Electrolytes + Acid Base Flashcards
What is Na+ used for?
Most abundant cation in extracellular fluid (135-145 mmol/L)
Used in transmission of nerve impulses
Influences contraction/relaxation of muscle
Maintains balance of fluids in body
Where is the one place that water does not follow sodium?
Ascending loop of Henle
- Countercurrent mechanism to concentrate urine
Give a four examples of Na+ and water imbalance:
Oedema - too much Na+ ( increase Na+ excretion)
Volume depletion - too little Na+ (reduce Na+ excretion)
Hyponatraemia - too much water (suppress ADH to increase water excretion)
Hypernatraemia - too little water (enhance ADH ro decrease water excretion)
Causes of hypernatraemia
Dehydration:
- Diuretics
- Diarrhoea
- Burns
- Surgery (drains)
- Intake restriction
Endocrine disturbance:
- Collecting duct abnormalities ()
- Hyperaldosteronism (Conn’s)
- Diabetes insipidus
- Cushing’s syndrome
High sodium intake
Symptoms of hypernatraemia:
Altered mental state
Irritability
Restlessness
Seizures
Muscle twitches
Hyperreflexia
GIT - nausea and vomiting
How does dehydration cause increased urea: creatinine?
In prerenal failure, urea increases disproportionately
Enhanced proximal tubular reabsorption of Na+ and water -> urea follows Na+
Approach to hyponatraemia?
Hypoosmolar (true)
Hypertonic
- Translocation of water from cells into ECF
- Pseudohyponatraemia ( increased lipids and protein) - normal/elevated serum osmolality
Dehydration?
Fluid overload?
Bulimia?
Approach to hypoosmolar hyponatraemia:
Hypovolaemic: Decreased total body water and Na
Euvolaemic: Increased TBW, no change in Na
Hypervolaemic: increased TBW and Na
What is hypovolaemic hypoosmloar hyponatraemia, and what are the causes?
Decreased TBW and Na
If U Na > 20, there will be dumping of Na
Causes:
Renal loss
- diuretics
- salt losing nephropathy (e.g., HIVAN)
- cerebral salt wasting
If U Na < 20, there will be retaining of Na (conc. urine)
Causes:
Volume depletion
- Vomiting
- diarrhoea
- 3rd space losses
- burns
- pancreatitis
- bowel lumen (e.g., ileus)
What is hypervolaemic hypoosmloar hyponatraemia, and what are the causes?
Increased TBW and Na
If U Na > 20
Causes:
- AKI
- CKD
If U Na <20
Causes:
-Nephrotic syndrome
-CCF
-Cirrhosis
-Primary polydipsia
What is euvolaemic hypoosmloar hyponatraemia, and what are the causes?
Increased in TBW, no change in Na
Causes:
SIADH
- Na dumping
- Low plasma Na (<130)
- Low plasma osmolality (<270)
- High urine:Na (>40)
- High urinary osmolality (>100)
Hypothyroidism
- Decreased ADH suppression
- Decreased GFR (Decreased clearance of ADH)
Addison’s
- Hyperpigmentation
- Low BP
- Low glucose
- Low Na
- Increased K + acidosis
What is SIADH?
Syndrome of inappropriate ADH secretion
- defect in osmoregulation due to ADH being inappropriately stimulated -> increased urine osmolality + urine Na
-Euvolaemic
-Normal thyroid function and cortisol levels
What are the causes of SIADH?
Respiratory:
- Paraneoplastic effect (e.g., small cell lung ca)
- Suppurative/cavitating lung disease
- Positive pressure ventilation
CNS:
- CVA/ cavernous sinus thrombosis
- meningitis/ encephalitis
- SOL
Drugs:
- Carbamazepine/ TCA/ SSRIs/ phenothiazines (incl maxalon)
What are the risks of managing low Na?
Risky!!
Acute hyponatraemia: ECF becomes hypotonic -> water drawn into cell -> oedema -> adapts -> osmoles pumped out -> oedema resolved
Acute hyponatraemia -> risk of cerebral oedema -> correcting too quickly -> ECF more hypertonic than ICF -> cells shrink -> central pontine myelinolysis
What is K+ used for?
Regulates intracellular enzyme function
Helps determine neuromuscular and cardiovascular excitability
Primarily found in muscle (most abundant cation)
