Summary Part 2 - THE COMPONENTS OF THE ECG

1. The atria
   • The normal resting heart rate in adults is 50-100 beats per minute, determined by the balance of sympathetic and parasympathetic influence. Particularly in the young, marked respiratory rate variation may be seen. A low heart rate is associated with good health.
   • The P wave reflects the electrical activity of the atria; the first part reflects the right atrium, the second the left one. The axis is 0-75°, duration ≤0.12 s, amplitude max 0.25 mV (2.5 mm). A P wave in V1 that has a deep negative tail (≥1 mm, ≥40 ms), suggests increased left atrial workload, as in severe mitral stenosis (now rare in the Western world) or reduced left ventricular compliance (diastolic dysfunction, common in hypertension and heart failure).
2. The conduction
   • The PQ interval (0.12-0.21 s) is determined mainly by the AV node.
3. Ventricular depolarization
   • The depolarization wave is reflected by the QRS: normally ≤0.10 s duration and axis -30° to +105°. QRS duration 0.10-0.12 with normal complex morphology is called slight intraventricular conduction delay. An infarct scar may be seen as pathological q waves in at least two neighbor leads (2 mm (40 ms) broad, at least 1mm deep and >¼ R in height). A tall R wave is seen in left ventricular hypertrophy (≥20 mm in standard, ≥30 mm in a precordial lead).
   • In bundle branch block the ventricles are not simultaneously activated, and QRS duration is 0.12 s or more. In left bundle branch block the last activated site is the apex (positive terminal deflection in V6), and in right bundle branch block this occurs in V1. A left bundle branch block usually reflects injury over a large area, whereas the injured area is more limited in right bundle branch block; RBBB may even be seen in heart healthy persons.
4. Ventricular systole
   • The ST segment starts at the J point, at baseline when the myocytes are synchronized. In ischemia, however, there is a current of injury from the ischemic towards the healthy muscles, causing an ST elevation above a transmural infarct (STEMI) and an ST depression above an area with subendocardial ischemia or infarct (angina pectoris, non-STEMI).
   • When the phase 1 current in the epicardium is stronger than usual, the ST take-off will come from an elevated J-point, shown as “early repolarization”. The ST slope then curves upwards, at variance from the dome-shaped ST segment in STEMI.
5. Ventricular repolarization
   • The T waves shall normally point to the same direction as their QRS; they are concordant. If two or more neighbor leads show discordant T waves, the cause is usually pathological.
   • When the AV conduction is abnormal (bundle branch block, preexcitation), or there are myocardial fibrosis or scars, the sequence of repolarization is reversed.
   • The QT interval reflects the duration of the action potentials (plus the inter- and intraventricular conduction time). The end point is defined as the crossing of the steepest tangent to the T wave and the baseline. The QT interval is short at fast, and long at slow heart rates. For this reason, we calculate the corrected QT interval, usually by Bazett’s formula QTc= measured QTinterval√ preceding RR interval (measures in seconds).