Final answer:
A person transitioning from normal sinus rhythm to sinus tachycardia will experience an increase in VO2 initially. However, when the heart rate exceeds 160 bpm, cardiac output may decrease due to a decrease in stroke volume, which could affect VO2.
Step-by-step explanation:
When a person transitions from a normal sinus rhythm to sinus tachycardia, which is an increase in heart rate, their VO2, or oxygen consumption, would typically increase. This is due to the higher demand for oxygen by the body's tissues as the heart rate rises. In a healthy young adult, when the heart rate reaches levels above 160 beats per minute, this can lead to a paradoxical situation where cardiac output may actually decrease as the stroke volume decreases more rapidly than the heart rate increases, due to less ventricular filling time in diastole.
Therefore, within certain limits, an increase in heart rate up to 160 bpm due to physical activity or other reasons can help maintain or even increase the cardiac output, but as the heart rate goes beyond this threshold, efficiency may reduce, and the cardiac output can start to decline. For someone transitioning to sinus tachycardia, the initial response of increasing VO2 would occur with the rise in heart rate; however, at very high heart rates, the cardiac output, and consequently, the VO2 might ultimately decrease if the rate exceeds the 160 bpm level where stroke volume begins to drop.