This lesson is about four numbers, and about the single decision they exist to inform: whether this is a call nine one one situation, a drive to urgent care situation, or a watch and reassess situation. Right now, most people make that decision on how bad somebody looks, which is a genuinely poor instrument. People look terrible and are fine, and people look fine and are decompensating, and the patient you know well is the one you are worst at assessing because you are frightened. Vital signs replace an impression with numbers you can act on, repeat, and hand to a paramedic. None of the four requires training beyond what is in this lesson, and the equipment for all four costs less than a phone. The catch, which we will return to, is that a number without a baseline is much weaker than people expect.

Three things you should be able to do by the end. First, state what normal looks like for pulse, breathing rate, temperature and oxygen saturation in an adult, so that a reading means something the moment you have it. Second, take all four at home with the right tools and in a repeatable order, because a routine you follow the same way every time is what makes the readings comparable. Third, and this is the one that changes outcomes, know what each abnormal reading should change about what you do next. A number you record and do nothing with has cost you time. The objective is not measurement. It is measurement that alters a decision.

A pulse oximeter is a clip for a fingertip that gives you two of the four vitals in under a minute, and reliable ones cost twenty to fifty dollars. Oxygen saturation in a healthy adult sits between ninety-five and one hundred percent. Ninety to ninety-four is a warning sign that warrants a call to a doctor. Below ninety is an emergency, so call nine one one. Buy a recognised brand rather than the cheapest listing, because inexpensive units are frequently inaccurate in exactly the range where accuracy matters. And know the limitations, because they are real. Cold fingers, poor circulation, nail polish and movement all produce false readings. Importantly, pulse oximeters can read higher than the true value on darker skin, which means a borderline reading deserves more concern rather than less. So treat the number as one input. If the reading looks reassuring and the person is struggling to breathe, believe the person.

Doing this in a fixed order is what makes it work under stress, so here is a routine. Sit the patient down and let them settle for a minute if the situation allows, because a reading taken thirty seconds after they walked upstairs tells you about the stairs. Clip the pulse oximeter on and leave it while you do everything else, since it needs a few seconds and gives you both saturation and pulse rate. Take a blood pressure with an upper-arm cuff, arm supported at heart height, cuff on bare skin. Then count breathing for a full thirty seconds and double it, and do it without announcing that you are doing it, because people change their breathing when they know it is being counted. Add a temperature. Then write all of it down with the time beside it. The written time is what makes the second set of readings useful, and it is the number people forget.

Take two minutes on this, because it is the question the whole lesson serves. Which single reading would change whether you call nine one one or drive to urgent care? There is no one right answer, and thinking it through is the exercise. For most people the honest answers are oxygen saturation below ninety, which is an ambulance; a pulse that is very fast or very slow in somebody who also looks unwell; and any altered mental state at all, which is an ambulance regardless of what the other numbers say. Then the harder half of the question. What is your own resting pulse, and your normal blood pressure? Most people do not know, and without that a reading of one hundred and ten is either alarming or entirely normal for that person, and you cannot tell which. Taking your household's vitals once, while everyone is well, is a ten-minute job that makes every future reading interpretable.

Recap. Vitals turn a guess, they look bad, into something you can act on, repeat ten minutes later, and hand to an arriving paramedic, and the trend across two sets is often more informative than either set alone. A pulse oximeter, an upper-arm blood pressure cuff and a thermometer cover all four numbers and belong in the home base layer. And know your own household's baselines, taken while everybody is healthy, because abnormal is defined relative to a person and not to a textbook. One closing caution: every one of these devices can be wrong, so if the numbers look fine and the person in front of you does not, trust the person and get help.