WHEN TO MEASURE PLASMA Na+
Although plasma Na+ is very frequently measured, there are only really three scenarios where it is likely to be of value:
1. Dehydrated patients or those with a history of fluid loss
2. Patients receiving fluids intravenously, especially infants, the elderly and the unconscious.
3. Patients with unexplained alteration in level of consciousness, confusion or irritability.
INTERPRETING A PLASMA Na+ RESULT
As should be clear by now, an isolated plasma [Na+] measurement is of little clinical value in the absence of additional information. For instance, a [Na+] of 125 could be due to:
• primary water overload (e.g. SIADH)
• diarrhoea or diuretic therapy, with replacement of water only
• diabetes mellitus, where glucose draws water from ICF into ECF, diluting the Na+
• gross lipidaemia.
Thus, to properly interpret a plasma Na+ result, one must consider factors including:
1. The history - has there been:
• a head injury to suggest SIADH?
• diarrhoea and/or vomiting?
• polyuria or polydipsia to suggest diabetes (mellitus or insipidis)?
• diuretic therapy?
2. The clinical examination
• level of consciousness?
• hydration status - are there signs of water overload (oedema, distended neck veins,hypertension, pulmonary oedema) or dehydration (loss of skin turgor, dry mouth, tachycardia, low BP,
• pigmentation to suggest Addisons disease?
• features of cardiac failure, liver cirrhosis or nephrotic syndrome, to suggest secondary hyperaldosteronism?
3. Urine findings
• appearance - dilute or concentrated?
• osmolality?
• [Na+] concentration?
• any glucose or protein?
4. Blood findings
• haemoconcentration (increased haemoglobin and protein)?
• renal function (urea and creatinine levels)?
• milky appearance (lipidaemia)?
• K+ abnormalities (Addisons or primary aldosteronism)?
• low cortisol or high TSH to account for SIADH?
OSMOLAR GAP
Osmolality is determined by the sum of all solutes present in a fluid, and is usually measured by freezing point depression. Plasma osmolality can generally be predicted quite accurately from the formula:
Osmolality = 2x [Na+] + [urea] + [glucose]
A significant disparity between measured and calculated osmolality is refered to as an "osmolar gap", and is usually due to the presence of an osmotically active substance, the commonest by far in clinical practice being ethanol.
SMALL GROUP TEACHING, LECTURE 4 DISORDERS OF WATER AND SODIUM BALANCE:
QUESTIONS
1. Explain and differentiate between the concepts of osmolarity, osmolality and tonicity.
i. How is the approximate molarity calculated?
ii. What is the osmolar gap?
iii. When is an increased osmolar gap of clinical significance.
2. Describe the mechanisms of urine dilution and concentration.
3. In a normal adult subject, what is the maximum and what is the minimum achievable urine osmolality?
4. Explain the water and electrolyte changes which occur on administering.
i. normal saline (0.9% NaCl 150 mmol/l);
ii. tap water orally;
iii. 5% dextrose i.v.
5. Outline your approach to the differential diagnosis of : i. hyponatraemia;
ii. hypernatraemia.
6. Write down the criteria necessary for diagnosing SIADH.
7. Explain the concept of pseudohyponatraemia. Should this condition be treated 8. Describe and explain the biochemical abnormalities which occur in :
i. osmotic diuresis ii. acute blood loss
9. Explain these results obtained on a 30 year old male patient Test Results Reference Range
Sodium 97 135 - 145 mmol/l
Potassium >10 3.0 - 4.7 mmol/l Chloride 86 99 - 112 mmol/l Total CO2 14 19 - 29 mmol/l Calcium 0 2.2 - 2.6 mmol/l Magnesium 0 0.6 - 1.0 mmol/l Phosphate 0 0.6 - 1.2 mmol/l Alkaline Phosphatase (ALP) 0 <120 U/l Lactate Dehydrogenase (LD) 0 <320 U/l Glucose 75 3.0 - 5.5 mmol/l
10. A 5 month old bottle-fed infant with severe diarrhoea has the following serum biochemistry:
Reference Range
Chloride 116 mmol/l (99-113) Total CO2 13 mmol/l (19-29) Urea 7.8 mmol/l (<5.5) Creatinine 44 µmol/l (<45) i. What is the likely fluid status of this child?
ii. Which constituents confirm this state?
iii. How would you manage the child?
iv. The normal range for creatinine in adults is up to 120mol/l. Why is this lower in a child?
11. A six week old male infant presented at Red Cross Hospital with projectile vomiting. Examination revealed a sausage-like mass in the epigastrium.
Serum: Reference Range Blood gases: Reference Range
Sodium 131mmol/l (135-145) pH 7.58 (7.37-7.43) ii. Which results confirm this ?
iii. Does alkalosis fit this picture of hydration ?
iv. How could hyponatraemia develop in this situation ? v. Why are U-Na and U-Cl divergent ?
12. A 60 year old man was admitted to hospital for evaluation of weakness, anorexia and haemoptysis (he gave a 20 year history of smoking 2 packs of cigarettes per day). The chest X-ray showed a left hilar mass. He had no oedema. Skin turgor appeared normal.
The following serum biochemistry results were reported:
Reference Range
Sodium 112mmol/l, Potassium 3.9 mmol/l, Chloride 84 mmol/l, Total CO2 21 mmol/l, Urea 1.9 mmol/l, Creatinine 60 µmol/l, Osmolality 240mmol/kg (280-295)
i. What is your diagnosis?
ii. What features are present that support your diagnosis and which would you like to know about that are not presented here?
iii. How would you treat his hyponatraemia?
13. A male aged 52 years has a 6 months history of loss of appetite, weight loss, fatigue and episodic abdominal pain. BP 105/60. Pulse 100/min.
Serum: Sodium 125 mmol/l, (135-145), Potassium 5.2mmol/l (3.3-5.2), Total CO2 16 mmol/l (19-29), Urea 8.2 mmol/l (2.6-8.0), Creatinine102 µmol/l (<120)
i. What is your differential diagnosis? Why?
ii. What would you expect his U-Na to be ?
iii. If his urinary sodium were low, how would you explain this?
iv. If high, how would you explain his urinary sodium?
v. How would you confirm your diagnosis?
vi. What is the treatment ?
SMALL GROUP TEACHING, LECTURE 4 DISORDERS OF WATER AND SODIUM BALANCE: ANSWERS 1.
i. Osmolarity is the number of osmoles dissolved in 1l of water and is expressed as mosmoles per litre: the total volume is thus 1l minus the solute volume. Osmolality refers to the number of osmoles per kilogram of water and is expressed as mosmol/kg: the total volume is thus 1l plus the solutes Tonicity refers to the effective osmolality i.e. that portion which is held on one side of a cell membrane and can thus cause fluid shifts. An isotonic fluid is always osmolar whereas the converse does not hold. A suspension of red blood cells in an iso-osmotic urea solution will haemolyse: urea will equilibrate pulling water into the RBC.
ii. Twice the sodium plus potassium plus glucose plus urea. Note that urea should be excluded to determine the effective osmolarity
iii. The difference between the measured osmolality and the calculated osmolarity iv. Poisoning e.g. methanol
2. Absorption of NaCl without water in the thick ascending limb of Henle for maximal dilution plus switch off ADH . For concentration NaCl absorption in the TAL to increase medullary osmolality plus the presence of ADH.
3. 50 mosmol/l and 1400 mosmol/l 4.
i. This will expand extracellular volume, but have no effect on the sodium concentration nor osmolality (iso-osmotic). There will be no movement across the cell membrane
ii. The addition of water will expand and dilute both the ICF and ECF
iii. This is a way of administering water to the body. 5%dextrose is iso-osmotic, but will be metabolised to cause net administration of water. Thus can cause hyponatraemia. Initially glucose will cause movement of water from the ICF, but in the presence of insulin will be taken up and metabolised leaving a net addition of pure water which will equilabrate across the cell membrane.
5. See lecture notes
6. Hypo-osmolar hyponatraemia. Inappropriately raised U-osmol. Normal potassium/acid base Normal renal, adrenal, thyroid function
7. Solutes as expressed in molar concentration actually 7% solute + 93% water ( 70ml solute plus 930ml water to make up a litre). In hypertriglyceridaemia and Hyperproteinaemia more water replaced with solute thus instead of 930ml containing proportionate amount of measured solutes, now lower volume of water with proportionately lesser amount of solute. Measurement error. Treat primary condition, not lab result
8.
ii. Loss is iso-osmotic/isotonic. Thus contraction of intravascular volume which does not cause fluid shifts. However lower GFR and diminished perfusion of the kidney.
9. Drip contamination with - 5% glucose + potassium 10.
i. Dehydrated
ii. Hypernatraemia, elevated urea with normal creatinine
iii. Stop milk feeds: oral rehydration ( “invented” by a pathologist in Zambia – Maurice King). IV rehydration if necessary. Encourage Mum that breast feeding would have been better iv. Creatinine is proportionate to muscle mass
11.
i. Dehydrated
ii. Urea >8.0 mmol/l and U-Cl < 6mmol/l
iii. Yes, due to secondary hyperaldosteronism caused by volume contraction iv. Replacement of electrolyte loss with water due to thirst stimulus
v. Obligatory loss of Na with bicarbonate, but maximum drive still present to conserve as much water in the proximal tubule as possible. Thus U-Cl reflects this component.
12.
i. SIADH due to lung Ca
ii. Hypo-osmolar hyponatraemia, normal potassium and bicarbonate, normal renal function.
Would like U-osmol >100 mosmol/l, U-Na > 20mmol/l, normal thyroid and adrenal function iii. Fluid restriction
13.
i. Addisons disease, diabetes . Hyperkalaemic acidosis, dehydration ( pre-renal failure), hyponatraemia
ii. High
iii. Dehydration causing maximum reabsorption of Na in the proximal tubule iv. Absence of aldosterone with delivery of sodium to the distal tubule
v. Synacthen test
vi. Replacement with mineralocorticoid (fludrocortisone) and glucocorticoid (hydrocortisone)