference in osmolarity + osmolality to see if OTHER particles are contributing to measured serum osmolality
> 10 indicates for e.g. alcohol, aspirin, manitol
What is the normal plasma osmolality maintained at + how sensitive is it to change?
Maintained at ~275-295 mOsm/kg of plasma H2O
Very sensitive to even 1-2% changes in osmolality
What is the main osmotic agent in ECF?
NaCl is the main osmotic agent in ECF - cannot alter one aspect of sodium/water balance without triggering homeostatic mechanisms which influence the other
How is plasma osmolality regulated?
Kidney function: H2O output in urine/salt output or reabsorption
Kidney function regulated by anti-diuretic hormone (ADH) (aka arganine vasopressin hormone/vasopressin)
- ADH produce in hypothalamus
- ADH stored in + released from posterior pit
Primary function of ADH = decrease water loss in kidneys = decrease conc of electrolytes in body fluid
Describe the ADH loop regulating osmolality
Increase in salt concentration;
- sensed by receptors in hypothala > signals post pit
- post pit increases ADH secretion
- blood reabsorbs more water from filtrate in kidney
- urine is more concentrated
- salt concentration of body fluid is decreased
Decrease in salt concentration;
- sensed by receptors in hypothala > signals post pit
- post pit decreases ADH secretion
- blood reabsorbs less water from filtrate in kidney
- urine is less concentrated
- salt concentration of body fluid is increased
Describe the state of sodium in the body + its relation to osmolality
Normal plasma osmolality = 270-295mmol/L due to Na+ and assoc anions
90% of all cations in the ECF
ECF = high Na+/low K+
ICF = low Na+/high K+
Cell balance is maintained by Na+/K+ ATPase ion pumps;
- 3 Na+ pumped out of cell in exchange for 2 K+ ions
- ATP>ADP
How is sodium measured?
ISE - routine in clinical lab Colorimetric - useful for POCT Enzyme modulation Flame emission spectroscopy (FES) Atomic absorption spectroscopy(AAS)
Some methods require large sample volume
Describe ISE as a method for sodium measurement
Uses semi-permeable membrane allowing particular ions to pass through;
- potential difference builds up b/n outside + inside of membrane depending on ion conc
- difference measured by 1 test electrode in respect to 2nd reference electrode
Measures ion activity not conc
ISE for sodium;
- ISE calibrated with Na+ soln of known concentrations
- 2 types of ISEs with no significant difference b/n results
- DIRECT ISE: reqs larger, undiluted sample
- INDIRECT ISE: reqs smaller, dilute sample (dilution error)
Advantages;
- cheap
- easily automated
- quick
Disadvantages;
- dilution error
- protein build up on membrane
- nonspecific
Discuss the possibility of dilution errors with ISE
Dilution + calculations to produce ISE depend on assumption sample contains normal fat levels
In v lipidemic samples = apparent hyponatremia even when biologically active sodium conc in aq phase is normal
What are the types of water balance disorders?
Dehydration
Hypotonic hydration
Describe dehydration, its signs + symptoms, and causes
Water loss> water intake
S+S;
- thirst
- dry/flushed skin
- oligouria (decreased urine production)
Causes;
- hemorrhage
- severe burns
- prolonged vomiting/diarrhea
- profuse sweating
- water deprivation
- diuretic abuse
Describe hypotonic hydration, its complications, and causes
Cellular overhydration/water intoxication
- diluted ECF: normal [Na+] but excess [H2O] = hyponatremia
- net osmosis into tissue cells (ECF>ICF)
Causes;
- renal insufficiency
- large volume water ingested
Complications;
- if not quickly reversed = severe metabolic disturbances, especially in neurons
What is referred to when discussing electrolyte balance in physiology?
Electrolytes = salts, acids + bases
But electrolyte balance usually refers to salt balance only
How can osmolality be used to classify + identify causes of hyponatremia?
Na+ = major contributor to osmolality therefore levels can be used to help identify cause of hyponatremia;
Hyponatremia with;
- low osmolality: increased Na+ loss/H2O retention
- normal osmolality: increased non-sodium cations/lithium excess/hyperlipidemia/ lots of others
- high osmolality: hyperglycemia, mannitol infusion
How does urea + electrolytes test help assess fluid status?
U+E = most commonly performed biochem lab test
Analytes;
- electrolytes: Na, K, Cl, HCO3
- markers of renal function: urea, creatinine
Provides info on renal function + dehydration
- can exclude serious imbalance of salts in blood
- creatinine levels = eGFR (gold std marker kidney health)
Abn very common in hospitalised pts
Conditions detected;
- hyper/hyponatremia
- hyper/hypokalemia
- metabolic acidosis/alkalosis (HCO3-)
- dehydration (increased urea/creatinine)
- GI bleed (increased urea)
- hyper/hypoglycemia (glucose)
Describe some of the main causes of fluid imbalance
Vomiting/diarrhea Unable to eat/drink Blood loss Kidney disease Water tablets for hypertension Heart disease Failure to pump fluid (blood) where it needs to be Iatrogenic disease (diag + therapeutic procedures)
When do U+E values need validated?
If result grossly abn
If an unlikely change b/n one day and next
What is hyponatremia?
Serum/plasma Na+ <135mmol/L
Common;
- most common disturbance in blood chem
- most common electrolyte disorder in hosp + non-hosp pts
Usually mild, self limiting condition
- pts asymptomatic
Severe can occur in serious conditions + associated with neurological symptoms
List the symptoms of hyponatremia
Depend on severity + speed of onset
120-125mmol/L = primarily GI <120mmol/L = nausea, vomiting, muscle weakness, headache, lethargy, ataxia, confusion
Severe symptoms = seizures, coma, resp depression
<120mmol/L for 48H or less = medical emergency
When should the ward be phoned for a hyponatremic pt?
<125mmol/L Na+
What are the 3 broad causes of hyponatremia?
- Increased sodium loss - loss of sodium rich fluid
- Increased H20 retention - failure to excrete fluid load
- Water imbalance - admin of hyponatremic fluids/failure to excrete water normally
Describe the causes of increased sodium loss
GI loss with inappropriate replacement
Decreased aldosterone production (hypoadrenalism)
Diuretics/medication
Salt losing nephropathy in renal tube disorders (renal salt excretion regulated primarily by tubular reabsorption rather than glomerular filtration)
Low K+: tubules conserve K+ and excrete Na+
Describe the causes of increased H20 retention
Failure to excrete water
Heart/liver failure
Dilution of serum/plasma Na+ in renal failure
Decreased plasma proteins = decreased colloid osmotic pressure = intravascular fluid (plasma) migrates to tissue causing edema
Failure to pump enough blood to kidney for excretion
Describe the causes of water imbalance
Syndrome of inappropriate AVP/ADH secretion (SIADH)
Pseudo-Na+ measured by indirect ISE in hyperprotein/lipidemic pts
Excess water intake
Inappropriate IV fluids
How is hyponatremia classified?
- True hyponatremia: serum Na+ conc truly less than normal (<135-145 mmol/L)
- Pseudo-hyponatremia: serum Na+ conc actually normal but erroneously reported low due to either hyperlipidemia or hyperproteinemia
What is pseudohyponatremia caused by hyperlipidemia?
Serum Na+ conc normal but falsely reported low due to hyperlipidemia
Not common, error usually small
Why does pseudo-hyponatremia occur?
Modern analysers used indirect ISEs to estimate electrolyte conc in serum;
- reqs 1:10 dilution ratio before measuring Na+
- assumes sample = 93% H2O
Water concentration is altered by presence of increased lipid/protein content = decreased water fraction in sample
- smaller sample in same dilution = increased dilution vs normal sample
- dilution step + calculation of conc by analyser then gives false low Na value
What is the problem with false low sodium value and how can it be avoided?
Pt at risk, especially if false result informs clinical decision
Avoided by;
Checking sample;
- hypertriglyceridemia usually grossly evident
- check protein level
- check osmolality: if Na+ low but osmolality normal then problem not with Na+ balance
Use direct ISE: no dilution step
- directly measures activity of Na+ in water phase rather than total Na+ conc
Calculate corrected value from serum water fraction if indirect ISE not available;
- equation for this
When should pseudo-hyponatremia be suspected?
Hyperproteinemia
Hyperlipidemia
Low sodium in diabetes mellitus with hyperlipidemia
Discrepancy b/n calculated + measured osmolality
What other investigations are carried out for hyponatremia after ISE serum?
Urinary Na+ and osmolality
How is hyponatremia treated?
Correct cause, either: increased H20 or decreased Na+
Depends on severity + onset
Strict fluid management, commonly fluid restriction + hypertonic saline
AVP/ADH receptor antagonists;
- conivaptan for treatment of euvolemic, hypervolemic hyponatremia
- blocks action of AVP in collecting ducts of nephron to decrease H2O reabsorption = pee out more water!
What is hypernatremia?
Hypernatremia = elevated serum Na+ conc (>145mmol/L)
Not usually caused by excess of Na+ but relative deficit of water so often coincides with dehydration
Less common than hyponatremia
List the symptoms of hypernatremia
Most commonly involve CNS;
- altered mental status
- lethargy
- irritability
- seizures
- muscle twitching
- nausea
- vomiting
Serum Na+ >160mmol/L = mortality rate of 60-75%
Pts obviously unwell with severe hypernatremia
When should the ward be called for a hypernatremic pt?
If Na+ >160mmol/L
What are the causes of hypernatremia?
Excess water loss;
- any condition causing excess water loss
Diabetes insipidous;
- deficiency of pit hormone ADH which regulates kidney function
Decreased water intake;
- pt with stroke/dementia
- increased urea/creatinine
Increased intake/retention;
- inappropriate IV prescribing
- hyperaldosteronism
How is hypernatremia diagnosed?
ISE
Measurement of urine osmolality needed to evaluate cause of hypernatremia;
<300mOsm/kg;
- diabetes insipidous (impaired secretion of ADH or kidneys cannot respond to ADH)
300-700mOsm/kg;
- partial defect in ADH release/response to ADH
- osmotic diuresis
> 700mOsm/kg;
- loss of thirst
- insensible loss of water (breathing/skin)
- GI loss of hypotonic fluid
- excess intake Na+
How is hypernatremia treated?
IV fluids to reduce Na+ levels at carefully controlled rate
Management: careful monitoring necessary as rapid reduction can cause cerebral edema, convulsions + permanent brain injury
List the important serum sodium measurements from hyponatremia to hypernatremia
Hyponatremia <135mmol/L Normal 135-145mmol/L Mild hypernatremia 140-149mmol/L Moderate hypernatremia 150-169mmol/L Severe hypernatremia >=170mmol/L
