Out-of-Hospital Ventilation for Acute Respiratory Failure
Out-of-Hospital Ventilation for Acute Respiratory Failure
In this study OOH treatment of ARF, irrespective of its origin, was managed more effectively with NIV compared with SMT. The brief period of OOH NIV seemed to contribute to improved outcome. With NIV oxygenation improved better and SMT failed to be effective for OOH treatment of ARF in five of 25 of the patients. In four of those patients, OOH NIV proved to be a sufficient rescue strategy. In cases of hypercapnia, the decrease in pvCO2 was more pronounced with NIV. Finally, NIV proved to be safe and feasible as no complications occurred and NIV was able to be continued until hospital admission in all patients.
Our findings agree with the previously reported finding that OOH NIV is able to improve vital parameters. However, those studies were limited to patients with ACPE only. Furthermore, some studies used pure CPAP and others used NIV with PEEP and pressure support14 for treatment, despite the use of the term NIV for both treatment modalities.
SMT has a limited effect on the treatment of ARF with hypoxaemia because it is not able to reduce intrapulmonary shunt. As seen in this study, SpO2 can improve with SMT, but an improvement of the oxygenation index was not seen. Pure CPAP may help recruit alveoli and prevent cyclic alveolar collapse. However, PEEP and pressure support may be needed to increase transpulmonary pressure further and thereby reduce intrapulmonary shunt and improve oxygenation. This is supported by the findings of this study, given that the majority (85.7%) of patients in the NIV group needed a pressure support level of at least 5 mbar. Although some authors favour pure CPAP for OOH treatment of hypoxaemic respiratory failure, we believe that OOH NIV should be applied in terms of PEEP and pressure support.
Although it is important to distinguish between ACPE, pneumonia and COPD for further treatment, the assignment of patients with ARF to only one certain group, as was done in previous studies, may not reflect clinical reality. Patients with ARF may have hypoxaemia, hypercapnia or both. In this study, 13 of 22 patients with a history of COPD were both hypoxaemic and hypercapnic (pvCO2 >Hg). Of 27 50 mm patients without a history of COPD, nine patients were 'only' hypoxaemic, whereas 12 patients were hypercapnic and hypoxaemic.
It is difficult to identify hypercapnic respiratory failure by clinical estimation only. OOH blood gas analyses are usually not available and end-tidal carbon dioxide does not reflect arterial carbon dioxide tension in patients with intrapulmonary shunt. In hypercapnic respiratory failure, patients have rapid shallow breathing with limited efficiency for removing carbon dioxide. When used with pressure support, NIV allows patients to take deeper breaths with less effort and to rest their respiratory muscles. PEEP counterbalances the effects of dynamic hyperinflation and reduces the work of breathing at the next inspiratory effort. With SMT alone, a patient with rapid shallow breathing will most likely not improve. Insufficient pressure support or even CPAP alone may increase the work of breathing. Therefore, pressure support ventilation, as was applied in this study, is considered to be a first-line treatment in those patients.
Two previous studies13 14 that applied OOH NIV also analysed patient outcomes. Plaisance et al13 found lower inhospital mortality while Weitz et al14 found no difference in mortality or the incidence of ICU treatment. In our study, mortality did not differ, but was lower than predicted. The outcome was better in the NIV group with regard to ICU treatment when compared with SMT because the patients were admitted to an ICU less often (p<0.05) and for a shorter period of time (p=0.03). We believe that there are two reasons for this improvement. NIV needs to be continued for at least 1 h before beneficial effects other than the improvement of vital parameters are seen.5 29 We agree with Plaisance et al13 that OOH NIV initiates cardiorespiratory recovery as supported by the measured variables, although the OOH treatment period is relatively short. This effect contributes to better outcome if NIV is continued immediately after hospital admission. In this study, OOH NIV seemed to promote the continuation of NIV therapy in the hospital, as inhospital NIV was applied more often (p<0.01) in the NIV group (n=14; 58%) compared with the SMT group (n=5; 20%). In addition, none of the patients in the SMT group were treated with NIV immediately after admission in contrast to patients in the NIV group, for whom NIV was continued after admission.
While the findings in this pilot study are encouraging, they need to be replicated in a multicentre trial with a larger number of patients. Taking the data from this pilot study as basis for a larger confirmatory trial, a power of 90% could be achieved with a sample size of 46 per group.
Furthermore, all emergency physicians in this study have been familiar with NIV. The application of NIV will require at least some training for paramedics and emergency physicians who are not familiar with NIV.
Discussion
In this study OOH treatment of ARF, irrespective of its origin, was managed more effectively with NIV compared with SMT. The brief period of OOH NIV seemed to contribute to improved outcome. With NIV oxygenation improved better and SMT failed to be effective for OOH treatment of ARF in five of 25 of the patients. In four of those patients, OOH NIV proved to be a sufficient rescue strategy. In cases of hypercapnia, the decrease in pvCO2 was more pronounced with NIV. Finally, NIV proved to be safe and feasible as no complications occurred and NIV was able to be continued until hospital admission in all patients.
Our findings agree with the previously reported finding that OOH NIV is able to improve vital parameters. However, those studies were limited to patients with ACPE only. Furthermore, some studies used pure CPAP and others used NIV with PEEP and pressure support14 for treatment, despite the use of the term NIV for both treatment modalities.
SMT has a limited effect on the treatment of ARF with hypoxaemia because it is not able to reduce intrapulmonary shunt. As seen in this study, SpO2 can improve with SMT, but an improvement of the oxygenation index was not seen. Pure CPAP may help recruit alveoli and prevent cyclic alveolar collapse. However, PEEP and pressure support may be needed to increase transpulmonary pressure further and thereby reduce intrapulmonary shunt and improve oxygenation. This is supported by the findings of this study, given that the majority (85.7%) of patients in the NIV group needed a pressure support level of at least 5 mbar. Although some authors favour pure CPAP for OOH treatment of hypoxaemic respiratory failure, we believe that OOH NIV should be applied in terms of PEEP and pressure support.
Although it is important to distinguish between ACPE, pneumonia and COPD for further treatment, the assignment of patients with ARF to only one certain group, as was done in previous studies, may not reflect clinical reality. Patients with ARF may have hypoxaemia, hypercapnia or both. In this study, 13 of 22 patients with a history of COPD were both hypoxaemic and hypercapnic (pvCO2 >Hg). Of 27 50 mm patients without a history of COPD, nine patients were 'only' hypoxaemic, whereas 12 patients were hypercapnic and hypoxaemic.
It is difficult to identify hypercapnic respiratory failure by clinical estimation only. OOH blood gas analyses are usually not available and end-tidal carbon dioxide does not reflect arterial carbon dioxide tension in patients with intrapulmonary shunt. In hypercapnic respiratory failure, patients have rapid shallow breathing with limited efficiency for removing carbon dioxide. When used with pressure support, NIV allows patients to take deeper breaths with less effort and to rest their respiratory muscles. PEEP counterbalances the effects of dynamic hyperinflation and reduces the work of breathing at the next inspiratory effort. With SMT alone, a patient with rapid shallow breathing will most likely not improve. Insufficient pressure support or even CPAP alone may increase the work of breathing. Therefore, pressure support ventilation, as was applied in this study, is considered to be a first-line treatment in those patients.
Two previous studies13 14 that applied OOH NIV also analysed patient outcomes. Plaisance et al13 found lower inhospital mortality while Weitz et al14 found no difference in mortality or the incidence of ICU treatment. In our study, mortality did not differ, but was lower than predicted. The outcome was better in the NIV group with regard to ICU treatment when compared with SMT because the patients were admitted to an ICU less often (p<0.05) and for a shorter period of time (p=0.03). We believe that there are two reasons for this improvement. NIV needs to be continued for at least 1 h before beneficial effects other than the improvement of vital parameters are seen.5 29 We agree with Plaisance et al13 that OOH NIV initiates cardiorespiratory recovery as supported by the measured variables, although the OOH treatment period is relatively short. This effect contributes to better outcome if NIV is continued immediately after hospital admission. In this study, OOH NIV seemed to promote the continuation of NIV therapy in the hospital, as inhospital NIV was applied more often (p<0.01) in the NIV group (n=14; 58%) compared with the SMT group (n=5; 20%). In addition, none of the patients in the SMT group were treated with NIV immediately after admission in contrast to patients in the NIV group, for whom NIV was continued after admission.
Limitations
While the findings in this pilot study are encouraging, they need to be replicated in a multicentre trial with a larger number of patients. Taking the data from this pilot study as basis for a larger confirmatory trial, a power of 90% could be achieved with a sample size of 46 per group.
Furthermore, all emergency physicians in this study have been familiar with NIV. The application of NIV will require at least some training for paramedics and emergency physicians who are not familiar with NIV.
