Pulse Oximeter Waveform, Plethysmography & Respiratory Rate: What the Wave Shape Tells You
Most people glance at the number on a pulse oximeter and move on. But the moving wave underneath that number, called the plethysmograph or "pleth," carries information that a single percentage cannot capture on its own.
That waveform can indicate poor circulation, an irregular heartbeat, fluid loss, or a compromised reading caused by skin tone or motion. Understanding what it shows helps clinicians and home users evaluate whether a reading is trustworthy, and when to seek further assessment.
This guide covers everything the waveform shows: what is normal, what is abnormal, how it connects to respiratory rate, and where its limits lie, including what a standard device cannot detect, like carbon monoxide poisoning.
What Is a Photoplethysmography (PPG) Waveform on a Pulse Oximeter?
Photoplethysmography (PPG) is an optical technique that detects blood volume changes in tissue by shining light through it. A pulse oximeter is, at its core, a PPG device: it emits red (660 nm) and infrared (940 nm) light through a fingertip, and a photodetector on the other side measures how much light passes through with each heartbeat.
As the heart pumps, a small surge of arterial blood enters the fingertip capillary bed. That surge absorbs slightly more light. The detector captures this rhythmic rise and fall and plots it over time, and that plot is the pulse oximeter waveform, also called the pleth or plethysmograph.
According to Allen J. (Physiological Measurement, 2007), the PPG waveform has two distinct signal components:
- The AC component: the pulsatile signal generated by each heartbeat. This is the wave visible on screen.
- The DC component: the non-pulsatile background signal from tissue, venous blood, and the constant portion of arterial blood.
The device subtracts the DC component and amplifies the AC signal. The ratio of red-to-infrared AC absorbance then feeds into the SpO2 calculation. The waveform itself is a direct visualization of that AC component, reflecting real-time arterial blood movement at the probe site.
The Pleth as a Signal Quality Indicator
The pleth is not decorative. It serves as a signal quality validator: the waveform tells you whether the SpO2 number above it is trustworthy. A strong, consistent wave with an even rhythm supports confidence in the numerical reading. A flat, jagged, or erratic trace indicates the number may not reflect the patient's actual oxygen saturation.
One caution worth noting: some pulse oximeters automatically rescale the Y-axis (vertical height) when the signal is weak, making a low-amplitude waveform appear taller than it is. This auto-scaling can lead a clinician or home user to believe the signal is stronger than it actually is. Checking the device's dedicated signal quality indicator alongside the waveform shape is the more reliable approach.
How to Read a Normal Pulse Oximeter Waveform
A healthy pleth has four consistent characteristics:
| Feature | What to Look For |
|---|---|
| Shape | Smooth asymmetric hump: steep rise, gradual fall |
| Amplitude | Consistent peak height across beats |
| Rhythm | Even spacing between peaks, matching heart rate |
| Dicrotic notch | Small secondary dip on the descending limb (may or may not be visible) |
The Ascending Limb
The sharp upstroke, called the anacrotic limb, reflects the rapid ejection of blood from the left ventricle during systole. It should be steep and clean.
The Peak
The crest of each wave corresponds to the point of maximum blood volume in the peripheral vessel. In a well-perfused fingertip, this peak is prominent and consistent across beats.
The Dicrotic Notch
A small secondary deflection on the descending limb, the dicrotic notch, corresponds to closure of the aortic valve. It is visible on some pleth displays and absent on others, depending on the device's filtering method and the patient's cardiovascular state. Its absence does not indicate a problem.
The Descending Limb
The gradual downslope represents blood draining from the peripheral capillary bed between beats. It should be smooth and continuous.
A waveform that repeats this pattern consistently, with even spacing and stable amplitude, indicates the probe has a reliable signal and the SpO2 reading is likely accurate.
Abnormal Waveforms: What Arrhythmia, AFib & Poor Perfusion Look Like
Deviations from the normal waveform pattern each carry distinct meaning. Here are the four most clinically significant patterns:
Low Perfusion: The Sine-Wave Pattern
When peripheral blood flow is reduced, from cold hands, vasoconstriction, or hypovolemia, the sharp peaks of a normal pleth flatten into rounded, low-amplitude humps resembling a sine wave.
According to Reisner A et al. (Journal of Trauma, 2008), changes in pulse oximeter waveform amplitude, width, and area correlate strongly with progressive reductions in stroke volume. These waveform changes appear before arterial blood pressure drops measurably, which supports the pleth as an early indicator of reduced blood volume.
When a sine-wave pattern appears: reposition the probe, warm the extremity, and check the device's signal strength indicator before relying on any SpO2 value generated under those conditions.
Motion Artifact: The Jagged Waveform
Patient movement, tremor, or shivering introduces high-frequency noise into the signal, producing an erratic, spiky trace. The device cannot reliably separate the motion signal from the pulse signal under these conditions, which can shift the SpO2 reading by several percentage points in either direction.
Masimo's Signal Extraction Technology (SET), described in Masimo's clinical white papers, was developed specifically to distinguish true arterial pulsation from motion artifact, a limitation that persists in many standard pulse oximeters.
When motion artifact appears: ask the patient to hold still, stabilize the probe, and wait for the waveform to settle before reading the number.
Irregular Rhythm: Uneven Peak Heights and Spacing
When the heart beats irregularly, as in premature ventricular contractions (PVCs) or atrial fibrillation, beats following a shorter interval produce less ventricular filling and therefore a smaller, weaker pulse. This appears on the pleth as irregular spacing between peaks and variable peak heights.
This pattern alone is not a diagnosis. Irregular pleth peaks that do not resolve with probe repositioning are a prompt for further cardiac evaluation.
What Does AFib Look Like on a Pulse Oximeter Waveform?
Atrial fibrillation (AFib) is a heart rhythm disorder in which the upper chambers of the heart beat chaotically instead of contracting in an organized rhythm. According to the American Heart Association, AFib affects more than 12 million Americans and is the most commonly diagnosed serious cardiac arrhythmia.
On a pulse oximeter waveform, AFib produces a recognizable pattern:
- Irregular intervals between peaks (no two peak-to-peak intervals are the same)
- Variable peak amplitudes (beats with shorter filling time produce smaller peaks)
- No consistent repeating baseline between waves
This irregular pattern contrasts with the even, repeating humps of a normal sinus rhythm.
A pulse oximeter cannot diagnose AFib. Diagnosis requires an electrocardiogram (ECG/EKG). The pleth can indicate that a rhythm is irregular, particularly when peak spacing is markedly inconsistent, but it cannot distinguish AFib from other irregular rhythms like frequent PVCs or second-degree heart block.
If a persistently irregular waveform pattern appears on a home pulse oximeter, speaking with a doctor about whether an ECG is appropriate is a reasonable next step. Some newer wearable devices with dedicated photoplethysmography algorithms carry FDA clearance for AFib detection, but a standard fingertip pulse oximeter does not.
Respiratory Rate from Pulse Oximetry: How It Works and How Accurate It Is
The Respiratory Signal in the Pleth
Breathing modulates the pleth waveform in two measurable ways:
- Amplitude variation: Inspiration lowers intrathoracic pressure, which transiently changes venous return and left ventricular stroke volume, causing the pleth amplitude to rise and fall with each breath.
- Baseline shift: The DC component of the PPG signal drifts rhythmically with respiration as thoracic blood volume changes.
By analyzing these rhythmic changes, newer pulse oximeters and monitoring systems can estimate respiratory rate (RR) from the pleth, a parameter sometimes labeled "PRb" (pleth-derived respiratory rate) or simply "RR" on clinical monitors.
Nilsson L et al. (Respiratory Physiology & Neurobiology, 2012) demonstrated that photoplethysmography-derived respiratory rate correlates reasonably well with capnography-measured respiratory rate under stable conditions, though accuracy degrades in patients with shallow breathing, arrhythmias, or poor peripheral perfusion.
The Pleth Variability Index (PVI)
A closely related metric is the Pleth Variability Index (PVI): the percent change in the perfusion index across a full respiratory cycle. A high PVI indicates that the patient's cardiac output responds significantly to breathing, a pattern associated with fluid responsiveness in mechanically ventilated patients. PVI is available on advanced clinical pulse oximeters and is not a feature of standard home devices.
Accuracy by Condition
| Condition | Effect on RR Accuracy |
|---|---|
| Atrial fibrillation | Degrades accuracy (irregular baseline) |
| Low perfusion / cold extremities | Degrades accuracy (weak signal) |
| Motion artifact | Severely degrades accuracy |
| Stable, resting patient | Reasonable correlation with true RR |
Pulse oximeter-derived respiratory rate is a trending tool, not a replacement for direct respiratory rate measurement or capnography in critically ill patients.
Devices That Display Respiratory Rate
Several consumer and clinical devices now display respiratory rate alongside SpO2 and pulse rate. The Masimo RAD-57 and similar co-oximetry platforms include respiratory rate as a monitored parameter. Some retail pulse oximeters marketed with respiratory rate features use proprietary algorithms whose clinical validation data varies by manufacturer. A doctor can advise on individual cases where respiratory rate monitoring is clinically relevant.
Pulse CO-Oximetry: How It Differs from Standard Pulse Oximetry
The Wavelength Limitation of Standard Oximetry
A standard two-wavelength pulse oximeter measures only the ratio of oxyhemoglobin (HbO2) to deoxyhemoglobin (Hb) at 660 nm and 940 nm. It cannot distinguish between hemoglobin species that absorb light similarly at those wavelengths.
Pulse CO-oximetry addresses this by using multiple wavelengths of light to separately identify and quantify additional hemoglobin species in real time.
What Pulse CO-Oximetry Measures
| Parameter | Standard Pulse Ox | Pulse CO-Oximeter |
|---|---|---|
| SpO2 (functional saturation) | Yes | Yes |
| SpCO (carboxyhemoglobin) | No | Yes |
| SpMet (methemoglobin) | No | Yes |
| Total hemoglobin (SpHb) | No | Some models |
| Respiratory rate | Some models | Some models |
The Masimo RAD-57 is the most widely cited FDA-cleared pulse CO-oximeter in clinical literature. It uses Masimo's Rainbow SET technology and is used in emergency departments, military medicine, and critical care settings to screen for carbon monoxide poisoning and methemoglobinemia without a blood draw.
Pulse CO-Oximeter vs Standard Pulse Oximeter: Key Difference
A standard pulse oximeter reports SpO2 as a functional saturation, meaning the fraction of hemoglobin carrying oxygen among only the hemoglobin species the device can detect. Because it cannot detect carboxyhemoglobin (COHb), it includes COHb in its SpO2 reading, producing a normal-appearing value even when carbon monoxide is occupying hemoglobin binding sites.
A pulse CO-oximeter reports SpO2 as fractional saturation and separately reports SpCO, giving clinicians a direct, non-invasive estimate of carbon monoxide load.
Can a Pulse Oximeter Detect Carbon Monoxide Poisoning?
A standard pulse oximeter cannot detect carbon monoxide poisoning. This is one of the most well-documented limitations of the technology.
Why Standard Devices Fail Here
Carbon monoxide binds to hemoglobin with more than 200 times the affinity of oxygen, forming carboxyhemoglobin (COHb). At the 660 nm red wavelength used by standard pulse oximeters, COHb absorbs light very similarly to oxyhemoglobin, so the device cannot distinguish between the two and reports a normal or near-normal SpO2.
Barker SJ & Tremper KK (Anesthesiology, 1987) demonstrated this directly: subjects breathing carbon monoxide maintained apparently normal SpO2 readings on standard pulse oximeters while their true arterial oxygen saturation declined. This study established the basis for understanding why carbon monoxide exposure cannot be evaluated using a standard fingertip oximeter.
The Waveform Does Not Indicate CO Exposure
Unlike arrhythmias or low perfusion, carbon monoxide poisoning does not alter the pleth waveform in a recognizable way. The waveform may appear entirely normal in terms of amplitude, rhythm, and peak shape, even when COHb levels are elevated.
When Carbon Monoxide Exposure Is Suspected
Symptoms of carbon monoxide poisoning include headache, dizziness, nausea, confusion, and shortness of breath, often in a setting of gas appliance use, vehicle exhaust exposure, or enclosed space fires. These symptoms in that context warrant evaluation regardless of what a pulse oximeter reads.
Confirmation requires either:
- Pulse CO-oximetry (SpCO measurement via a multi-wavelength device like the RAD-57), or
- Laboratory co-oximetry on an arterial blood gas sample.
A standard pulse oximeter reading should not be used to rule out carbon monoxide exposure. A doctor can advise on the appropriate assessment approach based on the clinical situation.
Clinical Applications of Waveform Analysis in ICU & Emergency Settings
The pulse oximeter waveform supports several monitoring applications in clinical environments beyond its role as a signal quality indicator.
Hypovolemia and Fluid Responsiveness Assessment
In mechanically ventilated patients, respiratory cycling creates predictable variation in left ventricular stroke volume. This variation is more pronounced when the patient has reduced blood volume. Measuring the percent change in pleth amplitude across the respiratory cycle (PVI or DeltaPOP) provides a non-invasive estimate of whether the patient will respond to intravenous fluids, a question central to ICU management.
Reisner A et al. (2008) showed that waveform features including amplitude, width, and area under the pulse curve tracked progressive blood volume loss in healthy volunteers before blood pressure changed, supporting pleth analysis as an early indicator of hypovolemia.
CPR Quality Monitoring
The pleth waveform can provide real-time feedback on compression quality during cardiopulmonary resuscitation (CPR). High-quality chest compressions that generate adequate cardiac output produce measurable pleth pulses, confirming that forward blood flow is occurring with each compression. An absent pleth signal during CPR indicates compressions are not generating effective perfusion.
Arrhythmia Pattern Recognition
In continuous monitoring settings, irregular pleth patterns serve as prompts for further ECG assessment. While the pleth cannot diagnose a specific arrhythmia, its continuous, non-invasive nature makes it a practical first-layer indicator, especially in step-down units where continuous ECG monitoring may not be in use for every patient.
Peripheral Perfusion Monitoring in Vascular Injury
Because the pulse oximeter measures perfusion at the probe site, it can be placed distal to an injured extremity to detect whether circulation is present or has been restored. This application is used in trauma settings when applying traction splints or monitoring for compartment syndrome.
Frequently Asked Questions
What does the waveform on a pulse oximeter mean?
The waveform, called the plethysmograph or pleth, is a real-time graph of blood volume changes at the probe site with each heartbeat. It indicates whether the SpO2 number on screen is based on a reliable signal. A smooth, consistent, evenly spaced wave supports the numerical reading. A flat, jagged, or irregular wave indicates the reading may not be accurate.
What does an abnormal pulse oximeter waveform look like?
Abnormal waveforms fall into four main patterns: a flattened sine-wave shape (low perfusion or poor circulation), a jagged erratic trace (motion artifact), irregular peak spacing and variable heights (irregular heart rhythm), and a waveform that appears normal in height but is auto-scaled by the device to mask a weak underlying signal. Each pattern has a different cause and a different clinical implication.
Can a pulse oximeter detect atrial fibrillation (AFib)?
A standard pulse oximeter can indicate an irregular heart rhythm by displaying inconsistently spaced peaks with variable amplitudes on the pleth. It cannot diagnose AFib. Diagnosis requires an ECG. Some dedicated wearable devices with FDA-cleared AFib detection algorithms can make that determination, but a standard fingertip pulse oximeter cannot.
Can a pulse oximeter detect carbon monoxide poisoning?
No. A standard pulse oximeter cannot detect carbon monoxide poisoning because carboxyhemoglobin (COHb) absorbs light at the same wavelength as oxyhemoglobin, causing the device to report a normal-appearing SpO2. A pulse CO-oximeter (which uses multiple wavelengths to measure SpCO directly) or a laboratory arterial blood gas with co-oximetry is required for accurate detection.
What is plethysmography in a pulse oximeter?
Photoplethysmography (PPG) is the optical technique that pulse oximeters use to detect blood volume changes in the finger. The device shines light through the tissue and measures how much is absorbed with each heartbeat. The rhythmic variation in light absorption is plotted as the waveform on the display. This waveform is the PPG signal and reflects the pulsatile flow of arterial blood at the probe site.
