The Risk of Opioids in Post-Surgical Settings

This is the fourth article in a series exploring the impact of pulse oximetry alarm thresholds in hospitalized patients.
In the first article, “Improving the Safety of Post-Surgical Care,” I introduced the concept that, although the current approach to physiologic threshold monitoring (triggering an alarm when oxygen saturation falls below 90%) works well in the OR, it is unreliable on post-surgical floors.


In the second post, “Pulse Oximetry False Alarms on Post-Surgical Floors,” I explored in more depth why the threshold for triggering a pulse oximetry alarm should vary depending on the site of care (OR vs post-surgical floor). The key to appreciating why this is the case is understanding that the clinical conditions that threaten oxygenation on post-surgical floors are different from the type of sudden, life-threatening airway compromise that occur in ORs. Those conditions often have an insidious onset and comprise sepsis, aspiration, congestive heart failure, pulmonary embolus, and two different types of opioid-associated respiratory depression.


In third post, “Detecting Deadly Post-Surgical Respiratory Dysfunction,” I reviewed the pattern of respiratory compromise that characterizes the conditions not related to opioid use. In this post, I will discuss the respiratory risk of opioids in post-surgical settings.
Type II – CO narcosis
The second pattern of respiratory dysfunction (Type II) is called carbon dioxide (CO narcosis. Adverse events associated with this pattern are considerable—an estimated 20,000 events per year occur in the USA that includes accidental death and severe anoxic brain injury. As illustrated in the chart below, it is not unusual to trigger a deteriorating, self-propagating process where both opioids and a rising carbon dioxide (PaCO ) contribute to an unstable central depression of patients’ ventilatory drives. The vicious cycle begins with a rise in carbon dioxide (PaCO ) due to neuro-inhibition of the brainstem by opioids.

Medications, like opioids, increase CO arousal thresholds. Even a slight additional arousal delay (for example from just one more PCA opioid administration) can permit critically low oxygen saturation (SPO ) levels to be reached.

Likewise, conditions that encourage ongoing cycling hypoxemia, such as chronically depleted oxygen reserves in both venous beds and lung from obesity, can have the same effect by accelerating desaturations to critically low levels even before normal PaCO arousal generating thresholds can be reached.

With both types of insults, arterial oxygen saturation in some patients will fall to the point where the brain no longer receives sufficient oxygen for a central arousal to occur. This is called the “Lights Out Saturation” (LOS). If resuscitation is not immediately provided, brain death follows within minutes.

This Type III pattern argues best for why all patients receiving parenteral opioids on post surgical floors should be continuously monitored during sleep. Yet, as we have already discussed , this can’t work when using oximetry threshold alarms set at 90%.

Type I and II patterns usually take many minutes to hours to evolve before a death occurs, providing staff multiple opportunities to detect trouble and intervene even without continuous monitoring. But with Type III patterns a patient can die in less than 10 unobserved minutes, while sleeping, without any visible or audible warnings unless some capable continuous electronic monitor is being used. Patients not on continuous electronic monitoring are left unobserved by professionals for far longer periods, and are therefore always at risk because of it.

Key take aways:

Opioids can cause two different types of respiratory dysfunction (Type II and Type III)

Type II is CO narcosis—it is caused by neuro-inhibition of the brainstem by opioids

Opioids promote airway collapse and obstruction, worsening the underlying problem of people with obstructive sleep apnea

Both types of respiratory dysfunction can lead to death if not recognized and appropriately intervened

Is there one pulse ox threshold solution for all three patterns?

The answer is yes. In the final post in this series, I will explain how it can be done.