maximal preoxygenation prior to induction of general anesthesia is essential in situations of difficult intubation / mask ventilation . however in certain clinical situations like fetal distress , lifesaving emergencies etc . , shorter preoxygenation would be more appropriate , if it is as effective as the traditional way of tidal volume breathing ( tvb ) of 100% o2 for 3 min . we aimed to compare the margin of safety following preoxygenation using 8 vital capacity breaths ( vcb ) in 1 min and tvb for 3 min , by assessing changes in arterial partial pressure of oxygen and apnea induced desaturation time . as the difference in the mean values of the two important parameters , pao2 ( 215.13  47.76 vs. 327.49  56.3 , p < 0.00005 ) and desaturation time ( 3.70  0.44 vs. 4.70  0.27 , p < 0.005 ) in the previous study by singh et al . was high , the minimum sample size when calculated initially , with 80% confidence and 90% power , was very small . hence , the sample size was finally calculated with 99% confidence and 99% power and each group required 10 patients to get statistically significant results . hence , 20 american society of anesthesiologist grade i and ii patients of both sexes , aged 20 - 60 years , scheduled for surgical procedures under ga with endotracheal intubation requiring invasive blood pressure ( bp ) monitoring , were included in this randomized clinical trial , which was conducted from september 2012 to may 2013 . patients with ischemic heart disease , chronic obstructive pulmonary disease obesity ( body mass index > 30 ) , anemia and anticipated difficult airway were excluded . approval of the hospital ethical committee and written informed consent from all patients were obtained . patients were randomly allocated to one of the two groups using computer generated sequence of random numbers . in group a , 3 min of tvb using oxygen flow of 5 l / min was used whereas in group b , patients were asked to take 8 vcb in 60 s using o2 flow of 10 l / min . after obtaining a venous access in the operation theatre a finger probe pulse oximeter , noninvasive the radial artery was cannulated ( 22 g ) under local anesthesia for arterial blood sampling and invasive bp monitoring . circle anesthesia system with a 3 l capacity reservoir bag and proper sized tight fitting face masks were used for preoxygenation . following preoxygenation with either method , induction of anesthesia was achieved with propofol 2 - 2.5 mg / kg body weight followed by succinylcholine 2 mg / kg intravenously ( iv ) . positive pressure ventilation was not administered , and the time at which patients became apneic was noted , that is , time at which there was the cessation of bag movement and/or etco2 showing a flat line . a quick and gentle laryngoscopy was done at 1 min following administration of succinylcholine and trachea was intubated with 7 - 8 mm i d cuffed endotracheal tube , which was kept open to atmosphere without administering positive pressure ventilation . vecuronium 0.1 mg / kg and midazolam 0.05 mg / kg were given iv to prevent the patients from regaining spontaneous respiration and development of awareness respectively . spo2 , heart rate ( hr ) , systolic bp ( sbp ) , diastolic bp ( dbp ) and mean arterial pressures ( map ) were closely monitored . the time at which saturation reached the study end point of 90% was noted , and the patients were immediately bag ventilated with 100% o2 . the point of maximum desaturation was also noted , and bag ventilation was continued till saturation became 98 - 100% . arterial blood gas samples were taken before induction while patients were breathing room air ( baseline ) , immediately after preoxygenation and also at 90% desaturation to note pao2 , paco2 and ph . at these three points of time spo2 , hr , sbp , dbp and map were also documented . during the study period if there was tachycardia ( hr > 100/min ) or hypertension ( sbp > 160 mmhg ) , plane of anesthesia was deepened with propofol 30 mg bolus iv . in the case of sustained tachycardia , hypertension or development of arrhythmias , even with saturation > 90% , patients were immediately ventilated and excluded from the statistical analysis . to test the statistical significance of the difference in mean values between the two groups student 's t - test was applied . as the difference of mean values at basal level was statistically significant for paco2 , for this variable , the two groups were compared for statistical significance with difference in the percentage change . distribution of age , sex , weight and height were comparable in both groups [ table 1 ] . comparison of spo2 at baseline , after preoxygenation and at maximum desaturation did not show any significant difference between groups [ table 2 ] . similar observations were made when sbp , dbp , map and hr were compared [ tables 3 and 4 ] . comparison of demographics comparison of sbp , dbp and map ( mmhg ) comparison of hr , pao2 and ph comparison of baseline pao2 among groups was comparable . however after preoxygenation group b had a significantly high pao2 value than group a ( 439.05  62.20 vs. 345.16  20.80 , p < 0.001 ) , but at 90% desaturation there was no significant difference between groups [ table 4 ] . baseline ph in both groups was comparable , but after preoxygenation group b had a significantly high ph as compared to group a ( 7.39  0.06 vs. 7.45  0.04 , p = 0.013 ) . comparison of paco2 at baseline showed a significant difference between groups with group a having a higher value ( 40.31  3.40 vs. 36.82  3.80 , p = 0.044 ) ) . so percentage changes of paco2 from the baseline value were calculated for comparing paco2 after preoxygenation . there was no significant difference between groups after preoxygenation and also at 90% desaturation [ table 5 and figure 1 ] . statistical analysis had shown that there was a significant difference between groups when time to desaturate to 90% was compared . group b showed a significantly high apnea induced desaturation time when compared to group a ( 6.87  1.78 vs. 3.47  0.38 min ) [ table 6 and figure 2 ] . comparison of changes in paco2 time ( in min ) to desaturate to 90% time to desaturate to 90% preoxygenation works by denitrogenation of the functional residual capacity , leading to an increase in oxygen stores which delays arterial desaturation during the apneic period . during tvb there but with vcb these areas will also get ventilated with 100% o2 leading to an increased reserve . though patients are usually preoxygenated with the commonly used anesthesia systems , circle absorber system was also found to be equally effective . to achieve maximal preoxygenation alveolar , vascular ( arterial and venous ) , and tissue compartments should be saturated with oxygen . failure to breathe a high inspired concentration of o2 or leak under the mask will affect the alveolar oxygenation . this is important as the effectiveness of preoxygenation depends greatly on tight face mask fit which may not be a very pleasant experience for a fully conscious patient . so reducing the tight face mask holding time with vcb technique for preoxygenation will improve patient comfort and acceptability . the alveolar oxygenation will not be maximally effective if there is inadequate time for preoxygenation . tvb for 3 - 5 min was found to be effective in attaining maximal preoxygenation compared to vcb for half a minute which resulted in suboptimal preoxygenation . it was observed that 3 vcb produced significantly inferior , 4 and 5 vcb equally effective and 6 vcb superior preoxygenation as compared to tvb for 3 min . therefore , 4 or more vcb was recommended as a reliable and rapid alternative to preoxygenation by 3 min of tvb . deep breathing yielded maximal preoxygenation when extended to 1.5 or 2 min , and only when high fresh gas flow ( fgf ) ( 10 l / min ) was used . taking this into consideration , in our study , patients were asked to take 8 vcb in 1 min using o2 flow of 10 l / min . the different fgfs used in our study ( 10 vs. 5 l / min ) can be further justified by the fact that , as tidal volume is one of the factors which decides fgf in different breathing circuits , if patient is taking vcb , to avoid rebreathing , higher flows will be required . previous studies have shown that 8 vcb preoxygenation in 1 min significantly delayed apnea - induced desaturation in comparison with tvb for 3 min . this observation will be of importance whenever we anaesthetize a patient with a difficult airway where difficulty and/or delay in intubation or ventilation is anticipated . the safe upper time limit is said to be 2 min as , if extended for more than 2 min undesirable side - effects of hypocarbia will set in . though few patients developed tachycardia and/or hypertension during the apneic period it could be controlled with propofol bolus . preoxygenation by 8 vcb in 1 min provides greater margin of safety , as it results in a significantly high arterial partial pressure of oxygen with an almost doubled apnea induced desaturation time , as compared to preoxygenation with tvb for 3 min .
background and aims : shortening the duration of efficacious preoxygenation would provide benefit in emergency situations like fetal distress etc . 
 this study aims to compare the margin of safety following preoxygenation using 8 vital capacity breaths ( vcb ) in 1 min and tidal volume breathing ( tvb ) for 3 min , by assessing changes in pao2 and apnea induced desaturation time.material and methods : patients were randomly divided into group a and b. in group a , 3 min of tvb using o2 flow of 5 l / min and in group b , 8 vcb in 60 s using o2 flow of 10 l / min were used . 
 anesthesia was induced in all patients with propofol followed by succinylcholine 2 mg / kg intravenously . 
 mask ventilation was not done and following intubation endotracheal tube was kept open to atmosphere . the time taken for the patients to desaturate to 90% 
 was noted and immediately ventilation was resumed . 
 arterial blood gas samples were taken while patients were breathing room air , immediately after preoxygenation and at 90% desaturation.results:baseline pao2 of both the groups were comparable . 
 after preoxygenation group b had a significantly high pao2 value than group a ( 439.05  62.20 vs. 345.16  20.80 ) . at 90% desaturation 
 there was no significant difference between groups . 
 group b showed a significantly high apnea induced desaturation time when compared to group a ( 6.87  1.78 vs. 3.47  0.38 min).conclusions : preoxygenation by 8 vcb in 1 min provides a greater margin of safety , as it results in a significantly high pao2 with an almost doubled apnea induced desaturation time , in comparison with tvb for 3 min .