What is the normal serum level of magnesium?

0.7–1.0 mmol/L.

What is the distribution of magnesium in the body?

Magnesium is the second most abundant intracellular cation after potassium. The total body magnesium is ∼25 g, with 65% being located in the bone. Only 1% of the body magnesium is found in the serum, so that the serum level is a poor ref lection of the total body store.

What purpose does magnesium serve?

Magnesium is an essential co-factor in a number of enzymes, notably in the transfer of phosphate groups, and protein synthesis. It is most conspicuously important for the normal function of the central nervous, neuromuscular and cardiovascular systems.

What is the relationship between magnesium and serum calcium?

High magnesium levels prevent calcium cellular uptake, and for this reason, hypermagnesaemia can lead to bradycardia and sluggish deep tendon ref lexes. What drug is used to reverse the effects of severe hypermagnesaemia? Calcium gluconate.

Which organ is largely responsible for magnesium homeostasis?

The kidney is the major site for magnesium balance. It is freely filtered at the glomerulus, and reabsorbed mainly at the proximal convoluted tubule and thick ascending limb of Henle.

What are the main causes of hypomagnesaemia?

Renal losses: any state of excess diuresis, e.g. diuretic use, diuretic phase of acute renal failure, hypercalcemia

Alcoholism

Gut losses/malabsorption: diarrhoea, inflammatory bowel disease, malnutrition, intestinal resection and bypasses

Endocrine disturbance: diabetes mellitus, hyperparathyroidism, hyperthyroidism

How common is hypomagnesaemia in the hospital setting?

Hypomagnesaemia occurs in over 60% of the critically ill, most commonly associated with the use of diuretics.

What is the normal level of the serum potassium?

3.5–5.0 mmol/l.

What is the distribution of potassium in the body?

98% of the total body potassium is intracellular. The intracellular concentration is ∼150 mmol/l compared to ∼4 mmol/l in the serum.

How is potassium regulated?

There are a number of influential factors on serum potassium

Dietary potassium: the ‘Western’ diet may contain 20–100 mmol of potassium daily

Aldosterone: this is a mineralocorticoid steroid hormone produced by the zona glomerulosa of the adrenal cortex. It stimulates sodium reabsorption in the distal convoluted tubule and cortical collecting duct through an active exchange with potassium, whose excretion is therefore promoted

Acid-Base balance: potassium and H+ are exchanged at the cell membrane, so that excess of one or the other leads to increased exchange. Thus, acidosis leads to hyperkalaemia and vice versa. Similarly, alkalosis can lead to hypokalaemia and vice versa. This also occurs at the kidney where reabsorption of one causes excretion of the other

Tubular fluid flow rate: increased flow rate promotes potassium secretion. This is one method by which some diuretics may cause hypokalaemia

Insulin: this stimulates potassium uptake into cells, reducing the serum level

What are the causes of hyperkalaemia?

Artefact: haemolysis in the blood bottle

Excess oral or i.v. administration

Redistribution:

Between the ICF and ECF due to injury: intravascular haemolysis, burns, tissue necrosis

Reduced cellular uptake: insulin deficiency, acidosis

Decreased excretion:

Renal origin: renal failure, potassium-sparing diuretics

Adrenal origin: Addison's disease

Mineralocorticoid resistance: systemic lupus erythematosus, chronic interstitial nephritis

What is pulse oximetry, and what does it measure?

Pulse oximetry is a non-invasive and continuous method of assessing arterial oxygen saturation (SaO2) and pulse rate.

Note that it is not a measure of the total oxygen content of the blood nor the PaO2. It does not assess ventilation, which requires a measure of the PaCO2.

By which principle does pulse oximetry work?

Pulse oximetry works on the principles of spectrophotometry. It contains a probe emitting light at the red (660 nm) and infra-red (940 nm) wavelengths, and a photodetector.

It relies on the differing amount of light absorbed by the saturated and unsaturated Hb molecules. The percentage oxygen saturation of the blood is calculated from the ratio of these two forms of the molecule.

What are its disadvantages and sources of error?

Problems encountered include

Diminished accuracy below a saturation of ∼70%

There is a delay of ∼20 s between actual and displayed values, limiting its use in the emergency setting

Poor peripheral perfusion leads to a poor signal quality

So does ambient light pollution

Abnormal pigments affect the results. External pigments include nail varnish. Internal pigments include bilirubin of the jaundiced patient, methaemoglobin, and carboxyhaemoglobin. Jaundice underestimates the true SaO2 and carbon monoxide poisoning overestimates the true SaO2

Abnormal pulsations, such as cardiac arrhythmia or venous pulsations of right heart valve defects may interfere with the signal

Note, however, that there is no interference from polycythemia or fetal haemoglobin

What is methaemoglobin?

This is a haemoglobin molecule that contains iron in the ferric (Fe3+) state within its haem portion, as opposed to the normal ferrous (Fe2+) state.