XII. ECG & Heart Function
Electrode leads on the chest wall are able to detect electrical impulses that are generated by the heart. Multiple leads provide many electrical views of the heart.
By interpreting the tracing, the physician can learn about the heart rate and rhythm as well as blood flow to the ventricles (indirectly).
Rate refers to how fast the heart beats. Normally, the SA node generates an electrical impulse 50-100 times per minute. Bradycardia (brady=slow + cardia=heart) describes a heart rate less than 50 beats per minute.
Tachycardia (tachy=fast + cardia=heart) describes a heart rate faster than 100 beats per minute.
Rhythm refers to the type of heartbeat. Normally, the heart beats in a sinus rhythm with each electrical impulse generated by the SA node resulting in a ventricular contraction, or heartbeat. There are a variety of abnormal electrical rhythms, some are normal variants and some are potentially dangerous. Some electrical rhythms do not generate a heartbeat and are the cause of sudden death.
XIII. Rhythms Originating from Sinus Node
A sinus rhythm is any cardiac rhythm where depolarization of the cardiac muscle begins at the sinus node. It is characterized by the presence of correctly oriented P waves on the electrocardiogram (ECG). Sinus rhythm is necessary, but not sufficient, for normal electrical activity within the heart.
Normal Sinus Rhythm
The term normal sinus rhythm (NSR) is sometimes used to denote a specific type of sinus rhythm where all other measurements on the ECG also fall within designated normal limits, giving rise to the characteristic appearance of the ECG when the electrical conduction system of the heart is functioning normally.
Schematic representation of normal sinus rhythm showing standard waves, segments, and intervals.
However, other sinus rhythms can be entirely normal in particular patient groups and clinical contexts, so the term is sometimes considered a misnomer and its use is sometimes discouraged.
Sinus tachycardia
Also colloquially known as sinus tach or sinus tachy, sinus tachycardia is a sinus rhythm with an elevated rate of impulses, defined as a rate greater than 100 beats/min (bpm) in an average adult. The normal resting heart rate in the average male adult ranges from 60–100 bpm and women 60-90bpm. Note that the normal heart rate varies with age, with infants having normal heart rate of 110–150 bpm, in contrast to the elderly, who have slower normals.
ECG readout of an individual with sinus tachycardia. Here the heart rate is around 150.
Tachycardia is often asymptomatic. If the heart rate is too high, cardiac output may fall due to the markedly reduced ventricular filling time. Rapid rates, though they may be compensating for ischemia elsewhere, increase myocardial oxygen demand and reduce coronary blood flow, thus precipitating an ischemic heart or valvular disease. Sinus tachycardia accompanying a myocardial infarction may be indicative of cardiogenic shock.
Sinus bradycardia
Sinus bradycardia is a sinus rhythm with a rate that is lower than normal. In humans, bradycardia is generally defined to be a rate of under 60 beats per minute.
The decreased heart rate can cause a decreased cardiac output resulting in symptoms such as lightheadedness, dizziness, hypotension, vertigo, and syncope. The slow heart rate may also lead to atrial, junctional, or ventricular ectopic rhythms.
Sinus bradycardia seen in lead II with a heart rate of about 50.
Bradycardia is not necessarily problematic. People who regularly practice sports may have sinus bradycardia, because their trained hearts can pump enough blood in each contraction to allow a low resting heart rate. Sinus bradycardia can also be an adaptive advantage; for example, diving seals may have a heart rate as low as 12 beats per minute, helping them to conserve oxygen during long dives.
Sinus bradycardia is a common condition found in both healthy individuals and those who are considered well-conditioned athletes.
Sinus arrhythmia
Sinus rhythm with a beat-to-beat variation in the P-P interval (the time between successive P waves), producing an irregular ventricular rate.
Its characteristics include: Variation in the P-P interval of more than 120 ms (3 small boxes). The P-P interval gradually lengthens and shortens in a cyclical fashion, usually corresponding to the phases of the respiratory cycle. Normal sinus P waves with a constant morphology (i.e. no evidence of premature atrial contractions). Constant P-R interval (i.e. no evidence of Mobitz I AV block).
Sinus arrhythmia is a normal physiological phenomenon, most commonly seen in young, healthy people. The heart rate varies due to reflex changes in vagal tone during the different stages of the respiratory cycle. Inspiration increases the heart rate by decreasing vagal tone. With the onset of expiration, vagal tone is restored, leading to a subsequent decrease in heart rate.
The incidence of sinus arrhythmia decreases with age, presumably due to age-related decreases in carotid distensibility and baroreceptor reflex sensitivity.
Sinus arrhythmia:
Normal sinus P waves (upright in leads I and II) with a constant morphology — albeit with an appearance suggestive of left atrial enlargement.
P-R interval is constant (no evidence of AV block).
The P-P interval varies widely from 1.04 seconds (heart rate ~57 bpm) down to 0.60 seconds (heart rate ~100 bpm); a variability of over 400ms.
For irregular rhythms such as this, the ventricular rate is best estimated by multiplying the total number of complexes in the rhythm strip by 6. This gives an overall rate of 12 x 6 = 72 bpm.
There are several other entities that cause sinus rhythm with an irregular ventricular rate:
Frequent premature atrial contractions
Second-degree AV block, Mobitz I (Wenckebach phenomenon)
Type I Sinoatrial Exit Block
Sinus Exit Block
Sinus exit block is a regular rhythm that may be normal or slow. It is followed by a pause that is a multiple of the P-P interval usually (2-4) . Conduction across the SA node is normal until the time of the pause when it is blocked.
Sinus rhythm (rate = 65/min) with Type II S-A (exit) block; (see laddergram). Note that the long cycles (191) are nearly identical to twice the short cycles (190).
Sinus Arrest
Sinoatrial arrest (also known as sinus arrest or sinus pause) is a medical condition wherein the sinoatrial node of the heart transiently ceases to generate the electrical impulses that normally stimulate the myocardial tissues to contract and thus the heart to beat. It is defined as lasting from 2.0 seconds to several minutes.
Since the heart contains multiple pacemakers, this interruption of the cardiac cycle generally lasts only a few seconds before another part of the heart, such as the atrio-ventricular junction or the ventricles, begins pacing and restores the heart action. This condition can be detected on an electrocardiogram (ECG) as a brief period of irregular length with no electrical activity before either the sinoatrial node resumes normal pacing, or another pacemaker begins pacing.
XIV. ATRIAL PROBLEMS
Premature atrial contractions (PACs), also known as atrial premature complexes (APC) or atrial premature beats (APB), are a common cardiac dysrhythmia characterized by premature heartbeats originating in the atria. ECG Features of Premature Atrial Complex (PAC) PACs usually have the following features: An abnormal (non-sinus) P wave is followed by a QRS complex. In this case, you will see an abnormal P wave that is not followed by a QRS complex (“blocked PAC”)
Supraventricular tachycardia (SVT) is an abnormally fast heart rhythm arising from improper electrical activity in the upper part of the heart. Specific treatments depend on the type of SVT. They can include medications, medical procedures, or surgery. Usually structural heart disease is not present in patients with SVT. Unless WPW is present, SVT is a nuisance and not life-threatening. Because of this, the treatment of SVT does not prolong life. The only other reason to treat SVT then is to prevent the patient from having symptoms from their tachycardia.
Paced Atrial Rhythm
The atria are being paced, but not the ventricles, resulting in an atrial paced rhythm. Accordingly the ventricular complex is delayed until the atrial signal has passed through the AV node. In the second image the ventricles are paced directly, resulting in a ventricular paced rhythm.
XV. VENTRICULAR PROBLEMS
Premature Ventricular Complex (PVC)
A premature beat arising from an ectopic focus within the ventricles.
PVCs have the following features:
Broad QRS complex (≥ 120 ms) with abnormal morphology.
Premature — i.e. occurs earlier than would be expected for the next sinus impulse.
Discordant ST segment and T wave changes.
Usually followed by a full compensatory pause.
Retrograde capture of the atria may or may not occur.
“Discordance” describes a pattern of repolarization abnormality (typically seen with left bundle branch block, paced rhythms, VT) in which the ST segment and T wave are directed opposite to the main vector of the QRS complex:
ST depression and T wave inversion in leads with a dominant R wave.
ST elevation with upright T waves in leads with a dominant S wave.
With a full compensatory pause, the next normal beat arrives after an interval that is equal to double the preceding R-R interval (see diagram below).
Retrograde capture describes the process whereby the ectopic impulse is conducted retrogradedly through the AV node, producing atrial depolarization. This is visible on the ECG as an inverted P wave (“retrograde P wave”), usually occurring after the QRS complex.
PVCs are said to be “frequent” if there are more than 5 PVCs per minute on the routine ECG, or more than 10-30 per hour during ambulatory monitoring.
An idioventricular rhythm is very similar to ventricular tachycardia except the ventricular rate is less than 60 beats per minute. When the ventricular rate is between 60 and 100 bpm, t is referred to as an accelerated idioventricular rhythm.
Ventricular signals are transmitted cell-to-cell between cardiomyocytes and not by the conduction system, creating wide sometimes bizarre QRS complexes (> 0.12 sec). The rate is usually 20-40 bpm. If the rate is >40 bpm, it is called accelerated idioventricular rhythm.
Ventricular tachycardia refers to a wide QRS complex heart rhythm — that is, a QRS duration beyond 120 milliseconds — originating in the ventricles at a rate of greater than 100 beats per minute. You can find links to many more examples of VT at the bottom of this review.
