Before they came the air was calm enough, Coming and going, breath by breath, without any fuss.
Apparently, before the tulips showed up, the air in the room was "calm." We're assuming it's the arrival of the tulips that makes everything go a bit wonky.
Also before the tulips, there was no "fuss." She could breathe easily enough. Sounds nice. So why'd those pesky tulips have to come in and mess everything up?
It's worth noting that there's a fight going on in this poem between calm images like this – moments of empty restfulness – and really scary moments where the speaker suddenly feels like she's drowning or dying.
Then the tulips filled it up like a loud noise.
Ever heard of something called synaesthesia? It's a big word, but it's worth knowing, because the idea is cool, and can help us understand this line. Basically, it's a kind of abnormal overlap in your senses. For example, someone experiencing synaesthesia might hear a sound and suddenly get a taste in her mouth, or read a word and suddenly see a color.
It's also a useful trick for creating memorable poetic images or similes, like this one. Flowers just don't make noise on their own. There's no getting around that. Here, though, the speaker tells us that these tulips were so bright, so intrusive, that they "were like a loud noise," as if the flowers were setting off fireworks in her clean, white hospital room.
Now the air snags and eddies round them the way a river Snags and eddies round a sunken rust-red engine.
This imagery is crazy cool, by which we mean it's both totally insane, and totally awesome.
She uses a simile to describe the air in the room flowing around the tulips as like a river running around a "rust-red engine." What a gorgeous line. The very air itself is swirling around, and she's building on the water imagery we've already seen: remember when she said she felt like a pebble in the water? Now it's the tulips that are submerged.
They concentrate my attention, that was happy Playing and resting without committing itself.
Ah, now we get the real issue. Finally. The real problem with the tulips is that they are interrupting the speaker's meditation. She's busy trying to enjoy the feeling of being empty and free of the world, but the tulips force her back into it.
How do they do this? They "concentrate her attention" which used to be free to wander.
There's an angry red splotch in her pure white room, and it's ruining just about everything. Stupid tulips.