Now, I understand that the QRS represents ventricular depolarization, but I’m still having trouble grasping how it records the electrical impulse.
For now I’m just talking about normal sinus rhythm.
Let’s start with the R wave. Does this represent the electrical impulse traveling from the AV node to the Bundle of His, or is the R wave after the impulse has reached the Bundle of His, and the ventricles have already started depolarizing?
Why is the QRS complex pointy? Is it just showing that between both ventricles the electrical current gets stronger and stronger until it peaks, then it abruptly gets weaker and weaker until there is no more electrical current generated by the ventricular tissue?
I’m having a hard time putting together what little I know from the dumbed down version of EKGs we learn in paramedic school, what I understand about how the heart contracts, and what I’ve learned about the Sodium/Potassium pump and how it effects the electrical charge of cells during depolarization. If anyone can shed some light on this it’d be greatly helpful.
Let’s start with the P wave. Normal rhythm originates in the sinoatrial (AV) node, and, as excitation spreads over both atria, it produces the P wave. After a delay at the atrionodal junction, the impulse passes through the atrioventricular (A-V) node, the bundle of His, and its right and left bundle branches which terminate in the subendocardial Purkinje network. This network conveys the impulse into the ventricular myofibrils. No electrical expression of this A-V conduction is seen, and it is not until the excitation spreads from the endocardial to the epicardial surface of each ventricle that electric activation reappears in the EKG and the QRS deflection is inscribed. Hence, an isoelectric P-Q segment follows the P wave. The QRS interval reflects the depolarization of cellular membrances of the ventricular myofibrils, and the T wave, which follows the QRS interval, reflects their repolarization. The P and QRS inscriptions precede, respectively, atrial and ventricular systole.
Thanks - that is one of the big explanations I was looking for. So in normal sinus rhythm, assuming there is normal mechanical capture, at what point in the QRS complex do the ventricles begin actually contracting?
I’m still confused about why the waves look the way they do. In Lead II (NSR) the R wave and S wave come together at a point. Is this because the excitation of the ventricles spreads inferiorly, then superiorly? Or is it just that the amount of electricity picked up by the positive electrode increases until it peaks, then abruptly decreases?
I’m told a positive deflection means that the impulse is going towards the positive electrode, so I’m picturing the excitation of the ventricles going towards the positive electrode in Lead II (left leg).
IANAMD, so I cannot answer all your questions. I’ve told you all I know about the EKG. Contraction, however, begins, of course, at depolarization, at the “Q” point.
No problem, I appreciate your help. As I’m getting deeper into cardiology it’s starting to make more sense. I’ll give this thread one last bump in case anyone else can help.