In first degree heart block, we did not see any loss of QRS complexes.It was only some delay in starting a QRS. But in second degree heart block, you will see some missed QRS complexes in the ECG.
Depending on how the pattern of missed QRS complexes is seen, it is of two types.
When there are missed QRS complexes in the ECG in-spite of having a p wave, we can call it second degree AV block. There are two types in 2nd degree heart block.
Type I (Wenckebach) – Here the PR duration gradually prolongs in the next beat till a point where one QRS goes missing altogether as shown in the image.
The 4th/7th/11th p waves are lacking a QRS complex after them.
If you see the PR interval, there will be gradual increase
Unlike type 1, there is no gradual change in PR interval. PR stays the same wherever P and R waves are present. But QRS complexes go missing all of a sudden.
In the following image, the 4th P wave does not have a QRS complex. If you notice the PR interval of other waves, you can see a constant PR interval.
Sometimes this missing QRS might show a pattern such as for every X number of p waves, 1 qrs complex is seen (x:1 block). Such as 2:1 or 3:1 or 4:1 etc See image at the bottom.
Except in case of fixed ratios, second degree blocks will have irregular R-R intervals.