PDA (Posterior Descending Artery) & Coronary Dominance-A MDCT Coronary Angiographic Analysis of Anatomic Variations and Clinical Importance

Introduction: There is considerable variation in the arterial sup ply of diaphragmatic surface of the heart. Diaphrag m tic myocardial infarction is one of the more common consequences o f coronary artery disease. The aim of this study wa s to assess the incidence of anatomic variants of origin and termination of Post erior Descending Artery (PDA) and to determine the type of coronary dominance in North Indian population. Subjects and Methods: This prospective study was carried out on 50 routin e subjects of different age groups who came to the Department of Radiodiagnosis, King Geor ge’s Medical University UP, Lucknow in the year 201 02011 with known or suspected coronary artery disease. All the cases we re investigated on a 64 slice Multidetector Compute d Tomographic (MDCT) scanner, using retrospective Electrocardiographic gating. En deavour was made to determine the incidence of site s of origin and termination of PDA as well as to determine the type of coronary domina nce. Results: PDA arose from Right Coronary Artery (RCA) in 39 ( 78%) cases and from Left Circumflex (LCx) artery in 11(22%) cases. The PDA was found to terminate in the upper 1/4 of P sterior Inter-Ventricular Groove (PIVG) in 18 (36%) cases, in upper 1/2 of PI VG in 19 (38%) cases, in upper 3/4 of PIVG in 10 (2 %) cases and at the apex of the heart in 1 (2%) case. None of the female had termin atio of PDA at the apex. Termination of PDA could not be determined in 2 (4%) cases. Right dominance was seen in 36 (72%) cases, Left do minance in 11 (22%) cases. and Co-dominance in 3 (6 %) cases. Conclusion: Posterior descending artery most commonly terminate d fter traversing the upper half of posterior inte r-v ntricular groove. The incidence of left coronary dominance was more in females, it was approximately twice the incidence found in male s. Co-dominance was observed only in male subjects.


Introduction
There is a wide range of variations within normal anatomic distribution of Posterior descending artery (PDA). PDA arises from Right Coronary Artery (RCA) in right dominant and co-dominant hearts, while it arises from Left Circumflex (LCx) artery in left dominant hearts. PDA supplies inferior wall of heart and inferior third of the interventricular septum. The artery that supplies the PDA and a posterolateral branch determines the coronary dominance. [1] PDA is also known as Posterior Inter-Ventricular Artery (PIVA). The site of origin of PDA is variable. In approximately 70% individuals, it arises from RCA near the crux of the heart and ends by anastomosing with the Anterior Inter-Ventricular Artery (AIVA) [Left Anterior Descending (LAD) artery] of Left Coronary Artery (LCA). In approximately 10% individuals, it arises either as a branch or continuation of LCx artery. The PDA extends along the posterior inter-ventricular groove (PIVG), from the crux of the heart toward the apex. PDA passes forward along the interventricular sulcus for a variable distance toward the apex of the heart. When the PDA has a premature takeoff and then courses toward the cardiac apex along the diaphragmatic surface of the right ventricle, the variant is called early takeoff of the PDA. The concept of RCA or LCA preponderance was first proposed by Schlesinger M.J. in 1940. The term 'dominant' coronary artery was introduced by Schlesinger who used it to indicate the areas of heart supplied by each artery. Coronary dominance is based on the arterial supply to the diaphragmatic surface of heart by either right or left coronary artery. [2] The dominant coronary artery is that which gives the posterior interventricular branch, traversing the posterior interventricular sulcus and supplying the posterior part of the ventricular septum and often part of the ISSN (0): 2455-5274; ISSN (P): 2617-5207

Tomar et al; PDA & Coronary Dominance
posterolateral wall of the left ventricle as well. [3] According to Dr. Matt A. Morgan coronary artery "dominance" is defined in terms of which coronary artery gives origin to the PDA and the Posterior Left Ventricular Branches (PLVBs). Most persons (approximately 89.1%) have a "right-dominant" system, which means that the RCA supplies these arteries. Approximately 8.4% of persons have a "left-dominant" system, which means that the LCx artery supplies these arteries. Approximately 2.5% persons have "co-dominant" system in which the RCA gives origin to the PDA and the LCx artery gives origin to the PLVBs. [4] The artery that supplies the posterior descending artery (PDA) and the posterolateral Branch (PLB) determines the coronary dominance. If the PDA and PLB arise from the RCA, then the system is said to be right dominant. If the PDA and PLB arise from the LCx artery, then the system is said to be left dominant. If the PDA comes from the RCA and the PLB comes from the LCx artery, the system is codominant. [5] Branches of both RCA and LCx arteries run in or near the PIVG in balanced pattern (co-dominance) of coronary distribution. [6] Dominance has important implications in myocardial ischaemia and infarction, imaging of the coronary arteries (CTCA and invasive coronary angiography) and the planning for coronary artery bypass grafting. [7]

Materials
In this study, digital copies of CT coronary angiograms of 50 subjects of both sex and different age groups [32 males (14-75 years), 18 females (12-70 years); mean age 51.36±14.07 years, age range 12-75 years] were analyzed after taking due permission from Institutional Ethical Committee.

Method
This was an observational study. The sites of origin and termination of PDA were observed. Coronary angiograms were also observed for origin of PLVB. Coronary Angiography (CA) was done on patients came to the Department of Radiodiagnosis, King George's Medical University UP, Lucknow in the year 2010-2011 with known or suspected coronary artery disease.

Inclusion criteria
Patients presenting with symptoms and signs of cardiovascular diseases e.g. Chest pain and Dyspnoea.
(3) Allergy to contrast agent. (4) Contraindication to radiation exposure (e.g.pregnancy). \ (5) Uncontrolled Heart rate.  Table 1). Pre-procedure precautions • The subjects were enquired, to rule out the presence of any drug allergy to avoid the occurrence of any untoward anaphylactic reaction during the procedure. • Two days prior to the procedure the subjects were advised to avoid the intake of fatty food. • They were advised to drink only water just prior to the procedure. • Blood urea and creatinine levels were evaluated.

Procedure
The subjects were laid supine. Their heart rate was stabilized with an oral dose of 50-100 mg Metoprolol one hour before the scan. If heart rate was not stabilized with an oral dose, then intravenous (IV) Metoprolol was given. Electrocardiogram (ECG) and pulse rate were monitored half an hour prior to the procedure. The subjects were counseled to reduce their anxiety. The subjects were connected to a cardiac monitor. For venous access, an upper extremity vein (antecubital vein) and a 20-gauge intravenous canula was used. 80-85 ml of non-ionic contrast Iohexol (Omnipaque, GE, GE Healthcare Ireland, Cork) containing iodine concentration of 350 mgI/ml, injected with a flow rate of 5.5ml/sec, followed by a 20 ml saline flush at a rate of 4ml/sec with a pressure injector (PSI-325). The scan timing was determined with automated bolus tracking technique by placing the region of interest over mid ascending aorta and setting the trigger threshold to 180 Hounsfield (Hu). The subjects were asked to lie still on the "scanning bed" for a period of 5-10 minutes. The instruction was given to the subjects to maintain an inspiratory breath hold during which CT data and ECG tracings were taken. CTCA was performed 5 seconds after aortic peak density. Scanning coverage was from the level of carina to the bottom of the heart. Raw spiral CT data of coronary arteries were reconstructed in various phases of cardiac cycle on a work station (Brilliance 64 version 4.5) to obtain images with the highest quality (without motion artefact).Reconstruction performed at 75% of R-R interval was found to be optimal for image analysis in most of the subjects. In some, if heart rate could not be stabilized properly, then reconstructions were performed at 45% of R-R interval. The images generated were reconstructed and viewed utilizing a separate workstation which enabled generation of the coronary arteries in the standard and in various other anatomical planes as and when required and were interpreted with the help of a cardiac radiologist. Subjects with previous bypass surgery and also those with suboptimal study due to breath hold artefacts were excluded. All images were reviewed first in axial projection and then with post processing tools such as Multiplanar Reconstruction (MPR), Curved Planar Reformation (CPR), thin-slab Maximum Intensity Projection (MIP), and Volume-Rendering Technique (VRT) with transparent background display. MIPs were obtained using various thicknesses (5-30 mm). Volume-rendered (VR) images were also obtained using various orientations. CTCA images were observed for the origin and termination of PDA and origin of PLVB. The statistical analysis was performed by using software SPSS (Statistical Package for Social Sciences) version 15.0. The values were represented in Number (%) and Mean ± Standard Deviation (SD).    Figure 3a, b, c & d, Table 3].

(c) 3D VR Image showing termination in upper 3/4 of PIVG (d) 3D VR Image showing termination at cardiac apex [PDA(arrow)]. RA-Right Atrium, RV-Right Ventricle, LA-Left Atrium, LV-Left Ventricle, Ao-Aorta
In the present study, the coronary dominance pattern results were: Right Dominance in 36 (72%) cases [24 (75%) males and 12 (66.67%) females] [ Figure 1a&b], Left Dominance in 11 (22%) cases [5 (15.63%) males and 6 (33.33%) females] [ Figure 2a&b] and Co-dominance in 3 (6%) cases [3(9.38%) males][ Figure 4]. None of the female had co-dominance [ Table 4].   In left dominant hearts, LCX artery passes around the left atrio-ventricular sulcus and gives PLV arteries. It then turns towards the apex of the heart as PDA. In codominant hearts, RCA reaches the crux of the heart where it turns towards the apex and continued as PDA. In these cases, PLV arteries originate from distal portion of LCX artery. [8] The concept of coronary dominance is reflected by considerable variation in the blood supply to the inferior and posterior portion of the left ventricular wall. [9] If the circumflex branch of the left coronary artery terminates in the posterior inter-ventricular sulcus, then the heart is left dominant. If the posterior septum is vascularised either by descending branches from both the RCA and the LCx arteries, or by a network of small branches from these two passing obliquely, so that there is no posterior interventricular branch. In such hearts the circulation is said to be 'balanced', as the posterior inter-ventricular branch is either bilateral or absent. [10] In the past, several studies were conducted to determine the coronary dominance [ Table 5 & 6]. In all studies, frequency of right dominance is more than that of left dominance and co-dominance except the study done by Fazliogullari Z et al. which showed the incidence of co-dominance is more than that of right dominance [ Table 5]. [11] The incidence of right dominance in the present study is similar to the findings of studies done by Allwork Table 5 & 6]. [10,[12][13][14][15][16] In most of the studies done in the past, the incidence of left dominance is more than the incidence of co-dominance. But some studies showed the greater incidence of co-dominance over left dominance [ Table 5 & 6]. [2,[11][12][13][17][18][19][20][21][22] The findings of a study done by Bharambe V K and Arole V U showed similar incidence of left dominance and co-dominance[ Table  5]. [23] The incidence of left dominance and co-dominance is same in studies of Allwork [19,26,27] Left dominant hearts are at an increased risk of coronary heart diseases. [15] No case of co-dominance was reported by James, Kalpana R, Anbumani TL et al, Khona P and Ashwini C., Gebhard C et al [  [15,24,25,33] In the present study, the incidence of codominance was 6%, which is similar to the reports of Kate G.J.R. ten et al. [34] In the present study, Co-dominance was seen only in male subjects. Knaapen M et al found that the prevalence of a left dominant or codominant coronary system decreased with increasing age. [22] Goldberg A et al hypothesized that the prognosis of patients with Acute Coronary Syndrome (ACS) with Left Dominance (LD) would be worse than that of patients with right or mixed dominance. They concluded that in patients with ACS, Left Dominance is a significant and independent predictor of increased long-term mortality. [35] In a CT angiographic study, Eren et al found that the coronary diseases and the number and rate of coronary artery variations are significantly higher among the individuals with left dominant circulation. [13] Kuno T et al. observed that among ACS patients who underwent Percutaneous Coronary Intervention (PCI), patients with left dominance had significantly worse in-hospital outcomes compared with patients with right dominance, and left dominant anatomy was an independent predictor of in-hospital mortality due to heart failure, cardiogenic shock or cardiopulmonary arrest. [36] Murphy ES et al, found in patients of aortic stenosis with left dominance, an increased risk of perioperative myocardial infarction if there is associated obstructive coronary artery disease. Patients with left dominance have a shorter left main coronary artery than patients with right dominance. Preoperative information about the coronary arterial anatomy and extent of coronary artery disease may be helpful in planning the use of coronary perfusion and other myocardial preservation techniques during surgery in order to reduce the incidence of myocardial infarction. [37] In cases where patients are being considered for right coronary bypasses, it is particularly important to call the attention of the surgeon to the presence of any of the anatomic variations. These variations affect the decision regarding the nature of bypasses to be constructed. Where the vessels do not conform to classical anatomic conceptions, it may be impossible, unless one is forewarned to the atypical configuration. In cases of dual supply of inter-ventricular septum, multiple stenoses might necessitate construction of two different bypasses to provide adequate vascularization. [8]

Conclusion
In the present study, PDA most commonly arose from RCA and frequency of origin of PDA from LCX Ar. was more in females.There is no significant difference in the dominance pattern of coronary circulation among males and females.Posterior descending artery most commonly terminated after traversing the upper half of posterior interventricular groove and there is no significant difference in the termination pattern of PDA among males and females.