Potassium and pH Flashcards
Role of potassium
key determinant of resting membrane potential
ENaC/ROMK channel function
Na+ comes in and K+ goes out
So it is important in regulating K+ elimination
Aldosterone activates these channels and inserts them into the membrane
K+ excretion is dependent on:
Plasma K+ concentration
Aldosterone
What is Na+/K+ pump is responsible for?
Maintaining the difference in electrolyte concentration between the ICF and ECF
What can Na+/K+ pump activity be influenced by?
beta adrenergic stimuli and thyroxine
Adrenaline effect on potassium
Adrenaline lowers plasma K+
Main causes of low K+:
Reduced intake (unlikely)
Renal losses
Intracellular shift
Causes of IC shift of low K+
Glucose load
Adrenaline
Alkalosis
Renal losses can cause low K+:
Mineralocorticoid excess (e.g. Conn’s syndrome)
Renal tubular disorder
Diuretics
What happens after glucose is ingested and enters the blood?
Stimulates a release of insulin, which drives K+ into cells
Most common cause of low K+
Diarrhoea + Vomiting
Overview of causes of high plasma K+:
Increased intake (unlikely)
Renal retention
Intracellular shift
Causes of renal retention of K+
Mineralocorticoid deficiency (Addison’s disease)
ACE inhibitors
Renal failure
Potassium sparing diuretics (e.g. spironolactone)
Causes of IC shift of high K+
Insulin deficiency
Acidosis
Exercise
Cell lysis
How does acidosis cause high K+?
When you’re acidotic, the H+ ions compete with the K+ ions so when you are acidotic you don’t get as good activity of that pump leading to high plasma K+
Main issue with high K+
Abnormal ECG
ECG abnormalities with hyperkalaemia:
Tented T waves
Loss of P waves
Broad QRS
BRADYCARDIA
Treatment of Hyperkalaemia
Calcium Chloride (IV) 50% Glucose Sodium Bicarbonate (NaHCO3)
Calcium Chloride as hyperkalaemia treatment mechanism of action
NO EFFECT on plasma potassium but it does reduce the effect of potassium on cardiac excitability and limits the ECG changes
It fixes the bradycardia
In other words, it makes your heart resistant to the effects of hyperkalaemia
50% glucose as hyperkalaemia treatment mechanism of action
As that glucose is taken up by cells, there is a shift of potassium from the ECF to the ICF
It will lower plasma K+ within half an hour and it will last for about 4-6 hours - the K+ will eventually leak back out again but it does buy you time
Sodium bicarbonate as hyperkalaemia treatment mechanism of action
This works if your patient is not fluid overloaded
This also affects the movement of potassium between ECF and ICF
Consequences of changes in pH:
Impaired ventricular function Arrhythmias, lower fibrillation threshold Vasodilation, catecholamine release Impaired oxygen delivery Bronchoconstriction Reduced hepatic/renal blood flow Impaired consciousness Respiratory muscle fatigue Protein catabolism Insulin resistance
What is HCO3- controlled by?
Kidneys
What is total CO2 controlled by?
Lungs
What occurs if you breathe faster
pCO2 decreases –> respiratory alkalosis
What occurs if you breathe slower
pCO2 increases –> respiratory acidosis
What happens if someone has lobar pneumonia?
The blood that goes to the bit that has pneumonia will NOT become fully saturated
The blood that goes to the normal parts of the lungs will be fully saturated
Lowers the oxygen content of the blood
What happens to blood going to damaged part of lung in lobar pneumonia
Blood going to the damaged region will not have its CO2 removed so the blood coming away from this part of the lung will have a higher CO2 concentration
What happens to blood going to healthy parts of lung in lobar pneumonia
The good regions of the lung will have abnormally LOW CO2 because you’re breathing a bit faster and removing more CO2
Oxygen and CO2 levels in focal lung disorders
With FOCAL lung disorders, the oxygen will go down BUT the CO2 will not increase
What causes type 1 respiratory failure
caused by FOCAL lung disorders
Pneumonia
Pulmonary embolism
Blood gases in Type 1 respiratory failure
Oxygen = LOW (< 8 kPa) CO2 = NORMAL or LOW
What can also cause Respiratory alkalosis?1.
hyperventilation
Why is there an abnormality of BOTH oxygen and CO2 in Type 2 respiratory failure?
Because the whole lung is not functioning properly
CAUSES of Type 2 Respiratory Failure:
Airways disease (mainly COPD) Pulmonary fibrosis
Blood gases in Type 2 respiratory failure?
pO2 = LOW pCO2 = HIGH
Common causes of metabolic acidosis
Renal failure
Mineralocorticoid deficiency
Diarrhoea
These causes are also associated with a HIGH CHLORIDE
Changes in other acids that cause metabolic acidosis
Lactic Acid
Ketoacids
What is the anion gap?
When the amount of Na+ (cation) should equal the amount of the total anions (Cl- and HCO3-) but there are some protein based anions that are not measured so there is always a little bit of a gap
Anion gap range
12 +/- 4
Anions that aren’t measured:
Phosphate
Proteins
Specific metabolic acid
Common causes of metabolic alkalosis:
Diuretics
Mineralocorticoid excess
Vomiting
Things that cause high K+ tend to cause which acidosis/alkalosis?
Metabolic acidosis
Things that case low K+ tend to cause which acidosis/alkalosis?
Metabolic alkalosis
Diarrhoea - more likely to cause acidosis/alkalosis?
Acidosis
Vomiting - more likely to cause acidosis/alkalosis?
Alkalosis
Unusual disorders that cause metabolic acidosis that are associated with a low K
Renal tubular disease
Urinary diversion
Conditions that cause acidosis but do NOT alter plasma K+
Ketones
Lactate
Poisons
Causes of Ketoacidosis
Diabetic Ketoacidosis (most common)
Starvation
Alcoholics
Causes of Lactic Acidosis
Occurs mainly when there is a lack of tissue perfusion (tissue hypoxia causes anaerobic respiration)
Shock
Liver failure
Short bowel
Low pH leads to:
Breathlessness
Causes strong respiratory drive
High pH leads to:
Cramps/tetany
Because pH affects calcium ionisation and that affects muscle excitability
Compensation for metabolic acidosis
Breathing faster - this lowers CO2 and restores the pH - this change occurs FAST
Compensation for respiratory acidosis
Develop a metabolic alkalosis to compensate - changing bicarbonate reabsorption release - this change occurs SLOWLY
