A 12-year-old boy presented to his paediatrician for a routine check-up.
The patient is a healthy child with no personal history of interest.
During physical examination, the pediatrician detects a cardiac systolic murmur 2/6 in relation to the left vibratory edge low phase, non-irradiated and modified with posture, with a second normal tone and arrhythmic tone.
Peripheral pulses are palpable and symmetrical.
The rest of the physical examination is normal.
Blood pressure was within percentiles for age and height.
The patient is asymptomatic, does not report syncope, precordalgia or syncope and has a good tolerance to physical exercise.
In the family, they do not report cardiovascular diseases or a history of sudden death.
When auscultation revealed arrhythmic tone, the pediatrician performed an electrocardiogram (ECG) in the consultation shown in Fig.
1.
What is the reading and interpretation of the electrocardiogram?1
Although the first thing that draws our attention is irregular rhythm, we began our systematic reading proving that it is a sinus rhythm (positive P wave in I and aVF) at a frequency of 55 bpm.
Then we analyzed the axis of the QRS following three steps: 1) locate the quadrant using leads I and aVF; 2) find the derivation with equiphasic QRS complex (quadrice of the R wave similar to the axis VR).
In our case: 1) the QRS is positive in both leads so that the axis will be between 0 and 90o; 2) the derivation with more equiphasic complex would be the I derivation, whose perpendicular QRS axis is 90o; therefore
The axis of the T wave is also normal because it is between 0 and 90o.
QRS complex duration was normal (0.04 sec).
We did not observe QRS morphology suggesting changes in intraventricular conduction (e.g., absence of rR' in V1) or findings suggestive of ventricular hypertrophy.
T wave and ST segment are normal; there are no changes in repolarization.
Then, we analyze the PR2: it is noteworthy that the PR interval (from the beginning of the P wave to the beginning of the QRS) varies throughout the recording, is not constant.
Figure 2 shows the tracing in DII from the fifth heartbeat of the basal ECG: the PR interval of the first heartbeat measures 4.5 quartiles or millimeters (0.04 x 4.5=0.18 sec).
We also see that after the fourth heartbeat there is a P wave (marked with a arrow) that is not followed by QRS complex.
This electrode recording (RR interval progressively lengthening until a P wave does not lead) reflects a second-degree AV block Mobitz type I. The QTc interval is normal (0.38 sec).
1.
Establishment of an atrial fibrillation, the pediatrician decides to refer the patient to the hospital and to the pediatric cardiology department for evaluation.
In the cardiology consultation an echocardiogram was performed which was normal, ruling out structural pathology.
On the new electrocardiogram, sinus rhythm was observed at 60 bpm with an enlarged PR (first-degree AVB), and the rest of the systematic reading was normal.
After performing light exercise to increase heart rate, the ECG was repeated, with normalization of the PR interval.
In summary, we are facing an asymptomatic patient from a cardiovascular point of view, with a cardiac murmur of functional characteristics, normal echocardiography and a casual finding of first-degree and second-degree AVB on ECG.
To extend the study, a 24-hour Holter-ECG was requested, which found a first-degree AVB with two second-degree AVB episodes Mobitz type I during night rest.
