This is a bit of fluff I wrote as a fill on Mass Effect Kink Meme in response to this prompt:

It always bothered me during ME3 where they go onto the Geth Drednaught that Shepard is not at all bothered by going in a corrider, no oxygen, multiple hull breaches, no gravity... when Shepard was actually spaced.

I don't know about anyone else, but I would at least be panicing inside my head, it would be too similar to the destruction of the original Normandy then the spacing itself.

So this anon wants to see Fem!Shepard actually starting to panic, at first it isn't noticable then EDI or Dr Chawkas who are monitoring her life signs (really they have the technology, wouldn't surprise me at all if any/all team members are monitored when they leave the ship) mention elevated heart rate. Shepard brushes it off and continues, manages to calm herself down only for the corridor to split. And the panic starts..

Bonus
Establised Fem!Shep/Garrus, Fluff maybe, comforting in his own Garrus-sie way
When the corridor splits its like Déjà vu when she was blown off her feet getting Joker into the escape pod

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The treble pitch of Shepard's voice and her sharp short breaths coming over the com link sounded just like they did when she was having one of her nightmares. Garrus twitched impatiently, listening to her struggle, unable to get to her.

Gasp. "Fuck ... I can't ..." Gasp. "Shit. Shit." Gasp.

When Shepard finally managed the docking mechanism, Garrus had his helmet off almost before the hold was pressurized. By the time the inner door opened, he had removed his gauntlets as well. He went straight to her and immediately pulled her hands away from where they were fumbling weakly with her helmet. His talons deftly popped the latches and helped her free her head. Her eyes were franticly skittering about, searching the area without really seeing anything until they lighted on his face. With serpent speed, Shepard's hands shot out and grabbed Garrus by the shoulders.

"Shh. Shh. I'm here. I'm not going anywhere. It's okay." He cooed at her. He put his hands on her hips and drew her as close as their bulky armor would allow.

Out of the corner of his eye, Garrus saw Tali slip off and find a terminal to keep occupied with, giving them some semblance of privacy. He thanked her in his head. The fact that Shepard was allowing him to coddle her this much with someone else present gave Garrus some measure of how bad it was.

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Garrus knew that Shepard felt like she couldn't breath when things got like this. It only made things worse; It felt like getting spaced. The first time this had happened, he had thought she really was suffocating. It was four days after returning from destroying the Collector base. They had docked at Omega and spent the interim working to get the ship repaired, the wounded tended and the med-bay restocked. It wasn't until day four that things had slowed down enough for them to take a moment to breath and celebrate the fact that they were still alive. Shepard had invited him up to her cabin for a private second time coming together was even better than the first. The nervous intensity had been replaced by relaxed joy. As they fell asleep tangled in the blankets together, Garrus felt contented and at peace.

The peace was short lived. She woke up in the night with a choked scream. She just scrabbled at her throat when Garrus asked what was wrong. He called for Chakwas, and told her Shepard was having an allergic reaction. When the doctor arrived, she made a quick assessment but administered an antihistamine shot just in case. The shot knocked Shepard out, effectively relaxing her and giving Chakwas a chance to do a more thorough examination.

"She's going to be okay," she informed the nervous turian.

"This is all my fault." Garrus groaned, mandibles twitching in anxiety.

"It's not an allergic reaction, Garrus."

"But- wait, you mean I didn't cause this?" He strode over too the doctor, looking down at the smaller woman and willing her to make sense.

Chakwas chuckled. "Oh, now I'm not sure I'd go that far, but it certainly isn't your fault. In a way, well, I wouldn't say this is a good thing, but... it's a good sign, maybe."

Garrus cocked his head to the side, expressing his confusion in a manner that translated easily between species.

"Have a seat. I'll explain." She side stepped around him and sank onto the sofa. "Oh, and if you could put some cloths on..."

A few minutes later, she started again. "It wasn't an allergic reaction, it was a panic attack."

"What?"

Chakwas tried to keep it simple. "Something triggered a fear response that was sudden and strong enough that she was overwhelmed."

"You're saying she's afraid of me." Garrus slumped.

"Stop being melodramatic. It's not you she's afraid of. Honestly, I've been expecting something like this for a while now. I'm surprised it took this long."

Garrus looked at her, still confused. "Please explain." He said flatly.

"When a ground crew is out, I watch their vitals on a screen in the med bay. I see when you're shields drop, how much damage you sustain, your heart rate and a dozen other things. I've learned to piece together a pretty good idea of how a battle is going using those stats. I notice that all too often one or all of you will lose your shields, start taking damage, but just keep fighting instead of taking cover."

"Yeah." Garrus said. "It's not always as easy as just taking cover. Sometimes the choice is stand and fight and maybe die or stop fighting and definitely die. If you kill all the enemies nearby, then you can take the time to find cover and tend your wounds."

"I know. In a way, Shepard's been wounded and fighting out of cover ever since Cerberus brought her back."

Garrus just looked at her, waiting for her to clarify.

"Do you remember what it felt like when you found out she had died?" Chakwas asked.

"Yes." Garrus didn't elaborate.

"Do you think dying was any less traumatic to her?"

He didn't respond but his mandibles and face plates drew in sharply.

"Facing grief like this isn't going to be easy. I think she's only now taking the time to tend her wounds." Chakwas said.

"With the Collectors dealt with, she has a break in the action." Garrus nodded, finally beginning to understand what the doctor had been talking around.

"Yes, but it's also more than that." Chakwas looked at him. "It wasn't until tonight, with you here, that she relaxed enough, even subconsciously, to face this. She had a break in the action. With you, she found cover."

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Back on the Geth dreadnought, Garrus held onto Shepard until she stopped shaking and her respiration slowed to a more normal pace. When she was ready, she drew back, not pulling away, just getting enough space that she could look at him.

"The CIC had been opened up like a tin can. When I was walking through it, I could see the stars out through the gaping hole and all I could hear was my own breathing."

Garrus had heard this story a dozen times now. It was part of the ritual. Shepard needed to face the thing that triggered the panic while in a rational mindset. She was too much of a badass to hide.

"God, Garrus." She breathed. "When that corridor split, it was as if I was back on the old Normandy just as the blast finally tore her in half and sent me hurdling into space."

He shuddered at the thought and pulled her close once again, running his talons through her hair and lightly scratching her scalp the way she liked. "Joker recommended a book on therapeutic massage. How about we kick some ass here, and when we get back to the ship, I'll show you what I've learned?"

Shepard chuckled and kissed the side of his neck. "You spoil me."

"No." Garrus replied. "I think you've earned it."