Magnesium and Cardiac Action Potential

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.

“Review Clinical disorders of magnesium metabolism.
Whang R
Compr Ther. 1997 Mar; 23(3):168-73.”

Common Hypomagnesemia causes include:

- Alcoholism

- Diabetic Ketoacidosis

- Malnutrition

- Digoxin

- 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)

hypomag-bmp

- 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

II

v2

- Flattened T waves

- Prolonged QT appearance due to prominent U wave

- Serum K+ = < 2 mEq/L

torsades

- Torsade De Pointes

Conclusion:

Although not every Hypomagnesemia case will present with ECG changes, these changes may be seen often and have similar Hypokalemic characteristics, as Magnesium plays a role in Potassium regulation.

2 Comments

  • Charlene says:

    Thank you for this information. Just yesterday I was asking my instructor about the MOA of Mg in Torsades and how it affects QTc . I didn’t exactly receive the most satisfactory answer. This makes it very clear.

    • Ivan Rios says:

      You’re welcome. I wish medic school went a little more in depth when it comes to pharmacodynamics and such. I’m glad this helped!

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EMS 12-Lead

Cardiac Rhythm Analysis, 12-Lead ECG Interpretation, Resuscitation
Comments
Sean V
Rate Related VS. Primary ST-T Changes:
Also forgot to mention decrease the FiO2, 3LPM is getting us a SpO2 of 98%, titrate down so we staying at or above 94%. No need to hyperoxygenate & create all those fun free radicals. I would also include using an EtCO2 nasal cannula, lets get another measure of our cardiac output.
2014-09-20 02:32:20
Sean V
Rate Related VS. Primary ST-T Changes:
Atrial Fibrillation w/ Rapid Ventricular Response. There appears to be possible Delta Waves, the most prominent being in aVL, also leads I, II, and V6. In the EMS 12-Lead there appears to be a fusion beat, 3rd in V2, slurred R-wave appears quite consistent with a Delta wave. I would consider WPW as the primary…
2014-09-20 02:28:16
Brian
Rate Related VS. Primary ST-T Changes:
Afib. There is widespread depression in most leads and aVR has some elevation...but I am skeptical about this ecg. If a quick fluid challenge of 500-1000cc did not slow down the HR I would give him some diltiazem (5mg increments is our protocol or 0.25mg/kg) and slow the rate down a bit and see if…
2014-09-19 21:02:48
Michael Schiavone
Rate Related VS. Primary ST-T Changes:
Isolated ST elevation in AVR with ST depression in several leads. Rapid, irregular rate suggests AFIB with RVR. I would provide entry note with this exact description and leave it to hospital whether or not to activate cath lab. My EMS treatment: IV access, 324 mg. ASA, NTG, Cardizem .25 mg/kg over 2 minutes, consider…
2014-09-19 20:30:35
Dayne
Rate Related VS. Primary ST-T Changes:
AF with RVR @167, LVH and prolonged QT. ST depression to I,II and V3-6 and reciprocal elevation to aVR equal to or >1mm highly suggestive of LMCA or 3-vessel disease. High specificity for proximal occlusion. Aspirin, GTN, IV access, Spo2 >95%, Transport to nearest PCI/Cath Lab facility ASAP
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