In Reply:
We read with a great interest the comments made by Dr Killen H Briones Claudett concerning adjustments of ventilatory settings during noninvasive ventilation (NIV) to treat subjects with hypercapnic coma. In a recent original article published in the December 2013 issue of Respiratory Care,1 we reported an overall intubation rate of 15% in a cohort of 242 subjects receiving NIV for acute hypercapnic respiratory failure of all origins. After adjustment, acidosis and severe hypoxemia after 1 h of NIV initiation were independently associated with NIV failure, whereas altered consciousness on admission and ventilatory settings had no influence on outcome. Altered consciousness was defined using the Richmond Agitation-Sedation Scale (RASS),2 and in all of the subjects who had encephalopathy at admission (defined as RASS < 0), the rate of intubation was only 23% (14/60). However, this rate reached 52% (16/31) in those who were comatose during the first 24 h (defined as Glasgow coma scale ≤ 8). It has already been found that NIV can be successful in subjects with hypercapnic coma, and NIV failure rates of only 20% have been reported.3 It is important to note that patients who succeed with NIV have a faster improvement of consciousness compared with those who need intubation.3 Dr Briones Claudett emphasizes that subjects with altered consciousness may require higher levels of pressure support than those with normal consciousness. To support this, Briones Claudett et al4 recently found a faster recovery from hypercapnic encephalopathy using ventilatory mode with average volume-assured pressure support compared with pressure support ventilation during NIV due to higher levels of ventilatory assistance and larger tidal volumes (VT).
In our cohort, of the 31 subjects who developed hypercapnic coma, 15 were comatose upon admission, and 16 developed coma during the first 24 h while receiving NIV.1 The rate of intubation was only 20% (3/15) in subjects who were comatose at admission, in line with the results found by Díaz et al.3 By contrast, the rate of intubation was 81% (13/16) in those who developed delayed coma during NIV (P = .001). Subjects with hypercapnic coma upon admission had similar VT values compared with other subjects (459 ± 175 vs 469 ± 142 mL, P = .82). However, they received higher pressure support levels (11.9 ± 2.7 vs 9.1 ± 2.5 cm H2O, P = .008), meaning that the level of ventilatory assistance had been correctly adjusted according to our protocol that aims to target predetermined VT. As we used exclusively ICU ventilators, inspiratory positive airway pressure, including pressure support and PEEP, reached 16.5 ± 2.8 cm H2O in subjects who were comatose at admission, a value close to that reported in the abovementioned studies.3,4
We agree that adjustment of an adequate pressure support level is the key setting to reverse hypercapnic coma, and we believe that our good results are due in part to our protocol of adjusting the pressure support level to target a minimal VT, as would average volume-assured pressure support. Altered consciousness at admission does not seem to increase the risk of NIV failure. By contrast, almost all subjects who developed delayed coma needed intubation, whereas their VT values and their pressure support levels adjusted at admission were similar to those of other patients (417 ± 142 vs 471 ± 142 mL, P = .27; 9.1 ± 7.4 vs 9.2 ± 2.6 cm H2O, P = .88). Therefore, NIV failure was probably due to a patient's worsening and/or failure of this treatment and not to inadequate adjustment of ventilatory settings.
Footnotes
The authors have disclosed no conflicts of interest.
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