Be sure that the atrial activity is visiblein the ECG

In an ECG tracing with narrow or broad QRS tachycardia, sometimes atrial activity is not observed because the atrial wave is hidden within the QRS complex (see Figure 15.13A). Sometimes, it could be useful to take an ECG during deep breathing (see Figure 15.5) or during carotid sinus compression. If, despite all these measures, the atrial wave is not seen (Bayés de Luna et al. 1978), it is useful to use voltage amplification techniques (see Figure 13.24) and, if possible, to apply the T wave filter (Goldwasser et al. 2011) (see Figure 25.16). However, the fact that no atrial activity is detected in the surface ECG even in the presence of slow heart rate is not conclusive evidence for atrial paralysis (see Figure 15.37), because the atrial rhythm may be concealed in the QRS complex or undetectable in the surface ECG (Figure 15.37). Atrial paralysis is only confirmed when no atrial activity is observed in intracavitary recordings.

Morphology and polarity

The atrial wave morphology suggests sinus or ectopic origin. In sinus rhythm, it is positive in leads V2–V6 and I, and negative in lead aVR; it is frequently ± in lead V1, whereas in rare cases it is ± in leads II, III, and aVF (Bayés de Luna et al. 1985; Bayes de Luna and Baranchuk 2017); finally, it is negative or ± I in aVL.

In the case of monomorphic atrial tachycardia of ectopic focus (MAT‐EF), the algorithm shown in Figure 15.10 allows us to localize the atrial origin of the ectopic P′ wave (Kistler et al. 2006). On the other hand, a very narrow P wave (<0.06 sec) is indicative of ectopic origin, although it should be pointed out that many ectopic P′ waves are wide (see Figures 15.7 and 15.9).

F waves of atrial fibrillation show low but variable voltages, being more evident in V1 (see Figures 15.25 and 15.26), whereas the typical common flutter waves display a sawtooth morphology with a predominant negative component in leads II, III, and VF (see Figure 15.33).

Figure 18.2 shows the morphology of atrial activation waves in the different supraventricular tachycardias with regular and monomorphic waves (see also Table 15.5) and Figures 18.3 and 18.4 show the different algorithms that, depending on whether atrial activity is present or not, allow us to determine the type of active supraventricular arrhythmia with narrow QRS and regular (Figure 18.3) or irregular RR (Figure 18.4).

Cadence

The cadence of atrial activity may be regular or irregular. In sinus rhythm, the cadence is regular, although it usually shows a little variability, especially during respiration (see Chapter 15, Sinus tachycardia). Ectopic atrial waves may show a regular or irregular cadence. The cadence of atrial activity is regular in all ectopic forms or reentrant tachycardias (of atrial or junctional origin) and in atrial flutter, whereas cadence is irregular in chaotic atrial tachycardia and atrial fibrillation (Figures 18.2 and 18.3). Relatively often, ectopic atrial tachycardias show some changes in the heart rate at the onset or end of the crisis (see below), in relation to some stimuli (exercise, etc.) or after some drugs administration (digitalis).

Rate

• Sinus rate during rest is not usually faster than 80–90 bpm, although this rate may be higher in sympathetic overdrive or in different pathologic situations (fever, acute myocardial infarction, hyperthyroidism, heart failure, etc.). Exercise and emotions (see Figure 15.4) lead to a progressive increase of up to 180–200 bpm in young people, whereas a progressive slowing down is observed when the stimuli disappear. The sinus rate during sleep or rest may be slower than 50 or even 40 bpm (see Chapter 17, Sinus bradycardia due to sinus automaticity depression). The rate is also increased in inappropriate sinus tachycardia and in sinus reentrant tachycardia (see Chapter 15, Sinus tachycardia).

  

  
  
• In MAT‐EF, the P′ wave ranges between 100 and 200 bpm. In some cases, its rate increases during exercise and at the onset of tachycardia (warming up) (see Figure 15.7).
  
• In MAT‐MR, the atrial rate is usually fast and does not change with exercise. If it reaches ~200 bpm, it may resemble an atypical flutter. In fact, both names probably correspond to the same arrhythmia (see Chapter 15, Atrial flutter).

  

  

  
  
• The rate in the two types of AV junctional tachycardias due to reentrant mechanism (JRT)—exclusively junctional and with accessory pathway—is usually high (~150–180 bpm).
  
• The rate of atrial rhythm in atrial fibrillation and flutter (A.Fl) and the ventricular rate is usually high (360–700 bpm and irregular in AF, and 200–300 bpm and regular in A.Fl), and the ventricular rate, depends on the grade of concealed AV conduction of f or F waves (see Chapter 15).
  
• A not very fast but fixed rate (100–130 bpm) both during the day and night supports diagnosis of ectopic rhythm (atrial tachycardia or 2 : 1 flutter instead of sinus rhythm) (see Figure 15.14).
  
• A slow regular atrial rate (<60 bpm) is indicative of sinus bradycardia, or, in rare cases, of a sinoatrial 2 : 1 block. In the latter case, the heart rate may double with exercise, whereas in sinus bradycardia the heart rate increases progressively.

Location of the atrial wave in the cardiac cycle (RR)

• The sinus P wave and most MAT P′ waves precede the QRS complex, with a PR < RP relationship (see Figure 15.13D).
  
• In paroxysmal junctional tachycardias, where the reentrant circuit exclusively involves the AV junction (atrioventricular nodal reentry tachycardia (AVNRT)), the P′ wave is located within the QRS complex (and is therefore not visible) or it is attached to its end, resulting in morphology distortion (see Figure 15.13A,B). If the reentry involves an accessory pathway (AVRT), the P′ wave follows the QRS complex closely so that P′R > RP′ (see Figure 15.13C).
  
• In junctional ectopic tachycardia when there is ventriculoatrial conduction, the P′ may be located before or after, or be hidden within the QRS complex (see Chapter 15, Atrioventricular junctional tachycardia due to ectopic focus).

Width and morphology

The QRS complexes may be narrow (<120 ms) or wide (≥120 ms).

• Narrow QRS complexes are the result of normal ventricular activation due to sinus rhythm if they are preceded by a sinus P wave, or due to different types of supraventricular tachyarrhythmias (see Tables 15.2 and 15.3). Narrow QRS complexes in the case of slow rates not preceded by sinus P waves correspond to junctional escape complexes or rhythm.
  
• Wide QRS complexes may be the result of intraventricular aberrant conduction, over an accessory pathway, or of ventricular origin (see below).

Cadence

In the presence of narrow QRS complexes, the RR cadence may be regular or irregular (see Table 15.3). Based on this premise, the different types of fast regular or irregular rhythms with narrow QRS complexes may be diagnosed using the algorithms of Figures 18.3 and 18.4. In addition, Tables 15.2 and 15.3 display the most important ECG aspects of paroxysmal regular supraventricular tachyarrhythmias with narrow QRS complexes. The presence of premature atrial or ventricular complexes may convert a regular rhythm into an irregular one.

All the types of slow regular or irregular rhythms, usually with narrow QRS complexes (sinus bradycardia, escape functional rhythm, etc.), are discussed in Chapter 17. A proposed algorithm for narrow complex tachycardias can be consulted for free at www.ecguniversity.org or by downloading for free a new i‐book (Baranchuk and Nadeau‐Rauther 2016; Baranchuk et al. 2018).

When the QRS is wide and the ventricular rate is fast and regular, a differential diagnosis between ventricular tachycardia (see Figures 16.12–16.14) and supraventricular tachycardia with aberrant intraventricular conduction (see Figure 16.19) or with anterograde conduction over an accessory pathway (see Figure 16.18) has to be performed. In most cases (when the substrate is ischemic), differential diagnosis may be established using the algorithm developed by Brugada et al. (1991). For more detailed information on the algorithm, visit www.ecguniersity.org.

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