Learn Spanish (Latin America): Controlamos la respiración, la cual es fundamental. - We control the breathing, which is fundamental.

While you could absolutely use que here, la cual belongs to a slightly more formal or written register. In non-defining relative clauses (clauses providing extra information that are separated by a comma), using el cual or la cual makes your Spanish sound more polished and precise.

How do I know whether to use el cual, la cual, or lo cual here?

These formal relative pronouns must match the gender and number of the specific noun they refer back to. Because the pronoun refers directly to la respiración (a feminine singular noun), you must use la cual.

Could I use lo cual in this sentence instead?

Yes, but it would change the meaning! If you said Controlamos la respiración, lo cual es fundamental, the neuter lo cual would refer to the entire previous action. It would mean that the act of controlling your breathing is fundamental, rather than the breathing itself.

Why is it es fundamental and not está fundamental?

We use the verb ser (here conjugated as es) because being fundamental is an essential, inherent characteristic of breathing. The verb estar is reserved for temporary states, locations, or conditions.