Calcium

Physiology

• Responsible for much of the bony skeleton's structure but not found much in soft tissue
• Blood coagulation, neuromuscular transmission, and smooth muscle contraction
• Abnormalities are fairly well-tolerated
• The most abundant electrolyte in the human body - but 99% in bone
• Present in three forms in plasma
  • Ionised (50%) - biologically active
  • Bound to albumin (40%)
  • Bound to plasma anions (10%)
• Most clinical laboratories assay all three forms - which is misleading in hypoalbuminaemia.  Although you can correct it against albumin level, the only truly accurate methodology is to measure the concentration of ionised calcium with specific electrodes.

Hypercalcaemia

• Aetiology
  • Most often paraneoplastic hypercalcaemia in critically ill - poor prognosis
  • Primary or secondary hyperparathyroidism
  • PHPT and malignancy make up 80% of causes
• Affects neural tissue and damages renal tubules, which leads to hypovolaemia
• Symptoms:
  • GIT - ileus, constipation, vomiting, pancreatitis
  • Cardiovascular - hypovolaemia, hypotension, shortened QT
  • Renal - stones and polyuria
  • Neurologic - confusion and depression
• Treatment:
  • Saline diuresis
  • IV bisphosphonate if that fails
  • Treatment of primary cause

Hypocalcaemia

• Remember to correct against the albumin level or confirm with an ionised level
• Aetiology
  • Hypoparathyroidism
  • Alkalosis
  • Blood transfusions
  • Drugs - aminoglycosides, heparin
  • Fat embolism
  • Magnesium depletion (promotes hypocalcaemia by inhibiting PTH secretion) - refractory to calcium replacement - just need to correct the magnesium
  • Pancreatitis
  • Renal insufficiency
  • Sepsis (unclear mechanism)
• Manifestations
  • Neuromuscular excitability - tetany, hyper-reflexia, paraesthesias, siezures. Chvostek's and Trousseau's signs are non-specific and non-sensitive respectively.
  • Cardiovascular - hypotension, decreased CO, ventricular ectopics
• Acute severe hypocalcaemia (tetany, laryngospasm, seizures)
  • calcium gluconate 10% 20 mL (4.4 mmol elemental calcium) in sodium chloride 0.9% 100 mL intravenously over 20 minutes (ensure good IV access as it will cause skin necrosis if tissued); repeat if required; followed by calcium gluconate 10% 100 mL (22 mmol elemental calcium) in sodium chloride 0.9% 900 mL by intravenous infusion at an initial rate of 50 mL/hour (1.1 mmol elemental calcium/hour). Titrate to maintain a corrected serum total calcium concentration of 2.0 to 2.3 mmol/L
  • Cardiac monitoring for duration of therapy
  • Measure calcium every 4 hours
• Moderate hypocalcaemia (muscle cramps, spasms, paraesthesiae):
  • calcium carbonate 1.25 to 1.5 g (elemental calcium 500 to 600 mg) orally, twice daily, with food (can also use calcium citrate if unavailable)
  • Also give oral colecalciferol (vitamin D)
  • Measure every 1-2 weeks
• Long-term monitoring
  • Aim for lower limit of reference range - avoid symptoms rather than necessarily get it into the reference range
  • Check levels every 6 months
• Treatment
  • Indications for IV replacement:
    • Symptoms
    • Prolonged QT interval
    • <=1.9mmol/L
  • Indications for PO replacement:
    • Chronic hypocalcaemia
    • Acute hypocalcaemia not meeting requirements for IV replacement