(1)
Pediatric Cardiology, Policlinico S.Orsola-Malpighi, Bologna, Italy
4.1 Heart Rate Calculation
4.1.1 In the Case of Regular Rhythm
- 1.
Identify a QRS complex that falls on a vertical line bordering a large square; the HR is calculated by dividing 300 by the number of large squares that separate it from the next QRS complex.
- 2.
By dividing 60,000 (milliseconds in 1 min) by the interval—in milliseconds—between two P waves (atrial rate) or between two QRS complexes (ventricular rate) (Fig. 4.1).
Fig. 4.1
Tobia Ravà. Existential codes
4.1.2 In the case of fast heart rate
- 1.
The HR is obtained by dividing 1500 by the small squares between two waves.
In the standard ECG, the paper speed is 25 mm/s: a large square of graph paper measures 200 ms, and in 1 min there are 300 large squares (40 ms for each millimeter). If the two complexes are separated by only one large square, the HR will be 300 bpm; if there are two squares, the HR will be 150 bpm, three squares 100 bpm, four squares 75 bpm, five squares 60 bpm, and so on (Figs. 4.2 and 4.3).
Fig. 4.2
The ideal PQRST complex and the seven questions (Portable Questionnaire Resolvable Simply Trying)
1. Is the paper speed 25 mm/s and the amplitude 10 mm/1 mV? For a fax or non-original copy: is the scale 1:1, i.e., do 10 small boxes of graph paper correspond to 10 mm?
2. Is the P wave ahead of the QRS? What polarity does P have in lead II? How tall is it? How long does it last? Is it definitely sinus? What is the atrial rate?
3. Is the PR normal? Can you exclude delta waves everywhere?
4. Are there as many QRS complexes as P waves? What is the direction of the electrical axis? Is the aVR as it should be (mostly negative apart from in the neonate), and can you exclude lead malpositioning? Is there a Q wave in V6? Is the amplitude of the R waves consistent with age and body weight?
5. Can the elevated ST segment in the precordial leads be explained by the early repolarization pattern?
6. Is the QT normal by applying the method of the tangent? At a glance, does the T wave end before the “center line” between two QRS complexes at the HR above 70 bpm? If so, the QT is normal.
7. Is the T wave negative in the precordial leads in a patient aged >12 years? This is not normal. Is the T wave negative in leads III and aVF? It could be normal.
Fig. 4.3
Heart rate at a glance
4.1.3 In the Case of Irregular Rhythm
- 1.
The HR is calculated by counting the number of QRS complexes in 6 s (equivalent to 30 large squares or 15 cm of paper) and multiplying it by 10.
Example: 21 QRS complexes in 30 large squares, the ventricular rhythm is 210 bpm.
4.2 QRS Axis
The ECG leads can be represented as axes lying on the same frontal plane, with the heart positioned at the center of it. The electrical activity of the heart can be reduced to a resultant vector which represents the instantaneous mean cardiac axis (hexaxial reference system) (Fig. 3.3). On the hexaxial reference system:
For simplicity the normal heart axis is between 0° and 90° (normal values vary according to age).
Right axis deviation: the cardiac axis moves to the right (more positive than 90°, between 90° and 180°).
Left axis deviation: the cardiac axis moves to the left (more negative axis than 0, between 0° and −90°).
For electrical axis calculation, a practical method may be to find the lead in the frontal plane with an isoelectric QRS (the algebraic sum of wave approaches zero): on the hexaxial reference system, the QRS axis will be perpendicular to this lead, directed toward one of the two leads which are located at right angles to the previous one and precisely toward the lead in which the positive component is predominant. In the absence of a perfectly isoelectric lead, it is necessary to make corrections of 15°. For example, if the “almost” isoelectric lead is aVF with R > S pattern and positive lead I, the cardiac axis will not be exactly at 0° (between the left two quadrants) but in the lower left quadrant at +15°; in the same way, in the case of aVF with R < S pattern and positive lead I, the cardiac axis will be not 0° but in the upper left quadrant at −15°. Another method is based on analysis of leads I and aVF, which in the hexaxial system are arranged perpendicularly (Figs. 4.4 and 4.5).
