I think that of your two possible ways of parsing this, the former is more likely: in other words it is:

CX dominada — con lesiones, and not
CX — dominada con lesiones

This seems to me to fit the sequence in your document, where it takes each vessel and then reports its state:

TC — sin lesiones.
DA — calcificada — sin lesiones significativas.
CX — dominada — con lesiones difusas en segmento proximal y medio.
CD — dominante — calcificada con suboclusión

So the phrases "sin lesiones" and "con lesiones" are reporting on the state of the vessels in question. The CX dominada has lesions.

Another point militating strongly against reading it as "dominada con lesiones" is that, as Liz says, it would be "dominada por" if it meant "dominated by". I can't find a single instance of "dominado/a con" in this sense and I don't think it would be said.

Speaking of an artery as "dominada" is very unusual, but not quite unprecedented. In this PowerPoint on locating the artery responsible for a infarction, which is pretty well incomprehensible to a layman like me, we do have reproductions of what I take to be ECG output (examples, not all from the same patient, I think) with features marked corresponding to different vessels, which include "circunfleja dominante" and "circunfleja dominada": not "dominada" by anything, just "dominada". It's slide 16 of 20:
http://cardioatriofiles.s3.amazonaws.com/Electrocardiografía...

So I think "dominada" and "dominante" are equivalent but antithetical terms. Obviously CX dominante would mean dominant circumflex. This patient, however, has a dominant RCA (as most people do), so he/she has a non-dominant CX, and I think that's most probably what "dominada" means. It's a little strange but not totally unreasonable, since a dominant RCA means that the RCA, not the CX, supplies the PDA, and in this situation the CX could, in a sense, be described as "dominated" by the RCA.

Anyway, there's no doubt that "non-dominant CX" or "non-dominant circumflex" make perfect sense in themselves, and there's hardly any need to quote examples.

"Coronary artery dominance
The artery that supplies the posterior descending artery (PDA) determines the coronary dominance.
If the posterior descending artery is supplied by the right coronary artery (RCA), then the coronary circulation can be classified as "right-dominant".
If the posterior descending artery is supplied by the circumflex artery (CX), a branch of the left artery, then the coronary circulation can be classified as "left-dominant".
If the posterior descending artery is supplied by both the right coronary artery and the circumflex artery, then the coronary circulation can be classified as "co-dominant"."
http://en.wikipedia.org/wiki/Coronary_circulation#Coronary_a...

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Note added at 11 hrs (2015-03-08 18:52:56 GMT)
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Another case of "dominada":

"En dicha prueba se detecta una estenosis distal en el Tronco común del 20%, una Arteria circunfleja dominada con una estenosis media del 70% y una Arteria coronaria derecha dominante con una oclusión proximal (Enfermedad de tres vasos)."
http://www.capitulodeflebologia.org/beca_leo/2010/1005030056...

Compare, for example:

" Tronco de la coronaria izquierda (CI) con lesión en cuerpo y distal [...].
• Arteria descendente anterior (DA) con lesión ostial en el segmento proximal [...].
• Arteria circunfleja (Cx) no dominante, sin lesiones significativas en el segmento proximal, con lesión ostial y proximal de la primera marginal [...].
• Coronaria derecha (CD) dominante, con lesión ostial del 60% [...]."
http://www.scielo.org.mx/scielo.php?pid=S1405-99402012000200...

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Note added at 11 hrs (2015-03-08 19:03:29 GMT)
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Hi Joseph. Well, I think it's plausible. It would be great to have a Spanish-speaking cardiologist's view.

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Note added at 11 hrs (2015-03-08 19:04:29 GMT)
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PS. Just noticed that my last ref. was found by Liz too.

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Note added at 341 days (2016-02-12 21:10:03 GMT) Post-grading
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That's great!