Tabby is still holding onto her teacher’s phone, though she’s not doing a great job of keeping it still. My current view as I cross the courtyard and take a left on Piccadilly is of the school crest on her royal blue sweatshirt and a segment of her plaited hair, long and blonde like mine.

‘Seventy-nine,’ Mrs Hopkins announces.

Fuck, fuck, fuck. Those oxygen levels are way too low. ‘Knees to your chest, sweetie. What were you doing when you started to feel bad?’ I ask Tabs as I speed-walk along Piccadilly. I’ll duck underground as soon as I’ve handed her over, but we’ll need to give the paramedics as much context as possible.

I get a shaky image of her pulling her bare knees up to her chest. By curtailing blood flow to the lower half of her body, her heart can focus on directing more of its limited supply to her lungs.

The TA’s voice sounds through my phone. ‘We were just rehearsing for the class assembly. The kids were doing a dance routine, and Tabby said she felt dizzy.’

Tabby’s condition, Tetralogy of Fallot, sounds like something from the Legend of King Arthur but is, in fact, a congenital heart defect. The pulmonary valve sending blood from her heart to her lungs was too narrow at birth, constraining her oxygen supply. She’s endured two open-heart surgeries to replace the valve, one at birth and one aged three, but she’s once again outgrown her current valve. Right now, it only takes a small amount of physical exertion to bring on what Tabs and I call a “blue spell”, where blood flow to the lungs decreases and oxygen levels plummet.

‘Got it. Mrs Hopkins, can you grab a fact sheet, please?’

‘Absolutely,’ I hear. ‘Don’t you worry. We’ve got this little pixie, don’t we, Tabs?’

Thank God for people like Mrs Hopkins, who treat everyone else’s kids with as much love and attention as their own. She keeps a stack of printed sheets that I’ve put together and filled with all of Tabby’s salient information—contact details for me, an overview of her condition and surgical history, her meds, and contact details for her cardiac team at Great Ormond Street Hospital, or GOSH, London’s preeminent and wildly under-resourced children’s hospital.

If my maths is right, we’ve handed over eight fact sheets since January, and it never gets easier.

The hours following my arrival at the hospital are the usual blur of doctors and machines and beeping. Tabby was given beta blockers and an oxygen mask when she was admitted, and both her oxygen saturation and heart rate have normalised, thank fuck. She’s had a chest X-ray as well as an ECG to assess the functionality of her pulmonary valve.