Dismiss Notice
Hey Texans—join us for a DFW meetup! Click here to learn more.

Can anyone explain why a T wave is not inverted?

Discussion in 'Step I' started by grw0o, Apr 26, 2007.

  1. grw0o

    grw0o Senior Member
    10+ Year Member

    Joined:
    Sep 23, 2003
    Messages:
    152
    Likes Received:
    0
    This is another thing that has been bothering me for quiet some time. I thought I understood it at one time why the T wave was not "negative" but for our cardiovascular block the doctor couldn't explain it.

    I know it has to do with the charges and how the electrodes of the ECG is set up. So an upstroke is depolarization from say the base to the apex of the heart. Then when the ventricles re polarize the the dipole is reversed - so I keep wanting to think uptroke of the right ventricle overpowers the downstroke of the left ventricle... or something to that effect.

    I am not sure if this is right or not - anyone have a quick explanation?

    Thanks!
     
  2. Note: SDN Members do not see this ad.

  3. SeventhSon

    SeventhSon SIMMER DOWN
    7+ Year Member

    Joined:
    Aug 23, 2005
    Messages:
    1,022
    Likes Received:
    0
    MDApps:
    Status:
    MD/PhD Student
    i never understood why the early precordial leads don't show concordance, but in general, the repolarization is essentially a "double negative equals a positive".

    The surface of the heart in depolarization is going from more positive to more negative along the axis of depolarization.

    In repolarization, the surface of the heart goes from more negative to more positive. This fact alone would invert the T-wave precisely on all leads if the geometry of the heart stayed the same throughout the contraction of the heart (which it obviously doesn't, and is probably the basis for lack of concordance in the early precordial leads). HOWEVER, the direction of repolarization moves (generally) from apex to base, so you get a "double negative".
     
  4. indo

    indo Feed me a stray cat
    7+ Year Member

    Joined:
    Sep 28, 2002
    Messages:
    7,270
    Likes Received:
    12
    Status:
    Resident [Any Field]
    I wrestled with this concept until I created an explaination I could live with.

    During the T wave the ventricle repolarizes backwards. The most lateral (highest) portion of the ventricular wall's outer surface becomes positive again, then the apex, then the interventricular area and the magnitude "arrow" points toward the positive charge. So, at the peak of the T wave the apex has become externally + and the superior septum is still negative.

    Hopefully this and the post above help you put it into your own terms.



    Why is Q inverted on the qrs complex?
     
  5. goodies

    goodies Member
    7+ Year Member

    Joined:
    Oct 8, 2005
    Messages:
    334
    Likes Received:
    0
    Status:
    Resident [Any Field]
    man, am i in big trouble if i dont understand any of this. :(
     
  6. anesthesiarocks

    2+ Year Member

    Joined:
    Feb 12, 2007
    Messages:
    201
    Likes Received:
    0
    Status:
    Medical Student
    god no.

    What you need to know about EKG's for step 1 is to be able to:

    1) look at them and pick up the patterns in Firstaid(Vtach, afib, etc)
    2) know what is occuring during each part of it(for example vent depolarization in QRS) and how to correlate this with what ions would be going in or out at that moment
    3) understand what some of the changes in a normal cycle are and what they mean(PR interval increased due to what and why, how hypokalemia affects it, ischemia, etc)

    worrying about anything beyond that is ridiculous. yeah there is always a chance they can ask you something about the electromagnetic orientation of leadX in relation to whatever....but they are far more likely to require that you know the PR interval will be increased with digoxin use due to AV block.
     
  7. TheMightyAngus

    5+ Year Member

    Joined:
    Jun 14, 2005
    Messages:
    1,193
    Likes Received:
    5
    Status:
    Medical Student
    The endocardium depolarizes first and repolarizes last. So the depolarization and repolarization are moving in opposite directions. They show up as similar deflections on an ECG because repolarization moving toward the lead produces the same voltage change as a depolarization moving away from the lead. Therefore, a QRS (depolarization) directed away from the apex will appear upright, as will a T-wave (repolarization) directed toward the apex.

    Source: Lilly
     
  8. Idiopathic

    Idiopathic Newly Minted
    Lifetime Donor Classifieds Approved 10+ Year Member

    Joined:
    Apr 21, 2003
    Messages:
    8,362
    Likes Received:
    16
    Status:
    Attending Physician
    'Double negative' giving a positive (i.e. in the same direction as the QRS) is the best answer. It depolarizes in the opposite direction, but also outside in, so the net effect of its ECG depiction is in line with the QRS.
     
  9. DocMorris

    Joined:
    Feb 11, 2013
    Messages:
    1
    Likes Received:
    0
    Status:
    Pre-Medical
    I know this is an old thread, but I had to complete the thought:

    This answer is correct: The missing piece however is why does depolarization move in the opposite direction? The septum and deep endocardial areas do depolarize first but they have a longer period of contraction. Therefore most of the outer surface of the ventricles, especially near the apex are the first to replolarize.
     
  10. g8orlife

    g8orlife chomp
    7+ Year Member

    Joined:
    Sep 15, 2009
    Messages:
    892
    Likes Received:
    27
    Status:
    Resident [Any Field]
    OMG! I'm several months removed from cardio, but this question drove me crazy every.single.day!
    I finally understand :idea: Thanks DocMorris for bumping this.
     
  11. Ravi Chaudhary

    Joined:
    Jun 29, 2018
    Messages:
    3
    Likes Received:
    0
    Sorry if it appear ridiculous, but if the repolarization starts first from the cell which is last to depolarize and in opposite direction then why the T wave is not the exact mirror image of QRS complex??

    Sent from my Nexus 5 using Tapatalk
     

Share This Page