7.2 Basic Concepts

Before interpreting cardiac rhythms, it is vital to understand the anatomy and physiology of the heart. Let’s begin with a basic review of the cardiovascular system. The heart is a fist-sized organ that pumps blood throughout the body. It is the primary organ of the circulatory system. The heart contains four main chambers made of muscle and powered by electrical impulses. The brain and nervous system direct the heart’s function. The electrophysiology of the heart will determine the rate and rhythm. The blood pressure is maintained by the contractility of the heart muscle.^[1] See Figure 7.1^[2] for an illustration of the heart.

The parts of the heart are similar to the parts of a house. Both the heart and a house have the following components^[3]:

• Walls
• Chambers (the rooms)
• Valves (the doors)
• Blood vessels (the plumbing)
• Electrical conduction system (the electricity)

Heart walls are muscles that contract (squeeze) and relax to send blood throughout the body. A layer of muscular tissue called the septum divides the heart walls into the left and right sides. Heart walls have three layers:

• Endocardium: The inner layer
• Myocardium: The muscular middle layer
• Epicardium: The protective outer layer

The epicardium is one layer of the pericardium. The pericardium is a protective sac that covers the entire heart. It produces fluid to lubricate the heart and keeps it from rubbing against other organs.^[4]

The heart is divided into four chambers. There are two chambers on the top (called the left and right atria) and two chambers on the bottom (called the left and right ventricles). Blood flows through the chambers of the heart in the following order:

• Right atrium: Two large veins called the superior vena cava and the inferior vena cava deliver oxygen-poor blood to the upper right chamber of the heart called the right atrium. The superior vena cava carries deoxygenated blood from the upper body. The inferior vena cava carries deoxygenated blood from the lower body. The right atrium pumps this blood through the tricuspid valve into the right ventricle.
• Right ventricle: This lower right chamber of the heart pumps the oxygen-poor blood through the pulmonary valves and then through the pulmonary arteries to the lungs. (Note that arteries usually carry oxygenated blood, but pulmonary arteries carry deoxygenated blood to the lungs.) The lungs reload blood with oxygen while removing carbon dioxide, and the pulmonary veins carry oxygenated blood back to the left atrium. (Note that veins usually carry deoxygenated blood, but pulmonary veins are the only veins in an adult that carry oxygenated blood.)
• Left atrium: The upper left chamber of the heart receives the oxygenated blood and pumps it through the mitral valve into the left ventricle.
• Left ventricle: The lower left chamber of the heart is called the left ventricle. It is slightly larger than the right ventricle because it pumps oxygen-rich blood through the aortic valve to the coronary arteries and out to the rest of the body.^[5] See Figure 7.2^[6] for an illustration of blood flow through the heart.

The heart valves are like doors between the four heart chambers that open and close to allow blood to flow through while preventing blood from moving backwards through the heart. The atrioventricular (AV) valves open between the atria and the ventricles (i.e., the upper and lower chambers of the heart). There are two AV valves^[7]:

• Tricuspid valve: The valve between the right atrium and right ventricle
• Mitral valve: The valve between the left atrium and left ventricle

Semilunar (SL) valves open when blood flows out of the ventricles. SL valves include the following^[8]:

• Pulmonary valve: The valve that opens when blood flows from the right ventricle into the pulmonary arteries (then to the lungs)
• Aortic valve: The valve that opens when blood flows out of the left ventricle to the aorta

The heart pumps blood through three types of blood vessels called arteries, veins, and capillaries^[9]:

• Arteries carry oxygen-rich blood from the heart to the body’s tissues. (As previously noted, the exception is the pulmonary arteries that carry deoxygenated blood to the lungs.) The aorta is a large artery that carries oxygen-rich blood from the heart to the rest of the body. The heart itself receives oxygen and nutrients through a network of coronary arteries that run along the heart’s surface.
• Veins carry oxygen-poor blood back to the heart. (As previously noted, the exception is the pulmonary veins that carry oxygenated blood from the lungs to the heart.)
• Capillaries are small blood vessels where the body exchanges oxygen and carbon dioxide in the blood at the cellular level.

The electrical conduction system of the heart is similar to the electrical wiring of a house. The heart has a network of electrical bundles and fibers that control the rhythm and pace of the heartbeat. The electrical conduction system includes these components^[11]:

• Sinoatrial (SA) node: The SA node is located in the upper part of the right atrium and is a major element of the conduction system. The SA node is often referred to as the heart’s natural pacemaker. The natural pacemaker rate for the SA node is 60-100 beats per minute. It sends the signals that make the heart beat with a normal rate and rhythm.
• Atrioventricular (AV) node: The AV node is located in the lower part of the right atrium. The AV node carries electrical signals from the SA node to the ventricles. If the SA node fails to send signals, the AV node takes over. The AV node is the “backup” pacemaker if the SA node fails. The AV node will pace the heart at a rate of 40-60 beats per minute.
• Bundle of His: A collection of cardiac cells found along the septum between the ventricles that sends electrical impulses from the AV node to the left and right bundle branches.
• Left bundle branch: Offshoots from the bundle of His that send electrical impulses to the left ventricle.
• Right bundle branch: Offshoots from the bundle of His that send electrical impulses to the right ventricle.
• Purkinje fibers: A network of thin filaments that carry electrical impulses that cause the ventricles to contract and pump blood out of the heart. If all other “backup” pacemakers in the heart fail, the Purkinje fibers will pace the heart at 20-40 beats per minute. See Figure 7.3^[12] for an illustration of the electrical conduction system of the heart.

Dysrhythmias

Occasionally, an area of the heart other than the SA node will initiate an impulse that will be followed by a premature contraction. Premature contractions simply mean these impulses happen too soon and may originate from a different place than a regular beat. These premature contractions can happen in the atrium (premature atrial contraction), ventricle (premature ventricular contraction), or AV junction (premature junctional contractions). Such a contraction is known as an ectopic beat, and the premature contraction causes an irregular heart rate and rhythm during that beat. The underlying heart rate and rhythm can be either regular or irregular. An ectopic focus may be stimulated by localized ischemia, exposure to certain drugs, abnormal electrolytes or acid-base balance, hypoxia, elevated stimulation by both sympathetic or parasympathetic divisions of the autonomic nervous system, or several diseases or pathological conditions. Occasional occurrences of dysrhythmias are generally transitory and not life-threatening. However, if the condition becomes a chronic deviation from the normal pattern of impulse conduction and contraction, it is referred to as dysrhythmia or arrhythmia. Severe arrhythmias can lead to cardiac arrest, which is fatal if not treated within a few minutes.

Electrocardiograms

Electrocardiograms (ECGs) use leads with electrodes attached to the client’s body to record the electrical activity of the heart on special graph paper or on a cardiac monitor. These electrodes detect the small electrical changes of cardiac muscle depolarization followed by repolarization during each cardiac cycle (heartbeat).

A paper rhythm strip is at least a 6-second tracing printed out on special graph paper that shows activity from one or two leads. See Figure 7.4^[14] for an example of an ECG rhythm strip. When interpreting a paper ECG, the vertical lines indicate voltage of a given waveform. The thin lines, thick lines, and boxes along the horizontal axis represent various amounts of time as the electrical signal is conducted through the heart tissue:

• Thin lines or small box (1 mm intervals): 0.04 seconds
• Thick lines or big box (5 mm intervals): 0.2 seconds

12-Lead ECG

A 12-lead electrocardiogram is a diagnostic test that uses 12 leads to record information through 12 different perspectives of the heart to display a complete picture of its electrical activity. Electrodes are placed on the surface of the client’s chest (i.e., leads V1, V2, V3, V4, V5, and V6), and four are placed bilaterally on their upper and lower extremities (i.e., RA, LA, RL, LL). In this manner, the heart’s electrical activity is measured from twelve different angles (referred to as “leads”) to capture each moment throughout the cardiac cycle. Accurate placement of leads to obtain a 12-lead ECG is further described in “Checklist: Obtain a 12-Lead ECG.”

A standard 12-lead ECG report displays a 2.5 second tracing of each of the twelve leads. The tracings are most commonly arranged in a grid of four columns and three rows. The first column is the limb leads (I, II, and III), the second column is the augmented limb leads (aVR, aVL, and aVF), and the last two columns are the precordial leads (V1 to V6). See Figure 7.5^[15] for an image of a 12-lead ECG with the various waveforms.

Each of the 12 ECG leads records the electrical activity of the heart from a different angle and, therefore, aligns with different anatomical areas of the heart:

• Inferior leads (II, III, and aVF): Inferior surface of the heart
• Lateral leads (I, aVL, V5, and V6): Lateral wall of the left ventricle
• Septal leads (V1 and V2): Septal surface of the heart
• Anterior leads (V3 and V4): Anterior wall of the right and left ventricles

When administered and interpreted accurately, an ECG can detect and monitor several types of heart conditions such as dysrhythmias, heart attacks (myocardial infarction), and electrolyte imbalances.

Telemetry

Telemetry refers to a portable device used to continuously monitor clients’ heart rhythms. While a client is on telemetry (also referred to as cardiac monitoring), their heart’s electrical patterns are displayed on a monitor. The patterns are continuously monitored by specially trained technicians and nurses who interpret the heart’s electrical activity. Nurses must be able to identify normal and abnormal heart rhythms displayed on a cardiac monitor. Health care agencies provide specialized training to nurses who work on units with clients on telemetry. See Figure 7.6^[16] for an image of a client on telemetry.

Artifact

The monitor or ECG strip typically displays the name of the cardiac rhythm the client is experiencing, but this display is not always correct due to artifact. Artifact occurs when the electrodes are not making good contact with the skin and/or if the client moves during the tracing. See an image of artifact on an ECG strip in Figure 7.7^[17]. Artifact may be interpreted by the monitor as ventricular beats or other abnormal cardiac patterns when, in reality, there are no cardiac abnormalities occurring. For this reason, it is important for nurses to observe the client to ensure what is displayed on the monitor is accurate according to the client’s condition.

Components of ECG Waveforms

There are five prominent components on an ECG waveform: the P wave; the Q, R, and S components (often referred to as the QRS complex); and the T wave. Each wave represents a specific electrical impulse in the heart with a specific appearance and normal ranges of measurements on the ECG graph paper. See Figure 7.8^[18] for an image of these components.

The small P wave represents the depolarization of the atria. The large QRS complex represents the depolarization of the ventricles, which requires a much stronger impulse because of the larger size of the ventricular cardiac muscle. The ventricles begin to contract as the QRS reaches the peak of the R wave, causing a “heartbeat” that is felt when assessing a client’s pulse. Lastly, the T wave represents the repolarization of the ventricle.^[19]

Intervals between waveforms are assessed on an ECG. The P-P interval represents the duration between atrial heartbeats. The R-R interval represents the duration between the ventricular heartbeats. Both of these intervals should be consistent if the heart rhythm is regular. However, the P-P interval and the R-R interval may not be the same if a client has a dysrhythmia with different atrial and ventricular heart rates. For example, in atrial flutter the atrial rate will be much faster than the ventricular rate.

The isometric line (also known as isoelectric line) is used to measure intervals. It is an imaginary line that can be drawn horizontally through the telemetry strip. This is also called the baseline and is used to determine where each component of the heartbeat starts and ends. It also helps to determine if the component has a positive or negative deflection. See Figure 7.9^[20] for an illustration of the isoelectric line.

The PR interval is measured from the start of the P wave to the start of the QRS complex. This is where the P wave starts to have a positive deflection off the isometric line to the first negative deflection. The QRS complex is measured from the first negative deflection (Q) through the upward spike (R) to the second negative deflection (S) once it returns to the isometric line. The QT interval is measured from the first negative deflection (Q) to the end of the T wave or where the T wave returns to the isometric line.

Table 7.2a reviews the characteristics of ECG components and intervals.

Table 7.2a Components and Intervals on an ECG Waveform