Magnesium (Mg++) is the second most abundant intracellular ion.
Normal Serum Mg++ is 1.8 to 2.5 mg/dL or .8 to 1.5 mmol/L (millimoles per liter)
(This values may vary depending on sources)
Keep in mind, the Mg++ concentration in the average adult is approximately 25g, but most of our Mg++ is found in bones and intracellular. Because this makes it hard to assess the true Mg++ concentration, true Mg++ measurement is often not performed, instead, Serum Mg++ levels are obtained. This measurement does not fully correlate with overall Mg++ because only a small amount is found in the serum, usually approximately 1% of all Mg++.
Mg++ has over 300 different physiologic functions, and it affects multiple phases of the cardiac AP.
- Mg++ acts as a physiologic Calcium (Ca++) Channel inhibitor by slowing slow L-Type Calcium channel during PHASE 2 of the AP
- This reduces further Ca++ release by the Sarcoplasmic Reticulum which leads to reduced automaticity, contractility and conductivity through cardiac tissue, including the AVN
Hypomagnesemia (Serum Mg++ < 1.8 mg/dL or .8 mmol/L)
- Mg++ mediates Potassium (K+) influx during PHASE 4 of the AP, therefore, during Hypomagnesemia, K+ influx is partially inhibited, which leads to delayed ventricular repolarization.
- Because Mg++ also is responsible for proper Na+/K+ pump, Hypomagnesemia leads to K+ loss which leads to Hypokalemia (serum K+ < 3 mEql/L).
Whang et al studied 46 Hypokalemic patients who also presented with Hypomagnesemia. In these cases, the Hypokalemia was only corrected when the associated Hypomagnesemia was fixed.
â€œReview Clinical disorders of magnesium metabolism.
Compr Ther. 1997 Mar; 23(3):168-73.â€
Common Hypomagnesemia causes include:
– Diabetic Ketoacidosis
– Diuretics (e.g. Thiazides, Loop Diuretics)
ECG Changes consistent with Hypomagnesemia:-
- ST segment depression (horizontal or downsloping ST segment)
- Tachycardia leading to bradycardia
- Diminished T wave amplitude or flattened T waves
- Presence of U waves (associated with Hypokalemia)
- Widened QRS complex >100ms (rare)
- Prolonged QTc (due to repolarization delay)
- Prolonged PR interval
- Torsade De Pointes (Polymorphic Ventricular Tachycardia)
– ST depression in V3-6 and Leads II and III
– Diminished T waves
– Serum Magnesium = 1.5 mg/dL
– Serum K+ = 3.7 mEq/L
– Flattened T waves
– Prolonged QT appearance due to prominent U wave
– Serum K+ = < 2 mEq/L
– Torsade De Pointes
Although not every Hypomagnesemia case will present with ECG changes, these changes may be seen often andÂ have similar Hypokalemic characteristics, as Magnesium plays a role in Potassium regulation.