A 27-year-old man is brought to the emergency department after he was accidentally hit by flying pieces of glass from a broken apartment window. His wounds were not severe and required a few stitches. The ECG was recorded when irregularity of his pulse was noted by the attending nurse:

• Sinus rhythm with marked sinus arrhythmia
• Ventricular rate 81/min
• Otherwise a normal ECG

There is marked respiratory sinus arrhythmia, a normal finding, particularly in young individuals. The P waves remain upright in leads 1, 2, 3, V5 and V6, but their morphology varies. Their amplitude and width increase at faster rates in the inferior leads as the location of the pacemaker in the sinus node changes.