What Are Pleth and SpO2 and What Do They Indicate?

Pleth is the continuously scrolling waveform you see on a bedside monitor or pulse oximeter, and SpO2 is the percentage displayed beside it. Together they tell clinicians two things: how blood is pulsing through your tissue (the pleth wave) and how much of your hemoglobin is carrying oxygen (the SpO2 number). Both come from the same underlying technology, photoplethysmography, which shines light through your skin and reads what comes back. The relationship between the waveform and the number is tighter than most people realize, and understanding one helps you make sense of the other.

How a Pulse Oximeter Actually Works

The word “pleth” is short for plethysmograph, a device that measures volume changes. In this case, photoplethysmography (PPG) is an optical technique that detects blood volume variations in living tissue by shining light into the skin and measuring how much is absorbed or reflected back.¹ A small sensor, usually clipped to your fingertip, contains two light-emitting diodes (one red, one infrared) and a photodetector on the opposite side. Every time your heart beats, a small surge of arterial blood expands the capillaries in your finger. That surge absorbs a bit more light than the baseline tissue around it, creating a pulsing signal synchronized with your heartbeat.²

The clever part is in how the device separates oxygen information from that pulsing signal. Oxygenated hemoglobin absorbs more infrared light, while deoxygenated hemoglobin absorbs more red light.³ By comparing the ratio of red-to-infrared absorption in the pulsing (arterial) portion of the signal, the device calculates SpO2.⁴ The pleth waveform is the raw visual output of that pulsing light signal. SpO2 is the processed number derived from it. They are two faces of the same measurement.

What SpO2 Tells You

SpO2 stands for peripheral oxygen saturation and represents the percentage of hemoglobin molecules in your arterial blood that are bound to oxygen. A reading of 98% means that roughly 98 out of every 100 hemoglobin molecules passing through the sensor are carrying oxygen. In healthy adults breathing room air, readings between 95% and 100% are considered normal. Values below 90% generally signal a problem that needs attention, and below 88% is typically classified as a threshold for clinical concern.

What SpO2 specifically reflects is “functional” hemoglobin saturation: the ratio of oxygenated hemoglobin to hemoglobin that is capable of carrying oxygen.⁵ This distinction matters because some forms of hemoglobin cannot carry oxygen at all. Carbon monoxide, for instance, locks onto hemoglobin so tightly that the molecule becomes useless for oxygen transport. A standard two-wavelength pulse oximeter cannot reliably distinguish carbon-monoxide-bound hemoglobin from oxygenated hemoglobin, which means SpO2 can look reassuringly normal even when a significant fraction of your blood is carrying carbon monoxide instead of oxygen. Specialized devices called pulse CO-oximeters attempt to detect carboxyhemoglobin noninvasively, but a systematic review found they have only moderate sensitivity and specificity, with wide limits of agreement compared to blood tests.⁶

The relationship between SpO2 and the actual amount of oxygen dissolved in your blood is not a straight line. The hemoglobin-oxygen dissociation curve, which maps the relationship between oxygen saturation and the partial pressure of oxygen in the blood, has an S-shaped curve.⁷ The practical consequence: at the top of the curve, where healthy people sit, a small drop in SpO2 (say, from 98% to 94%) reflects a relatively modest change in oxygen levels. But once SpO2 falls below about 90%, the curve steepens dramatically, and every additional percentage-point drop represents a much larger decrease in actual oxygen delivery. That is why clinicians treat a reading of 88% very differently from 93%, even though the numerical gap is small.

Reading the Pleth Waveform

The SpO2 number gets most of the attention, but the pleth waveform scrolling across the screen carries its own information. A healthy waveform has a crisp upstroke as the heart ejects blood, a rounded peak, and then a downslope that usually includes a small secondary bump called the dicrotic notch. That notch marks the moment the aortic valve closes and blood briefly bounces back against it. Researchers analyzing the dicrotic notch in fingertip plethysmograms of young adults found that its height and timing vary with age and sex, with the reflected-wave amplitude being larger in older subjects and the timing of the reflected wave arriving later in women.⁵

In practice, clinicians glance at the pleth waveform for quick clues about the patient’s circulation. A tall, well-defined wave with a clear dicrotic notch suggests good blood flow to the fingertip. A small, flattened, irregular wave often means poor perfusion, which could be caused by cold hands, low blood pressure, vasoconstriction from medications, or peripheral vascular disease. An erratic, noisy trace usually points to motion artifact, meaning the patient is moving the sensor. When the waveform looks bad, the SpO2 number becomes less trustworthy, because the device depends on a clean pulsatile signal to make its calculations.

Metrics Hidden Inside the Pleth Signal

Beyond SpO2, modern monitors extract additional measurements from the pleth waveform. Two of the most clinically useful are the perfusion index (PI) and the pleth variability index (PVI). The perfusion index quantifies the strength of the pulsatile signal relative to the non-pulsatile background. A higher PI means stronger arterial pulsation at the sensor site. PVI tracks how much PI fluctuates with the breathing cycle. In patients on mechanical ventilation, large swings in PVI can suggest that the patient would benefit from receiving more intravenous fluids, a concept called fluid responsiveness. A prospective study of spontaneously breathing post-surgical patients confirmed that PI correlates with stroke volume and that PVI can help predict fluid responsiveness.⁸

None of these derived metrics require an extra sensor or an arterial line. They come for free from the same fingertip clip, which is why pulse oximetry has evolved far beyond a simple oxygen number.

What Can Throw Off the Readings

Pulse oximeters are remarkably useful, but they are not infallible. A number of factors can push SpO2 readings higher or lower than the true oxygen saturation in your blood. Increased skin pigmentation, certain hemoglobin disorders, and high skin temperatures can cause the device to overestimate SpO2, while poor blood flow, cold fingers, nail polish, tattoos on the measurement site, severe anemia, and actual low oxygen levels can lead to underestimation.⁹

Nail polish is one people often ask about. A systematic review found that dark-colored polishes (black, blue, brown, and purple) did produce statistically significant drops in SpO2 readings, but the differences were small and generally not clinically meaningful.¹⁰ The exception was that one study reported a high proportion of completely failed readings from fingers painted with black polish (88% failure rate) and brown polish (36% failure rate). So while most polish colors are unlikely to cause a dangerously wrong reading, very dark shades can prevent the oximeter from working at all. If you are in a hospital and a nurse asks you to remove polish from one finger, that is why.

Motion is arguably the most common source of error in everyday clinical settings. When you move the finger wearing the probe, the sensor picks up light fluctuations that have nothing to do with your heartbeat, and the device can generate wildly inaccurate numbers or fail to produce a reading altogether. Advanced signal-processing approaches, such as Masimo’s Signal Extraction Technology, use adaptive filtering and digital signal processing techniques to separate the true arterial signal from motion noise.¹¹ These technologies have reduced false readings in restless patients, though no system eliminates the problem entirely.

The Skin Pigmentation Problem

One of the most important accuracy concerns has to do with skin color. Pulse oximeters were historically calibrated using study populations that were predominantly light-skinned, and the physics of how melanin interacts with red and infrared light means the devices tend to overestimate SpO2 in people with darker skin. This overestimation leads to a higher rate of “occult hypoxemia,” where the oximeter displays a reassuring number (say, 94%) while the patient’s actual arterial oxygen saturation has fallen below 88%, a level that would normally trigger clinical intervention.¹² This is not a small academic concern. It means that Black patients can be genuinely hypoxic without the alarm sounding or the number looking abnormal.

A prospective study looking at the combined effect of skin pigmentation and perfusion found that in lightly pigmented subjects, only low perfusion (weak pulse at the sensor) increased the bias of the reading. But in medium- and dark-skinned subjects, the bias worsened with both low perfusion and the degree of actual hypoxemia, creating a compounding error right when accuracy matters most.¹³ This has motivated research into improving pulse oximeter design and updating regulatory requirements to ensure devices are validated across a range of skin tones.¹⁴

Where You Put the Sensor Matters

The fingertip is the default location for a pulse oximeter sensor, and for good reason. When researchers compared finger probes to nose, ear, and forehead probes under conditions of poor perfusion, fingers came out on top. Some ear probes performed comparably, but nose and forehead probes performed poorly overall, and the study recommended finger probes in patients with weak peripheral circulation.¹⁵

A study of cardiac surgery ICU patients that compared finger, toe, forehead, and earlobe probes found that the finger had the highest correlation with arterial blood gas measurements, while the earlobe had the lowest correlation. Interestingly, though, the forehead showed the best agreement (closest match) with the blood-gas standard, while the earlobe again performed worst.¹⁶ The discrepancy between correlation and agreement is a subtle but real distinction: a probe can track changes faithfully (high correlation) while still reading consistently a point or two off (lower agreement), or vice versa. In practice, fingers remain the go-to site, with the forehead or earlobe used when the hands are unavailable, burned, or too cold to get a signal.

The Alarm Fatigue Problem

Anyone who has spent a night in a hospital knows the relentless beeping of monitors. Pulse oximeters are among the worst offenders. Alarm fatigue, the desensitization of healthcare workers caused by constant alerts, has been recognized as one of the most serious problems in the hospital environment, and excessive false physiologic monitor alarms are a major driver.¹⁷

In neonatal intensive care units, the problem is particularly acute. A quality improvement study at one NICU found that nearly three-quarters of pulse oximeter alarms (about 74%) in the baseline phase were false, occurring at a rate of roughly 0.39 alarms per patient per hour. The main causes were infant movement, inappropriate alarm thresholds set too narrowly, and loose probes.¹⁸ When nearly three out of four alarms are meaningless, the staff naturally starts to tune them out, which is dangerous when the one real alarm finally sounds. Hospitals are increasingly turning to smarter alarm algorithms, adjustable delay periods, and better probe fixation techniques to reduce the noise without silencing the genuine warnings.

Pulse Oximetry in Newborns

Neonates present a unique challenge for pulse oximetry because their blood contains a high proportion of fetal hemoglobin, which behaves differently from adult hemoglobin in how readily it binds and releases oxygen. The fetal form holds onto oxygen more tightly, which shifts the dissociation curve to the left. In practical terms, this means a given SpO2 reading corresponds to a different actual oxygen level in a newborn than it would in an adult.¹⁹ Clinicians caring for premature infants need to account for this shift, especially when setting the lower SpO2 alarm limits. Targeting the same numbers as for adults could expose a preterm baby to oxygen levels that are too low or, conversely, to supplemental oxygen levels that are too high, which carries its own risks for developing eyes and lungs.

Overnight Oximetry and Sleep Apnea

One of the most common outpatient uses of continuous SpO2 monitoring is screening for obstructive sleep apnea (OSA). When someone’s airway repeatedly collapses during sleep, each episode causes a brief dip in oxygen saturation. By wearing a finger probe overnight, patients generate a record of these desaturation events. The oxygen desaturation index (ODI), defined as the number of times per hour that SpO2 drops by at least 3% for more than ten seconds, is calculated from this recording. A study using high-resolution pulse oximetry categorized patients as having mild, moderate, or severe OSA based on ODI thresholds of 5–15, 15–30, and 30 or more events per hour, respectively, and explored whether ODI could serve as an accessible alternative screening parameter to the traditional measure used in full sleep studies.²⁰

Overnight oximetry is attractive because it is cheap, easy to do at home, and does not require spending a night in a sleep lab. It works well as a screening tool for severe OSA, where the desaturation events are frequent and deep. It is less sensitive for mild cases, where the drops may be subtle or the airway obstruction causes arousals without significant oxygen dips. Still, for many patients, a fingertip probe worn overnight is the first step toward diagnosis.

Blood Pressure Estimation From the Pleth Signal

Researchers have been exploring whether the pleth waveform can do even more than track oxygen levels and pulse quality. One active area of investigation is cuffless blood pressure estimation. The timing and shape of the pleth wave contain information about vascular stiffness and the speed at which the pulse wave travels through the arteries, both of which relate to blood pressure. A study using continuous-wave radar alongside a photoplethysmograph showed that measuring pulse transit time from the PPG signal could estimate systolic blood pressure, with correction for a physiological delay improving accuracy by about 10%.²¹ Other groups have built neural-network models that take a raw PPG signal as input and output a full blood pressure waveform, along with systolic and diastolic values.²²

None of these approaches has replaced the arm cuff in routine care yet. Calibration is tricky because the relationship between pulse transit time and blood pressure varies between individuals and changes with posture, stress, and medication. But the appeal is obvious: if a fingertip sensor or wristband could provide continuous, cuffless blood pressure readings alongside SpO2 and heart rate, it would transform monitoring for patients with hypertension, heart failure, and other chronic conditions. The pleth waveform that scrolls by mostly unnoticed on hospital screens may end up carrying more diagnostic weight than anyone imagined when Alrick Hertzman first observed the photoplethysmogram in 1937.⁵