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Tidal volume and non-invasive ventilation failure
Correspondence
Tidal volume and non-invasive ventilation failure I read with great interest the results of the post-hoc analysis by Jean-Pierre Frat and colleagues.1 Their results indicate that oxygen delivery through high-flow nasal cannula is associated with lower mortality and a lower risk of intubation compared with non-invasive mask ventilation in immunocompromised patients. Patients were treated with 7–10 mL/kg of expired tidal volume, but the amount to reach the lungs was probably not more than 5–8 mL/kg of bodyweight, because the average human being has 150 mL of dead space (roughly 2 mL/kg of bodyweight). Although possible, actual tidal volumes in the range of 7·5 mL/kg of predicted bodyweight (assuming dead space of around 2 mL/kg of bodyweight) are unlikely to lead to substantial ventilatorassociated lung injury so as to increase mortality independently. Perhaps a comparison of outcomes (intubation rates as well as mortality) between patients on non-invasive ventilation who generated expired tidal volume of less than 8 mL/kg and patients using high-flow nasal cannula and standard oxygen therapy would be worthwhile. Statistically insignificant difference
in outcomes between these groups would support the deleterious role of high expired tidal volume in producing negative outcomes among patients on non-invasive ventilation. High tidal volume could simply reflect the severity of the disease process. It might be just an association and not a cause of poor outcome. In fact, if this association is shown to be the case, then termination of noninvasive ventilation trials in patients that show high expired tidal volume can be considered, with a change to high-flow nasal cannula or standard oxygen therapy in anticipation of a non-invasive ventilation failure. Most hospitals have more non-invasive ventilation devices than high-flow nasal cannula setups and thus resource allocation might be an issue in units that don’t have enough high-flow nasal cannula devices. Interestingly, Guillaume Carteaux and colleagues concluded that expired tidal volume was one of the factors determining non-invasive ventilation failure in their study2—prior to Frat and colleagues’ analysis.1 In Carteaux and colleagues’ study,2 patients with a tidal volume above 9·5 mL/kg of predicted bodyweight had increased risk of non-invasive ventilation failure. This relationship between non-invasive ventilation success or failure and expired tidal volume was apparent only among
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patients with moderate-to-severe hypoxaemia (PaO2/FIO2≤200), and was not observed in patients with milder (PaO2/FIO2>200) degrees of hypoxaemia. A uniform effect over all subgroups would have provided stronger evidence for the putative effect of high expired tidal volume on mortality among patients treated with non-invasive ventilation. Finally, it would be interesting to discover the reasons for intubation in those patients for whom non-invasive ventilation therapy failed. Was it for tachypnoea or hypoxaemia? Non-invasive ventilation masks could probably add to a sense of claustrophobia and result in tachypnoea without worsening hypoxaemia. This unintended effect might be the cause of high intubation rates and have nothing to do with the proposed mechanism of ventilator-induced lung injury. I declare no competing interests.
Abhishek Biswas abiswas@ufl.edu Department of Pulmonary and Critical Care Medicine, University of Florida, Gainesville, FL 32610, USA 1
2
Frat J-P, Ragot S, Girault C, et al. Effect of non-invasive oxygenation strategies in immunocompromised patients with severe acute respiratory failure: a post-hoc analysis of a randomised trial. Lancet Respir Med 2016; 4: 646–52. Carteaux G, Millán-Guilarte T, De Prost N, et al. Failure of noninvasive ventilation for de novo acute hypoxemic respiratory failure: role of tidal volume. Crit Care Med 2016; 44: 282–90.
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Tidal volume and non-invasive ventilation failure I read with great interest the results of the post-hoc analysis by Jean-Pierre Frat and colleagues.1 Their results indicate that oxygen delivery through high-flow nasal cannula is associated with lower mortality and a lower risk of intubation compared with non-invasive mask ventilation in immunocompromised patients. Patients were treated with 7–10 mL/kg of expired tidal volume, but the amount to reach the lungs was probably not more than 5–8 mL/kg of bodyweight, because the average human being has 150 mL of dead space (roughly 2 mL/kg of bodyweight). Although possible, actual tidal volumes in the range of 7·5 mL/kg of predicted bodyweight (assuming dead space of around 2 mL/kg of bodyweight) are unlikely to lead to substantial ventilatorassociated lung injury so as to increase mortality independently. Perhaps a comparison of outcomes (intubation rates as well as mortality) between patients on non-invasive ventilation who generated expired tidal volume of less than 8 mL/kg and patients using high-flow nasal cannula and standard oxygen therapy would be worthwhile. Statistically insignificant difference
in outcomes between these groups would support the deleterious role of high expired tidal volume in producing negative outcomes among patients on non-invasive ventilation. High tidal volume could simply reflect the severity of the disease process. It might be just an association and not a cause of poor outcome. In fact, if this association is shown to be the case, then termination of noninvasive ventilation trials in patients that show high expired tidal volume can be considered, with a change to high-flow nasal cannula or standard oxygen therapy in anticipation of a non-invasive ventilation failure. Most hospitals have more non-invasive ventilation devices than high-flow nasal cannula setups and thus resource allocation might be an issue in units that don’t have enough high-flow nasal cannula devices. Interestingly, Guillaume Carteaux and colleagues concluded that expired tidal volume was one of the factors determining non-invasive ventilation failure in their study2—prior to Frat and colleagues’ analysis.1 In Carteaux and colleagues’ study,2 patients with a tidal volume above 9·5 mL/kg of predicted bodyweight had increased risk of non-invasive ventilation failure. This relationship between non-invasive ventilation success or failure and expired tidal volume was apparent only among
www.thelancet.com/respiratory Vol 4 November 2016
patients with moderate-to-severe hypoxaemia (PaO2/FIO2≤200), and was not observed in patients with milder (PaO2/FIO2>200) degrees of hypoxaemia. A uniform effect over all subgroups would have provided stronger evidence for the putative effect of high expired tidal volume on mortality among patients treated with non-invasive ventilation. Finally, it would be interesting to discover the reasons for intubation in those patients for whom non-invasive ventilation therapy failed. Was it for tachypnoea or hypoxaemia? Non-invasive ventilation masks could probably add to a sense of claustrophobia and result in tachypnoea without worsening hypoxaemia. This unintended effect might be the cause of high intubation rates and have nothing to do with the proposed mechanism of ventilator-induced lung injury. I declare no competing interests.
Abhishek Biswas abiswas@ufl.edu Department of Pulmonary and Critical Care Medicine, University of Florida, Gainesville, FL 32610, USA 1
2
Frat J-P, Ragot S, Girault C, et al. Effect of non-invasive oxygenation strategies in immunocompromised patients with severe acute respiratory failure: a post-hoc analysis of a randomised trial. Lancet Respir Med 2016; 4: 646–52. Carteaux G, Millán-Guilarte T, De Prost N, et al. Failure of noninvasive ventilation for de novo acute hypoxemic respiratory failure: role of tidal volume. Crit Care Med 2016; 44: 282–90.
e51