Basic intervals during sinus rhythm represent a quantitative assessment of the electrical activation of the heart. Normal atrial activation begins at the sinus node, spreads to the low atrium and atrioventricular (AV) junction, and then to the left atrium ( Fig. 6.1 ). Occasionally, the low-right atrium is activated slightly later than the atrium recorded at the AV junction. Activation of the left atrium is mainly through the region of the central fibrous trigone at the apex of the triangle of Koch but also occurs through Bachmann’s bundle, the midatrial septum at the fossa ovalis, and the coronary sinus. In the presence of a normal QRS duration, the normal activation times from the onset of ventricular depolarization to the electrogram recorded from the catheter placed near the right ventricular apex are 5 to 30 ms ( Fig. 6.2 ). The right ventricular (RV) free wall at the insertion of the moderator band into the anterior papillary muscle (apical third of the RV free wall) is typically the earliest site of RV activation and precedes the apical septum by 5 to 15 ms. This depends on if there is continuation of the right bundle branch (RBB) toward the apical septum (<25%) or if the apical septum is activated from Purkinje fibers coming off the RBB in the moderator band. Left ventricular endocardial activation begins at 0 to 15 ms after the onset of the QRS, and the duration of left ventricular endocardial activation ranges from 28 to 50 ms. The midseptum and the inferior wall adjacent to the midseptum are the earliest areas of left ventricular endocardial activation, followed by the superior-basal aspect of the free wall. Activation then spreads radially from these breakthrough sites to activate the apex and then the base at the inferoposterior wall.
Basic intervals during sinus rhythm are measured on the His bundle recording electrograms in sinus rhythm at the usual recording speed of 100 mm/s, although a speed of 200 mm/s may also be used for better accuracy ( Fig. 6.3 ). The most important criterion for reliable measurement is reproducibility. Care should be taken to ensure catheter stability during recordings by means of continuous verification against electrogram characteristics and, if needed, stored fluoroscopic or electroanatomic images
The AH interval represents conduction time from the low-right atrium at the interatrial septum through the AV node to the His bundle. It is measured from the earliest reproducible rapid deflection from baseline of the atrial activation to the earliest rapid deflection of the His bundle depolarization (see Fig. 6.3 ). Because the exact point within the atrial electrogram when the impulse encounters the AV node is not known, the AH interval is an approximation of AV nodal conduction time. The normal range is 55 to 150 ms. The AH interval is sensitive to autonomic tone. A prolonged AH interval may indicate AV nodal disease or high vagal tone, whereas a shorter than normal AH can occur during sympathetic activation.
The HV interval represents conduction time from the proximal His bundle to the ventricular myocardium. It is measured from the earliest rapid deflection of the His bundle depolarization to the earliest onset of ventricular activation recorded from multiple surface electrocardiography (ECG) leads. If only one ECG channel is available, a V1 or V2 lead should be used because the earliest ventricular activity is usually recorded in one of these leads in the presence of a narrow QRS. Occasionally, however, and especially in the presence of a septal infarction, ventricular activation can occur before the onset of the QRS. Because anatomically the proximal portion of the His bundle begins on the atrial side of the tricuspid valve, the most proximal His bundle deflection is that associated with the largest atrial electrogram. Thus even if a large His bundle deflection is recorded in association with a small atrial electrogram, the catheter must be withdrawn to obtain a His bundle deflection associated with a larger atrial electrogram. This maneuver can on occasion markedly affect the measured HV interval and can elucidate otherwise inapparent intra-His block. In the absence of preexcitation, the normal HV interval is 35 to 55 ms. Thus during sinus rhythm an apparent His deflection with an HV interval of less than 30 ms either reflects recording of a bundle branch potential or ventricular preexcitation. An apparent HV interval less than 35 ms in the absence of an atrial depolarization indicates that the presumed His bundle depolarization is actually an RBB potential, whereas a prolonged HV indicates infranodal AV conduction disease (see Chapter 9 ). Unlike the AH interval, the HV interval is not significantly affected by variations in autonomic tone.
The QRS duration is measured on the surface ECG from the beginning of the q or R wave to the end of the QRS. Normal QRS duration is 120 ms or less.
Incremental pacing and the introduction of programmed single or multiple extrastimuli during sinus or paced rhythms are essential for electrophysiology procedures. They are used to characterize the properties of the AV conduction system, detect arrhythmogenic substrates such as accessory pathways, assess inducibility of tachycardias, and provide clues to the facilitate the mechanism of tachycardia. Usually, ventricular pacing maneuvers are conducted before atrial pacing ones because the latter may induce atrial fibrillation. Autonomic manipulation with drugs such as isoprenaline or atropine may be needed to facilitate conduction and tachycardia induction.
Continuous pacing is delivered at a constant rate at a cycle length of 400 to 600 ms at atrial or ventricular sites.
Programmed electrical stimulation refers to pacing that is performed at a constant basic drive cycle length, most commonly 500 or 600 and 400 or 350 ms, for eight pulses (S1–S1), followed by the introduction of an extrastimulus (S2) and with stepwise reduction in the S1–S2 interval by 10-ms decrements until tissue (A or V or atrioventricular node [AVN]) refractoriness or S1–S2 of 200 to 220 ms is reached. Stimuli of 1 to 2 ms duration are delivered at twice the diastolic threshold. Double, triple, or more extrastimuli may be necessary for the induction of the clinical tachycardia.
Ramp pacing implies bursts of pacing with a gradual decrease of cycle length with each interval. It is used to induce or interrupt tachycardias.
The refractoriness of a cardiac tissue can be defined by the response of that tissue to the introduction of premature stimuli.
The effective refractory period (ERP) of a cardiac tissue is the longest interval of the input into a part of the conduction system that fails to propagate through that tissue. For example, the atrial ERP is defined by the longest S 1 S 2 interval that fails to capture the atrium.
The A 1 A 2 interval of an atrial pacing beat that fails to display a His or V electrogram represents the AV nodal ERP.
The relative refractory period (RRP) is the longest coupling interval of a premature impulse that results in prolonged conduction of the premature impulse relative to that of the basic drive. Thus the RRP of the atrium is the longest S 1 S 2 interval at which the S 2 A 2 interval exceeds the S 1 A 1 interval.
The functional refractory period (FRP) of a cardiac tissue is the minimum interval between two consecutively conducted impulses through that tissue. Because the FRP is a measure of output from a tissue, it is described by measuring points distal to that tissue. It follows that determination of the ERP of a tissue requires that the FRP of more proximal tissues be less than the ERP of the distal tissue; for example, the ERP of the His-Purkinje system can be determined only if it exceeds the FRP of the AVN. Thus the FRP of the AVN is the shortest H 1 H 2 interval in response to any A 1 A 2 interval.
Atrioventricular conduction curves
Atrial pacing maneuvers are used to study the properties of the AVN and to assess the inducibility of atrial arrhythmias.
Atrial pacing is most commonly performed from the high-right atrium in the region of the sinus node or from the coronary sinus. Programmed electrical stimulation is performed at a cycle length (S1–S1) of 500 to 600 and then 400 ms with an extra stimulus delivered after eight paced beats and at progressively shortened intervals (S1–S2), in 10- to 20-ms decrements, to a minimum of 200 ms or until atrial or AV nodal refractoriness is reached. Although there are several patterns of response to programmed atrial extrastimuli, characterized by differing sites of conduction delay and block and the coupling intervals at which they occur, the most common pattern is seen when the atrial impulse encounters progressively greater delay in the AVN without any change in infranodal conduction ( Fig. 6.4 ). Block eventually occurs in the AVN or the atrium itself. Thus the normal response to atrial pacing is for the AH interval to gradually lengthen as the cycle length is decreased until AV nodal block occurs. This is the typical decremental behavior of the AVN ( Fig. 6.5 ). Decremental conduction may also be seen with slowly conducting posteroseptal accessory pathways (APs), atriofascicular bypass tracts, and nodofascicular/nodoventricular bypass tracts.