A comparative examination of morphine and fentanyl: unravelling the differential impacts on breathing and airway stability

This study provides an in-depth analysis of the distinct consequences of the opioid drugs morphine and fentanyl during opioid-induced respiratory depression (OIRD). We explored the physiological implications of both drugs on ventilation and airway patency in anaesthetized mice. Our results revealed a similar reduction in respiratory frequency with equivalent scaled dosages of fentanyl and morphine, though the onset of suppression was more rapid with fentanyl. Additionally, fentanyl resulted in transient airflow obstructions during the inspiratory cycle, which were absent following morphine administration. Notably, these fentanyl-specific obstructions were eliminated with tracheostomy, implicating the upper airways as a major factor contributing to fentanyl-induced respiratory depression. We further demonstrate that bronchodilators salbutamol and adrenaline effectively reversed these obstructions, highlighting the bronchi's contribution to fentanyl-induced airflow obstruction. Our study also uncovered a significant reduction in sighs during OIRD, which were eliminated by fentanyl and markedly reduced by morphine. Finally, we found that fentanyl-exposed mice had reduced survival under hypoxic conditions compared to mice given morphine, demonstrating that fentanyl becomes more lethal in the context of hypoxaemia. Our findings shed light on the distinct and profound impacts of these opioids on respiration and airway stability and lay the foundation for improved opioid use guidelines and more effective OIRD prevention strategies.