Introduction to Sinus Rhythms

The family of sinus rhythms have one thing in common: They all start at the sinoatrial (SA) node. The purpose of this introductory page is to lay down some basic foundation and terminology that we will continue to use throughout this book. For now, don’t worry about understanding everything about each of the different sinus rhythms; we are going to review each of these in great detail in the coming pages.

Since, by definition, all of the complexes originate in the SA node, all the P waves for any one patient should be morphologically the same. In addition, since the impulses all travel down the normal electrical conduction system, the PR intervals should all be the same from complex to complex.

These are the two things common to all of the sinus rhythms ( Figure 8-1):

Figure 8-1 In all sinus rhythms, the P waves are identical.

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1. All the P waves are identical.
2. All the PR intervals are the same.

Other things like the rate, regularity, and morphology will vary depending on the actual rhythm involved and the patient’s individual anatomy, pathology, and variable neurohormonal state.

There are a couple of general terms we need to discuss at this point. The established normal rates for most people are from 60 to 100 beats per minute (BPM). When we encounter a rhythm strip with consistent morphology of the P waves and consistent PR intervals that has a regular heart rate between 60 and 100 BPM, we say that it is a normal sinus rhythm. This is a simplistic definition because there are some other criteria that we need to fulfill, but it will suffice for now. If the heart rate is irregular but meets the sinus rhythm criteria, we call it sinus arrhythmia (more on this later).

Bradycardia refers to slow rhythms, usually a rate of less than 60 BPM. Therefore, a sinus bradycardia refers to a rhythm that meets the sinus rhythm criteria and is less than 60 BPM. Tachycardia refers to fast rhythms, faster than 100 BPM, to be exact. Once again, a sinus tachycardia refers to a rhythm that matches the criteria we will outline for sinus rhythms but is faster than 100 BPM.

We want you to think of the regular sinus rhythms as a spectrum going from fast to slow (Figure 8-2). This is because, essentially, these three are exactly the same . . . just the rate is different. The clinical significance is highlighted by the color of the gradient. Green is clinically insignificant, whereas, red signifies possible danger zones. In general, when the rhythm is too slow or too fast, perfusion and cardiac function are compromised.

Figure 8-2 The regular sinus rhythms—sinus bradycardia, normal sinus rhythm, and sinus tachycardia—differ only in rate. Green is clinically insignificant and red signifies possible danger zones.

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Description

Characteristics of sinus rhythms are summarized in Table 8-1.

Table 8-1 Characteristics of Sinus Rhythms

Normal sinus rhythm	• 60 to 100 BPM • All P waves are identical. • All PR intervals are identical. • Regular heart rate
Sinus bradycardia	• Slower than 60 BPM • All P waves are identical. • All PR intervals are identical.
Sinus tachycardia	• Faster than 100 BPM • All P waves are identical. • All PR intervals are identical.
Sinus arrhythmia	• Can have a normal rate, a bradycardic rate, or a tachycardic rate • All P waves are identical. • All PR intervals are identical. • Irregular heart rate

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