hypocalcaemia

aetiology:

if plasma phosphate high:
- if plasma PTH low:
  - hypoparathyroidism - idiopathic/post-surgical
- if plasma PTH high:
  - chronic renal disease (secondary hyperPTH)
  - pseudohypoparathyroidism
plasma phosphate normal or low:
- malabsorptive & nutritional Vit.D defic. (osteomalacia & rickets) (PTH high)
- pancreatitis
- hypomagnesaemia
- over-hydration
if skeletal alkaline phosphatase raised
- osteomalacia/rickets/ch.renal failure
- “hungry bones” (eg. recent hyperthyroidism and now hypocalcaemia post-thyroidectomy with inadvertent parathyroidectomy, may require massive calcium/calcitriol therapy!!)
neonatal causes

ECG - look for prolonged QTc
take blood for U&E, Ca, PO4, albumen, Mg, looking for:
- corrected calcium level
- chronic renal failure
- hypomagnesaemia
if hypocalcaemic ¹⁾ on corrected calcium, then:
- if phosphate raised:
  - take blood for PTH
  - check renal function
- if phosphate normal or low:
  - take blood for alkaline phosphatase, +/- amylase, plasma 25-OH-vit D
  - excl. malabsorption and over-hydration
  - if plasma 25-OH-vit D very low → osteomalacia/rickets → skel. survey

adult bolus dose iv 10% calcium gluconate (10ml over 3min, rpt prn) is generally only warranted for Rx of hyperkalaemia in absence of digoxin toxicity and patient is in cardiac arrest or has significant ECG changes
in patients in cardiac arrest, calcium chloride boluses are preferred over calcium gluconate as 10% calcium chloride gives 3x the amount of elemental calcium as does 10% calcium gluconate

MAXIMUM DAILY DOSE SHOULD NOT EXCEED 15g CALCIUM GLUCONATE (ie 15 ampoules) unless special circumstances such as post parathyroidectomy patients with renal failure, where there is a substantial need for intravenous calcium sometimes up to 12g (>26 mmol) per 12 hours.

Rx choice should be governed by rate of change, rather than absolute levels
extravasation of calcium gluconate may lead to tissue necrosis
monitor for phlebitis
infusions of calcium gluconate MUST be given via an infusion device (eg. iMed)
use calcium gluconate cautiously in patients with impaired renal function, cardiac disease or sarcoidosis
increased fluid intake is recommended for patients prone to formation of calculi in the urinary tract
calcium gluconate may increase effects of digoxin and precipitate digoxin toxicity
corrected calcium does not necessarily reflect physiologic activity, an ionised calcium should be ordered if there is doubt clinically

ie. corrected serum calcium less than 1.8mmol/L with cramps, paraesthesiae, or prolonged QTc
take blood for U&E, Ca, PO4, albumen, Mg
ECG - look for prolonged QTc
cardiac monitor
check ionised calcium level on ABG or VBG and repeat 4-6hrly, particularly if albumin is low
iv rehydration
initial dosing: dilute 1g (2.2 mmols) calcium gluconate in 100 mL fluid (NS, Hartmann's or 5% dextrose) and administer over 15 minutes via a large bore peripheral cannula or via CVC
- alternatively, 3 ampoules 10% calcium gluconate (6.4mmol total) to be given over 30min, although this may cause flushing and there is a risk of hypotension, bradycardia, arrythmias and potentially cardiac arrest with rapid infusions and thus is rarely warranted, and should only be given under advice of a consultant for this indication.
ongoing symptoms, further infusion as for asymptomatic hypocalcaemia below
admit for Ix & ongoing Mx under endocrine unit

corrected serum calcium less than 1.8mmol/L
take blood for U&E, Ca, PO4, albumen, Mg
check ionised calcium level on ABG or VBG 12hrly
- remember acidosis increases ionised calcium whereas alkalosis decreases ionised calcium
ECG - look for prolonged QTc
cardiac monitor
iv rehydration
generally correct severe hypomagnesaemia or hypokalaemia first, although sulphate ions in magnesium sulphate may bind calcium exacerbating hypocalcaemia
dilute 5 g (11 mmol) calcium gluconate in 1 litre of fluid and administer via large bore peripheral line over 12 hours
- faster rates can be given on advice of endocrine, nephrology, ICU consultant
alternatively, for those patients with a CVC:
- dilute 5 g (11 mmols) calcium gluconate in 110 mL of fluid
  - withdraw 40 mL from 100 mL bag of appropriate fluid and add 50mL calcium gluconate to obtain a final volume of 110 mLs
- infuse at 1 mmol per hour = 10 mL /hour

corrected serum calcium 1.8mmol/L - 2.0mmol/L
oral Caltrate 600mg, two tablets bd away from meals is usually preferable to iv infusion unless hypocalcaemia is rapidly progressive or oral medications are C/I.

corrected serum calcium > 2.0mmol/L
calcium 5mmol 6h po
daily calcium levels
hypoPTH → calcium + calcitriol
- patients with hypocalcemia due to resistance to parathyroid hormone (PTH) generally will require long-term therapy with vitamin D and calcium supplementation
osteomalacia/rickets → Ix & Rx cause, ergocalciferol
CRF → Ix & Rx on merits, phosphate restriction +/- calcium & calcitriol;
- patients with hypocalcemia associated with chronic renal failure often require phosphate binders and vitamin D supplementation

¹⁾

hypocalcemic, hypocalcemia