8 yr old diagnosed with sinus arrhythmia. how high of a heart rate is too high? his heart rate is 150-165 bpm just putting shoes and coat on. he is alays out of breath. diagnosed asthma but wondering.

Check thyroid: Possibly from an overactive or cardiac issue(mitral valve). Need to see a cardiologist if asthma is under control.

Too high: A sustained heart rate of 150-165 with activities of daily living is too high and needs to be evaluated and treated asap. Assure he gets to see a pediatric cardiologist for a medical evaluation and the proper treatment. Best wishes to you.