*Note that the L and R prefixes are optional, but are sometimes used to indicate that a PL or PDA originates from the LCx or RCA, respectively (since they can arise from either).
Although a few different definitions are suggested to define dominance, dominance refers to which vessel (between the RCA and LCx) supplies the inferior myocardium, and in particular the PDA. Approximately 70% of people are right dominant, 10% left dominant, and 20% co-dominant.
Right dominant system
Left dominant system
Branch vessels supplying the lateral wall are called diagonal arteries if they arise from the LAD, and OMs if they arise from the LCx. In some patients, the left main coronary artery has a true trifurcation, in which case the middle vessel is referred to as ramus intermedius. If the very same vessel originates a few millimeters toward the LAD, it is called a diagonal. If it originates a few millimeters toward the LCx, it is called an OM.
An OM or a diagonal which originates very proximally on its parent vessel is sometimes referred to as a “high” diagonal/OM.
Some angiographers are very strict about calling ramus, and others less strict. For this reason, angiogram reports by different authors may appear at odds with one another when really it is just a matter of semantics.
Some angiographers might call the middle vessel ramus, and others might call it a high OM. Both are reasonable.
If an LCx branch is quite distal on the vessel, some angiographers prefer to call this a posterolateral branch, abbreviated (L)PL for left posterolateral (to distinguish it from an RPL branching off the RCA). There is no strict cut off for what defines a distal OM vs a PL.
