Scenario: This is a rhythm strip of lead V5 from a 24-hour Holter study of a patient who is scheduled for elective orthopedic surgery. He is a physically fit 24-year-old white male with no medical history. The Holter study was performed because during his preoperative evaluation he was noted to have an irregular pulse rate.

This rhythm is an example of marked respiratory sinus arrhythmia at 71 beats/min. In this rhythm, sinus rhythm is present along with beat to beat variations in the P-P interval (time measured between consecutive P waves). This produces an irregular R-R interval resulting in sinus arrhythmia. It is extremely noticeable so the word marked is added to the description. Sinus arrhythmia is a change in heart rate that is concurrent with the respiratory cycle, so that with inhalation the heart rate accelerates and with exhalation the heart...

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