First‐degree AV block

First‐degree AV block is characterized by a prolonged PR interval beyond what is considered normal for age. Each P wave is followed by a conducted QRS complex. This type of rhythm disturbance can be a normal variant in healthy individuals but can also be seen in a variety of disease states (e.g., structural heart defects associated with stretching of the atria, rheumatic fever). In general, a prolonged PR interval in an otherwise healthy child is a benign condition and requires no treatment.

Second‐degree AV block

There are four types of second‐degree AV block. The two predominant types, Mobitz type I (Wenckebach) and Mobitz type II, involve a periodic failure to conduct atrial impulses to the ventricle. In type I second‐degree AV block, the PR interval lengthens progressively until the next atrial impulse cannot be conducted to the ventricle. These failures of conduction manifest on the surface ECG as P waves without associated QRS complexes and concomitant shortening of the RR intervals (Figure 22.3). This rhythm disturbance can occur during periods of high vagal tone and is generally considered a benign phenomenon that requires no therapy.

In the less frequent type II second‐degree AV block, there is a constant PR interval before an atrial impulse that suddenly fails to conduct. This conduction abnormality is more concerning because of its potential for progression. It can be seen in patients after surgery for CHD and is thought to be due to damage to the His bundle or distal conduction system.

The third type of second‐degree AV block is two to one AV block (2:1). In 2:1, every second P wave is blocked. In most cases, this is a type of Mobitz type I or Wenckebach block and is secondary to high vagal tone. It is rare for 2:1 to progress to a higher degree of AV block or to require treatment.

The fourth type of second‐degree AV block is a high‐grade AV block, which is evidenced by two or more non‐conducted P waves in succession that would normally be expected to conduct. Temporary pacing and close patient observation may be warranted because hemodynamically significant bradycardia or continued progression of the conduction deficit may ensue.

Third‐degree (complete) AV block

Third‐degree AV block is characterized by the total failure of atrial impulses to be conducted to the ventricles. There is complete dissociation of the electrical activity between the atria and ventricles, and the ventricular rate is usually slow and regular. In third‐degree AV block, the ventricular escape rate may be narrow (if originating from the perinodal region) or wide (if originating from within the ventricle). The diagnostic feature on the ECG is that all atrial impulses that should be conducted to the ventricle fail to do so (Figure 22.4).

Complete AV block may be either congenital or acquired. Congenital complete AV block in infants with otherwise structurally normal hearts may be due to intrauterine exposure to maternal antibodies associated with collagen vascular diseases (e.g., lupus). Patients at high risk of complete AV block include those with congenitally corrected transposition (L‐transposition of the great arteries with ventricular inversion) and those with polysplenia (left atrial isomerism) [1]. Acquired postoperative AV block is thought to result from damage to the compact AV node or bundle of His and may be transient or permanent. The surgical procedures most commonly associated with the onset of complete AV block include repair of AVSD, closure of VSD, resection of obstructive subaortic tissue, and interventions in patients with congenitally corrected transposition [2]. In children, reported incidences of surgical AV block are as high as 2–4%. Normal conduction eventually recovers in more than 60% of patients, usually within the first 10 postoperative days [3, 4]. Acute treatment includes temporary pacing (either AV sequential pacing or ventricular pacing only). If the rate of the junctional escape rhythm is high enough to support stable hemodynamics, temporary pacing can be set as a backup with close monitoring of the underlying cardiac rhythm. Important considerations in patients with surgical AV block include careful surveillance for the return of AV conduction and frequent evaluation of temporary pacing wire thresholds. The ventricular output of the temporary pacemaker should be set well above the capture threshold to increase the margin of safety (please refer to the section on temporary pacing). Permanent cardiac pacing is generally indicated in patients who do not recover from complete AV block within 10–14 days after surgical intervention. A small minority of patients have late recovery of their native AV nodal conduction after surgically acquired complete AV block [3, 4].

When providing anesthetic care to a patient with complete AV block but no implanted pacemaker, the following should be considered:

• Drugs and resuscitation equipment. Immediate availability of emergency agents such as isoproterenol and epinephrine, in addition to resuscitation equipment, is essential.
  
• Transcutaneous cardiac pacing. Equipment (pacing pads and unit) should be available in case extracardiac pacing becomes necessary. Placing the pacing pads before anesthetic induction may be prudent.
  
• Access to temporary transvenous pacing. Although insertion of a temporary pacing catheter before anesthetic care has been suggested in children with complete AV block, a retrospective study of this approach showed that using this catheter routinely had no benefit [5].

Atrial electrogram

In the postoperative patient, an atrial electrogram (AEG) can be useful in both diagnosis and management of rhythm problems. This type of ECG recording is obtained from the temporary atrial wires placed toward the end of surgery (Figure 22.6). Typically, atrial wires emerge on the right side of the chest wall and ventricular wires on the left side, although this configuration may vary. Although both a standard 15‐lead ECG and an AEG record the same electrical cardiac activity, these electrical sequences display distinctly different configurations in different leads. On an AEG, the P waves are larger in amplitude, making them easily recognizable because the recording is obtained from wires attached directly to the atrial myocardium. Therefore, in situations in which P waves are not clearly identified on a surface ECG, an AEG may assist in defining atrial activity and the relationship between atrial and ventricular depolarization (Figure 22.7). Such recordings can help clinicians to differentiate between atrial and junctional arrhythmias [6]. For example, during junctional tachycardia, the AEG displays P waves that are either superimposed on the R waves or dissociated from them. In most reentrant SVTs, the PR interval is longer than the RP interval, whereas in sinus rhythm, the PR interval is shorter than the RP interval. An AEG is also helpful in defining the type of an AV block if present and can facilitate the differential diagnosis of sinus node dysfunction vs. various degrees of AV block.

AEGs can be obtained from bedside monitors (Figure 22.6) or standard 15‐lead ECG machines [7]. With a bedside or operating room monitor, it is best to use a rhythm strip with two or more channels so that AEG and ECG recordings can be viewed simultaneously. There are various ways to obtain an AEG along with a standard tracing, depending on various equipment‐related factors (e.g., the recorder, epicardial wires, lead configuration).

The following methods of obtaining an AEG require the use of a standard 15‐lead ECG machine:

• If two atrial wires are present, each lead is attached to the connectors that usually correspond to the right and left arm leads (an alligator clip can be used, if necessary). This allows for a bipolar AEG (large deflection of atrial depolarization with trivial or no signal representative of ventricular activity) to be recorded in lead I. The chest (precordial) leads provide ECG standard tracings. By evaluating the atrial activity as displayed by the AEG (lead I) and the ventricular impulses represented by the QRS complexes on the chest leads (V1–V6), the electrical sequence of cardiac events can be assessed.

  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  

  Features of the tachycardia	Automatic	Reentrant
  Onset and termination	“Warm‐up” at initiation, “cool‐down” at termination	Abrupt
  Mode of initiation	Spontaneous	Premature beats
  Ability to initiate/terminate with timed premature beats	No	Yes
  Variation in tachycardia rate	Wide	Narrow
  Response to catecholamines	Increased rate	None or slight rate increase
  Response to adenosine	None	Termination
  Response to drugs that increase refractoriness	Variable	Slowing or termination
  Response to overdrive pacing	Transient suppression, quick resumption	Termination
  Response to cardioversion	None	Termination

  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  

  Diagnosis	Electrocardiographic features
  Automatic tachycardias
  Ectopic atrial tachycardia or atrial ectopic tachycardia	Atrial rates of 90–330 bpm Incessant rhythm From atrial focus distinct from the sinus node Abnormal P‐wave morphology and/or axis Distinct P waves preceding QRS complexes No influence of AV block on tachycardia
  Junctional ectopic tachycardia	Narrow QRS tachycardia Incessant rhythm AV dissociation (often) Atrial rate slower than the ventricular rate Capture beats frequently seen (QRS complexes slightly earlier than expected from antegrade conduction of normal sinus impulses)
  Reentrant tachycardias
  Atrial flutter	Sawtooth pattern or more discrete undulating P waves (leads II, II, aVF) Variable rates of AV conduction seen (1:1, 2:1, 3:1, or 4:1)
  Atrioventricular reentrant tachycardia (accessory pathway‐mediated from concealed bypass tract or Wolff‐Parkinson‐White syndrome)	P waves immediately following the QRS complex, on ST segment or T wave AV block results in termination of tachycardia
  Atrioventricular nodal reentry tachycardia	P waves buried within QRS and not discernible AV block results in termination of tachycardia

  
  

  AV, atrioventricular

  
  
• If only a single atrial lead is available, this can be attached to one of the chest leads to obtain a corresponding AEG. In this case, the limb leads can be used to provide a reference for ventricular activity (Figures 22.6 and 22.7).
  
• An alternate lead configuration may utilize a single atrial lead and a skin lead as a substitute for the arm leads to obtain an atrial tracing in lead I, and other leads serve as a reference. A rhythm strip should be printed out so that the recordings can be examined.

In addition to assisting in arrhythmia diagnosis and the selection of appropriate treatment, temporary atrial wires can be used for rapid atrial pacing in attempts to terminate SVT due to reentrant mechanisms or to overdrive suppress an automatic focus.

General management principles for SVT

Management of SVT depends on the clinical status of the patient, the type of tachycardia, and the precise electrophysiologic mechanism that is causing SVT (Table 22.3). If the tachyarrhythmia is associated with significant hemodynamic compromise, emergent therapy is indicated. Synchronized direct‐current cardioversion (0.5–1.0 J/kg) should be considered for a patient with any acute tachyarrhythmia associated with low cardiac output, recognizing that this approach does not always restore normal sinus rhythm.

Atrial tachycardias

The automaticity of atrial tissue accounts for the majority of supraventricular arrhythmias in this group [8]. In general, these rhythm disorders are more recalcitrant and difficult to treat than reentrant ones.

Management principles for postoperative EAT

• General considerations. The management of postoperative EAT includes the treatment of fever if present, adequate sedation, correction of electrolyte abnormalities, and the withdrawal of medications that cause sympathetic stimulation (e.g., inotropic agents) or have vagolytic properties.
  
• Anti‐arrhythmic therapy. The institution of pharmacologic treatment is based on overall heart rate, the duration of the tachycardia, and the hemodynamic status of the patient. Treatment relies on clinical judgment and is influenced by ventricular function. There are no large studies on anti‐arrhythmic drug efficacy in postoperative EAT. Medications such as esmolol, procainamide, and amiodarone can be effective in slowing the tachycardia rate [12]. Oral agents (class I, II, and III drugs) can also be of benefit. Digoxin has a minimal effect on the atrial focus but can decrease the ventricular response by slowing AV conduction [13].

  

  
  
• Ablation therapy. In very few postoperative patients, EAT can be incessant and life‐threatening, and consideration should be given to transcatheter ablation of the atrial focus [14]. It has been reported that propofol anesthesia may not be appropriate for children undergoing catheter ablation of EAT, because administering this drug may terminate the tachycardia and prevent it from being induced by isoproterenol infusion (See Chapter 34) [15].
  
• Other modalities. Atrial pacing and cardioversion are unlikely to resolve EAT.

Junctional tachycardias

The automaticity of junctional tissue accounts for the majority of supraventricular arrhythmias in this group [8]. These rhythm disorders tend to be somewhat resistant to standard pharmacological therapy.

Management principles for postoperative JET

Numerous therapies have been advocated for JET [23, 24, 26, 32]. Strategies for acute care include the following:

• General considerations. Core temperature cooling (to 33–35 °C) in the younger patient by the use of cooling blankets, fans, or cold compresses has been shown to be of benefit in reducing the tachycardia rate [33–35]. Shivering, if significant, should be minimized to prevent potentially detrimental increases in oxygen consumption. Additional suggested approaches include withdrawing or decreasing vagolytic agents and any medications associated with catecholamine stimulation and correcting abnormal levels of electrolytes, especially magnesium, potassium, and calcium.
  
• Atrial pacing. Temporary atrial pacing at heart rates 10–20 bpm above the JET rate establishes AV synchrony and often improves hemodynamics. However, if the JET rate is faster than 180 or 190 bpm, overdrive atrial pacing often confers a little benefit.
  
• Anti‐arrhythmic medications. The two most widely used drugs for JET are amiodarone and procainamide [26, 32, 36–38]. Amiodarone has a longer onset of action and a longer half‐life than procainamide. The drug has been shown to reduce the heart rate in JET patients during the initial bolus infusion [36, 37]. Core cooling is often continued but is not generally needed for efficacy. Administering amiodarone may be a way to avoid having to evaluate clinical signs reflecting the adequacy of cardiac output (distal peripheral perfusion, skin temperature) in a patient with hypothermia and tachycardia. Amiodarone does not directly influence ventricular function and is generally thought to cause less likelihood of hypotension during the initial bolus infusion than procainamide does. However, the drug should be administered with caution, given its potential adverse events (hypotension, bradycardia, AV block). The benefits of procainamide are that it has a faster onset of action and a shorter half‐life than amiodarone. However, procainamide appears to be effective mainly when used with core cooling. It may also cause a decrease in systemic vascular resistance, with resultant hypotension, particularly during bolus infusions. Procainamide can also have negative inotropic properties. Usually, a fluid bolus or other volume expander should be given before or during procainamide therapy to maintain adequate hemodynamics. Both amiodarone and procainamide have been shown to be effective in the treatment of JET in published retrospective studies; however, the main determinant of drug selection in clinical practice is influenced by physician/institutional preference. Because amiodarone and procainamide can each cause QT prolongation and proarrhythmic side effects, these two drugs should not be administered concomitantly. Intravenous sotalol has been shown in case reports to be of use in the management of post‐operative JET. Further studies are needed to determine the safety and efficacy of sotalol in the management of postoperative JET [39]. Another novel anti‐arrhythmic agent that has shown promise in the management of JET is ivabradine (alone and in combination with amiodarone) [40–43]. Ivabradine is available only for oral administration, so its use is limited to patients that can tolerate oral medications. Anecdotal evidence suggests that digoxin loading can slow the JET rate, but this has not been well documented in the literature. ß‐blockers and calcium‐channel blockers can depress myocardial contractility, a feature that may limit their use in the immediate postoperative period. In this regard, a short‐acting agent such as esmolol may offer a larger margin of safety. The use of intravenous (IV) class IC agents such as propafenone and flecainide has been reported, but these drugs have not been studied extensively for JET [44–46]. The natural history of perioperative JET is that it resolves within 2–5 days after the surgical intervention. Long‐term anti‐arrhythmic therapy is usually not necessary. Rarely, extracorporeal membrane oxygenator (ECMO) support may be necessary for incessant hemodynamically significant JET not responding to other therapies.

  

  
  
• Cardioversion is generally considered ineffective in terminating JET.
  
• Catheter ablation of the junctional focus should be considered only as a last resort, because it can result in complete AV block, given that postoperative JET is usually transient [47, 48].

Reentrant supraventricular tachycardias

Reentry, also known as “circus” movement or reciprocation, implies that a single stimulus or excitation wave front returns and reactivates the same site or tissue from which it originated. Reentrant forms of SVT may or may not involve accessory pathways.

Atrial flutter

Atrial flutter is a rhythm disturbance confined to the atrial myocardium. The electrophysiologic basis for this arrhythmia involves reentry within the atrium itself. The typical or classic form of atrial flutter is characterized by a negative sawtooth P‐wave pattern and atrial rates that exceed 300 bpm (Figure 22.10). This form of atrial flutter is occasionally seen in otherwise healthy neonates but is relatively uncommon in children. In patients with CHD, slower atrial rates and varying P‐wave morphologies are more frequently seen than in those without CHD. This is due to anatomic abnormalities related to suture lines, scars, or fibrosis from a previous surgery involving atrial tissue. This form of “scar flutter” is commonly termed intra‐atrial reentrant tachycardia (IART) [49]. It is one of the most common arrhythmias in postoperative patients with structural heart disease and is considered the cause of significant morbidity after certain types of surgical interventions [50]. Procedures that involve extensive atrial suture lines, such as atrial redirection procedures (Senning or Mustard operations) and those associated with atrial dilation (Fontan surgery) pose a particularly high risk of atrial flutter. The possibility of atrial flutter is suggested by the abrupt onset of a rapid atrial rhythm that remains relatively regular over time. AV nodal conduction accounts for variability in the ventricular response rate. Rapid clinical deterioration is likely; fast ventricular rates frequently require prompt intervention.

Management principles for atrial flutter

• Adenosine will not terminate tachycardia but may assist in confirming the diagnosis by uncovering flutter waves during AV block.
  
• Atrial overdrive pacing is a safe and effective way to rapidly terminate atrial flutter by using a transesophageal or transvenous pacing catheter or epicardial wires [51]. After the atrial cycle length is assessed, rapid atrial stimulation is performed in short bursts to attempt an interruption of the reentry circuit.
  
• Synchronized cardioversion is the treatment of choice in any patient with unstable hemodynamics. Placement of the cardioversion pads over the front and back of the hemithorax may be necessary to provide a shock vector through the entire atrium, which is usually thickened in patients with CHD.
  
• Pharmacologic agents such as digoxin, procainamide, sotalol, and amiodarone can be used in urgent situations. Drugs for controlling the ventricular response in atrial flutter include ß‐blockers and calcium‐channel blockers. Important considerations regarding drug selection are patient age, underlying ventricular function, and whether there is sinus node dysfunction (a concomitant problem in patients with recurrent atrial flutter). Ibutilide and sotalol are available for rapid termination of atrial flutter and have been used in postoperative pediatric and adult patients with CHD [52–55].
  
• Long‐term drug therapy is frequently necessary in patients with CHD because of the potential for recurrence and associated rapid AV conduction.
  
• Pacemaker therapy, atrial anti‐tachycardia pacing, and radiofrequency ablation are additional modalities often used for long‐term management of atrial flutter.

Management principles for atrial fibrillation

• Its management principles are similar to those for atrial flutter except that atrial overdrive pacing is not effective in terminating the arrhythmia.
  
• Cardioversion is more likely to be required, and higher amounts of energy may be needed. As in the case of atrial flutter, the placement of cardioversion pads should be optimized by using a front and back configuration over the chest. Anticoagulation and consideration of transesophageal echocardiography for evaluating intracardiac thrombi are recommended before cardioversion if atrial fibrillation has been present more than 48 hours [56, 57].

Atrioventricular reentrant tachycardia and atrioventricular nodal reentrant tachycardia

Atrioventricular reentrant tachycardia (AVRT) is the most common type of SVT in infancy and childhood. It is mediated by an accessory pathway between the atrium and ventricle. The tachycardia circuit typically consists of conduction from the atrium, down the AV node, through the bundle of His and ventricles, and then up the accessory connection back to the atrium. This form of SVT is referred to as orthodromic SVT and occurs in patients with Wolff–Parkinson–White syndrome (WPW; Figure 22.11), concealed accessory pathways, and permanent junctional reciprocating tachycardia. In contrast, in antidromic SVT, conduction travels from the atrium, down the accessory connection, through the ventricles, up the AV node, and back to the atrium. The QRS complex in this form of SVT is wide. Antidromic tachycardia can occur in patients with WPW and other pre‐excitation variants (Mahaim tachycardia).

Atrioventricular nodal reentrant tachycardia (AVNRT), or reentry within the AV node, is most likely in adolescents or young adults. In AVNRT, the AV node has two physiologically distinct components, designated the “slow” and “fast” AV nodal pathways. The typical form of AVNRT consists of antegrade conduction (from the atrium to the ventricle) via the slow pathway, followed by retrograde conduction (back to the atrium) via the fast pathway.

Both AVRT and AVNRT have clinical characteristics typical of reentrant tachycardia mechanisms as listed in Table 22.1. The two can often be distinguished by closely evaluating the surface ECG during sinus rhythm and tachycardia. In orthodromic AVRT, the P wave can be seen immediately after the QRS complex or in the ST segment or T wave (Figure 22.12). The reason for this P‐wave location is that a set time is necessary for conduction to proceed from the ventricles through the accessory pathway back to the atrium. In contrast, in AVNRT, the P wave is buried in the QRS complex and is often not discernible (Figure 22.13). This is the case because the tachycardia circuit is within the AV node, and the atria and the ventricles are activated almost simultaneously. Patients with structurally normal hearts, as well as those with CHD, can have either AVRT or AVNRT. Ebstein malformation of the tricuspid valve is frequently associated with AVRT secondary to one or multiple accessory pathways. The accessory connections in this condition are usually right‐sided. Congenitally corrected transposition can be associated with an Ebstenoid left‐sided AV valve, and left‐sided accessory pathways can be identified in a subset of patients.

Management principles for AVRT or AVNRT

• Direct current synchronized cardioversion (0.5–1.0 J/kg) should be performed in hemodynamically unstable patients. A lower energy setting is adequate if paddles are used directly on the heart (epicardial paddles). Cardioversion should also be considered in stable patients when potential rapid clinical deterioration is anticipated or after an unsuccessful conventional therapy.
  
• In stable patients, tachycardia can be rapidly terminated with vagal maneuvers (Valsalva maneuver, coughing, gag reflex stimulation, ice to the face, Trendelenburg position), which enhance parasympathetic influences [58].
  
• Adenosine is the first‐line drug therapy for SVT [59–62]. Other agents (digoxin, edrophonium, ß‐blockers, calcium‐channel blockers, phenylephrine, dexmedetomidine, sotalol) have been used in acute treatment with variable results; however, serious adverse effects can be seen. Continuous ECG monitoring is recommended, as well as the availability of atropine and other emergency drugs is ensured, because transient bradycardia may follow tachycardia termination (Figure 22.14). During short‐term pharmacologic therapy, backup pacing may be appropriate, depending on the drug and clinical status of the patient.

  

  
  
• Rapid atrial pacing can be performed with a transesophageal electrode catheter or via temporary atrial pacing wires. Beforehand, one must ensure that there is no ventricular capture by the catheter or temporary wires at the desired output site. Rapid atrial pacing is performed by pacing the atrium at 10–20% faster than the SVT rate for a period of up to 15 seconds, which typically terminates the tachycardia. In a patient with a high catecholamine state, SVT can be successfully terminated but rapid recurrence is possible. In this case, a higher level of patient sedation and limiting catecholamine stimulation should be considered.
  
• Anti‐arrhythmic medication can be instituted once the tachycardia has terminated or if it terminates and then reinitiates. For perioperative patients unable to take oral medications, parenteral therapy may include procainamide, sotalol, or amiodarone. β‐blockers and calcium‐channel blockers may be less desirable in such cases because of their negative effects on myocardial contractility; a short‐acting agent, such as esmolol, may offer a larger margin of safety.
  
• Transcatheter ablation may be warranted in patients with a history of SVT who would prefer not to take medications, or in those with incessant tachycardia that cannot be controlled with medications. Success rates for transcatheter ablation exceed 95%.