Frequently, we encounter babies in the NICU with sudden wonkiness on the cardiac monitor. At first glance, many of these "funny looking beats" appear to be PVC's, but much more commonly they represent artifact: hiccups, preemie myoclonic jerks, or other abnormal body movements. Since things like hiccups can be signs of overstimulation in the preemie, it's nice if we can differentiate a hiccup from a PVC without lifting an isolette cover to peek inside.
The good news is that often we can make this distinction by carefully examining the rhythm, if we have an understanding of the relative and absoluterefractory periods. To best understand this stuff, first think about flushing a toilet. Right after the flush, what happens if you try to press the handle again? Nothing, right? The toilet is absolutely refractory to another flush since it has just been "depolarized." After a few seconds, though, if I depress the handle again, the toilet is only relatively refractory to another flush, and we get an abnormal, weak and funny-sounding "depolarization." Let's translate that to the heart.
The absolute refractory period stretches from the QRS complex to the first half of the T wave. No abnormal beats will be conducted during this interval. If I see a QRS and then another "beat" immediately after that QRS, I know that the second "beat" could not possibly be a cardiac depolarization. It's most likely a hiccup.
From the apex of the T wave to the end of the T wave, you have the relative refractory period. Remember our abnormal flush? Same thing in the heart, only with worse consequences. The abnormal depolarizations during this vulnerable period can lead to lethal arrhythmias. Look up the R-on-T phenomenon for an example of the badness that can ensue when an ectopic beat lands on the previous T wave.
Finally, following the T wave, there is a non-refractory period. This is where abnormal electrical foci most commonly cause PVC's. In this area, PVC's are usually not dangerous unless they are very frequent.
A Google Images search for "EKG refractory period" will turn up some neat diagrams that depict these periods and the corresponding parts of the EKG complex. And where did I get this crazy toilet analogy? I got it from an old medic, who had learned it from an even older medic. Based solely on its weirdness, I suspect that it did originate within EMS circles long ago.
The good news is that often we can make this distinction by carefully examining the rhythm, if we have an understanding of the relative and absoluterefractory periods. To best understand this stuff, first think about flushing a toilet. Right after the flush, what happens if you try to press the handle again? Nothing, right? The toilet is absolutely refractory to another flush since it has just been "depolarized." After a few seconds, though, if I depress the handle again, the toilet is only relatively refractory to another flush, and we get an abnormal, weak and funny-sounding "depolarization." Let's translate that to the heart.
The absolute refractory period stretches from the QRS complex to the first half of the T wave. No abnormal beats will be conducted during this interval. If I see a QRS and then another "beat" immediately after that QRS, I know that the second "beat" could not possibly be a cardiac depolarization. It's most likely a hiccup.
From the apex of the T wave to the end of the T wave, you have the relative refractory period. Remember our abnormal flush? Same thing in the heart, only with worse consequences. The abnormal depolarizations during this vulnerable period can lead to lethal arrhythmias. Look up the R-on-T phenomenon for an example of the badness that can ensue when an ectopic beat lands on the previous T wave.
Finally, following the T wave, there is a non-refractory period. This is where abnormal electrical foci most commonly cause PVC's. In this area, PVC's are usually not dangerous unless they are very frequent.
A Google Images search for "EKG refractory period" will turn up some neat diagrams that depict these periods and the corresponding parts of the EKG complex. And where did I get this crazy toilet analogy? I got it from an old medic, who had learned it from an even older medic. Based solely on its weirdness, I suspect that it did originate within EMS circles long ago.
