THRIVE , rapid sequence induction and oxygenation. A reply

We thank the correspondents above for their comments on our paper 1. They raise four main points: the validity of ‘pre-oxygenation’ with THRIVE; why an increased intubation time should exist between the THRIVE and facemask group; the potential for increased risk of pulmonary aspiration during this time; and the potential for awareness during this period. The concept of pre-oxygenation is not simply the achievement of fractional ETO2 > 90% as claimed by Dixit and Frerk. Measurements of pre-oxygenation have focused on ETO2 > 90% as this is most readily measurable, whereas a more accurate measure of the efficacy of pre-oxygenation is the arterial oxygen partial pressure itself. The efficacy of pre-oxygenation is dependent on the inspired O2 concentration, duration of pre-oxygenation, alveolar ventilation and functional residual capacity (FRC). What is perhaps even more important is the efficiency of pre-oxygentaion – the delay in desaturation during apnea. The efficiency of pre-oxygenation is dependent on the efficacy of pre-oxygenation, capacity for oxygen loading (i.e. decreased FRC, haemoglobin concentration, alveolar ventilation, cardiac output) and total body oxygen consumption 2. High-flow heated humidified nasal oxygen at 50 l.min−1 has been shown to provide a significant increase in arterial oxygen partial pressure levels (arterial blood gas) compared with standard face mask pre-oxygenation (100% oxygen flow and 12 l.min−1) at every time-period from start of pre-oxygenation to induction of anaesthesia at 5 min, and to tracheal intubation at 8.5 min 3. In contrast, studies looking at ETO2 4, 5 may have methodological issues because it is not possible to measure the ETO2 with nasal high flow. These healthy volunteer studies require the nasal high flow to be discontinued and then breathe out into a mouthpiece to measure ETO2. At 60 l.min−1 with the mouth closed, high-flow nasal oxygenation was shown to be as as effective as 3 min oxygen by face mask at 10 l.min−1 4. At 70 l.min−1 the median (IQR [range]) ETO2 at 180 s of pre-oxygenation was 86% (84–90% [78–92%]) 5. We found patients in the THRIVE group had an equivalent blood gas profile to facemask pre-oxygenation, in spite of a significantly longer apnoea time. It seems likely, therefore, that had the time taken to intubation been the same, the arterial blood gases and pH would have been more favourable in the THRIVE group. During pre-oxygenation with THRIVE, tidal volume, end expiratory lung volume and FRC are all increased and dead space reduced 6-9. Further, there is a seamless progression to apnoeic oxygenation and apnoeic ventilation when compared with face mask pre-oxygenation. If we regard pre-oxygenation as a technique to maximise apnoea time, then during spontaneous breathing, THRIVE with the mouth closed is superior to facemask pre-oxygenation 3 and after induction of anaesthesia apnoeic oxygenation and apnoeic ventilation occur, further increasing apnoea time 10. It seems likely that in the future our understanding of pre-oxygenation will not be limited to an ETO2 > 90%. Secondly, the increase in time to intubation with the THRIVE group compared to the facemask group was a surprising finding. Both patient groups were similar in ASA physical status, laryngoscopy grade and number of attempts at laryngoscopy. Following induction of anaesthesia, cricoid pressure was applied. In the THRIVE group, jaw thrust and flow of nasal oxygen continued whilst in the facemask group ventilation was discontinued. We see no obvious reason from the practical performance of THRIVE or face mask pre-oxygenation why there should be this difference. When using standard face mask pre-oxygenation, once induction takes place and cricoid pressure is applied, there is an implicit recognition that critical haemoglobin desaturation will occur in a relatively short period and that this period is even shorter as the patient becomes more ill, such as those presenting for rapid sequence induction 11. In contrast, the knowledge that THRIVE can prolong the safe apnoea time in these patients may have resulted in a more controlled decision around the optimal time for and performance of laryngoscopy and intubation. Thirdly, we know that minimising the time to secure the airway with a cuffed tracheal tube during RSI minimises the risk of pulmonary aspiration, and yet in our study patients in the THRIVE group had a mean apnoea time of 248 s compared to 123 s in the face mask group. We speculate that perhaps part of the drive for early immediate intubation in an RSI is the knowledge that sick patients will very quickly desaturate rather than overwhelming concerns around the exposure to potential aspiration as a result of an extra 1 or 2 min during which cricoid pressure is providing protection. Finally, any increase in apnoea time should be paralleled with an understanding that there is the potential for awareness during a RSI with longer apnoea times. This is no different to any other procedure (e.g. difficulties encountered during RSI laryngoscopy following pre-oxygenation with a facemask) in which adequate anaesthesia should be maintained during airway management.