Normal heart rate varies with age: the younger the child, the faster the heart rate. Therefore the definitions used for adults of bradycardia (fewer than 60 beats/min) and tachycardia (above 100 beats/min) have little significance for children. A child has tachycardia when the heart rate is beyond the upper limit of normal for age and bradycardia when the heart rate is slower than the lower limit of normal (see Table 2.1 ).
I. Rhythms Originating in the Sinus Node
All rhythms that originate in the sinoatrial (SA) node (sinus rhythm) have two important characteristics.
- 1.
A P wave is present in front of each QRS complex with a regular PR interval. (The PR interval may be prolonged, as in first-degree AV block).
- 2.
The P axis is between 0 and +90 degrees, often a neglected criterion. This produces upright P waves in lead II and inverted P waves in aVR (see Figure 2.6 , Figure 2.7 ).
A. Regular Sinus Rhythm
- 1.
Description: The rhythm is regular and the rate is normal for age. Two characteristics of sinus rhythm (as described previously) are present ( Fig. 16.1 ).
Fig. 16.1
Normal and abnormal rhythms originating in the sinoatrial node.
From Park, M. K., & Guntheroth, W. G. (2006). How to read pediatric ECGs (4th ed.). Philadelphia: Mosby.
- 2.
Significance: This rhythm is normal at any age.
- 3.
Treatment: No treatment is required.
B. Sinus Tachycardia
- 1.
Description: The characteristics of sinus rhythm are present. A rate above 140 beats/min in children and above 170 beats/min in infants may be significant. In sinus tachycardia, the heart rate is usually lower than 200 beats/min (see Fig. 16.1 ).
- 2.
Causes: Anxiety, fever, hypovolemia, circulatory shock, anemia, CHF, catecholamines, thyrotoxicosis, and myocardial disease are possible causes.
- 3.
Significance: Increased cardiac work is well tolerated by the healthy myocardium.
- 4.
Treatment: The underlying cause is treated.
C. Sinus Bradycardia
- 1.
Description: The characteristics of sinus rhythm are present. A rate below 80 beats/min in newborn infants and below 60 beats/min in older children may be significant (see Fig. 16.1 ).
- 2.
Causes: Sinus bradycardia may occur in trained athletes. Vagal stimulation, increased intracranial pressure, hypothyroidism, hypothermia, hypoxia, and drugs such as digitalis and β-adrenergic blockers are possible causes.
- 3.
Significance: Some patients with marked bradycardia do not maintain normal cardiac output.
- 4.
Treatment: The underlying cause is treated.
D. Sinus Arrhythmia
- 1.
Description: There is a phasic variation in the heart rate, increasing during inspiration and decreasing during expiration, and the two characteristics of sinus rhythm are maintained (see Fig. 16.1 ).
- 2.
Causes: This normal phenomenon is due to a phasic variation in the firing rate of cardiac autonomic nerves with the phase of respiration.
- 3.
Significance: There is no hemodynamic significance.
- 4.
Treatment: No treatment is indicated.
E. Sinus Pause
- 1.
Description: In sinus pause , there is a momentary cessation of sinus node pacemaker activity, resulting in the absence of the P wave and QRS complex for a relatively short duration (see Fig. 16.1 ). Sinus arrest lasts longer and usually results in an escape beat (such as junctional escape beat).
- 2.
Causes: Increased vagal tone, hypoxia, digitalis toxicity, and sick sinus syndrome (see next section). Well-conditioned athletes may have bradycardia and sinus pause of greater than 2 seconds due to prominent vagal influence.
- 3.
Significance: Sinus pause of less than 2 seconds is normal in young children and adolescents. Sinus pause usually has no hemodynamic significance.
- 4.
Treatment: Treatment is rarely indicated except in sick sinus syndrome and digitalis toxicity.
F. Sinoatrial Exit Block
- 1.
Description: A P wave is absent from the normally expected P wave, resulting in a long RR interval. The duration of the pause is a multiple of the basic PP interval. An impulse formed within the sinus node fails to propagate normally to the atria.
- 2.
Causes: Excessive vagal stimulation, myocarditis or fibrosis involving the atrium, and drugs such as quinidine, procainamide, or digitalis.
- 3.
Significance: It is usually transient and has no hemodynamic significance.
- 4.
Treatment: The underlying cause is treated.
G. Sinus Node Dysfunction (Sick Sinus Syndrome)
- 1.
Description: The sinus node fails to function as the dominant pacemaker of the heart or performs abnormally slowly, producing a variety of arrhythmias. The arrhythmias may include profound sinus bradycardia, sinus arrest with junctional (or nodal) escape, and ectopic atrial or nodal rhythm. When these arrhythmias are accompanied by symptoms such as dizziness or syncope, sinus node dysfunction is referred to as sick sinus syndrome. Long-term electrocardiogram (ECG) recording (such as Holter) is usually required in documenting overall heart rate variation and the prevalence of abnormally slow or fast rhythm.
- 2.
Causes: Extensive cardiac surgery involving the atria (e.g., the Fontan operation), myocarditis, pericarditis, antiarrhythmic drugs, hypothyroidism, CHD (such as sinus venosus ASD, Ebstein anomaly), and occasionally idiopathic occurring in an otherwise normal heart.
- 3.
Significance: Bradytachyarrhythmia is the most worrisome rhythm. Profound bradycardia following a period of tachycardia (overdrive suppression) can cause syncope and even death.
- 4.
Treatment:
- a.
For severe bradycardia:
- (1)
Acute symptomatic bradycardia is treated with intravenous (IV) atropine (0.04 mg/kg IV every 2 to 4 hours) or isoproterenol (0.05 to 0.5 μg/kg IV) or transcutaneous pacing. Temporary transvenous or transesophageal pacing can be used until a permanent pacing system can be implanted.
- (2)
Permanent implantation of pacemaker is the treatment of choice in symptomatic patients, especially those with syncope. Most patients receive atrial demand pacing. Patients with any degree of AV nodal dysfunction receive dual-chamber pacemakers. Ventricular demand pacemakers may be used.
- (3)
Asymptomatic patients with heart rate under 40 beats/min or pauses longer than 3 seconds are less clear indications for permanent pacing.
- b.
For symptomatic tachycardia:
- (1)
Antiarrhythmic drugs, such as propranolol or quinidine, may be given to suppress tachycardia, but they are often unsuccessful.
- (2)
Digoxin or amiodarone may help to decrease AV conduction of rapid tachycardia.
- (3)
Catheter ablation of arrhythmia substrates (often requiring concomitant surgical revision of previous surgeries) may be indicated.
- (4)
Patients with tachycardia–bradycardia syndrome may benefit from antitachycardia pacemakers.
- (1)
- a.
II. Rhythms Originating in the Atrium
Atrial arrhythmias ( Fig. 16.2 ) are characterized by the following:
- 1.
P waves of unusual contour (abnormal P axis) and/or an abnormal number of P waves per QRS complex.
- 2.
QRS complexes of normal duration (but with occasional wide QRS duration caused by aberrancy).
A. Premature Atrial Contraction
- 1.
Description
- a.
In PAC the QRS complex occurs prematurely with abnormal P wave morphology. There is an incomplete compensatory pause—that is, the length of two cycles including one premature beat is less than the length of two normal cycles.
- b.
An occasional PAC is not followed by a QRS complex (i.e., a nonconducted PAC) (see Fig. 16.2 ).
- c.
A nonconducted P AC is differentiated from a second-degree AV block by the prematurity of the nonconducted P wave (P′ in Fig. 16.2 ). The P′ wave occurs earlier than the anticipated normal P rate, and the resulting PP′ interval is shorter than the normal PP interval for that individual. In second-degree AV block, the P wave that is not followed by the QRS complex occurs at the anticipated time, maintaining a regular PP interval.
- a.
- 2.
Causes: PAC appears in healthy children, including newborns. It also may appear after cardiac surgery and with digitalis toxicity.
- 3.
Significance: Isolated PAC has no hemodynamic significance.
- 4.
Treatment: Usually no treatment is indicated except in cases of digitalis toxicity.
B. Wandering Atrial Pacemaker
- 1.
Description: Gradual changes in the shape of P waves and PR intervals occur. The QRS complex is normal (see Fig. 16.2 ).
- 2.
Causes: This is seen in otherwise healthy children. It is the result of a gradual shift of impulse formation in the atria through several cardiac cycles.
- 3.
Significance: There is no clinical significance.
- 4.
Treatment: No treatment is indicated.
C. Ectopic (or Autonomic) Atrial Tachycardia
- 1.
Description
- a.
There is a narrow QRS complex tachycardia (in the absence of aberrancy or preexisting bundle branch block) with visible P waves at an inappropriately rapid rate.
- b.
The P axis is different from that of sinus rhythm (see Fig. 16.2 ). When the ectopic focus is near the sinus node, the P axis may be the same as in sinus rhythm.
- c.
The usual heart rate in older children is between 110 and 160 beats/min, but the tachycardia rate varies substantially during the course of a day, reaching 200 beats/min with sympathetic stimuli. Holter monitoring may demonstrate a characteristic gradual acceleration of the heart rate, the so-called warming up period, rather than abrupt onset and termination seen with re-entrant AV tachycardia.
- d.
It represents about 20% of SVT. This arrhythmia is sometimes difficult to distinguish from the re-entrant AV tachycardia and thus it is included under the “Supraventricular Tachycardia” section later.
- a.
- 2.
Causes : This arrhythmia originates from a single focus in the atrium. It is believed to be secondary to increased automaticity of nonsinus atrial focus or foci. Myocarditis, cardiomyopathies, atrial dilatation, atrial tumors, and previous cardiac surgery involving atria (such as Fontan procedure) may be the cause. Most patients have a structurally normal heart (idiopathic).
- 3.
Significance : CHF is common with chronic cases. There is a high association with tachycardia-induced cardiomyopathy.
- 4.
Treatment : It is refractory to medical therapy and cardioversion. Drugs that are effective in re-entrant atrial tachycardia (such as adenosine) do not terminate the tachycardia. Cardioversion is ineffective because the ectopic rhythm resumes immediately.
- a.
The goal may be to slow the ventricular rate (using digoxin or β-blockers) rather than to try to convert the arrhythmia to sinus rhythm.
- b.
Long-term oral antiarrhythmic drugs (such as flecainide or amiodarone) are the mainstay of therapy in patients not undergoing radiofrequency ablation.
- c.
Radiofrequency ablation may prove to be effective in nearly 90% of cases. In children, the foci are found in the LA near the pulmonary veins and the atrial appendage (in contrast to the RA found in adults).
- a.
D. Multifocal (or Chaotic) Atrial Tachycardia
- 1.
Description: There are three or more distinct P-wave morphologies. The PP and RR intervals are irregular with variable PR intervals. The arrhythmia may be misdiagnosed as atrial fibrillation.
- 2.
Causes: Most patients with the condition are infants; it is very rare after 5 years of age. Thirty percent to 50% have respiratory illness. Myocarditis and birth asphyxia have been described. This arrhythmia may occur with or without CHDs. The mechanism of this arrhythmia has been poorly defined.
- 3.
Significance: CHF may develop. Sudden death has been reported in up to 17% while receiving therapy. Long duration of the arrhythmia may cause LV systolic dysfunction. Spontaneous resolution frequently occurs.
- 4.
Treatment
- a.
Adenosine is ineffective in terminating the tachycardia (this is a useful diagnostic sign of the condition). This arrhythmia is also refractory to cardiac pacing and cardioversion.
- b.
Drugs that slow AV conduction (propranolol or digoxin) and those that decrease automaticity (such as class IA or IC or class III) have not been very effective. However, they may control the heart rate.
- c.
Amiodarone (IV followed by oral [PO]) appears to be the current treatment of choice.
- a.
E. Atrial Flutter
- 1.
Description: Atrial flutter is characterized by a fast atrial rate (F waves with saw-tooth configuration) of about 300 (ranges 240 to 360) beats/min, the ventricle responding with varying degrees of block (e.g., 2:1, 3:1, 4:1), and normal QRS complexes (see Fig. 16.2 ).
- 2.
Causes: Structural heart disease with dilated atria, myocarditis, thyrotoxicosis, and previous surgery involving atria (such as Senning or Fontan operation) are possible causes. However, most fetuses and neonates with atrial flutter have a normal heart.
- 3.
Significance: The ventricular rate determines the eventual cardiac output; a too-rapid ventricular rate may decrease the cardiac output. Thrombus formation may lead to embolic events. Uncontrolled atrial flutter may lead to heart failure.
- 4.
Treatment
- a.
In acute situations, synchronized cardioversion is the treatment of choice. Adenosine is not effective.
- b.
For long-standing atrial flutter or fibrillation (of 24 to 48 hours) or those with unknown duration, it is important to rule out intracardiac thrombus by echo (preferably transesophageal echo) before cardioversion because it may lead to cerebral embolization. If a thrombus is found or suspected, anticoagulation with warfarin (with international normalized ratio [INR] 2 to 3) is started and cardioversion delayed for 2 to 3 weeks. After conversion to sinus rhythm, warfarin is continued for an additional 3 to 4 weeks.
- c.
For control of the ventricular rate, calcium channel blockers, propranolol, or digoxin may be used.
- d.
For prevention of recurrence, class I (quinidine) and class III (amiodarone) antiarrhythmic agents may be effective in some cases.
- e.
For refractory cases, antitachycardia pacing or radiofrequency ablation may be indicated.
- a.
F. Atrial Fibrillation
- 1.
Description: Atrial fibrillation (AF) is characterized by an extremely fast atrial rate (F wave at 350 to 600 beats/min) and an irregular ventricular response with narrow QRS complexes (see Fig. 16.2 ).
- 2.
Causes: AF usually is associated with structural heart diseases with dilated atria, such as seen with mitral stenosis and regurgitation, Ebstein anomaly, tricuspid atresia, ASD, or previous intraatrial surgery. Thyrotoxicosis, pulmonary emboli, and pericarditis should be suspected in a previously normal child who develops AF.
- 3.
Significance: AF usually suggests a significant pathology. Rapid ventricular rate and the loss of coordinated contraction of the atria and ventricles decrease cardiac output. Atrial thrombus formation is quite common.
- 4.
Treatment: Treatment of AF is similar to that described under the “Atrial Flutter section.
- a.
If AF has been present more than 48 hours, the patient should receive anticoagulation with warfarin for 3 to 4 weeks to prevent systemic embolization of atrial thrombus, if the conversion can be delayed. Anticoagulation is continued for 4 weeks after restoration of sinus rhythm. If cardioversion cannot be delayed, heparin should be started, and cardioversion performed when activated partial thromboplastin time (aPTT) reaches 1.5 to 2.5 times control (in 5 to 10 days), with subsequent oral anticoagulation with warfarin.
- b.
Propranolol or digoxin may be used to slow the ventricular rate.
- c.
Class I antiarrhythmic agents (e.g., quinidine, procainamide, flecainide) and the class III agent amiodarone may be used but the success rate in rhythm conversion is disappointingly low. These agents may prevent recurrence.
- d.
In patients with chronic AF, anticoagulation with warfarin should be used to reduce the incidence of thromboembolism.
- e.
In the Cox maze procedure (or the “cut-and-sew-maze”), multiple surgical incisions are made in the right and left atria that are then repaired in an attempt to minimize the formation of a re-entrant loop. The procedure showed greater than a 96% cure rate 10 years after the surgery in adult patients.
- f.
Radiofrequency ablation to electrically isolate the pulmonary veins from the left atrium or directly ablating the ectopic focus within the pulmonary veins has shown better results than pharmacologic agents in rhythm control in adults. Stenosis of the PV(s) is a significant complication of the procedure.
- a.
III. Rhythms Originating in the AV Node
Rhythms originating in the AV node ( Fig. 16.3 ) are characterized by the following:
- 1.
The P wave may be absent, or inverted P waves may follow the QRS complex.
- 2.
The QRS complex is usually normal in duration and configuration.
Junctional rhythm describes an abnormal heart rhythm resulting from impulses coming from a locus of tissue in the area of the AV node, the “junction” between atria and ventricles. Since the node-His (NH) region of the AV node is the only part of the AV node with demonstrable ability to pace the heart, some authorities prefer the term “nodal” over “junctional.”
A. Junctional (or Nodal) Premature Beats
- 1.
Description: A normal QRS complex occurs prematurely. P waves are usually absent, but inverted P waves may follow QRS complexes. The compensatory pause may be complete or incomplete (see Fig. 16.3 ).
- 2.
Causes: Usually idiopathic in an otherwise normal heart but may result from cardiac surgery or digitalis toxicity.
- 3.
Significance: Usually no hemodynamic significance.
- 4.
Treatment: Treatment is not indicated unless caused by digitalis toxicity.
B. Junctional (or Nodal) Escape Beat
- 1.
Description: When the sinus node impulse fails to reach the AV node, the NH region of the AV node will initiate an impulse (nodal or junctional escape beat). The QRS complex occurs later than the anticipated normal beat. The P wave may be absent (see Fig. 16.3 ), or an inverted P wave may follow the QRS complex.
- 2.
Causes: It may follow cardiac surgery involving the atria (e.g., the Fontan operation) or may be seen in otherwise healthy children.
- 3.
Significance: Little hemodynamic significance.
- 4.
Treatment : Generally no specific treatment is required.
C. Junctional (or Nodal) Rhythm
- 1.
Description: If there is a persistent failure of the sinus node, the AV node may function as the main pacemaker of the heart with a relatively slow rate (40 to 60 beats/min). P waves are absent or inverted P waves follow QRS complexes (see Fig. 16.3 ).
- 2.
Causes: It may be seen in an otherwise normal heart, after cardiac surgery, in conditions of an increased vagal tone (e.g., increased intracranial pressure, pharyngeal stimulation), and with digitalis toxicity. Rarely, it may be seen in children with polysplenia syndrome.
- 3.
Significance: The slow heart rate may significantly decrease the cardiac output and produce symptoms.
- 4.
Treatment: No treatment is indicated if the patient is asymptomatic. Atropine or electric pacing is indicated for symptoms. Treatment is directed to digitalis toxicity if caused by digitalis.
D. Accelerated Junctional (or Nodal) Rhythm
- 1.
Description: In the presence of normal sinus rate and AV conduction, if the AV node (NH region) with enhanced automaticity captures the pacemaker function (60 to 120 beats/min), the rhythm is called accelerated nodal (or AV junctional) rhythm. P waves are absent or inverted P waves follow the normal QRS complexes.
- 2.
Causes: Idiopathic, digitalis toxicity, myocarditis, or previous cardiac surgery.
- 3.
Significance: Little hemodynamic significance.
- 4.
Treatment: No treatment is necessary unless caused by digitalis toxicity.
E. Junctional Ectopic Tachycardia (Nodal Tachycardia)
- 1.
Description: The ventricular rates vary from 120 to 200 beats/min. P waves are absent (see Fig. 16.3 ) or inverted P waves follow the QRS complexes. The QRS complex is usually normal, but aberration may occur. Junctional tachycardia is difficult to separate from other types of SVT. Therefore the arrhythmia is grouped under SVT.
- 2.
Causes: Enhanced automaticity of the junctional area is the suspected mechanism. There are two types: postoperative and congenital.
- a.
The postoperative type is more common than the congenital type. This transient disorder is seen after open heart surgery, and lasts 24 to 48 hours. Trauma, stretch, or ischemia to the AV node and electrolyte imbalance may be responsible for the rhythm disorder.
- b.
The rare congenital type may occur with or without associated CHDs.
- a.
- 3.
Significance
- a.
In the postoperative type, a loss of AV synchrony in the presence of a fast rate (nearly 200 beats/min) compromises cardiac output, leading to a fall in blood pressure (BP). Increased endogenous catecholamine levels and administered inotropic support (to maintain adequate BP and renal perfusion) may result in peripheral vasoconstriction, leading to a rise in the core temperature. The rising core temperature exacerbates the tachycardia, worsening ventricular performance.
- b.
In the congenital form, most patients present before 6 months of age, usually with CHF (with overall mortality rate of 35%).
- a.
- 4.
Treatment
- a.
For the postoperative type: Heart rate <170 beats/min is well tolerated but rates >170 to 190 beats/min need to be slowed.
- (1)
Atrial overdrive pacing (typically 10 beats/min higher than the rate) often restores AV synchrony.
- (2)
Mild systemic hypothermia is induced, usually a core temperature of 34°C to 35°C. At a core temperature below 32°C, ventricular function may be impaired.
- (3)
Cardiac output is maximized by carefully titrating fluid and electrolyte balance, inotropic support, and pain management.
- (4)
IV amiodarone appears to be the drug of choice as antiarrhythmic therapy. In the past, procainamide IV drip was widely used with good success. Digoxin is no longer used in this situation.
- (5)
Extracorporeal membrane oxygenation (ECMO) can be used as an alternative in selected patients.
- (1)
- b.
For the congenital type, amiodarone appears to be the drug of choice. Amiodarone in high dose was effective in 85% of the patients with almost 75% survival rate. If amiodarone is not effective, ablation therapy may be tried.
- a.
IV. Supraventricular Tachycardia
SVT refers to any rapid heart rhythm originating above the ventricular tissue, including atrial and junctional tachycardias. SVTs are caused by two mechanisms: re-entry and automaticity.
- 1.
Re-entry: Most cases of SVTs are due to re-entrant (or reciprocating) AV tachycardia. Only re-entry AV tachycardia is discussed in this section.
- 2.
Automaticity: SVTs caused by increased automaticity of a single focus in the atria or the AV node are infrequent. Examples of this entity include atrial ectopic tachycardia and junctional (or nodal) ectopic tachycardia (as discussed in earlier sections).
A. Mechanism of Re-Entry Tachycardias
In SVT caused by reentry, two pathways are involved:
- 1.
The anatomic accessory pathway, such as the bundle of Kent, produces AV re-entry tachycardia (AVRT) ( Fig. 16.4A, B ). WPW preexcitation is frequently present on the ECG.
