W.J.Wajszczuk, M.S. Moskowitz, T. Bauld, T. Pałko, J. Przybylski, P. Dabos, R. Weiss, M. Stopczyk, R. Żochowski, M. Rubenfire. Proceedings of “BIOSIGMA 78”, International Conference on Signals and Images in Medicine and Biology, Paris, April 24-28, 1978. Session C.IV: Non-aggressive methods for data acquisition, Communication C.IV.2

ABSTRACT

Mobile instrumentation and method were developed which allow noninvasive recording and instantaneous read-out of cardiac conduction system activity. Amplified and filtered precordial signal is digitized and averaged over 128 or 256 consecutive cycles. QRS signal triggers the acquisition and transfer of signal occur­ring during the preceding P-R interval. Conduction system potentials had amplitude of 0.5 to 4.0 μV. Instrumentation noise was less than 0.1 to 0.3 μV. Invasive recordings in animals and patients showed good correlation between the major His bundle deflections, but external recording showed additional activity deflections including small pre-P potentials. Bipolar anterior lead was used routinely, but X, Y, Z reference system provided supplemental information and X, Y plotting may facilitate pattern recognition in pathology.

INTRODUCTION

External recording of electrical activity originating in the cardiac conduction system was described indepen­dently in 1973 by Berbari et al.(l), Flowers et al.(2) and Stopczyk et al.(3). Subsequent studies by other investigators (4-6) as well as in our laboratory (7,8) confirmed feasibility of such recordings.

The amplitude of the potential from the bundle of His as obtained from the precordium is generally less than 10 μV. This is the same order of magnitude as the noise in the system arising from the combined influence of sources such as muscular activity, electrical noise from the environment and noise inherent within the instrumentation. Therefore, to extract the signal of interest, the His bundle potential, from the signal obtained from the precordium, high gain low noise signal amplification, filtering and averaging process have been employed.

The averaging process requires that the signal of interest be synchronous to other electrical activity in the cardiac cycle and averaging is triggered by a temporally stable signal for each of the averaged beats. Pacemaker spike or esophageal lead (3,5) deflections were initially used for stable triggering, but more recently adequate triggering with precordial QRS signal has been demonstrated.(5-11) Recently, we have also shown that in addition to detecting externally from the precordium the low amplitude signals origina­ting from the His bundle and its branches, the method allows intra-atrial recording of low voltage pre-P (S-A node region) activity. (12,13)

The purpose of this communication is to review the technique of QRS-triggered averaging, experimental correlations, lead selection and potential clinical applications as well as describe portable instrumen­tation and clinically applicable method which could be used for sequential studies.

METHOD AND INSTRUMENTATION

The method we employed utilizes the QRS complex following the His potential to trigger the acquisition and averaging sequence. The instrumentation is shown in schematic form in Figure 1.

Figure 1. Block diagram – see in the original publication

The signal from the precordial electrodes is amplified by the low noise differential amplifier (A-Princeton Applied Research, Model 113) by a factor of either 5,000 or 10,000. The bandpass filter contained within the amplifier is set for a 30-300 Hz band for rejection of both low fre­quency and high frequency components while allowing sufficient bandwidth for inclusion of the His signal with minimal distortion. The second identical pre-amplifier (B) with bandpass settings of 10-30 Hz and a gain of 1,000 to 5,000 is used to provide the trigger signal for acquisition of the data. The 10-30 Hz network acts as a differentiator and together with the threshold adjustment on the transient recorder (C-Biomation, Model 805 Waveform Recorder) provides a consistent trigger. The circuit triggers when the first derivative of the input ECG (QRS) exceeds the threshold value. The principle of QRS triggering and pre-trigger data acquisition is illustrated in Figure 2.

Figure 2. Principle of QRS triggering and of pre-trigger data acquisition - see in the original publication

The transient recorder (C) continuously digitizes data with a 4 μV resolution (referred to input) at a 5 KHz rate and stores the data digitally. Upon detection of the QRS trigger signal, acquisition process is halted and the contents of the digital memory containing data acquired during the preceding P-R interval are trans­ferred to a digital signal averager (D-Nicolet, Model 1072) with capacity for 1,024 18 bit words. After 128 or 256 beats are acquired and averaged, the process is terminated and the averaged signal is displayed on an oscilloscope and photographed with a Polaroid camera.

Figure 3.  Recording equipment in a mobile cart - see in the original publication

The gain  in the  final  display  is  generally 0.5  to 5 μV per vertical division of the oscilloscope and has a resolution of better than  0.1 μV.  It  should be noted that  the resolution of the averaged  signal  is  improved by the presence of some degree of noise  in the  input signal. The reason for this is that signal averaging is a statistical process based on the assumption of synchronization of the His potential with the QRS trigger signal and the lack of synchronization of all other noise sources with the trigger. Thus, although the initial digitization has only 4 μV resolution, averaging 256 repetitions allows for better than 0.1 μV resolution in the overall system. The instrumentation in its mobile cart ready for use at the patient's bed­side is shown in Figure 3.

EXPERIMENTAL CORRELATIONS

A-V Conduction System. Studies were performed in anesthetized dogs (Pentobarbital 35 mg/Kg) supported with a respirator. After mid-sternal thoracotomy, the heart was exposed and a multipolar electrode catheter was introduced via an incision in the tip of the right atrial appendage and advanced to the apex of the right ventricle under control of gentle palpation. A distal pair of electrodes were connected to the oscilloscope in a bipolar arrangement.

in a bipolar arrangement. Catheter was then gradually withdrawn. Upon appearance of deflections originating from the conduction system, recording was obtained and location of the electrode was verified by gentle palpation. At the end of the experiment, the right ventricle was opened and approximate course of catheter and location of electrodes were reproduced to verify the sites of recordings. No medications were given during the experiment. On occasion, transient prolongations of the P-R and A-H intervals were seen on the oscilloscope, most likely due to direct pressure over the area of the atrio-ventricular (A-V) node. The H-V intervals remained stable.

Figure 4. External recording and direct intracardiac recordings in dog

Figure 4 illustrates an example of catheter recordings along the course of the conduction system in the right ventricle. Upper panel includes 3 tracings: l)precordial reference bipolar low gain recording (top tracing) with the P wave in the center of the recording and the beginning of the QRS complex along the right edge of the illustration; 2) the averaged external high gain precordial His bundle recording (EHB) (middle tracing) shows multiphasic deflections originating from the conduction system; 3) the same precordial recording (bottom tracing) photographed with lower gain for identification of the P and QRS waves. The three lower tracings display bipolar recordings obtained along the course of the conduction system from locations indica­ted in the drawing of the heart on the left. The deflections of the direct recordings correlate with major deflections of the precordial averaged recording. The uppermost deflection (#1), representing activity of the proximal portion of the A-V conduction system, is projected on the downslope of the external recording. The middle tracing deflection (#2) corresponds to small deflection on the horizontal portion of external recor­ding and represents activity of the His bundle. The lowermost recording deflection (#3) coincides with negative deflection preceding the onset of the QRS in the reference lead and most likely represents activation of the terminal portions of the ventricular conduction system including His-Purkinje-myocardial junctions. Relatively long duration of deflections in bipolar recordings is related to inter-electrode distance of 1 cm.

Pre-P (S-A node region) activity. In a separate series of experiments, a multi-electrode patch with an inter-electrode distance of 4 mm was sutured in the area of the S-A node over the posterior aspect of the right atrium. Bipolar leads were studied for identification of earliest epicardial activity in the S-A node area. External averaged recording was obtained together with epicardial recording which displayed earliest activity (Figure 5A). An early low-amplitude deflection pre­ceding the P wave in the external recording (middle tracing) corresponds to early epicardial activity deflection (bottom tracing). External recordings in humans (Figure 5B) displayed on occasion similar early pre-P deflections. Since in most of our recordings the initial portion of the P wave was not included in the study, their incidence of detection is at present unknown. It appears that these deflections originate in the S-A node region but it is not known at present whether they represent activation of the S-A node itself or activation of the myocardium in the immediate vicinity of the S-A node. (14)

Figure 5. A - External recording and direct epicardial recording of pre-P (S-A node?) activity in dog. B - External recording of pre-P activity in man

LEAD SELECTION

 In most of our animal and human studies, a bipolar precordial lead (Y) was used, with electrodes located along the sternum, in the third right (negative) (3RICS) and fourth left (positive) intercostal space (4LICS) a few centimeters from the sternal edge. This lead approximates the course of the His bundle. On occasion, the positive electrode was moved farther away in the same direction and towards the apex (Y-1 lead). Since the A-V conduction system has a three-dimensional distribution, it was only logical to study it with the perpendicular system X, Y, Z. The X lead electrodes were located on both sides of the chest in the mid-axillary line at the level of the 4 or 5ICS. The Z lead electrodes were applied antero-posteriorly from the 4LICS location parasternally. Examples of recor­dings are shown in Figure 6. In this normal young subject, the X lead appears to have sensitivity superior to the other leads, but similar deflections can be identified in all leads.

Figure 6. External recording of cardiac conduction system activity with an array of perpendicular bipolar trans-thoracic leads (X,Y,Z)

Table I summarizes our experience in 84 patients in whom three (or at least two) orthogonal leads were studied. There is no clear-cut superiority of any of the individual leads. The Y or Y-1 lead appears to have the best yield if used alone, but significant supple­mental information is gained from other leads in most cases. On occasion lead Y+90°(perpendicular to Y over the anterior chest surface) allowed better detection of very early deflections following closely the end of the P wave (representing the A-V node?).  In cases of bundle branch blocks, the horizontal plane leads (X and Z) may be superior, possibly due to the fact that disturbance of conduction alters the sequence of activation to the highest degree in this plane. Frontal (Y+X) or sagittal (Y+Z) plane leads, because of their Y lead component approximating the course of the His bundle, may be best for detection of the abnormality of the A-V conduction (below the A-V node). On theoretical grounds, the Y+90° lead may be appropriate to study the A-V node since it approximates its anatomical course.

Explanation: The denominator indicates the total number of patients studied with this lead. The numerator indicates the number of patients in whom the best re­cording was obtained with this particular lead. 1°A-V = first degree A-V block, IRBBB = incomplete right bundle branch block, RBBB = right bundle branch block, LAFB = left anterior fascicular block, LBBB = left bundle branch block, Misc. = myocardial infarction with atypical intraventricular conduction delay.

As theoretically expected (due to the continuous nature of the conduction system) and in contrast to direct intracardiac recordings, it was noted that the external recordings frequently contained numerous deflections. Progress of activation which follows its three-dimen­sional course becomes altered and more complex in the conditions of intra-ventricular blocks.  It is postu­lated, and studies are in progress to determine whether the three-dimensional display (plotting of lead pairs in perpendicular planes), will facilitate grouping of curves into patterns typical for each pathological condition. Examples of displays obtained for a normal individual and a patient with acute myocardial infarc­tion and 2:1 A-V conduction block are presented in Figure 7.

Figure 8. External recordings during the course of experimenta1 myocardial ischemia in a dog.

Another example is presented in Figure 9.

Before direct recordings could be obtained, transient RBBB was produced inadvertently during placement of endo-cardial electrodes and was associated with sinus tachvcardia. The time interval between the end of the P wave and the onset of altered early QRS forces became shortened. After administration of propranolol, heart rate decreased and additional deflections are visual­ized prior to QRS. Above examples indicate the obvious need for detailed correlations with mapping of the conduction system activity, to allow precise identifi­cation of deflections and diagnosis.

Figure 9. External recordings in experimental right bundle branch block in dog

DISCUSSION AND SUMMARY

External recording allows detection of the activity of the cardiac A-V conduction system on the surface of the body. It is not known whether activity of the A-V node can be detected externally or separated from forces of atrial activation. Similarly, activation from the atrial pacemaker site can be demonstrated, however, it is not known whether activity of the S-A node proper can be detected.

Triggering with QRS appears to provide adequate syn­chronization without significant loss of information. Noise level originating from the instrumentation, environment and muscle activity can be effectively reduced well below the level of conduction system potentials.

The major problem now concerns proper identification of deflections, in particular in pathologic conditions of conduction disturbances. Experimental studies with simulation of pathology are needed to provide answers and correlations. Accordingly, new norms for A-H and H-V intervals will have to be developed.

It appears that due to the three-dimensional distribu­tion of the conduction system in the heart, a similar three-dimensional approach to recording of its poten­tials externally is most appropriate to prevent loss of significant information. The information appears to be supplemental in individual orthogonal leads. Due to the complexity of the anatomical structure and its distribution in the heart, scalar recordings may be difficult to interpret. Plotting from pairs of leads in three orthogonal planes may facilitate pattern recognition, in particular, in pathology of conduction (intra-myocardial blocks).

BIBLIOGRAPHY

1. Berbari, E.J., Lazzara, R., Samet, P., Scherlag, B.J.: Noninvasive technique for detection of elec­trical activity during the P-R segment. Circulation 48:1005, 1973.

2. Flowers, N.C,, Horan, L.G.: His bundle and bundle-branch recordings from the body surface. Circulation 7-8 (suppl IV):.IV-102, 1973.

3. Stopczyk, M.J., Kopeć, J., Żochowski, R. J., Pieniak, M.: Surface recording of electrical heart activity during the P-R segment in man by computer averaging technique. Int. Res. Com. Syst. (73-8) 11, 21, 2, 1973.

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13. Stopczyk, M.J., Wajszczuk, W.J., Żochowski, R.J., Rubenfire, M.: Human and Canine pre-P (sino-atrial node) activity recording from the right atrial cavity by signal averaging. Circulation (submitted for publication), 1978.

14. Masuda, M.O., Paes de Carvalho, A.: Sinoatrial transmission and atrial invasion during normal rhythm in the rabbit heart. Circulation Res 37:414, 1975.