This is defined as an total serum calcium level below the normal range (this will vary between labs, with 2.1-2.6 mmol/L a guide). A low ionised calcium level may also be defined as this, given that this is the active state. However, it is important to note that ‘pseudohypocalcemia’ may exist in states of low albumin. This is because there is less calcium in the blood overall as nearly half is usually bound to albumin. As it is the free ionised calcium which has the physiological effects, this is not true hypocalcaemia. There is often a ‘corrected calcium’ level on blood results to take this into account.
A good understanding of the normal physiology of calcium is important to understand the pathology. This is discussed elsewhere.
As calcium is such an important ion for many cellular processes, low levels start to lead to a failure of these. The keys ones involve:
Calcium acts as a stabiliser for sodium channels in a number of excitable tissues, such as neurons and the muscles. The reduction in levels therefore tends to lead to over excitability of these cells. This results in the manifestation of tetany, seizures and cardiac conduction abnormalities which form part of the clinical picture.
The causes of hypocalcaemia can be thought of as:
Decreased entry to the blood
Increased loss from the blood
Decreased Entry into Blood
This is most commonly due to loss of parathyroid hormone (hypoparathyroidism):
Congenital disease e.g. DiGeorge’s syndrome
Vitamin D deficiency may also be a cause:
Vitamin D resistance
This may lead to a secondary hyperparathyroidism state.
Increased Removal From Blood
Massive blood transfusion
In cases of major tissue trauma e.g. tumour lysis syndrome, the large levels of cell death leads to phosphate release which binds calcium.
This may depend of the speed and severity of the decrease. Rapid changes e.g. with hypoventilation, can produce more notable symptoms. The history can provide a guide to the possible causes. The features can be considered on a systems basis.
Paraesthesias (especially perioral, and peripherally)
Chvostek’s sign - tapping of the facial nerve (just below the zygomatic arch) provokes facial twitching
Trousseau’s sign - inflation of a BP cuff (over SBP) and left for a few minutes triggers spasm in the forearm/hand
Progression to VT
ABG/VBG - ionised calcium
PTH/Vitamin D levels
Urinary calcium - may be persistently high in hypoparathyroidism
This can be differentiated into:
Some patients will need IV replacement:
Calcium gluconate 10ml 10% = 2.3 mmol
Calcium chloride 10ml 10% = 6.8 mmol
This will include:
Ionised Ca2+ < 0.8 mmol/L
High CVS support
Calcium channel blocker overdose
Oral replacement may be suitable in stable patients