Restore life support
You need to run to the security station - the only difficulty here is the lack of oxygen. Leave the office, run through the ER and dispatch the crawling necromorphs - again, it's time to use the line gun's alternative fire or the flamethrower. When you reach the room with invulnerable monster don't fight - just use stasis on him and his buddies, use the Oxygen recharger and leave the room.
Run to the security station without looking back. To the left from the save station you'll find a room with life support console.
Create the poison
You will now need to go from the security station into the Imaging Diagnostics lab.
Activate the console and pick up the capsule. After a few seconds, the door to the right will open. Enter, and check the two crates in the dead end. Proceed along the corridor to the Cryogenics Lab. Use the Save station and help yourself to the things you'll find in the lockers to your left.
Go to Hydroponics
During the dialog be sure to set Isaac in the most advantageous position - your interest is the cryo chamber. When the regenerating necromorphs gets inside shoot at his groins, then attack its two remaining limbs in order to immobilize it for some time. You can also use stasis to slow him down even further.
Ignore the remaining necromorphs and head to the room separated from cryo chamber by thick glass and activate cryo chamber thus freezing the necromorph! In the very same room you will find a Power node. Now's the time to deal with the remaining necromorphs. Also be sure to check the containers around the area.
You will reach the tram station by going through the room with the console you used to activate cryo chamber. Off to the next chapter!