Final answer:
A real-time gas analyzer is necessary for breath-by-breath exhaled gas analysis to determine O2 concentration in an individual's breath.
Step-by-step explanation:
To perform breath-by-breath exhaled gas analysis to determine O2, the necessary equipment is a real-time gas analyzer. This is essential in measuring the oxygen concentration in the exhaled breath of an individual. The Douglas bag method mentioned provides an alternative way to collect and analyze exhaled air, allowing for the determination of both oxygen consumption and CO2 production, but for real-time, continuous monitoring of exhaled gases, a real-time gas analyzer is required. Neither an ECG monitor, blood pressure cuff, nor the direct measurement of tidal volume is specifically necessary for the determination of O2 in this context.