This section reviews dysrhythmias with normal heart rates – in other words, non‐sinus rhythms with rates in the normal range of 60–100 bpm. These dysrhythmias are commonly encountered during the delivery of emergency care and not infrequently cause signs and symptoms that lead patients to seek assistance.

Healthcare providers have many reasons to obtain a surface electrocardiogram (ECG) during the provision of care, which could be a single‐lead ECG (rhythm strip), multilead monitoring, or a 12‐lead ECG. In some cases, the ECG may be obtained in order to seek a specific etiology of the patient’s complaint (i.e. chest pain or shortness of breath); in other cases, it may be performed as part of routine evaluation and monitoring of an ill or injured patient.

Not infrequently, patients present with normal heart rates, defined here as 60–100 bpm for adults and within defined, age‐specific ranges for children (Table 5.1). A normal heart rate neither rules in, nor rules out, serious underlying conditions. Certain “normal rate” ECG tracings can immediately indicate a diagnosis – for instance, the 12‐lead ECG that reveals normal sinus rhythm with ST segment elevation indicative of a ST segment elevation myocardial infarction (STEMI) or single‐lead rhythm strip that shows atrial fibrillation with a controlled ventricular response (i.e. heart rate <100 bpm). However, in many cases, “normal rate” single‐ and multilead ECGs merely provide additional information to the emergency care provider without indicating a specific diagnosis.

Normal sinus rhythm is a common finding. This rhythm (Figure 5.1) is identified by its rate, P wave morphology, PR interval, QRS complex, relationship of the P wave to the QRS complex, and regularity of the QRS complexes. The normal rate for an adult is between 60 and 100 bpm; of course, children demonstrate age‐related differences in rate. Normal P wave morphology refers to a single, consistent P wave, which is upright in the limb leads I, II, and III. The PR interval (0.12–0.20 s) and QRS complex (<0.12 s) are normal in width. Each P wave is associated with a QRS complex, and every QRS complex has a preceding P wave. The rhythm is regular as determined by consistent and identical P–P and R–R intervals. It is important to emphasize that by itself, a finding of normal sinus rhythm neither confirms nor rules out any acute process or injury.

Sinus arrhythmia has all the hallmarks of normal sinus rhythm except that while the P waves and QRS complexes are normal and identical from beat to beat, the P–P and R–R intervals vary (Figure 5.2). This variation is often caused by changes in intrathoracic pressure and may be seen during the normal respiratory cycle. This rhythm is frequently found in healthy young patients and does not necessarily indicate pathology.

Atrial fibrillation is a rhythm that is most commonly described as “irregularly irregular.” In atrial fibrillation, multiple electrical foci in the atria discharge simultaneously and in rapid succession, producing “electrical chaos.” While the atrial rate may approach 600 bpm, it is not possible to observe this many discharges on the ECG. Rather, there is an absence of a consistent P wave and the chaotic electrical activity in the atria is manifested through the presence of a disorganized baseline that is observed between QRS complexes.

The atrioventricular (AV) node allows only a certain number of atrial impulses per unit time to pass through to the ventricular conduction system, thus protecting the ventricle from excessively rapid rates. At any given time, only a single electrical impulse is transmitted through the AV node, leading to depolarization of the ventricles. In atrial fibrillation, this transmission of the atrial impulses is irregular in occurrence, producing no discernible pattern and resulting in irregularly occurring QRS complexes. Therefore, the R–R interval is highly variable from beat to beat. Unless the patient has underlying conduction system abnormalities, the QRS duration in atrial fibrillation will be normal and narrow (<0.12 s). Thus, atrial fibrillation is defined by the absence of discernible P waves and irregularly irregular QRS complexes (Figure 5.3).

In many cases of atrial fibrillation, the ventricular rate is rapid with a rate greater than 100 bpm; this is generally referred to as atrial fibrillation withrapid ventricular response (RVR). In other situations, the ventricular rate may be rather slow, less than 60 bpm, and is termed atrial fibrillation with slow ventricular response. In situations involving coexistent conduction system disease and/or the use of AV‐node‐blocking medications (e.g. β‐adrenergic and calcium‐channel‐blocking agents), the ventricular rate may be within the normal range for the adult patient (i.e. ventricular rates of 60–100 bpm). In such situations, the rhythm is termed atrial fibrillation with controlled (or normal) ventricular response. Refer to Box 5.1 for further clinical and management issues for atrial fibrillation with a normal heart rate.

Atrial flutter is another common atrial dysrhythmia. Unlike atrial fibrillation, atrial flutter is most frequently regular and paroxysmal; that is to say that it is often intermittent and rarely lasts for more than a few hours at a time. In atrial flutter (unlike the atrial electrical chaos seen in atrial fibrillation), a single ectopic atrial focus produces rapid atrial depolarizations. A single and consistent P wave is noted, which is typically regular in occurrence, occurring at a rate of 250–350 bpm, and is termed a flutter wave. Flutter waves have a distinct morphology with a uniform “saw‐tooth” pattern that is evident between QRS complexes (Figure 5.4). These “saw‐tooth” waves are most evident in the inferior leads (leads II, III, and aVF).

As with atrial fibrillation, the ventricular response in atrial flutter is controlled by the atrioventricular node (AVN). Several flutter waves occur for each conducted impulse, which produces a QRS complex. The ratio of flutter waves to QRS complexes may vary widely but is most commonly seen in ratios of 4 : 1, 3 : 1, or to 2 : 1. Because the flutter waves are so regular, it is assumed that a flutter wave is also occurring simultaneously with the QRS complex. Therefore, by convention, the ratio of flutter waves to QRS complexes is defined by the number of flutter waves seen between the QRS complexes plus one. This ratio may change within even a short rhythm strip (Figure 5.5), leading to QRS complexes with varying R–R intervals; this is referred to as atrial flutter with variable conduction. It is also possible for atrial fibrillation and atrial flutter to be evident in a single rhythm strip. This entity may be caused by one dominant ectopic atrial focus competing with the impulses from several other ectopic foci, leading to an irregular rhythm that demonstrates characteristics of atrial fibrillation and atrial flutter in different beats (Figure 5.6).