Need help with ECG interpretation

[GQGomer]

Questions are:

(1) P-wave - what EXACTLY causes the down slope of the P-wave? I understand the P-wave is atrial depolarization, so UPWARD portion/slope of the P-wave makes sense, BUT if depolarization is still occurring during the P-wave, WHY would the slope then go DOWNWARD? I would think that (a voltage drop as indicated by the downward slope on the 2nd half of the P-wave) indicates REpolarization (ie voltage drop), BUT repolarization of the atrium doesn't occur until later during QRS (masked by depolarization of the ventricles). Basically, it seems there should be NO downward slope on P.

(2) QRS - same question: WHY the downward slope (on the second half) if in DEpolarization? (ie since repolarization of vetricles doesn't occur until T)

(3) why the UPward slope on the T-wave if T-wave is REpolarization? (Would think REpolarization would be DOWNward since voltage difference gets greater/ie drops again during repolarization)

Thanks in advance

[GQGomer]

Thanks to all who answered. Your responses have definitely clarified the issue. Knowing it's relative to the leads, it makes sense now. :)

[Class2011]

The short answer to all three questions is that cardiac cells behave differently and repolarize at different rates based on location/function within the heart.

For example, the SA node cells begin repolarizing right away whereas the ventricular cells have a variable plateau phase due to long opening calcium channels that keep the cells depolarized. Simboka's post shows this plateau in ventricular cells nicely. Once those calcium channels close, the cells begin to repolarize due to potassium channels that were previously opened.

A neat thing about the ventricle is that the variable opening time of these calcium channels allows for a repolarization wave that moves in the opposite direction of the depolarization wave; that is, the last cells to depolarize are the first cells to repolarize. This helps prevent heart cells from prematurely depolarizing (prevents R on T). Now the ion charges are also reversing their flow, with K+ moving out rather than Na+ moving in. So you're reversing the direction of the repolarization wave and reversing the current flow--two negatives make a positive--thus the T wave is positive.

The longer answer involves a thorough understanding of the ECG and Vectors. Unfortunately the topic is usually self taught over time and I don't have any favorite references, but there are many sources available online.

As others suggest, it is crucial to remember that the ECG is measuring the wave of depolarization/repolarization as it travels through the heart; that is, it measures net flow parallel to the direction of the lead in question; it cannot measure depolarization or electrical potential of an individual cell. Also as you suggested with the hidden repolarization of the atria, lesser events will not be detected.

FWIW: The Ventricles first depolarize down the septum via the bundles parallel with lead II and a positive R wave is recorded in lead II. At the apex of the heart, the depolarization wave turns and travels upward and through the walls of the ventricles via the perkinje fibers; this is the negative S wave in lead II. The plateau phase is idealized by the isoelectric ST segment and it's where contraction occurs. Ejection continues through the T wave which is why it is crucial not to elicit another AP until the T wave completes and the ventricles relax.

[remifentanil]

I'm an ICU nurse..but that's still a little too technical for me. I just know how to interpret it and how to treat it. Your best bet would be to ask an EPS doctor or look on an EPS website.

Why ask a doctor? There has been at least ONE answer that was dead on... You need to quit thinking doctors are the only ones who know this stuff.... please tell me you are not interested in becoming a CRNA. ...becuase if you do.. will WILL have to know this stuff at least was well as a all knowing " doctor".

This is nothing more that stuff that is covered in depth in advanced physiology.

[GilaRRT]

Probably not as well as an EP physician however. In addition it is quite technical, especially when you try to appreciate the fundamental aspects of this topic. For example, what exactly is charge? How do you really explain this concept in any sort of intuitive way? I can describe forces exerted with Coulombs law and it's various derivatives. However, beyond saying opposites attract and likes repel, the fundamental aspects of this stuff are abstract and ultimately not all that intuitive IMHO.

[airina390]

you are welcome.

[BigRed86]

In depth lectures on ECG fundamentals:

A rhythm interpretation simulator: ECG Simulator | SkillStat Learning - Fast, Fun and Effective!

[sweetdreame, BSN, RN]

This book has very simple explanations for many ECG conundrums! I bought it as a new CV nurse and it was a required text for one of my courses in CRNA school. Taking physiology really helps, but this book breaks it down for you, even if you don't have a physiology background. It would be my gift to any new cardiac nurse. :)

Oops, the picture was supposed to be of the bright orange "Rapid Interpretation of EKG's" By: Dale Dubin.

Good luck!