SIGNIFICANCE OF VECTORCARDIOGRAM
IN CARDIOLOGY DIAGNOSIS
IN 21st CENTURY
Andres Ricardo Perez Riera et al.
Andrés Ricardo Pérez Riera1; Celso Ferreira Filho2; Adriano Meneghini3; Celso Ferreira 4; Edgardo Schapachnik5; Sergio Dubner6; Augusto H Uchida7, Paulo Moffa8.
1) Chief of the Sector of Electro-Vectorcardiography of the Discipline of Cardiology, School of Medicine, ABC Foundation – Santo André – São Paulo – Brazil.
2) Full Professor of the School of Medicine of Santo Amaro – UNISA – São Paulo – Brazil; Assistant Professor of the Discipline of Cardiology, School of Medicine, ABC Foundation – Santo André – São Paulo – Brazil.
3) Full-time Assistant Professor of the Discipline of Cardiology, School of Medicine, ABC Foundation – Santo André – São Paulo – Brazil; Chief of the Ergometer Sector of the Discipline of Cardiology, School of Medicine, ABC Foundation – Santo André – São Paulo – Brazil.
4) Full Professor of the Discipline of Cardiology, School of Medicine, ABC Foundation – Santo André – São Paulo – Brazil; "Livre Docente" Professor of the Federal University of São Paulo – São Paulo – Brazil.
5) Chief of the Department of Chagas Disease of the Dr. Cosme Argerich Hospital, Buenos Aires, Argentina.
6) Director of Arrhythmias and Electrophysiology Service, Clinical and Maternidad Suizo Argentina, Buenos Aires, Argentina.
7) Assistant MD of the Electrocardiology Service of the "Instituto do Coração" – HCFMUSP – São Paulo – Brazil.
8) Associate Professor of Cardiology and Director of the Electrocardiology Service of the "Instituto do Coração" – HCFMUSP
Address for correspondence:
Sebastião Afonso, 885 CEP: 04417-000
Jardim Miriam – São Paulo - Brazil
Phone: (11) 5621-2390
Fax: (11) 5506-0398
Key words: vectocardiogram – corrected orthogonal leads – sensitivity – specificity – accuracy.
Until the middle 80s, it was believed that the vectocardiogram presented a greater specificity, sensitivity and accuracy in comparison to the conventional electrocardiogram, in the diagnosis of the different heart diseases. However, more recent studies revealed that both methods present similar diagnostic abilities. The vectocardiogram still is superior to the electrocardiogram in very specific situations, such as in the evaluation of electrically inactive areas, in intraventricular conduction disorders combined and/or in association to inactive areas, in the identification and location of ventricular pre-excitation, in the differential diagnosis of patterns varying from normal of electrical axis deviation, in the evaluation of particular aspects of Brugada Syndrome, and in the estimation of the severity of some enlargements, among others.
With the advent of computerized vectocardiography, a technology that improves the determination of the onset and the end of loops, establishes a ratio between the length and width of the T waves and the spatial areas of the vectors; a future still promising is expected for this methodology.
The vector cardiogram is the spatial representation of electromotive forces generated during cardiac activity in the three spatial planes (horizontal, frontal and sagittal) 1. ( Figure 1)
P, QRS AND T – LOOPS OF VECTORCARDIOGRAM ON THREE PLANES
An instantaneous electric dipole (difference of potential between two points, of equal magnitude and opposite charges) is formed each moment during ventricular depolarization. The addition of all individual dipoles generates the resulting dipole of the cardiac electrical activity, moment to moment, represented by a vector.
Spatial vectocardiography is the form of electrocardiography that tries to describe the electromotive force developed by the heart each instant as a single vector, while all the successive instantaneous vectors have a common point of origin2.
Vector: Measurement unit that has direction or orientation and module, magnitude and intensity, used in electrovector cardiography to represent the dipole of depolarization and repolarization. All vectors have an onset and an end called origin and end. Vulgar terms such as "tail" or "arrow" to refer to the origin or end of a vector must be avoided.
The size of a vector determines the magnitude, the orientation and the direction in the electric field represented by it, while the point of the vector always indicates its positive side (as was originally advocated by Einthoven). Therefore, vector cardiographic loops represent the position of all the instantaneous vectors, at each moment, during cardiac repolarization, obtaining different loops for the P, QRS, T and U waves1.
Vector cardiography is based on the concept of the dipole as an approximation equivalent originated in the heart, and uses corrected orthogonal leads, which determine three spatial planes: frontal (FP), horizontal (HP) and left sagittal (LSP) or right sagittal (RSP). The term orthogonal originates in the fact that the axes of the three planes are perpendicular to each other, and corrected because technical devices of resistance and multiple connections that correct the deficient homogeneity of the electric field that surrounds the heart are used. These three corrected orthogonal leads of Frank's system, as well as the three planes determined by them, cross each other at a central point called E point, thus forming a 90º angle between each other.
Conventionally, the horizontal lead that stretches from left (0º) to right (+/- 180º) is called X orthogonal. The axis of the corrected X orthogonal lead corresponds approximately to the bipolar DI lead and the V6 precordial lead. This lead forms the HP and the FP.
The vertical lead is known as the Y orthogonal lead, and it stretches from down (+90º) to the top (-90º) and it approximately corresponds to the unipolar aVF lead of ECG, which has its positive pole in +90º. It provides information about the infero-superior orientation of the vectors. The Y lead forms the FP and the LSP or the RSP.
Finally the axis of the sagittal orthogonal lead known as the Z axis, stretches from the back (+90º) to the front (-90º) or postero-anterior orientation, with its posterior part being positive and its anterior part being negative. The Z orthogonal lead corresponds approximately to the precordial V2 lead of the conventional ECG and it forms the HP and the LSP or the RSP. ( Figure s 2 and 3 )
THE THREE ORTHOGONAL CORRECTED LEADS AND THE THREE PLANES ON VECTORCARDIOGRAPHY
Figure 3LOCATION OF ELECTRODES TO PERFORM VCG
BY THE FRANK METHOD
Advantages of VCG compared with ECG
1) VCG provides three-dimensional information of the electric activity of the atria and the ventricles, showing in a clearer way than ECG, the spatial orientation and the magnitude of the vectors at every moment3.
2) VCG has a greater sensitivity than ECG in detecting atrial enlargements4 and greater sensitivity and specificity than ECG in the diagnosis of left ventricular enlargement (LVE).
3) Abbott-Smith et al., made vectocardiograms in 100 patients carriers of left
ventricular enlargement (LVE) confirmed in the necropsy5 and concluded
that VCG was capable of diagnosing 50% of the cases, with 11.7% of false
positives. These figures point out a greater sensitivity of VCG over ECG in
the diagnosis of LVE;
4) VCG may clear doubts in the cases of suspicion of electrically inactive area in the septal or antero-septal wall of the left ventricle (LV), when the LVE of the systolic type is present, observed in ECG with QS pattern in V1; V1 and V2 or V1, V2 and V3. In absence of electrically inactive area, the "dashes" of the initial 10 to 20 ms of the QRS loop, are recorded without delay, while in the presence of electrically inactive area, the dashes of the initial 40 ms are very close to each other6.
5) VCG presents a greater correlation with the echocardiogram, when compared with ECG, in determining the left ventricular mass7, and it appears as superior to ECG and echocardiogram in the diagnosis of chamber enlargement, associated to electrically inactive areas8.
1) VCG presents a greater diagnostic sensitivity in comparison to ECG in acute myocardial infarction (AMI), when associated to left anterior fascicular block (LAFB). In the presence of AMI of the LV inferior wall, VCG may bring additional information, which ECG does not reveal, such as the association with LAFB10.
2) VCG presents a greater sensitivity and specificity than ECG in the diagnosis of strict dorsal AMI, and it enables a more appropriate differentiation with other causes of prominent anterior forces, such as normal hearts with counterclockwise rotation of the longitudinal axis and shift to the right of the transition area in precordial leads, right ventricle enlargement, complete right bundle branch block (CRBBB), hypertrophic cardiomyopathy both in its obstructive and non-obstructive form (increase in the magnitude of the septal vector), diastolic enlargement of the left ventricle with dislocation of the transition area to the right, ventricular pre-excitation of the Wolff-Parkinson-White type with anomalous bundle, in a parallel way to posterior location (WPW type A), Duchenne-Erb myopathy or malignant in childhood and other causes11; 12.
3) VCG has more sensitivity than ECG for the diagnosis of multiple infarctions, associated to LAFB13.
4) VCG has a greater accuracy than ECG in the diagnosis of inferior infarction10, however, about this topic, there is no consensus since there are studies that concluded that VCG is not superior to ECG in the diagnosis of isolated diaphragmatic infarction14. Edenbrandt et al, compared the diagnostic value of both methods in 65 patients with inferior AMI proven by hemodynamic study and gammagraphy with Thallium 201. The authors observed that the sensitivity of the VCG was 69%, and the ECG was 43%, with this difference being statistically significant (p<0.001). In the control group, only three false positives occurred15.
1) The method improves sensitivity in the diagnosis of inferior infarction extended to the LV anterior wall16 (called deep septal infarction).
2) VCG is of great significance for the diagnosis of left septal fascicular block (LSFB)17; 18; 19. This type of left fascicular block was shown in numerous publications and, in an unexplainable way, the Anglo-Saxon literature does not acknowledge it.
3) VCG is superior to the ECG in the cases of atypical CRBBB associated to LAFB (bifascicular block) called by Rosenbaum "standard masquerading bundle branch block". In these cases, in the presence of CRBBB associated to a high degree of LAFB, the DI lead presents small or non-existent s wave, with a pure R wave appearing in this lead, characteristic of CLBBB (pseudo CLBBB). This situation translates the presence of CRBBB associated to LAFB, LVE and block located in the left ventricle20.
In some cases, a CRBBB pattern is observed in the right precordial leads and CLBBB in the left precordial leads. This situation was called "masquerading bundle branch block". This pattern defines the presence of CRBBB associated to severe LVE, a block located in the antero-lateral wall of the left ventricle and usually LAFB21.
4) VCG is very useful to differentiate the rare CLBBB with extreme deviation of SAQRS to the right in the FP (to the right of +90º). According to the location of SAQRS in the FP, the CLBBB was divided into 4 types:
a) CLBBB with SAQRS not deviated: between –30º and +60º. It represents 65% to 70% of the cases;
b) CLBBB with SAQRS with extreme deviation to the left: beyond –30º. It represents 5% of the total;
a) CLBBB with SAQRS deviated to the right between +60º and +90º. It represents 4% of the total;
b) CLBBB with SAQRS presenting extreme deviation to the right: >+90º. This group represents less than 1% of the total of CLBBB and was called "paradoxical type" by Lepeschkin.
CLBBB with SAQRS located to the right of +90º in the FP, may have SAQRS located in the right inferior or right superior quadrant. Lepeschkin called them "paradoxical CLBBB" or type IV (SAQRS between +90º and +135º). We could add a type V when SAQRS is located to the right of +135º (CLBBB of congenital heart diseases). In these cases,VCG is superior to ECG in determining the possible cause:
a) If CLBBB is associated to severe RVE;
b) If fascicular CLBBB (LAFB + LPFB) by a higher degree of block in the left posterior fascicle;
c) If CLBBB is associated to lateral electrically inactive area.
1) The technique known as Continuing Vectocardiography Monitoring (CVM) carried out during elective angioplasty, proved to be a promising tool to detect patients with an increased risk of developing AMI related to the procedure. Guo et al22, used the method in 169 patients, which started 5 minutes before the procedure and was interrupted 30 minutes after the first insufflation of the angioplasty balloon. Considering the ST segment elevation to determine the AMI, the sensitivity of the CVM to detect increased risk of acute myocardial infarction related to the procedure was 93%, the specificity was 56% and the negative predictive value 99%.
1) VCG presents a greater diagnostic sensitivity than ECG to determine the severity of congenital aortic valve stenosis. Thus, the presence of the maximal vector in the horizontal plane to the left (LMSV) with a voltage greater than 4 mV, heading to the left and backward around –56º, represents a significant marker of severe aortic stenosis (left intraventricular pressure > 200 mmHg); the presence of the maximal vector to the left with a voltage near 2.2 mV and around –19º, indicates mild congenital aortic stenosis23.
2) In patients carriers of congenital pulmonary valve stenosis, VCG has a good correlation between the value of the systolic pressure of the right ventricle and the presence of the maximal spatial vector to the right of the HP: "Maximal Spatial Voltage directed to the Right" (RMSV). Thus, a right intraventricular pressure >100 mmHg has a RMSV >2.3 mV24.
3) VCG is superior to ECG to identify and locate the anomalous bundle in pre-excitation of the Wolff-Parkinson-White. The method presents a high sensitivity and accuracy. This fact is relevant to guide the electrophysiologist, pointing the most appropriate site to apply radiofrequency energy25. The diagnostic specificity is not increased when compared to an ECG in this case3.
4) VCG presents greater sensitivity and specificity than ECG in the diagnosis of end conduction delay by the fascicles of the right branch (blocks of the right branch: fascicular, zonal or of the free wall). VCG enables to rule out or confirm the cases where ECG presents a doubt when there is association of end delay through the right branch with electrically inactive areas, both of the inferior and the anterior walls26.
5) VCG optimizes the differential diagnosis of right fascicular blocks with left fascicular blocks27.
1) V ectocardiogram is very useful in the diagnosis of Brugada syndrome when ECG shows extreme deviation of SAQRS to the left in the FP (9.5% of the cases)28. We showed that in this entity, the extreme deviation of SAQRS to the left might be the consequence of LAFB and of end conduction delay through the superior or subpulmonary fascicle of the right branch, which goes through the right ventricle outflow tract, the area affected in this entity29.
2) VCG has a great value in the analysis of electrical modifications that are the consequence of septal percutaneous ablation of the obstructive form of severe hypertrophic cardiomyopathy, not responsive to drugs and with incapacitating symptoms (functional class II and IV), by injection of absolute alcohol. The result of septal or antero-septal infarction generates a pattern of CRBBB in almost all cases, unlike myotomy/myectomy surgery, which promotes CLBBB in approximately 80% of the cases30.
With the use of computerized VCG, obtaining and processing graphs is easier, and the problems of measuring the loops are eliminated, since it is possible to determine with greater accuracy, where each one begins and ends, establishing in a precise way, the ratio of length and width of T waves, and the estimation of the areas of the loops. In comparison with the traditional recording method, computerized VCG has a greater accuracy in measurement, besides a great processing velocity31;32.
In spite of the studies that show that VCG and the 12-lead ECG have a very similar diagnostic capacity, when ECG has a specialized interpretation33, VCG is still evolving and it will always have didactic usefulness to teach electrocardiology due to its three-dimensional representation, besides representing a simple and low-cost method, with great diagnostic value in different situations where conventional electrocardiographic recording is doubtful34.