To the Editor:
We read with interest the comments from Sarc et al1 about our previous study on delivered by noninvasive ventilation (NIV) compared with long-term oxygen therapy at the same flow.2 We want to give some precision in response to their comments. Sarc et al1 state that our result of a decrease in delivered by NIV at the same oxygen flow delivered by nasal cannula is not surprising. Indeed, it is an expected result related to the dilution of oxygen by the NIV flow. To our knowledge, this had not yet been clearly demonstrated in real-life home conditions.
Their main remark concerns the lack of data on , stating that cannot be directly translated into and suggests that the decrease in would be compensated by the increase in alveolar ventilation; in other words, the decrease in . We are not totally agreed with this point. is the main determinant of alveolar oxygen pressure (): = – / R = (PB – PH2O) – ( / R). In the the studies by Murphy et al3 and Köhnlein et al,4 the change in associated with NIV use was 5 mm Hg and 6.75 mm Hg, respectively. With a commonly accepted respiratory coefficient of 0.82, variation would be from 6.1 to 8.2 points. In our study, the change related to the decrease in (from ∼31% with daytime oxygen therapy to 25% on average with NIV) would be ∼42.8 points. Therefore, the drop in cannot compensate for the drop in (linked to the drop in related to compensation for leaks). This results in a drop in and, therefore, in . In addition, a study by Storre et al5 demonstrated that significant leaks influence both and . In our study2, monitoring was usable in 5 subjects; in all the subjects, we noticed a drop in when the leaks increased and the decreased (Fig. 1).
The consequences of nocturnal hypoxemia are multiple, and nocturnal oxygen supplementation can improve arrhythmias and reduce blood pressure surges.6,7 The benefits for sleep quality are not well established.8,9 The association between nocturnal oxygen desaturation and the development of chronic pulmonary hypertension remains unclear. Data on mortality and its association with nocturnal oxygen desaturation in COPD are scarce and have not demonstrated a survival benefit when corrected by oxygen supplementation. However, the 2 studies that looked at this topic were carried out on small cohorts and concerned subjects with low hypoxemia (mean of 76 mm Hg and 62.7 mm Hg).10,11
Finally, Sarc et al1 state that oxygen flow should be titrated to . However, this is difficult to achieve in the daily practice at the patient's home. We believe that it is simpler in daily practice to determine the oxygen flow according to the delivered. Like the servo flow regulator that titrates oxygen flow based on pulse oximetry feedback,12 technologic innovation would be the introduction of an oxygen flow regulator based on the flow of the home ventilator turbine and, therefore, of the leaks, so that the increase in the oxygen flow would limit the drop in linked to leaks.
Footnotes
- Correspondence: Michael Cardinale MD, Fédération d’Anesthésie – Réanimation, Hôpital d’Instruction des Armées Sainte-Anne, Boulevard Sainte-Anne, BP 20545 - 83041, Toulon Cedex 9, France. E-mail: mickaelcardinale{at}hotmail.fr
Dr Goutorbe discloses a relationship with Breas Medical, in addition, Dr Goutorbe has patent systems and methods for automatically adjusting a determined supply of generated from a CPAP, noninvasive ventilation, or other ventilator systems issued. Drs Cardinale, Esnault, Cungi, and Meaudre declare no conflict of interest.
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