3rd Degree AV Blocks...HELP!


Specializes in Telemetry, Case Manager, Delegating RN.

Hi everyone!

I am in my last semester of nursing school and we are learning about how to read EKGs. I am confused on how to differentiate 3rd Degree AV Blocks :eek:. This particular EKG reading doesn't appear to have anything special about it which is making it hard for me to pick it out from a set of four different EKG readings. Does anyone have any tips or advice about this!:confused:

Thanks :-)

Esme12, ASN, BSN, RN

Specializes in Critical Care, ED, Cath lab, CTPAC,Trauma. Has 41 years experience.

Electrocardiogram (ECG, EKG) library

I think you will find this site extremely helpful. Remember in 3rd degree heart block the "P" waves and the "QRS" and no correlation at all........is is a slow AV dissociation. The R to R intervals may be regular, the PR interval will be wildly inconsistent. But it is important to realize that not all patients with AV dissociation have complete heart block. For example, patients with ventricular tachycardia have AV dissociation, but not complete heart block; in this example, AV dissociation is due to the ventricular rate being faster than the intrinsic sinus rate. On electrocardiography (ECG), complete heart block is represented by QRS complexes being conducted at their own rate and totally independent of the P waves

Third-degree AV block, also known as complete heart block, is a medical condition in which the impulse generated in the SA node in the atrium does not propagate to the ventricles. Because the impulse is blocked, an accessory pacemaker in the lower chambers will typically activate the ventricles. This is known as an escape rhythm. Since this accessory pacemaker also activates independently of the impulse generated at the SA node, two independent rhythms can be noted on the electrocardiogram (ECG).

Differentiating AV Blocks - YouTube

  • The P waves with a regular P to P interval represents the first rhythm.
  • The QRS complexes with a regular R to R interval represent the second rhythm. The PR interval will be variable, as the hallmark of complete heart block is no apparent relationship between P waves and QRS complexes.

One of the pathognomonic characteristic of this block is the absolute absence of the opportunity for atrial impulses to enter and capture the ventricles(unlike AV dissociation,with functional block)due to organic nature of block(eg, fibrosis, calcification, or infiltration of the node) . It means that, in the presence of complete heart block, fusion or capture beats will never be seen.

Patients with third-degree AV block typically experience bradycardia (an abnormally low measured heart rate), hypotension, and at times, hemodynamic instability.

Medscape: Medscape Access (requires registration but it is free and a great resource!)

I hope this helps. :loveya:

aw, esme, ya beat me to it again!

op, remember that an ekg shows you the path the electrical impulse takes as it travels through the heart. take a look at the diagram of the normal conduction system. that will help you visualize better.

most muscle cells cannot contract without some sort of electrical stimulus telling them to. (imagine the havoc if all the muscle cells in your thigh could contract willy-nilly whenever the mood struck them.) cardiac muscle cells are a little different; they can contract on their own and generate their own little electric jolt and pass it along cell-to-cell to their neighbors if the normal impulse doesn't come through the normal conduction pathway often enough. remember that for later.

1)the normal impulse starts in some specialized cells in the sinoatrial (sa) node, a little patch of tissue that has the ability to do this by itself 'way up in the atria.

2) this impulse spreads thru conduction pathways in the atria, making the muscle cells contract as it goes, in a nice even pattern that empties the atria thru the tricuspid valve (right heart) and mitral valve (left heart) into the ventricles to give them something to do. that's diastole. this electricity looks like a nice round little bump, the p wave, on ekg.

3) there's a teeny pause while the impulse is gathered up in the atrioventricular (av) node, then spreads in a nice pattern thru the ventricles, their muscle wringing like a washcloth. (the electrical signature of this action is the qrs, the big spiky deflection on the ekg.) the pressure thus developed closes the mitral and tricuspid valves but opens the pulmonic valve (right side) and aortic valve (left side) and blood gets pushed into the pulmonary artery and aorta. that's systole, and we have...a blood pressure.

if the tissue at the av node is on strike for some reason, like it's dead after infarct (good reason), when the impulse comes down to it from the atria, it's unable to pass it along to the ventricular conduction pathway, so there is no longer a nice p wave->qrs, p->qrs, p->qrs happening. after a bit the ventricles notice that they are not getting any direction from up above. they are big and strong, but not that smart, so they only get it together to generate their own contraction slowly after one of their cells takes it upon itself to contract. because the impulse driving them does not come down that nice dedicated pathway but has to spread cell-to-cell from there, it takes longer and doesn't look like it knows where it's going, so the qrs is wider and funny-looking.

now if you look at the tracing for this, you see a nice regular march of p waves, indicating the atria are working they way they are supposed to, and then, at a totally different rate and not playing nice and holding hands with their friends, the ventricles tooling along on their own, slower rate. it may be fairly regular but it won't have any relationship at all to the p waves. that's complete heart block (3rd degree av block).

Esme12, ASN, BSN, RN

Specializes in Critical Care, ED, Cath lab, CTPAC,Trauma. Has 41 years experience.

:lol2: We should set up an online tutoring or school instead of giving it up for free....;).

Anyhow, It keeps me sane....


Specializes in Telemetry, Case Manager, Delegating RN.

Thanks so much! I really appreciate your advice! I hope you continue to share knowledge.. It's helpful for those who cannot afford anymore schooling than what they're in. lol. Thanks ladies :-)


Specializes in Gerontological, cardiac, med-surg, peds. Has 16 years experience.

Esme12 and Grn Tea - A warm THANK YOU to both of you for all you contribute here in this forum! It is GREATLY appreciated! :) :yelclap::redbeathe:nurse::yeah::up::w00t:

Esme12, ASN, BSN, RN

Specializes in Critical Care, ED, Cath lab, CTPAC,Trauma. Has 41 years experience.

I was just teasing.....;)...you're welcome!

well, thank you very much, vickyrn!:thankya:

personally, i love students (they are our future, after all) and definitely consider the source :nurse:.

(does that mean esme and i get cheers:smokin:?

rah, rah, r[color=#00ff00]ah!

DixieRedHead, ASN, RN

Specializes in ED/ICU/TELEMETRY/LTC. Has 20 years experience.

Hillbilly version: You got p waves? You got QRS? If the answer to both is yes, then ask do bear any relationship to themselves or each other? If the answer is no, you got yourself a third degree block.

turnforthenurse, MSN, NP

Specializes in ER, progressive care. Has 7 years experience.

How I remember a 3rd degree block: If your P's and QRS' don't agree, you've got yourself a 3rd degree. 3rd degree block is also known as AV-dissociation. The P's are off doing their own thing and the QRS' are off doing their own thing - think of them as "not being married to each other."


Notice how the PR-interval is all of the place. You may also have P's buried in the QRS or in the T waves (as in this example).

...Compared to a Mobitz II (second degree type II) where the PR-interval is consistent but you will have a dropped QRS.



Specializes in critical care, PACU. Has 2 years experience.

I like the Hillbilly version