Saturday, January 11, 2014

Low Voltage in Precordial Leads

A patient with was resuscitated from respiratory and cardiac arrest of uncertain etiology, but because she was very difficult to ventilate with BVM ventilation, and there were no ultrasonographic slidings signs, pneumothorax was suspected and bilateral needle thoracostomies were placed. This ECG was recorded:
There is sinus tach and very low voltage in the precordial leads.  It is otherwise unremarkable, especially for a patient who was in cardiac arrest.


There was a previous ECG available:
The low voltage is indeed new, and there is no change in limb lead voltage



Why is there low voltage?








Although she was thought to probably have severe asthma, she underwent a CT pulmonary angiogram to rule out pulmonary embolism.  Here it is:
Air surrounds the heart (pneumopericardium and pneumothorax)


The air explains the low voltage, but why is the voltage not diminished in the limb leads? 

(I don't know the answer, and I'll add that the CT scan showed air surrounding the entire heart, in all planes.)


The QRS is said to be low voltage when:
  • The amplitudes of all the QRS complexes in limb leads are less than 5 mm; or
  • The amplitudes of all the QRS complexes in precordial leads are less than 10 mm

There are several etiologies of low voltage:
  1. Obesity
  2. Emphysema
  3. Pericardial effusion, Pleural Effusion
  4. Severe hypothyroidism (myxedema)
  5. Subcutaneous emphysema
  6. Pneumothorax or Pneumopericardium
  7. Old large MI
  8. End-stage dilated cardiomyopathy
  9. Infiltrative/restrictive diseases such as amyloidosis or hemochromatosis.

2 comments:

  1. Steve...

    I just came across this presentation of yours from 2 years ago while preparing a lecture on QS complexes in the right precordial leads and I saw that there had been no comments or responses to your puzzlement over why the precordial leads had low voltage but not the limb leads.

    When air affects the ECG, it does so by attenuating the voltage of the "y axis" in the frontal plane (i.e., Lead I) and augmenting the voltage of the "x axis" (Lead aVF). The air occupying the lungs on each side of the heart is a very poor conductor while the myocardial tissue occupying the mediastinum (recall that in COPD the heart is usually pretty stretched out vertically) is a very good conductor. So what you would expect to see is smaller voltage in Lead I (perhaps in aVL also because it's the next nearest lead, only 30 degrees from Lead I) and larger voltage in the more vertical leads (II, III and aVF). This is seen very commonly in COPD.

    But here's something that few people discuss or even think about: if Lead I is the "y axis" then so, too, is Lead V6. Each axis is actually the axis for TWO planes - not just one - and the "y axis" of the frontal plane also serves as an axis for the horizontal plane (except we call it "V6" instead of "Lead I"). Since there is no verticality to result in augmented voltage, virtually all of the horizontal plane has an attenuated voltage and Lead V6 (analogous to Lead I) has the most attenuated voltage of all. Also, in the limb leads, Lead I has lower voltage than the other leads with aVL perhaps the next lowest voltage.

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