Read an Excerpt
A Visual Guide for Nurses, Techs, and Fellows
By Paul D. Purves, George J. Klein, Peter Leong-Sit, Raymond Yee, Allan C. Skanes, Lorne J. Gula, Andrew D. Krahn
Cardiotext Publishing, LLC Copyright © 2012 Paul D. Purves
All rights reserved.
Unit 1: The Basics
In this unit we look at catheter placement, the computer system, signal processing, signal sequence in sinus rhythm, basic conduction intervals, and two basic but critical tissue characteristics:
Understanding these concepts is critical to understanding the mechanisms of most tachycardias.
Starting on page 18, you will find our methodology and the sequence of cardiac stimulation we use in a routine diagnostic study for SVT. Examining the normal sequence of signal conduction and measuring various basic intervals establishes a baseline for the patient. These measurements will vary depending on the clinical problem.
UNIT 1 OUTLINE
1: Catheter Placement 6
2: The Computer System 8
3: Signal Processing 10
4: Signal Sequence in Sinus Rhythm 12
5: Basic Conduction Intervals 14
6: Tissue Conduction 16
7: Supraventricular Tachycardia Diagnostic Study 18
1. Catheter Placement
Multi-electrode catheters are exclusively used in clinical E P. Electrical signals are detected and recorded from the individual electrodes and from adjacent electrodes (bipole). By convention, the most distal electrode (at the tip of the catheter) is numbered 1. Subsequent electrodes are numbered in a sequential fashion, as shown in the coronary sinus (CS) decapolar catheter here (labeled in green).
The high right atrial (HR A) catheter is labeled in red. It will record an atrial (A) signal on the red HR A channel on our tracings throughout this book. The H IS catheter is labeled in yellow. It is positioned in the region of the atrioventricular (AV) node guided by both the signals and fluoroscopic images. Since the AV node is in close proximity to both the atrium and the ventricles, it will have both an A signal as well as a ventricular (V) signal. In addition, the H IS catheter will have a third electrogram (E GM ) representing the bundle of His, which is the electrical conduit from the AV node to the specialized conduction system within the ventricles. This E GM is often refer red to simply as an "H." So the properly positioned H IS catheter will display three EGMs — atrial (A), HIS (H), and ventricular (V) — on our yellow HIS channel. The right ventricular apex (RVA) catheter is labeled in magenta. It will display a V signal on our magenta RVA channel.
The CS catheter is labeled in green. The CS is the vein situated between the left atrium and the left ventricle. Therefore, this catheter, for the most part, identifies signals from the left atrium and left ventricle when properly positioned within the CS. It is our custom that CS 9-10 should be positioned at the left edge of the spine in the left anterior oblique (LAO) projection. This "neutral" position places these poles at the junction of the right atrium and the CS opening. CS 1-2 will display signals from the lateral aspect of the left atrium. Most CS signals will have two components, A and V, on our green CS channel. Currently, we use a 10-pole (decapolar) CS catheter.
Commentary: Atrial activation sequence proceeds from proximal to distal in the normal heart assuming a normally positioned (neutral or central) CS catheter. If the CS catheter is not in the neutral position, atrial activation during ventricular pacing may appear eccentric and thus be very misleading (ie, mimicking APs). If the CS catheter is advanced too far into the CS, the signals may appear as a chevron (ie, CS 1-2 activation is as early as CS 9-10 activation). This early activation is due to detection of Bachman's bundle.
2. The Computer System
All intracardiac catheters are plugged into patient interface blocks via connecting cables. The electrical signals detected from the catheters are sent to these blocks. From here, the signals are sent to the amplifier and then on to the recording system and monitors.
How your computer/recording system displays these signals is quite customizable. The colors of individual E GM s, signal location on the screen, order of the catheters displayed on t he screen, gains, and filters are entirely modifiable.
In this image, we have our CS catheter plugged into the interface block on the far right. To see these signals, we instruct the computer to display Block A >> pins 1 to 10 >> and color them green. We then position these bipole signals (CS 1-2, CS 3-4, CS 5-6, CS 7-8, CS 9-10) at the bottom of our screen.
When troubleshooting, keep in mind that problems often stem from a "connectology" issue. Be sure catheters are connected securely to the connection cables, cables to the interface boxes, and boxes to the amplifier. Also check that channel gains are appropriate, filters have not been changed, and someone didn't turn off the signal display. It is not uncommon to accidentally plug the catheters into the wrong pins on these blue interface boxes and thus display no signals at all.
Commentary: Filtering of signals is explained in greater detail on page 10.
3. Signal Processing
How we see the signals on the monitor is a result of various processing procedures. You need to understand the concepts of gain, clipping, high-pass filters, and low-pass filters. This image shows the settings for electrocardiogram (ECG) filtering (channels 1–12) and three intracardiac channel settings (channels 33 – 48).
Gain simply increases the amplitude, or size, of the signal. It only amplifies whatever is on that channel, including (unfortunately) noise. Clipping is simply constraining the size of a signal to a geographic location on the monitor such that two adjacent signals don't overlap each other.
High-pass and low-pass filters are often confused. The high-pass filter allows any signals higher than the preset frequency (Hz) to be passed through to the monitor. Therefore, it filters out low-frequency signals, such as a wandering baseline. In this example, any intracardiac signal with a frequency higher than 30 Hz will be allowed to pass through to the monitor and any signal lower than 30 Hz will be blocked. The low-pass filter allows any signals less than the preset frequency to be passed through to the monitor. It, therefore, filters out high-frequency signals. In this example, any intracardiac signal with a frequency lower than 500 Hz will be allowed to pass through to the monitor and any signal higher than 500 Hz will be blocked.
The total effect is to allow signals between 30 and 500 Hz to be displayed. Signals outside this range are blocked from view.
The notch pass is a special and a relatively specific 60-Hz noise filter.
Commentary: Changing the high-pass filter on the ECG can markedly change its morphology. Try it and see the result. Highly filtered signals look "cleaner" but remove information, possibly important information. There is always a compromise between obtaining a clean signal and maintaining the information required. The filter settings displayed here are nominal and consistent across most recording systems.
4. Signal Sequence in Sinus Rhythm
1st: The HRA A electrogram (AEGM) is the earliest since this catheter is closest to the sino-atrial (SA) node. Note that it corresponds to the onset of the surface P-wave.
2nd: H IS A is the next A E GM to appear in sequence. The wave of atrial depolarization has successfully arrived at the AV node.
3rd: CS A signals appear next as the wave spreads from the proximal CS distally into the left atrium.
4th: The H deflection on the HIS catheter is next. This indicates that the wave has propagated over the atrioventricular (AV) node and arrived at the bundle of His.
5th: The RVA V electrogram (VEGM) is generally next as it is near to the right bundle branch exit, which is usually the first part of the ventricles to be activated. Note that it is before the HIS V or CS V signals since the wave of depolarization travels down the bundle branches, past the apex, and arrives at the base of the ventricles last.
6th: The HIS channel V and the CS V's are the last signals to occur. Remember that the H IS and CS catheters are positioned at the base of the ventricles.
Commentary: Take careful note that the RVA V signal is ahead of the HIS V and CS V signals in sinus rhythm since the right ventricle is activated early in normal depolarization without bundle branch block. On the other hand, the HIS or CS V signal may precede the right ventricle with right bundle branch block (RBBB), where the left ventricle will be activated before the right. This relationship could change if an AP is present and may give a clue as to the insertion site of the AP into the ventricles.
5. Basic Conduction Intervals
Routine baseline measurements should be made and recorded for every patient undergoing an EP study. The names and normal ranges are as follows:
P-A interval: Measure from the onset of the P-wave on the surface E CG to the rapid deflection of the A-wave on the H IS channel. (See the red calipers.) The P-A interval is usually about 35 to 45 msec. This is the trans-right atrial conduction time, that is, the approximate time it takes for the electrical signal to travel from the SA node to the AV node.
A-H interval: Measure on the H IS channel as the A signal to the onset of the H deflection. (See the white calipers.) The A- H interval is usually about 70 to 80 msec. This is the trans-nodal conduction time, or the time it takes for the electrical signal to travel through the AV node.
H-V interval: Measure from the onset of the H IS deflection to the earliest onset of ventricular activation on any channel available, either intracardiac or E CG . (See the green calipers.) It is usually the onset of the QR S complex. The H -V interval is about 35 to 45 msec. This is the His to ventricular activation time, that is, the time it takes for the electrical signal to travel from the His bundle to the ventricles.
Therefore, on a surface E CG , these three measurements make up the PR inter val:
PR interval = P- A + A- H + H -V
= 40 + 80 + 40
= 160 msec
Commentary: Careful measurement of these basic intervals is critical. You must know the baseline A-H in order to recognize subsequent AV nodal decrement and potential "jumps." Additionally, a long H-V interval indicates distal His-Purkinje disease.
6. Tissue Conduction
The key concepts to understand about tissue conduction are:
Tissue conduction velocity
Tissue conduction velocity refers to the speed at which the electrical signals travel between cells in a specific part of the heart. In contrast, the refractor y period measures recover y time of the tissue before it can conduct electrical signals again.
Tissues with rapid conduction time may or may not have short refractory periods and vice versa. The usual measurement we cite is the effective refractory period (ERP). For example, the ERP of the AV node is the longest atrial extra-stimulus (or A1-A2) that fails to conduct to the His bundle as the extra-stimuli are gradually decremented, that is, brought progressively closer to the last paced beat (last A signal of the eight-beat drive cycle). Another common measure that also essentially gauges refractoriness is the block cycle length. The block cycle length is determined via incremental pacing (pacing progressively faster in small increments) and is the longest cycle that blocks. This is usually referred to as the Wenckebach cycle length. A short block cycle length indicates tissue capable of sustaining rapid rates during a tachycardia.
Commentary: This image is a schematic illustration of the extra-stimulus technique using the example of an atrial stimulation site. Time is represented horizontally, and the passage of the impulse through the heart is represented vertically. The heart is paced at a defined rate (generally eight beats), designated S1. In successive runs, an extra-stimulus (S2) is made progressively more premature to the last S1 and thus "scans" the cardiac cycle. S2 is progressively decreased until it fails to capture the atrium.
The top row shows a drive with conduction occurring over a "fast" AV nodal pathway. The second row shows block of the impulse at the fast pathway with conduction over the "slow" AV nodal pathway. In the third row, the S2 is sufficiently premature to block to the slow pathway. The fourth row shows an S2 that is sufficiently premature to prevent atrial capture. In this way, the ERPs of the various structures can be determined (ie, the longest coupling interval that doesn't make it to the next level).
A = conduction through the atrium, AVCS = AV conduction system,
V = conduction in the ventricle, FP = fast pathway, SP = slow pathway,
NC = no capture (the atrial ERP)
7. Supraventricular Tachycardia Diagnostic Study
Incremental Ventricular Pacing
Our diagnostic study begins with incremental ventricular pacing to establish the retrograde block cycle length (Wenckebach cycle length) of the A -V conduction system. Pacing begins 100 msec faster than the patient's intrinsic rate and decreases by 10 to 20 msec every eight beats until 1:1 ventriculo-atrial (V-A) conduction is no longer maintained. We do not pace faster than 250 msec because ventricular tachycardia (VT) or ventricular fibrillation (VF) may be induced at higher rates. There are four basic questions to consider:
1. Did the pacing capture the ventricle? Inspect the E CG and E GM s to confirm that the pacing stimulus did indeed capture the ventricle.
2. Is there V-A conduction? Look at the RVA channel (magenta) and the HR A channel (red) and establish the relationship between ventricular activation and atrial activation. This example shows each VEGM followed by an AEGM, or 1:1 conduction (arrows). At any given pacing rate, there may be no V-A conduction at all (the A signals will have no relationship to the V signals) or there may be V-A conduction that is 2:1 or some other ratio.
3. What is the pattern of retrograde atrial activation? In the normal heart, retrograde conduction to the atrium occurs via the AV node so that atrial activation will occur here first. This is referred to as "central" or "concentric" atrial activation and is seen as the yellow H IS A being the first A seen following the large V. It is generally slightly ahead of any CS A . The proximal CS is close to the AV node and is thus activated next. That is, the CS AEGMs activate at CS 7-8 first, followed by CS 5-6, CS 3-4, and finally CS 1-2. This central pattern is the expected pattern when retrograde conduction is through the AV node. When the earliest A signal is not at the H IS A, the pattern is referred to as "eccentric" atrial activation, which suggests that there may be another connection between the ventricles and the atrium other than the AV node (ie, an AP).
4. Does the V-A time stay constant or become prolonged (decrements) as we increase the pacing rate? The expected behavior of the AV node is that conduction slows at higher pacing rates. Therefore, at higher rates, the V-A time should be longer if conduction is going through the AV node (due to its decremental properties). This is readily obvious on the CS channels. Most APs do not decrement. Therefore, the absence of V-A time prolongation at higher pacing rates suggests conduction using an AP.
Commentary: The fifth question is "What is the V-A time?" A long V-A time could indicate a retrograde slow pathway or a decremental AP. Since both demonstrate a long V-A time, making an accurate diagnosis of the tachycardia circuit may be quite challenging! More pacing maneuvers are required.
Excerpted from Cardiac Electrophysiology by Paul D. Purves, George J. Klein, Peter Leong-Sit, Raymond Yee, Allan C. Skanes, Lorne J. Gula, Andrew D. Krahn. Copyright © 2012 Paul D. Purves. Excerpted by permission of Cardiotext Publishing, LLC.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.