Lecture 3: Electrolytes and Fluid Balance II Flashcards
What organ maintains sodium intake and output?
Kidneys
Where do we gain sodium?
Diet
How do we loose sodium?
Kidneys, sweat, faeces, urine
What controls urinary output of sodium?
Aldosterone
Vasopressin
Atrial natriuretic peptide
What secretes aldosterone? Why?
Adrenal cortex (glands on top of kidneys)
In response to changes in the volume of the ECF compartment
What does aldosterone do?
stimulates the reabsorption of sodium ions in the kidneys in exchange for potassium and hydrogen ions, which are excreted in the urine
What secretes atrial natriuretic peptide? What does ANP do?
secreted by cells in the right atrium of the heart
increases urinary sodium excretion.
Regulation of electrolytes and fluid in the kidneys
(see figure)
What is Hyponatraemia?
Sodium concentration less than 135 - 145 mM
Caused either because of sodium loss or water retention
Patients don’t usually show peripheral edema
What diseases cause loss of sodium?
Renal tubular necrosis
Gastrointestinal loss due to diarrhea, vomiting, and intestinal fistulae.
Aldosterone deficiency
Drugs that affect aldosterone action/secretion
What causes water retention?
Deficiency in water excretion (drug induced, over secretion of vasopressin, trauma, surgery)
Infusion of intravenous hypotonic solution
Compulsive water intake (very rare occurs when there is dehydration followed by increased intake of water)
What is hypernatraemia?
Sodium above 145 mM
Causes of hypernatraemia
Water loss from decrease intake or excessive loss (comatose, elderly patients not eating well)
Deficiency in vasopressin secretion or action
Sodium gain or salt poisoning (high intake of sodium feed, sodium bicarbonate to correct for acidosis or drowning in salt water)
Hyperaldosteronism (Conn’s syndrome; over secretion of aldosterone)
Parenteral infusion of hypertonic sodium solution
Excessive sweating, vomiting, and diarrhea where more water is lost than salt (rarely this happens!)
What does sodium depletion cause?
loss of osmotic pressure of the ECF, and hence loss of water.
This results in low blood volume (hypovolaemia) and blood pressure
What is tachycardia usually accompanied by?
Low blood volume and low blood pressure
Clinical symptoms of sodium depletion
Weakness, dizziness, and general tiredness
Renal failure can result due to low glomeruli pressure due to hypotension in the kidneys, oliguria (low urine output usually less than 400 mL/24 h), and tachycardia ( the heart beats faster to correct for the low
blood pressure).
Symptoms of excess sodium
Related to increase in osmotic pressure in ECF, leading to expansion of ECF
Peripheral edema
decrease in ICF (water moves from ICF to ECF)
Hypertension
Venous congestion
Weight gain
Difficulty breathing (edema creates pressure)
How is ingested potassium excreted?
Mostly by the kidneys
Some through the faeces
Small amount through sweat
Normal concentration of serum potassium
4.5 mM (even small variation of water amount can change concentration significantly)
What is cellular uptake of potassium stimulated by?
Insulin
**MAKE SURE TO ALSO GIVE PATIENT GLUCOSE OR ELSE THEY WILL BECOME HYPOGLYCEMIC
Relationship between K+ and H+
Reciprocal relationship to maintain electrical neutrality
Hyperkalemia
is associated with acidosis
hydrogen ions is taken up into the cells (ICF) and potassium is displaced from the cells into the ECF (it increases in concentration in the ECF)
Hypokalaemia
is associated with alkalosis;
hydrogen is displaced from the ICF into the ECF; in contrast potassium is taken up into the ICF from the ECF (concentration of potassium decreases in the ECF).
K+ conc. for hypokalemia
below 4 mM
Why does hypokalemia arise?
- Gastrointestinal losses (vomiting, diarrhea, fistulae)
- Renal tubular disease
- Administration of hypotonic solutions
- Increased aldosterone (more sodium reabsorption means more K+ and H+ excretion)
- Drug
- Alkalosis (can result in movement of K+ frrom ECF to ICF
K+ concentration for hyperkalemia
K+ exceeds 5 mM
Reasons for hyperkalemia
- Renal failure (glomerular filtration is low)
- Deficiency in aldosterone secretion or action or aldosterone agonists (sodium reabsorption and K+ excretion are reciprocal)
- Acidosis (movement of K+ from ICF to ECF to maintain electrical neutrality)
- Potassium release from damaged cells (bone damage and fractures, muscle crush injuries)
What does potassium depletion cause?
Hyperpolarization of excitable membranes, which causes:
Decreased excitability of membranes
Irritability
Shallow respiration
Confusion and Drowsiness
Disturbances in neuromuscular activities: muscle weakness, lethargy, and irregular heart rate (arrhythmias), tachycardia (fast heart rate).
Decrease bowel movements, resulting in constipation; decrease in nausea and vomiting in contrast to hyperkalemia.
Alkalosis
Why is hyperkalemia so dangerous?
Increases RMP, which is very dangerous for the heart
Other signs of hyperkalemia
Decrease cardiac contractility
Dysrhythmias (irregular rhythm of heart rate)
Irritability & anxiety
Muscle twitches and cramps
Diarrhea, abdominal cramping, nausea and vomiting
Low blood pressure Acidosis
Management of Hyperkalemia
IV insulin (promotes uptake of K+)
IV glucose (so that patient doesn’t become hypoglycaemic due to insulin)
Bicarbonate solutions for acidotic patients (to neutralize acid)
Renal dialysis (gets rid of potassium)
Causes of electrolyte and fluid imbalance
- Hormonal imbalance
- Adverse drug effects
- Cancer
- Disease of the Kidneys
- Water retention
- Water loss
- IV hypertonic/hypotonic solutions
Guidelines for IV fluid therapy to correct imbalances
- Clinically assess the primary cause for the imbalance
- Assess the biochemical reason for this imbalance.
- Pay particular attention to cardiac, renal, and liver functions.
What are patients with hyponatraemia and edema likely to show in chronic state?
Water and sodium overload
What is the usual cause of hypernatraemia?
Water loss because patients are unable to drink as well as insensitive loss of water through breathing and the skin
Best treatment for hypokalemia
IV of potassium solution, take care not to cause hyperkalemia
