Abstract
Dear Editor:
The letter writer raises several important questions. Does it matter which opioid is used to treat dyspnea and at what dose? What might be clinical or disease-specific predictors of opioid response to dyspnea? Despite some progress in our understanding of the basic physiologies of dyspnea and the establishment of a broad evidence base for the utility of opioids in the treatment of dyspnea, these questions highlight real gaps in our understanding of opioid use. As the writer notes, most studies have focused on the use of morphine in the treatment of dyspnea. Studies have demonstrated the effectiveness of other commonly used opioids, but to the best of my knowledge good comparative studies of opioids have not been done. Nor is there a clear physiologic basis for recommending one opioid over another. Lacking such evidence, opioid choice is best driven by consideration of other factors—opioid metabolism, route of administration, and cost, among others—in the individual patient.
Similar caveats exist for the use of opioids in different disease processes, chronic obstructive lung disease or cancer, for example. Within any particular disease, multiple physiologies might or might not be at work. For example, patients might or might not have anxiety as a component of their dyspnea. The balance between “work-of-breathing dyspnea” and “suffocation dyspnea,” as I discuss might also vary significantly within a given disease. More sophisticated therapies ideally would target the specific physiologies at work in the individual patient, regardless of formal disease classification.
Experience has suggested to many clinicians that lower opioid doses are typically needed to treat dyspnea rather than pain. I am not aware of studies that explain the reason for this in terms of factors that may affect potency of action, such as differing opioid receptor affinity. Responses to opioids for both pain and dyspnea exist within complex matrixes of positive and negative feedback loops that can significantly affect the net response. Significant differences exist in the nature of the feedback loops modulating pain and dyspnea. Oxygen, carbon dioxide levels, and feedback from mechanoreceptors involved in respiration, for example, affect dyspnea responses, but not pain. It seems plausible that differences in overall dose responses to opioids reflect differences in the nature of the response matrixes of pain and dyspnea, as much as any differences in the specific actions of opioids.