This was a prospective, randomized, crossover study comparing two preoxygenation techniques: eight vital capacity (VC) breaths vs. three minutes tidal volume (TV) breaths in 270 patients undergoing elective surgery under general anaesthesia. It was a single centre study conducted at P. D. Hinduja National Hospital and Medical Research Centre, Mumbai. The primary outcome is to identify which of the two techniques of preoxygenation is effective across the study group. The secondary outcome is to identify the effectiveness of either technique of preoxygenation in overweight and obese patients. As it was a crossover study all patients underwent both techniques of preoxygenation. However, the patients were randomized to either perform the TV or VC maneuver first. This randomization into two groups was done by a computer generated sequence of random numbers. All the patients were given a 25⁰ head up position for both the techniques. Patients assigned to Group One were instructed to breathe at normal TV for 3 min through a close fitting mask. Over the next ten minutes the patients breathed room air to allow the PAO₂ to come back to baseline before crossing over to VC breathing. Patients assigned to Group Two were first instructed to take eight VC breaths, followed by ten minutes of room air breathing before they crossed over to the TV breathing. Preoxygenation was carried out with 15 L/min of FGF with FiO₂ of 1.0 using the circle system. The ETO₂ achieved was sampled at the patient end, on completion of each technique. After data collection, data entry was done in Excel. Data analysis was done using SPSS Software version 20.0.