4. Heart Rate: A Mandatory Part of an EKG Interpretation

In this material

Self-study learning goals

Define and Identify the following:
- Sinus rhythm
- Sinus bradycardia
- Sinus tachycardia
- Sinus arrhythmia
Correctly measure the heart rate in any regular rhythm.
Correctly measure the heart rate in atrial fibrillation.
Define the pathophysiology of Sinus tachycardia Sinus bradycardia.
List possible underlying causes of Sinus tachycardia Sinus bradycardia.
Describe findings on physical examination or laboratory testing that would clarify the etiology.

The EKG provides a skilled reader with a wealth of information about the heart. One of the most basic yet important pieces of information the EKG provides is the heart rate (HR).

The heart rate is defined as the number of times the heart beats per minute.
It is a vital sign.
It is always clinically relevant.

Calculating the heart rate is easy because the EKG is always recorded on graph paper that measures time as it travels horizontally through the machine. Ignoring the heart rate on an EKG is the single most common mistake in learning to read an EKG!

Measurement of the Heart Rate

The most accurate way to measure heart rate is by measuring the R-R interval. The R-R interval is the distance from one R wave to the next R wave. When measuring the R-R interval, take the beginning of one QRS complex, and count the number of “little boxes” up to the beginning of the next QRS complex. Divide this number into 1500. This method of calculating the heart rate is valid if the heart rate is regular. Table 4.1 at the end of this chapter offers a convenient method for obtaining the heart rate without doing the calculations. It is a good idea to carry such a table in your pocket or on your iPhone for quick reference and accurate determination of the heart rate.

The Sinus Rhythms

The sinus node normally sets and controls the heart at a rate of 60 to 100 beats per minute (bpm). This is called sinus rhythm or normal sinus rhythm (Figure 4.3, rate 65 bpm). When the sinus rate is below 60 bpm, the rhythm is called sinus bradycardia (Figure 4.4, rate 58). When the sinus rate is greater than 100 bpm, it is called sinus tachycardia (Figure 4.5, rate 125 bpm). The upper limit for sinus tachycardia in a given patient is estimated by a formula: (Max HR = 220 – patient’s age in years). Thus, the maximum HR a person can achieve decreases with age. Sinus rhythms, regardless of rate, are typically regular. When the heart rate varies by more than 10%, the rhythm is called sinus arrhythmia.

The sinus node: Master and servant

The sinus node normally sets and controls the rate of depolarization and contraction of the rest of the heart. In this sense, the sinus node is the master of the heart. Yet, the sinus node is itself under the control of two parts of the nervous system—the sympathetic and parasympathetic nervous systems. These two forces are always active, with either one able to predominate depending on the clinical situation.

Critical concepts

Sinus tachycardia

Pathophysiology

Sinus tachycardia represents a relative imbalance in the normal sympathetic/parasympathetic balance of the heart. There are two basic causes of sinus tachycardia:

Increased sympathetic activity
Decreased parasympathetic activity.

Increased sympathetic activity is by far more common clinically. Increased sympathetic activity is part of the ancient “flight or fight” protective emergency system of the body. It is analogous to someone inside the patient’s body calling a “code” or pulling a fire alarm. It is that serious, and that significant. The underlying cause should always be determined. It will typically be very relevant clinically.

Causes of increased sympathetic activity include:

Shock.
Heart failure.
Infection.
Bleeding.
Pain.
Pulmonary embolism.
Hypoxia.
Hypoglycemia.
Anxiety.
Drugs.

All are major disorders. Commonly used drugs that cause increased sympathetic activity include bronchodilators, inotropic infusions, and pressors. From the EKG, diagnose sinus tachycardia, and then evaluate the patient to determine its cause. Decreased parasympathetic activity is a much less common cause of sinus tachycardia and is most commonly related to atropine administration or poison ingestion. The most common big mistake EKG readers make is ignoring the presence of sinus tachycardia. A return to the bedside frequently provides the answer!

Sinus bradycardia

Pathophysiology

Sinus bradycardia represents a relative imbalance in the normal sympathetic/ parasympathetic balance of the heart. There are two basic causes of sinus bradycardia:

Decreased sympathetic activity.
Increased parasympathetic activity.

Parasympathetic activity is frequently the “relax and take your time” signal that counterbalances the sympathetic nervous system. Increasing parasympathetic activity slows down the heart rate. Decreased sympathetic activity does the same and is more common clinically. It is the result of the current common practice of using drugs that block the sympathetic nervous system in the treatment of hypertension, coronary artery disease, and heart failure. It is also possible to directly stimulate the parasympathetic nervous system. A common cause of increased parasympathetic activity is the vagal response. The vagal response can occur secondary to gastrointestinal (GI) stimulation during nausea and vomiting, drug treatment, the carotid reflex, or with direct therapeutic vagal stimulation for seizures or depression.

The heart rate reference table

Table 4.1 provides a quick reference for accurate determination of the heart rate when the patient is in a regular rhythm.

memorize this

Clinical significance of heart rates

The heart rate is a vital sign and conveys critical information.

Causes of sinus tachycardia	Causes of sinus bradycardia
Shock	Beta blocker therapy
Heart failure	Vagal response (increased parasympathetic tone)
Bleeding	Hypothyroidism
Infection	Athlete’s conditioning
Hyperthyroidism
Pulmonary embolism
Sympathomimetic drug therapy
Anxiety
Hypoglycemia
Hypoxia
Asthma