How to read a patient monitor, step by step

If you want the meaning of each number first, start with what the numbers on a hospital monitor mean. This page is about the routine: how to take in the whole screen without missing anything.

1. Look at the patient before the screen

A monitor measures signals, not people. Someone talking to you in full sentences does not have a saturation of 70, whatever the display says. Start with colour, breathing and how awake they are. Then turn to the screen and ask whether it agrees with what you just saw.

2. Check that every curve is a real signal

Every number on the right is calculated from a curve on the left. If the curve is rubbish, so is the number. Run down the waveforms quickly:

• ECG: a steady baseline with recognisable complexes, not a wandering or fuzzy line.
• Pleth: a regular pulse wave for every heartbeat. Flat or chaotic means the SpO2 can't be trusted.
• Arterial line: a sharp upstroke and a small notch on the way down. A soft, rounded curve can mean the line is damped and the systolic reads too low.
• Capnography: a squarish shape with a plateau for every breath.

3. Rate and rhythm

Now the ECG. Fast or slow, regular or irregular, and is there a P wave in front of each QRS? Then compare the heart rate with the pulse in the pleth or arterial curve. They should match. If the ECG counts 110 and the pulse waves come unevenly with some barely visible, as in the picture below, think atrial fibrillation with a pulse deficit.

4. Oxygen and breathing

SpO2 tells you about oxygenation, respiratory rate and EtCO2 about ventilation, and they fail in different ways. A patient on a lot of oxygen can keep a perfect saturation while CO2 climbs because they are barely breathing. A rising EtCO2 or a falling respiratory rate is often the early sign, and the saturation drops last.

5. Pressure and perfusion

Blood pressure, and above all the MAP. First check which pressure you are looking at: a continuous arterial line or a cuff reading that may be twenty minutes old. Then connect it to the rest of the screen. A low pressure with a fast heart rate and a weak pleth tells a different story from a low pressure with a slow heart rate. The two kinds of pressure are compared in NBP vs ABP.

6. Read the trend, not the snapshot

One value says less than its direction. A heart rate of 105 is nothing special on its own, but it is if it was 80 two hours ago and the blood pressure has dropped 20 points over the same time. Most monitors have a trend view showing the last few hours as a graph. Use it, especially at handover.

7. Alarms

Find out what set the alarm off before you silence it. Many alarms are artefacts from movement or a loose sensor, which is exactly why it is dangerous to get used to them. Check the patient, check the signal, then decide whether the alarm limits suit this particular patient.

Practise the routine

The order becomes automatic through repetition, and a simulator lets you repeat it without a real patient at risk. The instructor changes one value at a time and the student talks through the screen from top to bottom. The simulator here can also let values drift slowly, say a blood pressure falling from 120 to 80 over ten minutes, which is closer to how real deterioration looks than a sudden jump.