This is the third 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 occurs 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 this post, “Detecting Deadly Post-Surgical Respiratory Dysfunction,” I will review the pattern of respiratory compromise that characterizes the conditions not related to opioid use. This post is a bit technical but bear with me. It is crucial to understand this type of respiratory dysfunction so that it can be detected and the patient is treated as early as possible in order to save lives.
Understanding Hyperventilation Compensated Respiratory Failure
The first pattern (Type I) pertains to all the conditions but two opioid associated respiratory dysfunctions. It is called Hyperventilation Compensated Respiratory Failure (HCRF). Although this is a mouthful, you will see that it accurately describes what is happening to the patient.
HCRF reflects a clinically evolving process comprised of microcirculatory failure induced by sepsis, heart failure, aspiration, and pulmonary embolism. Although these conditions can have an insidious onset, this type of respiratory failure can devolve quickly and unexpectedly lead to death if it is not detected very early and managed appropriately.
As shown in the illustration below, the Type I pattern is signaled first by a rising minute ventilation (Ve) accompanied by mild elevations in respiratory rate (RR). This causes carbon dioxide (PaCO2) levels to fall.
It is critically important to be cognizant of the fact that the oxygen saturation (SPO2) remains unchanged. This is because respiratory alkalosis (a change in the blood pH) caused by hyperventilating immediately shifts the oxyhemoglobin dissociation curve leftward (Bohr effect), thus maintaining normal appearing oxygen saturation values even as oxygen partial pressures drop because of accumulating lung injury.
The decline in oxygen saturation (SPO2) begins later in the disease, along with more rapid rise in minute ventilation (Ve). At this point, the rise in respiratory rate (RR) becomes severe (>30 breaths per minute) and results in marked additional falls in carbon dioxide (PaCO2).
If supplemental oxygen is being provided at progressively increasing flow rates using nasal cannulas or O2 masks to maintain oxygen saturations at some preselected value above 90%, this does not treat the condition. It only conceals the patient’s advancing deterioration and the accelerating injury taking place in the patient’s lungs by temporarily elevating oxygen saturation values seen on a pulse oximeter.
Why a 90% Pulse Oximetry Threshold isn’t Good Enough
By the time the 90% pulse oximetry threshold is breached and the alarm is set off, the patient is critically ill and well beyond the golden period for providing optimal rescue. Unfortunately, no oxygen saturation threshold breach can detect Type I (HCRF) patterns early enough for timely intervention.
The best marker available for early detection is taking patient complaints of shortness of breath seriously. Bedside clinicians must assume that any complaints or signs of dyspnea are bad until proven otherwise regardless of normal oxygen saturation values (SPO2) found early on. Unwarranted delays in immediate, thorough evaluation and treatment enable deadly progression of all these possible diseases and are unconscionable.
- Hyperventilation Compensated Respiratory Failure (HCRF) is characterized by an increased respiratory rate, low CO2, and initially, normal appearing oxygen saturation value.
- Patients with HCRF can decompensate quickly and die if not treated early.
- Pulse oximetry threshold alarms are not helpful in guiding early treatment.
- Patients on post-surgical floors who complain of shortness of breath must be evaluated thoroughly to enable early intervention.
Coming next, I will take a deep dive into the two remaining patterns of respiratory dysfunction commonly seen on post-surgical floors.