Cardiac dominance is determined by coronary aery:

In 85% of patients the right coronary aery (RCA) is said to be "dominant" because it supplies circulation to the inferior poion of the interventricular septum the right posterior descending coronary aery/posterior interventricular aery.In these cases the RCA travels to the cross-section of the AV groove and the posterior interventricular (IV groove). Here, it gives rise to the right posterior descending coronary aery (PDA) branch which travels in the posterior IV groove and gives off several septal perforator branches (SP). The SP supply blood to the lower poion of the IV septum. Generally, the dominant RCA also gives rise to the AV nodal branch which supplies blood to the AV nodeThe dominant RCA also provides the right postero-lateral (PLA) branch to the lower postero-lateral poion of the left ventricle.The sinus or sino-atrial (SA) node branch originates in the proximal poion of the RCA in 60% of cases and as a left atrial branch of the Cx in the remaining 40% of cases. This is unrelated to whether the aery is "dominant" or not.