Dr G Sathananthan 109/2 Bluegrey Avenue, Stonefields, Auckland 1072, New Zealand (firstname.lastname@example.org )
First published online April 15, 2015
D ata have suggested that in vivo cardiac orientation has the greatest effect on the cardiac electric field, and, thus, surface electrical activity. We sought to determine the correlation between in vivo cardiac orientation using cardiac computed tomography (CT) and the electrical cardiac axis in the frontal plane determined by surface electrocardiogram (ECG).
Patients aged between 30 and 60 years old with a normal body mass index (BMI), who underwent CT coronary angiography between July 2010 and December 2012 were included. Patients with diabetes, hypertension, arrhythmias, structural heart disease or thoracic deformities were excluded. In vivo cardiac orientation was determined along the long axis and correlated with the electrical cardiac axis on surface ECG.
There were 59 patients identified, with 47% male, mean age of 49.9 years and a mean BMI of 22.39 kg/m 2. The mean cardiac axis on CT was 38.1 ± 7.8°, while the mean electrical cardiac axis on ECG was 51.8 ± 26.6°. Bi-variate analysis found no correlation between the two readings (Pearson r value 0.12, p=0.37).
We conclude, there is no simple relationship between the anatomical cardiac axis and the ECG determined electrical axis of the heart. The electrical axis of the heart, however, showed more variability, reflecting possible underlying conduction disturbances.
Due to the asymmetry of the heart, it has long been described in what is known as the ‘Valentine’ position, in which the heart is oriented vertically downwards. It defines the heart as a solitary organ and provides no reference point for its location within the chest. This description has since been found to be inaccurate, as we know the heart is positioned in a direction extending from the right shoulder to the left hypochondrium. The in vivo orientation of the heart takes into account its surrounding bony structures and is the best definition of true anatomical heart position. 1,2
Figure 1. Pathway of cardiac electrical activation
In 1951, Fowler and Braunstein noted a significant association between electrocardiographic and anatomical positions of the heart about the antero-posterior and longitudinal axes, but notably not along the transverse axis. This study used X-ray and angiocardiogram to assess anatomical cardiac position. 3 As seen in figure 1. cardiac electrical activation is generated from the sinus node located high in the right atrium and spreads throughout the atria to the atrioventricular node in the infero-posterior region of the interatrial septum. It then enters the base of the ventricle at the bundle of His and follows the left and right bundle branches along the interventricular septum. The pathway of activation described, in approximation, travels along the long axis of the heart. Therefore, an association between the electrocardiographic axis and the anatomical position along the long axis seems feasible. 4,5 The electrical cardiac axis on electrocardiogram (ECG), however, represents the mean direction of the electrical action potential during ventricular depolarisation.
A small canine study in 2005 by Arteeva et al. supported Fowler’s findings, and concluded that the orientation of the heart within the thorax affected the formation of the cardiac electric field on the body surface much more than the torso geometry. 6 The same year, Engblom et al. conducted a study using cardiac magnetic resonance imaging (CMR) that suggested there was no simple relationship between the electrical and anatomical axes of the heart. 7 Therefore, it remains largely unclear whether the electrical cardiac axis and the anatomical cardiac axis are entirely separate entities or whether they are, in fact, related.
We sought to identify in vivo cardiac orientation
along the long axis using computed tomography (CT) to calculate an individual’s true anatomical axis. We then subsequently assessed its correlation with their electrical cardiac axis determined on ECG in the
This was a retrospective study that included patients who underwent CT coronary angiography (CTCA) at three private radiology centres across Sydney between July 2010 and December 2012.
A Siemens SOMATOM Definition Flash dual-source 128-slice and a General Electric Lightspeed VCT 64-slice CT scanner were both used across the three radiology centres. Six CTs were performed using the 64-slice CT scanner while the remaining were all performed using the 128-slice CT scanner.
There were 59 patients identified as appropriate for the study. Patients aged between 30 and 60 years old with a normal body mass index (BMI) were included. Normal BMI was defined as between 18.5 kg/m 2 and 24.99 kg/m 2 according to the World Health Organization (WHO) guidelines. 8 Patients with diabetes, hypertension, arrhythmias, structural heart disease or thoracic deformities were excluded. Diabetes and hypertension were defined as any individual on one or more diabetic or anti-hypertensive medications, respectively. Arrhythmias referred to tachyarrhythmias only, and patients were excluded regardless of whether they were paroxysmal or chronic in nature. Patients who had previously had ablative procedures for these tachyarrhythmias were also excluded. Those with congenital or acquired structural heart disease, or thoracic abnormalities were also excluded.
Figure 2. Multi-planar reconstruction (MPR) view of the cardiac long axis
The CT scanning technique used was at the discretion of the radiographers and cardiologists present at the time of the scan. This often adhered to the scanning protocols available at each of the individual radiology centres.
In vivo cardiac orientation was determined along the long axis using Osirix image analysis. Multi-planar reconstruction (MPR) views were used to mark a point at the left ventricular apex and the centre of the mitral annulus. The line connecting these points was marked as the cardiac long axis, as seen in figure 2 .
A point at the most distal aspect of the sternum was marked as the xiphisternum. 3D volume-rendered images and axial views were used in conjunction to mark points along the vertebral column. Points were marked along the most posterior aspect of the vertebral foramen, as well as at both junctions of the ribs with the vertebral body at each vertebral level (figures 3 and 4 ).
Figure 3. Vertebral points
The angle of the long axis was measured and subtracted from the bony landmarks to give in vivo cardiac orientation along the long axis.
A standard 12-lead ECG performed in the frontal plane was obtained for each patient. The majority of patients had ECGs performed within 12 months of the CTCA. Three patients did not have recent ECGs and their most recent ECG was approximately five years prior to their CTCA.
The electrical cardiac axis was determined using the formula below. This formula uses leads I and aVF. The correlation factor in the formula counteracts the inaccuracy created by the difference in the electrical strength of aVF, which is a bipolar lead and lead I which is a unipolar lead. 9,10
chest with multiple bony and cardiac
Bi-variate analysis was performed using SPSS, version 21. The results are all presented as mean ± standard deviation (SD), followed by the range of values in brackets. All axes, both anatomical and electrical are displayed in degrees. The anatomical axis was calculated on Osirix image analysis in radians and subsequently converted to degrees using the formula below: