Morphine for management of refractory dyspnoea

Editor—Abernethy et al identify that hypercapnia and respiratory depression are impediments to the use of opiates in patients with chronic obstructive pulmonary disease, yet they do little to allay those concerns in their report.¹

Most (88%) of their patients were reported to have chronic obstructive pulmonary disease. This is a diagnosis based on spirometric assessment,² but they present no such data. Spirometry would give confirmation of the diagnosis, prognostic information and perhaps selection criteria for the intervention. We disagree that measurement of pulmonary function would be neither generalisable nor ethical as spirometry is an easily performed, non-invasive procedure that is widely available.

The authors say that no respiratory depression was identified, but they present no data to support this. Oxygen saturation and respiratory rate, which are reported, may not identify respiratory depression. Alveolar hypoventilation, the result of respiratory depression, is a function of respiratory rate and tidal volume. The only valid parameter to assess respiratory depression is arterial carbon dioxide (Paco₂), and while we agree that arterial blood gas sampling is an invasive procedure, minimally invasive procedures such as capillary blood sampling or capnography would have provided useful results as increases in Paco₂ are associated with adverse outcomes.

This is particularly relevant since it seems from the discussion that some patients used non-invasive ventilation, presumably for hypercarbic respiratory failure. We recommend that for refractory dyspnoea opiates continue to be used with extreme caution, in chronic obstructive pulmonary disease at least.