Pulse Oximetry and Non-Invasive Monitoring

Objective. The student will obtain an accurate SpO2 reading, identify sources of artifact and error, and interpret SpO2 trends in conjunction with patient clinical signs to make a documented oxygen titration recommendation.

Pulse oximetry is one of the most common monitoring tools you will use as a respiratory therapist. It gives you a continuous, non-invasive estimate of arterial oxygen saturation—called SpO2—by passing two wavelengths of light through perfused tissue and measuring how much each wavelength is absorbed by oxyhemoglobin versus deoxyhemoglobin. Understanding both what the device tells you and what it cannot tell you is essential for safe patient care.

How the Device Works

A pulse oximeter probe contains a light-emitting diode on one side and a photodetector on the other. The device analyzes only the pulsatile (AC) component of the signal—the part that changes with each heartbeat—so that venous blood and tissue do not skew the reading. The result is displayed as SpO2, the estimated percentage of hemoglobin saturated with oxygen, along with a plethysmographic waveform and pulse rate. A strong, consistent waveform is your first sign that the reading is reliable.

Obtaining an Accurate Reading

Site selection. Finger probes are standard for adults and older children. The index or middle finger is preferred. Use the ear lobe when peripheral perfusion is poor, since the ear warms faster and responds more quickly to saturation changes. Pediatric and neonatal probes are sized to match smaller digits or to wrap around a foot. Always confirm the probe fits snugly but not so tight that it impairs circulation.

Waveform assessment. Before you document an SpO2 value, look at the plethysmographic waveform on the monitor. A sharp, regular waveform that matches the displayed pulse rate tells you the sensor is picking up a true arterial pulsation. A dampened, irregular, or noisy waveform signals artifact—do not chart that number as clinical fact.

Documentation. Record the SpO2 value, the FiO2 or liter flow the patient is receiving, the probe site, the time, and any relevant clinical context (position, activity level, whether the patient just had a coughing episode). This context is what turns a data point into actionable information.

Sources of Error and Artifact

Knowing the limits of pulse oximetry protects your patients from harm caused by false reassurance or unnecessary intervention.

Motion artifact. Patient movement causes the sensor to detect non-pulsatile signals as pulsatile, producing false readings or signal dropout. Immobilize the probe site when possible; many modern devices have motion-rejection algorithms, but these are not foolproof.
Nail polish and artificial nails. Dark or opaque nail polish (especially blue, green, and black) can absorb light at similar wavelengths to deoxyhemoglobin, falsely lowering SpO2. Remove polish or rotate the probe to the side of the finger if removal is not possible.
Poor peripheral perfusion. Vasoconstriction from cold, shock, or vasopressor use reduces pulsatile flow to the fingertip, weakening the signal or making it undetectable. Moving to the ear or using a reflectance probe on the forehead can help.
Dyshemoglobinemias. Standard two-wavelength pulse oximeters cannot distinguish carboxyhemoglobin (COHb) or methemoglobin (MetHb) from oxyhemoglobin. In carbon monoxide poisoning, SpO2 will read falsely normal even when functional oxygen delivery is critically reduced. Suspect this in any patient with possible CO exposure and obtain co-oximetry from an arterial blood gas.
Ambient light interference. Strong external light sources (surgical lights, fluorescent lamps, infrared warmers) can overwhelm the photodetector. Cover the probe with an opaque wrap if interference is suspected.
Anemia. SpO2 measures saturation, not hemoglobin content. A severely anemic patient can have a normal SpO2 yet have dangerously reduced oxygen-carrying capacity. Always interpret SpO2 in the context of the full clinical picture.

Interpreting SpO2 Trends Alongside Clinical Signs

A single SpO2 number is less meaningful than a trend. Watch for direction of change, rate of change, and correlation with other vital signs and clinical appearance.

Maintain current oxygen: SpO2 is within the prescribed target range (commonly 92–98% for most adults; 88–92% for patients at risk for hypercapnia, such as those with chronic hypercapnic COPD), the waveform is strong, and the patient is comfortable with stable respiratory rate and heart rate.
Titrate oxygen up: SpO2 is below the prescribed target and/or the patient shows signs of hypoxemia—increased respiratory rate, use of accessory muscles, restlessness, cyanosis, rising heart rate. Increase the FiO2 or liter flow in appropriate increments per protocol, reassess, and document.
Titrate oxygen down: SpO2 is consistently above the upper limit of the prescribed target. Excess oxygen carries its own risks (absorption atelectasis, worsening hypercapnia in susceptible patients, oxidative stress). Wean in small steps and reassess after each change.
Escalate care: SpO2 fails to respond to supplemental oxygen increases, or the patient shows clinical deterioration—altered mental status, severe work of breathing, hemodynamic instability, or impending respiratory failure. Notify the physician immediately and prepare for advanced interventions.

Safety Points

Never rely on pulse oximetry alone when CO poisoning, methemoglobinemia, or severe anemia is possible.
A technically unreliable waveform means the SpO2 number is unreliable—treat the patient, not the monitor.
Prolonged probe placement on the same site can cause pressure injury or burns, especially in neonates. Rotate probe sites per facility policy.
Oxygen titration decisions must remain within your scope of practice and follow physician orders or approved protocols. When in doubt, communicate with the care team before acting.

Key terms9

SpO2The estimated percentage of hemoglobin saturated with oxygen as measured non-invasively by a pulse oximeter using light absorption.

Plethysmographic waveformThe graphical display of pulsatile blood flow detected by the pulse oximeter; used to assess signal quality and probe placement before trusting an SpO2 reading.

Motion artifactA false or erratic SpO2 reading caused by patient movement that disrupts the sensor's ability to isolate the true arterial pulsatile signal.

DyshemoglobinemiaA condition in which abnormal forms of hemoglobin (such as carboxyhemoglobin or methemoglobin) are present; standard pulse oximeters cannot distinguish these from oxyhemoglobin, leading to falsely elevated SpO2 readings.

Co-oximetryA laboratory measurement performed on an arterial blood sample that uses multiple wavelengths of light to identify and quantify all hemoglobin species, including carboxyhemoglobin and methemoglobin.

FiO2Fraction of inspired oxygen; the proportion of oxygen in the gas mixture a patient is breathing, expressed as a decimal (e.g., 0.21 for room air, 1.0 for 100% oxygen).

Oxygen titrationThe process of adjusting supplemental oxygen delivery up or down in measured steps to maintain a patient's SpO2 within a prescribed target range.

Peripheral perfusionThe adequacy of blood flow to distal tissues such as fingers and toes; poor peripheral perfusion weakens the pulse oximeter signal and can produce unreliable readings.

Absorption atelectasisLung collapse that can occur when high concentrations of oxygen replace nitrogen in the alveoli, causing rapid oxygen absorption and alveolar closure; a risk of excessive supplemental oxygen.

Worked examples3

1. A patient on 2 L/min nasal cannula has a documented SpO2 of 88%. The plethysmographic waveform is strong and regular. The prescribed SpO2 target range is 92–98%. The patient is alert, reporting mild shortness of breath, and has a respiratory rate of 22 breaths/min. What is the appropriate first action, and how should it be documented?

Step 1 – Confirm the reading is reliable. The waveform is strong and regular, so the SpO2 of 88% is a valid measurement.
Step 2 – Compare to the prescribed target. The target is 92–98%; 88% is below the lower limit, indicating hypoxemia that needs to be corrected.
Step 3 – Assess clinical signs. Respiratory rate of 22 breaths/min (elevated), patient-reported dyspnea, and SpO2 below target all support the need to increase oxygen.
Step 4 – Titrate oxygen up per protocol. Increase nasal cannula flow from 2 L/min to 4 L/min as an initial step, then reassess SpO2 and clinical signs after 2–5 minutes.
Step 5 – Reassess. If SpO2 rises to 92–98% and symptoms improve, maintain the new flow. If SpO2 does not improve or the patient deteriorates, escalate care and notify the physician.
Step 6 – Document. Record time, SpO2 value (88%), probe site and waveform quality, previous flow (2 L/min), new flow (4 L/min), reason for change, patient symptoms, and plan to reassess.

Answer: Increase nasal cannula flow to 4 L/min, reassess in 2–5 minutes, and document the indication, intervention, and reassessment findings. If SpO2 does not reach target or the patient worsens, escalate care.

2. A patient is brought to the ED after being found unconscious in a garage with a running engine. SpO2 reads 99% on room air and the patient's skin appears pink. Should you reassure the team that oxygenation is adequate? Explain your reasoning.

Step 1 – Identify the clinical context. Possible carbon monoxide (CO) poisoning from enclosed-space engine exhaust.
Step 2 – Recall the limitation of standard pulse oximetry. A two-wavelength pulse oximeter cannot distinguish carboxyhemoglobin from oxyhemoglobin; both absorb red light similarly, causing the device to report COHb as SaO2.
Step 3 – Interpret the SpO2 reading correctly. The 99% reading is falsely elevated because it is counting COHb as oxyhemoglobin. The patient's true functional oxygen saturation may be critically low.
Step 4 – Recognize that pink skin color is also misleading in CO poisoning; cherry-red appearance is a classic but unreliable sign and does not indicate adequate oxygenation.
Step 5 – Recommend the correct diagnostic test. Order co-oximetry from an arterial blood gas to measure actual COHb level and functional SaO2.
Step 6 – Immediate treatment. Administer 100% oxygen via non-rebreather mask while awaiting results; this accelerates CO elimination.

Answer: No. The SpO2 of 99% is unreliable in suspected CO poisoning because standard pulse oximeters read carboxyhemoglobin as oxyhemoglobin. Co-oximetry from an arterial blood gas is required to determine actual functional saturation, and 100% oxygen should be applied immediately.

3. During a lab exercise, a classmate applies a finger probe and the monitor shows SpO2 of 79% with an irregular, noisy waveform. The classmate looks completely well, has pink nail beds, and is speaking normally. What is the most likely explanation, and what should be done?

Step 1 – Assess the waveform first. An irregular, noisy waveform indicates a poor-quality signal—the device is not reliably detecting a true arterial pulsation.
Step 2 – Consider common causes. The probe may be loose, the finger may be moving, there may be nail polish, or the probe is misaligned.
Step 3 – Compare the reading to clinical appearance. A healthy person who is speaking normally and has pink nail beds would not have an SpO2 of 79%; this reading is inconsistent with the clinical picture.
Step 4 – Correct the problem. Reposition or replace the probe, ensure the finger is still, remove or work around nail polish, and confirm the probe is fully seated.
Step 5 – Reassess after correction. A reliable reading will show a strong, regular waveform whose pulse rate matches the patient's palpated or displayed heart rate, and SpO2 consistent with a healthy individual (typically 95–100%).

Answer: The low SpO2 of 79% is almost certainly artifact from a poor-quality signal, as indicated by the noisy waveform and the mismatch with the patient's healthy clinical appearance. Reposition the probe, reassess the waveform, and obtain a reliable reading before drawing any clinical conclusion.

In class: Lab exercise: students apply pulse oximeter probes to their own finger and to a simulation arm, introduce controlled variables (motion, nail polish simulator, vasoconstriction model, poor perfusion state) and document how each affects waveform quality and SpO2 reading. · Case interpretation exercise: students receive eight printed patient strips showing SpO2 trends alongside heart rate, respiratory rate, and brief history; they write a one-paragraph recommendation for each case (maintain, titrate up, titrate down, or escalate care). · Peer teaching: each student prepares a two-minute explanation of one SpO2 error source (assigned randomly) and presents to the group, fielding one question from classmates.
Materials: Pulse oximeters (finger, ear, and pediatric probes), simulation arm with perfusion control, printed SpO2 trend strips, clinical scenario packets, Egan's Fundamentals (monitoring chapter), AARC pulse oximetry guideline summary
Check: Case interpretation exercise graded with answer rubric; pass mark 75%; instructor observation of probe placement and artifact identification during lab: all critical steps correct (waveform assessment, site selection, documentation) required to pass.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.