A doctor looks at three records. In record 1, a normal P wave is followed by a normal QRS complex…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A doctor looks at three records.
- In record 1, a normal P wave is followed by a normal QRS complex and T wave. In which chambers are the excitation and the contraction normal?
- In record 2, there are regular P waves, but some of them are not followed by a QRS complex. What does this suggest about the signal?
- In record 3, the QRS complexes come s apart. What is the heart rate?
- Where does the doctor look first for the cause in record 2?
Need a reminder? The key ideaShow
The P wave is the depolarisation of the atria. The QRS complex is the depolarisation of the ventricles, and their contraction starts soon after Q. The T wave is the repolarisation of the ventricles, and its end marks the end of the systole. Sympathetic signals and the hormones of the adrenal medulla raise the heart rate and the output. Parasympathetic signals lower them.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. (a) The atria are excited first (P), and the ventricles follow (QRS). Then the ventricles return to normal (T). The sequence is normal.
Step 2. (b) The atria are excited, but the signal does not always reach the ventricles. The path from the atria to the ventricles, by the AVN and the AV bundle, is faulty.
Step 3. (c) beats a minute.
Step 4. (d) The doctor looks at the AVN and the AV bundle.
Answer: (a) normal; (b) the signal is blocked between atria and ventricles; (c) beats a minute; (d) the AVN and the AV bundle.
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