6 Flashcards
WHat is the most abundant intracellular cation?
Potassium
What is the range of normal serum potassium?
3.5-5.5 mmol/L
What problems may occur if serum potassium is outside the normal range?
Will cause nerve dysfunction and cardiac arrest
What two system excrete potassium and how much do they secrete?
- kidneys excrete 80% K+
- bowel excretes 20%
Why is it important to maintain a relatively low extracellular [K+]?
-to maintain the steep K+ ion gradient of excitable and non-excitable cells
WHat metabolic disturbances can occur if you have extremely low ECF [K+]?
- inability of kidney to form concentrated urine
- tendency to develop metabolic alkalosis; and, closely related to this acid base disturbance
- a striking enhancement of renal ammonium excretion
What is the normal range for intracellular [K+]?
130-140mmol/L
How will an ECG show different stages of hyperkalaemia?
- > 6.0 mmol/L: tall T wave
- > 7.5 Mmol/L: long PR interval, wide QRS duration, tall T wave
- > 9.0 mmol/L: absent P wave, sinusoidal wave
What problems can occur at the different stages of hypokalaemia?
2.5-3 mmol/L: AF, muscle weakness, muscle cramps, constipation
<2.5 mmol/L: increasing weakness, cardiac conduction abnormalities, cardiac arrest
When giving diuretic drugs to promote diuresis, why is it more effective to give a combination of 2 diuretics?
- synergistic effect
- one nephron segment can compensate for altered sodium reabsorption at another nephron segment; therefore, blocking multiple nephron sites significantly enhances efficacy
Where in the nephron is majority of K+ filtered out?
At glomerulus and PCT
What hormone stimulates potassium secretion into the lumen?
Aldosterone
What does spironolactone do?
Stops K+ from being excreted which causes serum [K+] to increase
In what instance will carbonic anhydrase inhibitors be used as a treatment?
- Glaucoma to reduce the formation of aqueous humour
- also in some unusual types of infantile epilepsy
On which areas of the nephron do diuretics act on?
- thick ascending loop of Henle
- early DCT
- CT and ducts
What are some causes of hyperkalaemia?
Lack of excretion
- ACE inhibitors
- aldosterone deficiency
Release from cells
- acidosis
- cellular breakdown: ischaemia, toxins, chemo, rhabdomyolysis
Excess administration
- potassium containing fluids or medications
- blood transfusion
How can we treat hyperkalaemia?
Immediate
- insulin: shifts K+ into cells, lasts 6 hours
- salbutamol: same action as insulin, lasts 6 hours
- calcium stabilizes cardiac membrane potential
Later
- low potassium diet
- calcium resonium to bind potassium in gut
- stop offending medications
- furosemide: enhances potassium loss in urine
What are some causes of hypokalaemia?
- potassium entering cells: insulin, alkalosis, beta 2 agonists
- extra renal losses: diarrhoea, laxatives
- decreased intake
- renal losses: diuretics, renal tubular acidosis, DKA
- increase GI loss: vomiting, diarrhoea
- magnesium deficiency
How can we treat hypokalaemia?
- treat cause
- give potassium replacement
- oral: bananas, orange, Sando-K
- IV: saline +40mmol KCl, Dextrose +40mmol KCl, Central concentrated KCl
- potassium sparing diuretics: spironolactone, amilorides, etc.
What 3 things predominantly increase 3Na/2K ATPASE activity?
- K+ concentration in plasma
- insulin
- noradrenaline effect on B2-adrenoceptors
What things inhibit Na/K ATPase activity?
- digitalis
- chronic disease (heart failure, CKD)
In what ways is potassium excreted?
- kidney
- small amounts lost in faeces and sweat
Whereabouts in the nephron is potassium reabsorbed?
- 65 to 70% reabsorbed in PCT
- 20 to 25% reabsorbed in thick ascending loop
How can hyperkalaemia and hypokalaemia symptoms affect the membrane potential?
- low serum K+ leads to bigger K+ gradient between intracellular and extracellular compartment
- depolarisation leads to increased excitability (risk of arrhythmia)
- high serum K+ leads to smaller K+ gradient
- decreased membrane excitability (risk of cardiac arrhythmias)
- risk of symptoms depends on both degree and rapidity of change
Why is it hard to treat babies and elderly for infection?
- babies have 75% TBW so too much fluid
- elderly have 50% TBW, ability to hold on to water and not excrete is poor
What are some differences between hypernatremia and hyponatremia?
Hypernatremia
- cell shrinkage
- confusion/seizures
Hyponatremia
- cell swelling
- cerebral oedema
- headache/seizures
What is a 1000mL 5% dextrose solution used for?
- used when fluids are lost from body and need carbohydrates
- contains 50g/L glucose
- osmolarity = 278 mOsm/kg
- glucose is taken up by cells rapidly
- hyperglycemia will occur if infusion rate is quicker than uptake and metabolism
- H2O reduces osmolarity of all compartments
What is a 1000mL 0.9% saline solution used for?
- needed to replenish sodium levels
- contains 154mmol Na+ and 154mmol Cl-
- osmolarity= 308mOsm/kg
- Na+ remains in the ECF, no change in osmolarity
What is 1000mL Hartmann’s solution used for?
- used for replacing fluids and electrolytes in those who have low blood volume or low BP
- contains 131mmol Na, 111mmol Cl, 5mmol K+, 2mmol Ca, and 29mmol lactate
- osmolarity = 280mOsm/kg
- majority retained in the extracellular space as osmolarity maintained with effective osmoles sodium, potassium and calcium
Why would a patient need IV fluids?
- nil by mouth
- malfunctioning GI tract
- dehydration
- fluid losses
- abnormal electrolytes
Always ask yourself:
- whether a pt. Needs IV fluids
- whether you need to do anything to try and stop fluid loss
How do you determine which IV fluid to give?
- What does the patient need as maintenance?
- give maintenance fluids if unable to take orally
- has the patient lost any additional fluids?
- what are they losing?
- just replace as close to that as possible
What do maintaineance fluids do?
Replace standard daily losses
How do we determine the amount of fluids to give?
- look at size and age of pt and make relative measurements
- older: give more
- younger: give less
- decide which fluids you want to give for 24 hours
