What muscles does morphine (opioid analgesic) relax to facilitate breathing?

From the Guidelines

Morphine relaxes the upper airway muscles, which can cause airway obstruction and lead to respiratory depression, particularly in patients with obstructive sleep apnea (OSA) 1. When administered, morphine acts on mu-opioid receptors in the brainstem's respiratory centers, specifically the medulla oblongata, decreasing their sensitivity to carbon dioxide and reducing the neural drive to breathe. This results in slower, shallower breathing patterns and decreased respiratory rate. The relaxation of these muscles is not beneficial for breathing but rather a concerning side effect that can lead to respiratory depression. At therapeutic doses, morphine can reduce respiratory rate from the normal 12-20 breaths per minute to fewer than 12, and in overdose situations, breathing may become dangerously slow or even stop. This respiratory depression is dose-dependent and represents the most serious potential adverse effect of morphine and other opioids. Patients receiving morphine should be monitored for signs of respiratory depression, particularly when initiating therapy or increasing doses, and naloxone should be available as an antidote in case of severe respiratory depression 1. Key points to consider when prescribing morphine include:

• Monitoring for signs of respiratory depression, such as reduced respiratory rate or oxygen saturation
• Using the lowest effective dose to minimize the risk of respiratory depression
• Avoiding concomitant use of other central nervous system depressants
• Having naloxone available as an antidote in case of severe respiratory depression 1. It is essential to weigh the benefits of morphine against the potential risks, particularly in patients with pre-existing respiratory conditions, such as OSA or chronic obstructive pulmonary disease (COPD) 1.

From the Research

Morphine's Effect on Muscles for Breathing

• Morphine is known to cause respiratory depression, which can lead to a decrease in respiratory rate and tidal volume 2.
• The preBötzinger complex, located in the medulla, is the site responsible for the decrease in respiratory rate following systemic administration of opioids, including morphine 2.
• Neurons in the preBötzinger complex expressing neurokinin-1 receptors are selectively inhibited by opioids, leading to a decrease in respiratory drive 2.
• Morphine can relax the diaphragm and other respiratory muscles, leading to a decrease in respiratory function 3.
• The relaxation of these muscles can lead to a decrease in lung volume and an increase in respiratory resistance, making it more difficult to breathe 3.

Specific Muscles Affected by Morphine

• The diaphragm is the primary muscle responsible for breathing, and morphine can decrease its contractility and relaxation 3.
• The intercostal muscles, which help to expand the chest cavity during inhalation, can also be affected by morphine, leading to a decrease in respiratory function 3.
• The accessory muscles of respiration, such as the sternocleidomastoid and scalene muscles, can also be relaxed by morphine, leading to a decrease in respiratory function 3.

Clinical Implications

• The use of morphine can lead to respiratory depression, which can be life-threatening in severe cases 2, 4.
• The risk of respiratory depression is increased when morphine is combined with other sedatives or opioids 4.
• Patients receiving morphine should be closely monitored for signs of respiratory depression, including decreased respiratory rate and tidal volume 4.
• The use of opioid antagonists, such as naloxone, can help to reverse respiratory depression caused by morphine 2, 4.