Abnormally high heart rate - Series 3

Start by checking the fit of your watch band, Wearing your Apple Watch - Apple Support and remember the band may need to be very slightly tighter if you are engaged in vigorous exercise.

Your screenshot might be a clue, with one point showing about 80 and 150 in the same short time interval. That seems unlikely for a real heart rate problem, and more likely an artefact from a noisy signal. You have to be the judge and decide which is more likely, and the risk to your health.

If the band is too slack the watch can move slightly on your arm, and generate false spikes in heart rate. You can demonstrate this yourself while resting in your armchair and make rapid forearm movement for about 30 seconds. I found I could fake the spikes with a shoulder height cocktail shaking action or rapid forearm/hand rotation like using a screwdriver, but you might find other actions. Clearly your real heart rate will not change much, but the watch might detect anything up to about 200 bpm.

Better be safe than sorry, it should always be best to consult with a medical professional. On a side note, Apple Watch readings are most accurate when the watch is worn close to the skin, you may use the linked guides to figure out the right fit and wait another day, however, do go have it checked out nonetheless. Hopefully it's nothing.

Wearing your Apple Watch - Apple Support

On another note, this is from: Get the most accurate measurements using your Apple Watch - Apple Support

"Many factors can affect the performance of the Apple Watch heart rate sensor. Skin perfusion (or how much blood flows through your skin) is one factor. Skin perfusion varies significantly from person to person and can also be impacted by the environment. If you’re exercising in the cold, for example, the skin perfusion in your wrist might be too low for the heart rate sensor to get a reading.

Permanent or temporary changes to your skin, such as some tattoos, can also impact heart rate sensor performance. The ink, pattern, and saturation of some tattoos can block light from the sensor, making it difficult to get reliable readings.

Motion is another factor that can affect the heart rate sensor. Rhythmic movements, such as running or cycling, give better results compared to irregular movements, like tennis or boxing.

If you’re not able to get a consistent reading because of any of these factors, you can connect your Apple Watch wirelessly to external heart rate monitors such as Bluetooth chest straps. Learn how to pair Bluetooth accessories.

Heart rate is one of many factors that Apple Watch uses to measure your activity and exercise. Depending on your workout, it selects the most appropriate inputs for that activity. For example, when you’re running indoors, it also uses the accelerometer. Learn more about how your Apple Watch uses GPS and the heart rate sensor when you use the Workout app."