Hi,this is how the hits look from the net. Notice that a hyphen isn’t used.

1
ST segment depression is a nonspecific abnormality that must be evaluated in the clinical context in which it occurs. In a patient with angina pectoris
ST depression usually means subendocardial ischemia and, unlike ST elevation, is not localizing to a particular coronary artery lesion.

2
Correlation of remote [ST segment depression] and coronary anatomy during acute coronary occlusion

This is an example of an ATTRIBUTIVE NOUN.

From Concise Oxford Companion to the English Language.
An ATTRIBUTIVE NOUN is a noun that modifies another noun: steel in steel bridge; London in London house. Nouns used in this way are sometimes said to be adjectives or to behave like adjectives. They are generally not used predicatively: the bridge is steel is possible, *the house is London impossible. Phrases with attributive nouns are common, and are similar to COMPOUND NOUNS like teapot and coffee jug. However, such phrases have the same stress patterns as adjectival phrases (equal stress: a bíg hóuse, a Lóndon hóuse), but compounds take contrastive stress, emphasizing the first element in a pair: the WHITE Hóuse in Wáshington, rather than the whíte hóuse next dóor; COFfee pót as opposed to stéel brídge. In answer to questions or for emphasis, however, phrases with ATTRIBUTIVE nouns are contrastively stressed and sound like compounds: What kind of bridge is it?—A STEEL brídge. Noun compounds and phrases with attributive nouns can usually be paraphrased in the same way: A steel brídge a bridge made of steel; a coffee pot a pot for (making) coffee (in). An attributive noun can modify a compound (china in a china téapot) and one compound can modify another attributively (STRIKE commíttee modifying POLicy decísion in a STRIKE commíttee POLicy decísion). Such phrases are numerous and unpredictably creative, especially when they incorporate one or more loanword: alfresco staircase, glissando laugh, goy Zionist, Uzbek mafia. See EPONYM.