ECG help, atrial rate?

[meepmeep]

Hi there, I need help. I'm working through the "ECG Workout" book by Jane Huff in preparation for going back to school in September. I'm in the chapter that discusses junctional arrhythmias and AV blocks and for the life of me, I can't seem to understand how the book calculates the atrial rates for second (mobitz I/Wenckebach) and third degree AV (mobitz II) blocks. I tried to Google how to do it, but I still don't understand and most sites just describe the typical heart rate calculations.

I know I'm dumb. I assume you count the small squares between each P wave and divide that number by 1500 which is what I do for the ventricular rate. But the number the book arrives with don't match for all the examples and some rates like 123 for atrial rate aren't possible according to the conversion table for heart rate in the back. (i.e. 1500/12 squares = 125, 1500/12.5 squares = 120, 1500/13 squares = 115) So how'd they get 123??? Then I wondered if it was some weird average, but still that doesn't make sense. I don't see anywhere in the book that mentions how to calculate it. So I'm missing something here and must be using the wrong method to figure out atrial rate? Any help is greatly appreciated.

[Lev, MSN, RN, NP]

Count the number of P waves in a 6 second strip. Multiply by 10. That is your atrial rate.

Edited to add: watch out for P waves hidden in the QRSs when dealing with the heart blocks.

[RescueNinjaKy]

Use a caliper, measure p to p. Then put it against the boxes and you can calculate the time between the p's. Then so 60 divided by whatever number you get.

[chare]

I know I'm dumb. I assume you count the small squares between each P wave and divide that number by 1500 which is what I do for the ventricular rate. But the number the book arrives with don't match for all the examples and some rates like 123 for atrial rate aren't possible according to the conversion table for heart rate in the back. (i.e. 1500/12 squares = 125, 1500/12.5 squares = 120, 1500/13 squares = 115) So how'd they get 123

No, you're not dumb. You calculate the atrial rate the same way that you calculate the ventricular rate.

1500 ÷ 12.195 = 123.001123001; that's how you get a rate of 123. However, in my opinion this is just a little over the top. As is including a conversion table using 0.5 of the smaller square on the ECG paper. If you need to be that accurate in determining the heart rate, you should run a 60 second strip and count the complexes or count the pulse for 60 seconds.

[meepmeep]

Thanks for the tips. The calipers did make identifying the distance between P waves easier and I managed to get the same atrial rate as the book for the mobitz I blocks. However, for the mobitz II blocks, the book seems to give weird numbers that would require math with decimals which I find odd and is too much trouble for me to bother with. Wonder why there is this inconsistency.

[nurseprnRN, BSN, RN]

Thanks for the tips. The calipers did make identifying the distance between P waves easier and I managed to get the same atrial rate as the book for the mobitz I blocks. However, for the mobitz II blocks, the book seems to give weird numbers that would require math with decimals which I find odd and is too much trouble for me to bother with. Wonder why there is this inconsistency.

In the real world nobody will ever test you on the difference between 123 and 125 or require decimals, even for NCLEX.