Our 1-month-old went to the ER because h

If his heart rate is running fast again, or he is not feeding, not waking properly, floppy, pale, blue around the lips, or breathing fast — go to the nearest children's emergency department now, or call your local emergency number; do not wait for the cardiology appointment and do not wait to see if it settles on its own. Which country or city are you in?

Why this is the first line. In a baby of this age a fast heartbeat that lasts, or a baby who "is not right" in any of those ways, is treated as needing assessment now. The same applies to the return signs the ER team usually sends you home with. The fact that he was seen and discharged is not a reason to sit it out tonight.

(1) What you already tried vs. what works better

Relying only on a home monitor. It is worth keeping, but on its own it does two things badly. It can read high from movement, fussing and poor skin contact — and either way it cannot tell you *what kind* of fast rhythm this is. That distinction is exactly what decides the next step, and only an ECG can make it. So treat the monitor as a diary, not a verdict.

Better, and about the same effort:

• Keep the log, bring it to cardiology. One line per event: time, what the monitor read, how long it lasted, what he was doing just before (feeding, crying, asleep, waking, after a feed, dirty nappy), and whether he looked unwell during it. Photos of the monitor screen after an episode are worth more than the reading alone. This is the single most useful thing you can hand the team, and it costs no clinic time to collect.
• Record a strip, not just the number. If the clinic has already given you a plan for when to record, follow it exactly — that plan belongs to them, not to us. If they have not, ask them at the next visit what they want recorded and for how long.
• Bring the monitor itself to the appointment, plus the box, so the team can check whether it is reading the baby correctly. A misreading device is a common finding and worth ruling out.
• Write down the questions while the night is fresh. The three that decide the next six months: what exactly was the rhythm, what is the plan if it happens again, and what must bring us straight back in. Take the answer in writing.

What cardiology will usually sort out — the same sequence for most newborns with a fast rhythm: a rhythm recording on paper, a longer recording if the episodes are intermittent, blood pressure, blood tests, an ultrasound of the heart, and a decision about whether anything needs to be done at all or only watched. Whether the fast rate starts in the heart itself or the heart is speeding up because of something elsewhere (fever or infection, low blood sugar, dehydration, anaemia, or a structural heart problem) is the question the tests exist to answer. I cannot tell you which applies to your son — that is the diagnosis, and it belongs to the team examining him.

(2) Constraints and avoid list

• No medicine changes at home. Nothing to start, nothing to skip, nothing to "give a bit extra" if a rate climbs, and nothing from anyone else, online or from a previous child. If a medicine is prescribed, changes go through cardiology.
• No powders, no supplements, no "natural" drops for a newborn with a heart rhythm question. Not from a shop, not online, not from a well-meaning relative.
• Do not buy prescription medicines online or share anyone else's. That includes antibiotics and anything described as being "for the heart".
• Do not let the monitor make you change anything — not feeds, not fluids, not rest, not whether you go to hospital. It is evidence to hand over, not a decision to make on.
• Do not restrict or push fluids or feeds to "test" what happens. Change nothing deliberately to provoke or test a rate.
• Do not use a home monitor reading as permission to stay home.
• Keep the household calm and smoke-free — no smoking or vaping anywhere near the baby or in the house, and keep other people from handling the baby if anyone has a chest infection this week.
• Do not chase the number on your own schedule. If you are alone with the school run and the elder or sibling needs you, that is a logistics problem, not a reason to delay a trip to hospital — ask for help with the other care rather than delaying.
• Don't re-check repeatedly. Re-reading the monitor every few minutes raises your anxiety and changes nothing; log it and stop, unless something looks wrong, in which case go.

(3) Decision tree — when to stop and get clinical help

Go to the emergency department now, or call your local emergency number:

• The fast heart rate comes back and has not settled within a few minutes of you picking him up and settling him.
• Any blue or grey colour around his lips, face or tongue.
• He is floppy, limp, hard to wake, or does not react to you the way he normally does.
• Breathing is fast, laboured, grunting, or he has pauses in breathing.
• He refuses feeds, or you cannot get him to feed at all.
• Fewer wet nappies than usual over the day, a dry mouth, or no tears.
• A fever of 38 °C or above, or a temperature that keeps dropping.
• Repeated vomiting, or vomiting that is green.
• He has a fit, or is stiff or floppy after being left alone.
• He looks pale and sweating, or the skin is mottled or blotchy.
• Anything at all that makes you say "that is not him". That instinct is on the list — you do not need permission to use it.

Do not go to the emergency department for: ordinary fussiness with normal feeding, normal colour, normal breathing, and him alert between episodes. Record it and put it in front of cardiology instead.

Bring with you if you go in: the monitor and its log, a note of the times and what he was doing, current weight if you know it, the number of wet nappies in the last 24 hours, immunisation status, and every medicine given including anything given before you left home.

What success looks like in the next 24 hours

Not a cure — a plan. In the next day you want:

• A written answer from cardiology to three questions: what the rhythm was, what to do if it recurs, and what brings you straight back.
• A complete event log handed over, so nobody re-asks you for a timeline you already lived through.
• A named door and a phone number for the follow-up, and a plan for who you call when the clinic is closed.
• A decision on whether the home monitor is worth continuing — it may not be, and that is a normal outcome, not a setback.
• You eating and drinking something and asking someone else to take one thing off your plate.

If those five are true tomorrow, the night went well, whatever the rhythm turns out to be.

I am not a doctor, and nothing here diagnoses your son or changes his treatment — everything above is home care, recording, and communication. Which country or city are you in, so I can point you to the right local door next time you need one?