During a cardiopulmonary stress test using breath-by-breath gas analysis, a pulmonary function technologist notices that the VO2 suddenly decreases. Which of the following may explain this change?
1. The patient has achieved anaerobic threshold.
2. The measurement of the expired gas volumes is inaccurate.
3. O2 analyzer "phase delay" has increased.
4. There is a leak in the tubing or mouthpiece.