top of page
Post: Blog2_Post

Signal Through Noise


We have all heard the phrase “treat the patient, not the monitor.” Even though this quote is overused and often misapplied, it stems from the fact that the equipment we use to assess and construct a field diagnosis is sometimes .. well, just flat-out wrong. Every provider can probably think of a time they had to go back and fix some vital signs imported from the monitor that did not make sense in the patient care report.


This may be due to the pulse ox being on the same finger as the blood pressure cuff, showing a drastic drop in SPO2, or the vibrations from the helicopter giving an obscure blood pressure.


As clinicians, we attempt to weed out the noise and find the signal of what is actually going on with the patient. In this blog, we will look at several sources of noise that can be a distraction.


Blood Pressure

The automatic blood pressure uses something called an oscillometric technique. This means that as the cuff begins to release, small amounts of blood will begin to go through the cuff, causing oscillations. The maximal amplitude of oscillations will be at the mean arterial pressure. As pictured below:

The monitor will take that MAP and try to extrapolate systolic and diastolic pressures based on proprietary algorithms that will estimate at which % away from the peak oscillation systolic and diastolic can be observed.

My main takeaway from this study is that the point of maximal oscillations corresponds with the MAP, which is the most pronounced part of the blood pressure reading. The oscillation points correlating to systolic and diastolic are less pronounced and not always as accurate.


What does this mean for my clinical practice?

My personal takeaways are for one, that It is important to trend your MAP's as opposed to the systolic and diastolic with automatic cuffs. Consider a hypotensive patient with these vitals.

Systolic or MAP & When?

There are a lot of guidelines that predicate or trigger interventions based on a systolic parameter. It is important to understand the essence of that particular guideline. Most of these can be seen in the critical care transport setting, particularly neuro. For example, in head bleeds, two components of blood pressure are important: cerebral perfusion and shearing force. The cerebral perfusion pressure can be calculated by subtracting the ICP from MAP, so the oscillometric blood pressure works pretty well for this. However, the shear force relates to the increase in arterial volume after the heart contracts during systole. This pressure can cause rupture or further dissection of vulnerable vessels. To get a true systolic, The patient will need either a manual blood pressure taken or an arterial line placed. Check out this conversation I had with Bryan Boling. Bryan is an NP and co-host of the Critical Care Scenarios podcast.


ECG Artifact

Nothing is more frustrating than a noisy ECG. Considering the small amount of electricity our heart generates in a world powered by technology, it is not hard to imagine why it could be difficult to block out ambient interference. Cardiac monitors have come a long way in an attempt to isolate biological signals. This is primarily done utilizing filters. I found an interesting article on how difficult this process can be by Peter Selvey from MEDTEQ.


He states:

"Ideally, a filter should remove noise without affecting the signal we are interested in. Unfortunately, this is rarely possible. One reason is that the signal and noise may share the same frequencies. Mains (powerlines) noise (50/60Hz), muscle noise, and drift in dc offsets due to patient movement all fall in the same frequency range as a typical ECG."


One interesting way the ECG filters out interference is by utilizing the right leg lead as a reference. The monitor can filter out unnecessary noise by evaluating common environmental noise (powerlines) and comparing them to the right leg. This is why the right leg lead coming unattached from the patient can cause a wandering baseline or artifact.

Because the electrodes will reject common interference, it is important for them to all have similar contact with the skin. If the contact of one electrode is poor due to placement over hair or dried-out electrodes, the interference will be different from that of the other electrodes and not "common"; therefore, it will remain.


While researching this topic, I found it interesting how the filters do not often have hard-cut-off filters. They can be dynamic and adjust over time. This is why it is not uncommon to have an ECG that looks very noisy shortly after attaching the electrodes, and then a few minutes later, it looks nice and clean.


SPO2

The pulse oximeter typically uses two light sources: one red light and one infrared light. The device emits these lights and passes through a part of your body, often a fingertip or earlobe. The light absorption pattern changes when your heart pumps oxygen-rich blood through your arteries. Oxygenated hemoglobin absorbs more infrared light, allowing more red light to pass through. Deoxygenated hemoglobin absorbs red light, allowing more infrared light to pass through. By comparing the amounts of light absorbed by each wavelength, the pulse oximeter can determine the oxygenated to deoxygenated hemoglobin ratio in your blood.

One of the pitfalls a provider can fall into when interpreting the SPO2 reading is not paying attention to the pleth wave. The pleth wave represents the arteries dilating during systole and allows the pulse oximeter to be a better differentiator of oxygenated hemoglobin in veins than arteries.

Because non-oxygenated hemoglobin absorbs more red light and oxygenated hemoglobin absorbs more infrared light, the pleth looks at how much light makes it through when the vessel dilates (systole) compared to when it is relaxed (diastole).

Because the pleth waveform measures the distance the light travels between pulsations, it is prone to artifact from moving. This can be seen in most helicopters during the initial ascend or in the back of an ambulance on Milwaukee streets.

The probe and sensor are detecting variable distances that are not consistent. Even though the SPO2 is within an acceptable range, the reading should be scrutinized, and attempting to remove the artifact should be the next step. This may be as simple as placing a rolled-up blanket or pillow under the arm.


In addition to movement, another form of "noise" is ambient lighting. Fluorescent lights have a wavelength very similar to that of red light. Because more red light passes through oxygenated hemoglobin, the fluorescent could trick the pulse oximeter into thinking that the SPO2 is higher than it actually is. The LED from the pulse oximeter records the ambient lighting in between the flickers of its own light and determines the "dark signal."


Similar to how the ECG utilizes the right leg electrode to detect and filter out consistent noise, The lighting that exists when the pulse ox light is off is assumed to be ambient lighting.


The problem is when the ambient lighting is not consistent. This may happen with the flicking of the sun through the rotor blades in a helicopter or the sun passing through the ambulance window during transport.


Nail Polish (False Low)

Nail polish, particularly black, blue, or green, can reduce the amount of light that is detected by the sensor and underestimate oxygenated hemoglobin. This would cause a false low, meaning the oxygenation is higher than we think. The best practice is to remove the nail polish. However, if nail polish removal is unavailable, turning the SPO2 sensor to transmit through the side of the finger instead of through the nail bed can help improve accuracy.


Melanin Concentration (False High)

While underestimating someone's oxygenation is not ideal, overestimating oxygenation can lead to delayed detection of hypoxia. During COVID-19, a study found that populations with high melanin concentrations (black, Hispanic, and Asian) overestimated oxygenated hemoglobin, causing a delay in treatment. This is because melanin absorbs infrared light & UV light, similar to oxygenated hemoglobin. Does this mean that populations with high melanin concentrations should always get oxygen? Of course not, but with this information, the clinician should err on the side of caution and always ask if the numbers from our tools match the patient's clinical presentation.

Conclusion

Technology has provided the modern-day clinician with valuable tools to aid in assessing and quickly identifying life-threatening conditions that require timely intervention. A clinician who knows how to weed through the noise and find a signal of truth amongst the array of diagnostics available will serve their patient best.



References:

Babbs C. F. (2012). Oscillometric measurement of systolic and diastolic blood pressures validated in a physiologic mathematical model. Biomedical engineering online, 11, 56. https://doi.org/10.1186/1475-925X-11-56


Fluck, Robert R., et al. "Does ambient light affect the accuracy of pulse oximetry?."Respiratory care 48.7 (2003): 677-680.


Sinex, James E. "Pulse oximetry: principles and limitations."The American journal of emergency medicine 17.1 (1999): 59-66.


Interested in learning more? Check out FOAMfrat Studio!


Comments


bottom of page