Understanding Cardiac Arrhythmias

Jon W. Atkinson

THE AMERICAN ACADEMY OF SLEEP MEDICINE MONITORING AND REPORTING GUIDELINES

In 2007, shortly after the first edition of Fundamentals of Sleep Technology was published, the American Academy of Sleep Medicine (AASM) published The AASM Manual for the Scoring of Sleep and Associated Events: Rules, Terminology and Technical Specifications (1). This manual contains information about the technical aspects of recording the electrocardiogram (ECG) as well as recommendations for reporting abnormal cardiac events. Little change in the cardiac section of the scoring manual has occurred through the different versions, including the latest version 2.5 (2) except a slight revision of the definition of atrial fibrillation.

Technical Considerations

The AASM manual provides the following technical specifications for the recording of ECG during polysomnography:

The sampling rate should be 500 Hz desirable, 200 Hz minimum, with a digital resolution of 12 bits per sample.
The maximum electrode impedance for ECG is not specifically stated; however, 5 kΩ is appropriate.
Recommended low-frequency filter is 0.3 Hz and high-frequency filter 70 Hz.
The recommended recording derivation is a modified lead II with the right shoulder negative and the left lower torso positive.

Notes provided suggest that the recommended filters may be prone to various artifacts such as movement, perspiration, and muscle. Employing standard ECG electrodes instead of electroencephalogram (EEG) electrodes may make these problems less likely to occur. Additional ECG electrodes may be placed at the discretion of the practitioner.

Scoring Rules

The AASM manual recommends indicating the presence of the arrhythmias listed in the next paragraph, which should be acknowledged by responding “yes” if they occur and providing the heart rate (HR) or the duration of the pause.

1. Sinus tachycardia (ST) during sleep for sustained (>30 seconds duration) sinus HR greater than 90 bpm for adults

Report (yes/no); report highest HR observed

2. Bradycardia during sleep for sustained HR of less than 40, ages 6 through adult

Report (yes/no); report lowest HR observed (defined as HR <40 bpm)

3. Asystole for pauses greater than 3 seconds duration for ages 6 through adult

Report (yes/no); report longest event

4. Wide complex tachycardia for a rhythm of 3 or more consecutive beats at a rate greater than 100 bpm with a QRS duration of more than or equal to 0.12 seconds

Report (yes/no); report highest HR observed

5. Narrow complex tachycardia for a rhythm of 3 or more consecutive beats at a rate greater than 100 bpm with a QRS duration of less than 0.12 seconds

Report (yes/no); report highest HR observed

6. Atrial fibrillation for an irregularly irregular ventricular rhythm where the normal, consistent P waves have been replaced by rapid oscillations of varying timing, size, and shape

Report (yes/no)

The notes in the cardiac scoring section of the manual state that significant arrhythmias should be reported if the single lead recorded is sufficient for accurate scoring. This would include type II and type III atrioventricular (AV) blocks, escape rhythms, atrial flutter, and so on. Ectopic beats should also be reported if deemed clinically significant. Included in this group are frequent premature ventricular contractions (PVCs), patterned PVCs, ventricular couplets, and frequent atrial or junctional extrasystoles. These events may be reported similar to the aforementioned:

7. Other arrhythmias (yes/no); report the arrhythmia (such as heart block or significant ectopy)

ANATOMY AND PATHWAYS

This portion of the chapter is reproduced with modification from Atkinson JW, with permission (3). There are numerous other excellent resources for arrhythmia interpretation (4, 5, 6).

Arrhythmia recognition and identification is a fairly logical process that is based on a good working knowledge of the anatomy and pathways of the conduction system of the heart. Within the myocardium, there are areas of specialized tissue that have the primary function of generating electrical impulses or transmitting these impulses to other areas of the conduction system and finally to the cardiac muscle mass. The main areas of interest are the sinoatrial (SA) node, the AV node, the AV bundle or bundle of His, the left and right bundle branches, and the Purkinje system (fibers).

The normal sequence of events is as follows:

The cycle begins with a discharge of the SA node, which causes the atria to depolarize.
The AV node receives input through the internodal pathways and holds the signal for a brief period (a brief pause between atrial and ventricular depolarization that facilitates ventricular filling).
The impulse is passed from the AV node to the AV bundle.
The signal is then sent down the right and left bundles (the left bundle divides into an anterior and a posterior fascicle).

Figure 40-1 The conducting system.
The left and right bundles distribute the impulse through the Purkinje system to the ventricular myocardium such that all the cardiac muscles contract simultaneously as a unit.

See Figure 40-1 for a graphic illustration of the conducting system.

CARDIAC CYCLE

The cardiac cycle is the repetitive sequence of atrial depolarization, atrial repolarization, ventricular depolarization, and ventricular repolarization. The corresponding electrophysiologic events appear as the main components of the ECG. These are identified as the P wave, the QRS complex, and the T wave. Furthermore, there is a relationship between the physiology of the cardiac cycle and the waveforms and intervals of the ECG. Table 40-1 shows the physiologic event and related ECG component. Figure 40-2 shows a graphic representation of the events in Table 40-1.

NORMAL PARAMETERS

Any presentation of arrhythmias requires an understanding of the ECG in its normal state. The normal ranges for key parameters needed to diagnose arrhythmias are shown in Table 40-2.

The basis for observing and identifying arrhythmias is the normal sinus rhythm (NSR). The following samples are presented in both a 30-second window and a 10-second window for comparison.

Table 40-1 Physiologic Event and Related Electrocardiogram Component

Physiologic Event	Electrocardiogram Waveform
Atrial depolarization	P wave
Atrial repolarization	None seen (hidden by the timing and magnitude of the QRS complex)
Pause between atrial depolarization and ventricular depolarization	PR interval (the last portion following the P wave, PR segment)
Ventricular depolarization	QRS complex
Ventricular repolarization	T wave

Figure 40-2 Graphic representation of the events in Table 40-1.

One of the better features of digital recordings is the ability to change time scales without changing how the recording is saved to the hard drive. For the purpose of arrhythmia analysis, it is frequently beneficial to view the ECG utilizing a 10-second screen (Fig. 40-3¹) rather than the usual 30-second window (epoch; Fig. 40-4) used for sleep staging. This functionality demonstrates a basic concept: If you want to view rapidly occurring activities (ECG, EEG, etc.), spread the display out; if you want to view slowly occurring events (apneas, hypopneas, limb movements, etc.), compress the display.

Table 40-2 The Normal Adult Ranges for Key Electrocardiogram Parameters Needed to Diagnose Arrhythmias

Key Parameter	Value Range
Heart rate	60-100 bpm, awake 40-90 bpm, during sleep
Rhythm	Regular
PR interval	0.12-0.20 s
QRS interval	0.04-0.11 s
Sinoatrial node discharge rate	60-100/min
Atrioventricular node discharge rate	40-60/min
Ventricular discharge rate	20-40/min

Features of the NSR

P wave: Present, each appears the same

QRS complex: Present, each appears the same

PR interval (start of P wave to start of QRS complex). See Figure 40-2: 0.12 to 0.20 seconds

QRS interval (start of Q wave to end of S wave). See Figure 40-2: 0.04 to 0.11 seconds

P to QRS ratio: 1:1

Rhythm: Regular

Rate: 60 to 100

ARRHYTHMIA BASICS

An ECG arrhythmia occurs if there is a difference from NSR in impulse formation, impulse conduction, or both. Heart beats that originate outside of the SA node, such as other areas in the atrium, the AV node, or the ventricle, are some examples of impulse formation abnormalities. These abnormal beats will not have a sinus P wave. Abnormal beats with only conduction abnormalities will exhibit a sinus P wave but then show prolonged conduction across the AV node or through the ventricular conduction pathway. The AV blocks and bundle branch blocks are good examples of impulse conduction abnormalities. When both impulse formation and impulse conduction are affected, neither a normal sinus P wave nor a normal QRS complex is present. Any of the ventricular arrhythmias demonstrate both impulse formation and impulse conduction abnormalities. Essentially, an arrhythmia is present if the rate is too fast or too slow, the rhythm is irregular, the site of origin
is abnormal, or the movement of the impulse through the conductive system is abnormal.

Figure 40-3 Normal sinus rhythm, 30-second display.

Figure 40-4 Normal sinus rhythm, 10-second display.

ARRHYTHMIA ANALYSIS

Far too often, there is the temptation to “guess” the type of arrhythmia that is presented on the polysomnogram. This section presents a systematic method to accurately analyze cardiac rhythm disturbances. If possible, a second ECG derivation should be added to the montage. This is easily accomplished unless the equipment manufacturer has only a dedicated, two-input amplifier for recording the ECG. Utilizing a second ECG channel may reveal changes in P wave or QRS complex configuration that are difficult to ascertain with a single-channel recording. When examining the ECG for detail, use a 10- or 6-second window to examine the intervals or subtle changes in morphology.

Routine use of the following methodical evaluation system will improve the accuracy of identifying arrhythmias.

Step 1. Examine the P wave.

Is the P wave absent or present? Absence of a distinct P wave would indicate that the arrhythmia is not of atrial origin (except atrial fibrillation or atrial flutter). Do the P waves all look the same (have the same morphology)? A basic concept that should be remembered is as follows: If an ECG complex starts from the same location and takes the same pathway, it will always look the same. This is the rule of identical morphology. The corollary to the rule of identical morphology is as follows: If the complex originates from a different location or takes a different pathway, the appearance will be different. This is the rule of differing morphology.

Step 2. Examine the QRS complex.

Check for absence or presence and similar appearance of the QRS complex. Absence of the QRS indicates some type of second- or third-degree AV block, or severe ventricular disturbance such as ventricular fibrillation or asystole. Differing morphologies of the QRS indicate a shift from a supraventricular origin of the abnormal beat(s) or a different pathway in the ventricle, such as bundle branch block or multifocal ventricular origin.

Step 3. Examine the P and QRS relationship.

Ask the following questions. Is there a P wave for every QRS complex? Is there a QRS complex for every P wave? Is there a 1:1 P to QRS ratio? A P to QRS ratio of more than 1 (more P waves than QRS complexes) indicates some sort of AV block. A P to QRS ratio of less than 1 (more QRS complexes than P waves) indicates a junctional or ventricular arrhythmia.

Step 4. Measure the intervals.

Is the PR interval normal, or is it too long or too short? An abbreviated PR interval may indicate a junctional beat (retrograde P wave) or an accessory pathway (such as in Wolff-Parkinson-White syndrome). A prolonged PR interval indicates that some type of AV block is occurring. Is the QRS interval normal or too long? (Seldom, if ever, will it be too short.) A widened QRS complex likely indicates bundle branch block, a beat of ventricular origin, or, in some cases, an aberrantly conducted beat of supraventricular origin (the beat originates before the ventricular conductive pathway is repolarized during the relative refractory period).

Step 5. Regular or irregular rhythm.

Examine the P-P and the R-R intervals. If the intervals are constant, the rhythm is regular. If they vary, the rhythm is irregular. Using calipers on a computer monitor is not advised, so either print the screen image and measure the intervals with calipers on paper or use something like a 3 × 5 index card. Make pencil marks corresponding to a succession of two or more P waves, while holding the card against a stationary screen display. Move the card from one set of P-P intervals to the next and see if they fall on the marks. Do the same for the R waves.

Step 6. Determine the rate.

An accurate determination of HR is obtained from the ECG. Do not rely on the readout from a pulse oximeter. Using the ECG display, HR can be determined in a variety of fashions:

Times two method: Count the number of beats in a 30-second screen display and multiply by two. This can be a bit cumbersome but is quite accurate. Make sure you do this in a frozen screen view.

Times four method: Count the number of beats in a 15-second display and multiply by four. Again, use a stationary screen.

Times six method: Count the number of beats in a 10-second display and multiply by six.

Interval method: Measure the active screen width in millimeters. If you have a 30-second window, multiply this number by two to get the number of millimeters in 60 seconds. If you are using a 10-second window, multiply the active screen width by six to obtain the number of millimeters in 60 seconds. Measure the R-R interval with a ruler and divide into the number of millimeters in 60 seconds.

Figure 40-5 Premature atrial contraction, 30-second display.

Figure 40-6 Premature atrial contraction, 10-second display.

COMMON ATRIAL ARRHYTHMIAS

Premature atrial contractions (PACs)
Sinus bradycardia (SB)
Atrial tachycardia (AT); ST; supraventricular tachycardia (SVT)
Sinus arrhythmia
Sinus pause
Paroxysmal atrial tachycardia
Atrial fibrillation
Atrial flutter

PREMATURE ATRIAL CONTRACTION

Features of PACs

P wave: Present, appearance of the P wave of the abnormal beat will be different (it arises from a different location)

QRS complex: Present, each appears the same

PR interval: 0.12 to 0.20 seconds, may vary slightly with the abnormal beat

QRS interval: 0.04 to 0.11 seconds

P to QRS ratio: 1:1

Rhythm: Irregular because of the premature beat; P-P will be different, so will be the R-R

Rate: 60 to 100

Comments: Note the different appearance of the P wave (origin is from a different source and travels a different pathway) at the arrows in Figures 40-5 and 40-6. There may also be subtle PR interval changes. The QRS and the T wave are normal in appearance, unless the ectopic atrial focus fires so early that it captures the ventricle, whereas the bundle branches or the Purkinje system is still relatively refractory to conduction (PAC with aberrancy).

SINUS BRADYCARDIA

Features of SB

P wave: Present, each appears the same

QRS complex: Present, each appears the same

PR interval: 0.12 to 0.20 seconds

QRS interval: 0.04 to 0.11 seconds

P to QRS ratio: 1:1

Rhythm: Regular

Rate: Less than 50, some authors say less than 60 awake. Bradycardia during sleep is regarded as a sustained HR of less than 40 bpm (1).

Comments: Sinus bradycardia (Figs. 40-7 and 40-8) looks like NSR except the rate is slower. This is not uncommon in athletic individuals and patients on beta-blockers.

ATRIAL TACHYCARDIA; SINUS TACHYCARDIA; SUPRAVENTRICULAR TACHYCARDIA

Features of Supraventricular Tachyarrhythmias

P wave: Present, each appears the same, may not be well defined in SVT

QRS complex: Present, each appears the same

PR interval: 0.12 to 0.20 seconds

QRS interval: 0.04 to 0.11 seconds

P to QRS ratio: 1:1

Rhythm: Regular

Rate: Greater than 100 bpm. ST during sleep is regarded as a sustained sinus rhythm of more than 90 bpm (1).

Figure 40-7 Sinus bradycardia, 30-second display.

Figure 40-8 Sinus bradycardia, 10-second display.

Comments: Supraventricular tachyarrhythmias are rapid, regular rhythms with normal-appearing QRS complexes. ST will have a normal sinus P wave. In AT, the P wave will have a morphology different from that of the sinus P wave (Figs. 40-9 and 40-10). SVT will not have a clearly defined P wave. It is difficult to distinguish one from the other on the surface ECG. Electrophysiologic studies may be necessary to determine the precise focus. Sustained tachyarrhythmias will decrease cardiac output. The patient should be assessed for the level of consciousness, chest pain, and blood pressure and the physician notified. The AASM recommends reporting ST during sleep if sustained. Otherwise, brief runs of these arrhythmias are reported as narrow complex tachycardia.

SINUS ARRHYTHMIA

Features of Sinus Arrhythmia

P wave: Present, each appears the same

QRS complex: Present, each appears the same

PR interval: 0.12 to 0.20 seconds

QRS interval: 0.04 to 0.11 seconds

P to QRS ratio: 1:1

Rhythm: Irregular

Rate: 60 to 100

Comments: Sinus arrhythmia (Figs. 40-11 and 40-12) is frequently seen in infants and children. It is often seen in obstructive sleep apnea as a result of the vagal effect of intrathoracic pressure fluctuations. Minor fluctuations of HR associated with inspiration and expiration are quite normal and indicate a “healthy” autonomic control of HR. Lack of HR fluctuation may indicate an underlying pathologic condition such as diabetes mellitus.

Only gold members can continue reading. Log In or Register to continue