Abstract
A time period of 2–4 weeks is required before a state of “stable chronic hypercapnia” is likely to be present following an acute COPD exacerbation http://bit.ly/2D4lOok
To the Editor:
We read with interest the “European Respiratory Society guidelines on long-term home non-invasive ventilation for management of COPD” [1]. In recent years, several studies have attempted to improve outcomes by using home non-invasive ventilation in patients with end-stage COPD who suffer from chronic hypercapnic respiratory failure (CHRF). As noted in the guidelines, the definition of “stable chronic hypercapnia” following an acute exacerbation has historically been ambiguous, which has probably contributed to the inconsistent findings across clinical trials of home non-invasive ventilation [1].
From a physiological point of view, one may argue that the respiratory acidosis associated with chronic hypercapnia must be fully compensated to be considered “stable”. In various prediction rules, equations and algorithms for diagnosing acid-base disorders by arterial blood gas analysis, respiratory acidosis is often stated to be fully compensated within 5 days [2]. This compensation involves an increase in the renal tubular reabsorption of bicarbonate, mainly in the proximal tubule and collecting ducts. This is partly based on findings from studies in dogs conducted in the 1950s and 1960s, which suggested that renal compensation to respiratory acidosis reached a near steady state within the first 6 days [3, 4]. However, a remarkable study by Schaefer et al. [5] indicated that a true steady state may take several weeks to develop [5]: they studied 21 healthy subjects in which respiratory acidosis was induced by exposure to a carbon dioxide-rich environment (inspiratory carbon dioxide fraction (FICO2) 1.5%) for 42 days aboard a submarine. The authors found that full renal compensation was not reached until on average 24 days of exposure. In a more recent study on goats exposed to a hypercapnic environment (FICO2 6.0%), full renal compensation was not reached until after 30 days [6].
To our knowledge, no studies have yet specifically investigated the time required for full renal compensation in patients with COPD. Based on data from the above-mentioned studies, we posit that a time period of several weeks is required before a state of “stable chronic hypercapnia” is likely to be present. Thus, we suggest that future studies on home non-invasive ventilation in CHRF be conducted on COPD patients who are at least 2 to 4 weeks out from their acute exacerbation event. Operationally, it would be reasonable to define recovery as the time when the patient is deemed clinically stable without severe decompensated acidosis (e.g. pH >7.30). This physiologically based definition of “stable chronic hypercapnia” should be considered when designing future clinical trials, as well as when interpreting results from previous and current studies on patients with CHRF.
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Footnotes
Conflict of interest: C. Hedsund has nothing to disclose.
Conflict of interest: T.R. Wilcke has nothing to disclose.
Conflict of interest: R.M.G. Berg has nothing to disclose.
- Received November 8, 2019.
- Accepted November 14, 2019.
- Copyright ©ERS 2020