Pulse oximetry and capnography are the two most widely used continuous monitoring modalities in acute care nursing. Together they give the nurse a real-time window into two distinct but inseparable physiological processes: oxygenation and ventilation. SpO2 tells you whether the blood is carrying enough oxygen; end-tidal CO2 (EtCO2) tells you whether the patient is ventilating effectively enough to eliminate carbon dioxide. Understanding what each measurement actually reflects — and crucially, what it does not — is what separates competent monitoring from genuinely safe monitoring.
This guide covers the physiology behind both modalities, how to interpret values and waveforms, where common errors occur, and how to apply both tools across the clinical scenarios you will encounter most often: post-intubation confirmation, procedural sedation, CPR, and ventilated ICU patients. Twenty NCLEX high-yield tips close the article.
Pulse oximetry: how it works and what it measures
The physics of SpO2
A pulse oximeter probe contains two light-emitting diodes that transmit light at 660 nm (red) and 940 nm (infrared) through a vascular tissue bed to a photodetector. Oxyhemoglobin and deoxyhemoglobin absorb these two wavelengths at different ratios. By comparing the pulsatile (arterial) component of absorbance at both wavelengths, the device calculates the percentage of hemoglobin that is bound to oxygen — the functional arterial oxygen saturation (SpO2).
The key distinction nurses must understand is functional vs fractional saturation. Functional saturation is the percentage of hemoglobin capable of carrying oxygen that is actually saturated. Fractional saturation is the percentage of total hemoglobin that is saturated — and total hemoglobin includes dyshemoglobins like carboxyhemoglobin (COHb) and methemoglobin (MetHb). Standard two-wavelength pulse oximeters measure functional saturation only, which is why carbon monoxide poisoning produces a falsely normal SpO2 (see Limitations below).
The oxyhemoglobin dissociation curve
The relationship between SpO2 and the partial pressure of oxygen (PaO2) is not linear — it follows the S-shaped oxyhemoglobin dissociation curve. Three inflection points are clinically essential:
- PaO2 ~100 mmHg → SpO2 ~98–99%: Normal healthy adult at rest. Increasing PaO2 above 100 mmHg adds very little additional saturation — the curve has plateaued. This is the “flat” upper portion.
- PaO2 ~60 mmHg → SpO2 ~90%: This is the critical inflection point at the “shoulder” of the curve. Above this point, large drops in PaO2 cause only small drops in SpO2. Below it, the curve steepens sharply — small further drops in PaO2 cause disproportionately large drops in saturation. A patient at SpO2 90% is at the cliff edge, not in a stable zone.
- PaO2 ~40 mmHg → SpO2 ~75%: Normal mixed venous oxygen level — the saturation at which tissues return deoxygenated blood to the right heart.
The curve shifts right (reduced hemoglobin affinity for O2, promotes offloading) with acidosis, hyperthermia, hypercapnia, and increased 2,3-DPG. It shifts left (increased affinity, impairs offloading at tissues) with alkalosis, hypothermia, hypocapnia, fetal hemoglobin, and carbon monoxide poisoning. In clinical practice, the right-shift is a compensatory adaptation during hypoxia; the left-shift in CO poisoning is one reason why tissue hypoxia is so severe despite a normal-appearing SpO2.
For a full discussion of the arterial blood gas values that quantify PaO2 and SaO2 directly, see the guide to ABG interpretation.
Probe placement
The finger is the default site — well-perfused, simple to apply, and comfortable. The probe should be placed on the patient’s dominant-hand index or middle finger with the nail bed facing the light source. Avoid the thumb (arterial pulsations can be irregular) and little finger (smallest digit, higher artifact rates).
Earlobe: Faster response time than the finger — approximately 20–30 seconds vs 60+ seconds. Preferred in low-perfusion states (shock, vasoconstriction) because earlobe perfusion is maintained longer. Useful during rapid clinical deterioration when you need the most current value.
Forehead (reflectance probe): Measures SpO2 via reflection rather than transmission. Better signal in low-perfusion states. Useful in patients with cold, poorly perfused extremities.
Nasal alar / nose: Reflectance probe; used when other sites are inaccessible.
Toe / foot: Used in neonates and infants, or when upper extremity sites are unavailable. Note that toe response time is slower than finger and earlobe.
Plethysmographic waveform quality
Most monitoring systems display a plethysmographic (pleth) waveform alongside the numeric SpO2 value. This waveform represents the pulsatile variation in blood volume as detected by the photodetector. A good signal shows a regular, tall, rounded waveform with a clear dicrotic notch — the small notch on the downstroke that represents aortic valve closure. A poor signal is flat, irregular, or noisy with no identifiable dicrotic notch. Never trust a SpO2 numeric value accompanied by a poor pleth waveform — it is the device reporting noise, not physiology.
When the waveform is poor, reposition the probe, try the earlobe, warm the extremity, or dry the probe site before documenting the reading.
Interpreting SpO2 values
Normal ranges and clinical thresholds
| SpO2 range | Interpretation | Nursing action |
|---|---|---|
| ≥95% | Normal in healthy adults | Routine monitoring; ensure good waveform quality |
| 92–94% | Mild hypoxemia — borderline; may be acceptable in some populations | Assess for cause; increase O2 if warranted; increase monitoring frequency |
| 90–91% | At the inflection point — risk of rapid deterioration below this threshold | Notify provider; supplement O2; position patient upright; prepare for possible escalation |
| <90% | Hypoxemia — intervention required | Immediate O2 titration; provider notification; ABG if not already ordered; assess cause |
| <85% | Severe hypoxemia — critical | Urgent escalation; rapid response activation if no physician present; prepare for intubation |
| 88–92% (COPD target) | Acceptable range in patients with chronic hypercapnia | Do not supplement beyond this target without medical order — see COPD section below |
Population-specific targets: COPD and chronic hypercapnia
Patients with severe COPD and chronic CO2 retention have a blunted central chemoreceptor response to hypercapnia. Their primary respiratory drive stimulus becomes peripheral chemoreceptor response to hypoxia — sometimes called hypoxic drive. Over-oxygenation in this population can suppress ventilatory drive, worsen CO2 retention, and precipitate hypercapnic respiratory failure.
The target SpO2 in COPD exacerbation is 88–92%. Titrate supplemental oxygen to achieve and maintain this range — not higher. An SpO2 of 97–100% in a COPD patient on supplemental oxygen should prompt the nurse to reduce the flow rate, not feel reassured. A full discussion of oxygen delivery devices and titration is in the guide to oxygen therapy nursing.
Limitations and sources of error
Understanding where pulse oximetry fails is as important as knowing normal values. A falsely normal SpO2 in a deteriorating patient is dangerous; a falsely low reading in a stable patient triggers unnecessary interventions. The table below covers the most clinically significant sources of error.
| Factor | Effect on SpO2 reading | Nursing response |
|---|---|---|
| Dark or blue/black nail polish | Falsely low reading — absorbs red light wavelength | Remove polish with acetone wipe or reposition probe to earlobe; avoid acrylic nails when possible |
| Poor peripheral perfusion (shock, hypothermia, vasoconstriction) | Unreliable or unobtainable — inadequate pulsatile flow for detection | Move probe to earlobe or forehead; warm extremity; consider arterial line for continuous SaO2 |
| Motion artifact | Spurious readings — random high or low values | Verify probe is secure; use motion-tolerant probe if available; interpret in context of clinical picture |
| Anemia (severe) | Falsely normal SpO2 despite low oxygen content — hemoglobin that is present is saturated, but there is very little of it | Check hemoglobin/hematocrit; calculate oxygen delivery (DO2); SpO2 alone does not reflect oxygen content in severe anemia |
| Carbon monoxide poisoning | Falsely normal — COHb and OHb absorb red light similarly; device reads COHb as OHb | If CO poisoning suspected, do NOT rely on standard pulse oximetry; order co-oximetry (ABG with co-oximetry panel) which measures COHb directly |
| Methemoglobinemia | Reads approximately 85% regardless of true saturation — MetHb absorbs both wavelengths equally at high concentrations | If methemoglobinemia suspected (exposure to oxidizing agents, dapsone, benzocaine), order co-oximetry; prepare for methylene blue administration per order |
| Dark skin pigmentation | Emerging evidence of systematic overestimation of SpO2 vs SaO2 on co-oximetry in patients with darker skin tones — clinically meaningful in borderline hypoxemia | Use lower thresholds for escalation in patients where bias is suspected; ABG co-oximetry provides ground truth; maintain heightened vigilance |
| Venous pulsations | Falsely low readings — elevated venous pressure (tricuspid regurgitation, right heart failure) creates venous pulsation that the device may detect | Evaluate probe site; consider alternative location; correlate with clinical assessment |
| Supplemental O2 masking hypoventilation | SpO2 may remain normal while PaCO2 rises dangerously — patient on O2 can hypoventilate significantly before SpO2 falls | Add capnography in any patient on supplemental O2 where ventilation is a concern (post-op, sedation, opioids); do not rely on SpO2 alone to assess ventilation |
| Excessive ambient light | Spurious readings from fluorescent or fiber-optic light entering the probe | Cover probe with opaque material if bright light source cannot be moved |
Continuous vs spot-check monitoring
Continuous monitoring is indicated for any patient at risk of sudden respiratory deterioration: post-operatively, during and after procedural sedation, on opioid infusions or patient-controlled analgesia, receiving supplemental oxygen for acute illness, or with known cardiorespiratory instability.
Spot-check monitoring (periodic measurement, typically q4h with vital signs) is appropriate for stable patients without active respiratory conditions and not on therapies that suppress ventilation.
The monitoring frequency should escalate with clinical acuity, and the decision to upgrade from spot-check to continuous must be individualized to the patient’s risk profile — it cannot be driven solely by the current SpO2 value.
Capnography: waveform analysis and clinical use
What EtCO2 actually measures
Capnography measures the concentration of carbon dioxide in exhaled gas, expressed as a partial pressure (mmHg) or percentage. End-tidal CO2 (EtCO2) is the CO2 concentration at the end of a complete exhalation — the point at which exhaled gas most closely approximates alveolar CO2 and, by extension, arterial CO2 (PaCO2).
In a patient with normal lung physiology and normal cardiac output, EtCO2 runs approximately 2–5 mmHg below PaCO2. This gradient exists because exhaled gas is diluted by dead-space ventilation (gas from airways that never participates in gas exchange). Normal EtCO2 is 35–45 mmHg. When pulmonary dead space increases — as in pulmonary embolism, severe COPD, or low cardiac output — the EtCO2-PaCO2 gradient widens. In these patients, EtCO2 underestimates PaCO2 more significantly, and the absolute EtCO2 value alone is less useful than the trend.
Two types of capnography are in clinical use:
- Mainstream capnography: Sensor sits directly in the airway circuit. Fast response, no sample lag. Used on intubated patients.
- Sidestream capnography: Gas is aspirated from the circuit or nasal cannula sampling line to an external sensor. Slightly slower response; works on non-intubated patients via nasal cannula or nasal-oral sampling cannula.
Normal capnogram waveform: four phases
A normal capnogram is rectangular with sloped transitions and has four distinct phases. Every nurse monitoring ventilated patients or patients under sedation should be able to identify these on sight.
Phase I (baseline / anatomical dead space): EtCO2 = 0 (or near 0). This is the early part of exhalation — CO2-free gas from the anatomical dead space (trachea, bronchi) exits before alveolar gas reaches the sensor. Normally flat at zero.
Phase II (expiratory upstroke): EtCO2 rises rapidly as alveolar gas displaces dead-space gas at the sensor. Should be steep and linear on a normal capnogram.
Phase III (alveolar plateau): EtCO2 reaches its maximum and forms a near-horizontal plateau as alveolar gas — relatively uniform in CO2 concentration — is continuously exhaled. The plateau should be flat or only slightly upward-sloping. The value at the end of Phase III is the EtCO2 reading. The slight upward slope normally seen reflects heterogeneity in alveolar emptying times.
Phase IV (inspiratory downstroke): CO2 rapidly drops back to zero as fresh inspired gas flushes the sensor. Should be steep and return to baseline (zero). The angle between Phase III and Phase IV is called the alpha angle; between Phase II and Phase III is the beta angle.
Abnormal capnogram patterns
| Pattern name | Appearance | Clinical cause | Nursing action |
|---|---|---|---|
| Normal rectangular | Flat Phase I, steep Phase II, flat Phase III plateau, steep Phase IV return to zero | Normal lung physiology | Continue routine monitoring |
| Shark fin waveform | Phase II upstroke is slowed and merges with Phase III — no clear plateau; the whole curve looks like a shark fin or steep slope | Bronchospasm, severe COPD, asthma exacerbation — airways obstruction causes uneven alveolar emptying, so CO2-rich gas arrives at the sensor gradually rather than in a defined wave | Notify provider; administer bronchodilator per order; assess breath sounds; monitor EtCO2 trend for improvement |
| Sloping Phase III plateau | Phase III has a pronounced upward slope rather than being flat | Obstructive disease (COPD, asthma) — heterogeneous alveolar emptying; also seen with incomplete bronchodilator response | Assess for bronchospasm; correlate with auscultation; document for provider |
| Absent / flat waveform (EtCO2 = 0) | No waveform — flat line at zero | Esophageal intubation (most critical); apnea; circuit disconnection; sensor malfunction | Immediately verify ETT placement if intubated — assess for chest rise, auscultate, arrange chest X-ray; if apneic patient, stimulate or initiate rescue breathing; check all circuit connections |
| Sudden drop to near zero | Previously present waveform drops abruptly to near zero mid-monitoring | Airway disconnection, circuit leak, ETT dislodgement, or cardiac arrest (EtCO2 drops with loss of pulmonary perfusion) | Inspect circuit immediately; check ETT position; assess patient responsiveness and pulse; initiate CPR if cardiac arrest confirmed |
| Rising baseline (above zero) | Phase I does not return to zero — baseline drifts upward with each breath | CO2 rebreathing — exhausted CO2 absorber (in anesthesia circuits), insufficient fresh gas flow, or partial circuit obstruction | Check CO2 absorber; increase fresh gas flow; notify anesthesia or respiratory therapy; replace circuit components as needed |
| Sudden rise followed by sustained elevation | EtCO2 rises abruptly, often to 35 mmHg or above, during cardiac arrest with CPR | Return of spontaneous circulation (ROSC) — restored cardiac output dramatically increases CO2 delivery to the lungs | Notify provider immediately — check pulse and blood pressure; prepare post-ROSC care; EtCO2 rising to ≥35 mmHg during CPR is the most reliable non-invasive indicator of ROSC |
Clinical applications of capnography
Post-intubation confirmation
Continuous waveform capnography is the gold standard for confirming endotracheal tube (ETT) placement in the trachea. When the ETT is correctly placed in the trachea, CO2 is exhaled with each breath and a normal rectangular waveform appears immediately. When the tube is in the esophagus, no CO2 waveform is detected — the EtCO2 reads zero (or near zero after the first 1–2 breaths, as any CO2 from carbonated beverages or gastric gas is rapidly exhausted).
Do not rely on chest rise, epigastric auscultation, or misting in the tube alone to confirm placement — these are adjuncts, not confirmatory tests. Chest X-ray confirms tip position relative to the carina but cannot be obtained fast enough to serve as the primary confirmation method. The capnogram waveform is immediate, continuous, and unambiguous. Full protocols for ETT placement confirmation are covered in airway management nursing.
Procedural sedation monitoring
Capnography detects hypoventilation and apnea 60 or more seconds before SpO2 drops in a patient breathing supplemental oxygen. This detection lag is the most clinically critical reason to use capnography during procedural sedation.
When a sedated patient begins to hypoventilate or becomes apneic, they continue to absorb oxygen from their lungs for approximately 60–90 seconds before SpO2 begins to fall — especially if they received supplemental O2. During this window, capnography has already detected the problem: EtCO2 has stopped cycling (no waveform = apnea) or the value is rising (hypoventilation with CO2 accumulation). Waiting for SpO2 to drop before intervening means a full minute of apnea has gone undetected.
For nurses supporting procedures with moderate or deep sedation, capnography via nasal sampling cannula is standard of care. Assess and document EtCO2 waveform and value at the same intervals as SpO2, heart rate, and blood pressure. See conscious sedation nursing for the full monitoring framework.
CPR and cardiac arrest
During cardiac arrest, cardiac output falls to zero or near zero, and pulmonary blood flow ceases. Without blood flow through the pulmonary vasculature, CO2 cannot be transported to the lungs for exhalation — EtCO2 drops precipitously, typically to less than 10 mmHg or absent waveform.
EtCO2 serves two distinct functions during resuscitation:
-
Compression quality feedback: EtCO2 correlates with cardiac output during CPR. Effective chest compressions that generate adequate cardiac output will produce higher EtCO2 values. An EtCO2 consistently below 10 mmHg despite ongoing CPR indicates inadequate compression depth or rate and should trigger immediate reassessment of technique.
-
ROSC detection: When spontaneous circulation returns, cardiac output is restored, CO2 is rapidly transported to the lungs, and EtCO2 rises sharply — often from <10 mmHg to ≥35 mmHg within 1–2 breaths. This rise typically precedes any palpable pulse and is the most sensitive non-invasive ROSC indicator available. Always check EtCO2 trend before calling ROSC, and consider ROSC when a sustained rise is seen even if a pulse is not immediately palpable.
For a full discussion of defibrillation and post-ROSC care, see cardioversion and defibrillation nursing. Capnography protocols in code situations are covered in rapid response and code blue nursing.
Ventilated ICU patients
In mechanically ventilated patients, EtCO2 provides a continuous, non-invasive approximation of PaCO2 and enables real-time ventilator management without requiring serial ABGs for every adjustment. The key clinical uses include:
- Trending PaCO2: Once the EtCO2-PaCO2 gradient is established from an ABG (normal: 2–5 mmHg), EtCO2 trends reliably track PaCO2 changes. A rising EtCO2 in a ventilated patient indicates CO2 retention — increase respiratory rate or tidal volume per order.
- Detecting circuit leaks and disconnections: A sudden drop in EtCO2 to zero or near zero in a ventilated patient is a circuit disconnection or accidental extubation until proven otherwise.
- PEEP adjustment guidance: Changes in EtCO2 following PEEP adjustments can reflect changes in dead-space ventilation and alveolar recruitment.
- Detecting acute changes in pulmonary perfusion: A sudden decrease in EtCO2 in a ventilated patient (without ventilator changes) can indicate a new pulmonary embolism or acute decrease in cardiac output — both reduce CO2 delivery to the lungs.
Ventilated patient management, weaning criteria, and ventilator settings are covered in depth in mechanical ventilation nursing.
Non-intubated patients via nasal sampling cannula
Nasal capnography cannulas deliver supplemental O2 from one port while aspirating exhaled gas samples from nasal prongs for EtCO2 analysis. This enables combined O2 delivery and capnography monitoring in spontaneously breathing patients without an artificial airway. Signal quality is generally good for patients breathing through their nose; mouth-breathing patients require an oral-nasal sampling cannula.
Indications include: procedural sedation, post-operative monitoring in PACU, patients on opioid infusions, any spontaneously breathing patient in whom ventilation adequacy cannot be assumed from SpO2 alone.
Monitoring during patient transfer
Portable capnography is essential for any high-acuity patient transfer — whether inter-unit (floor to ICU) or inter-facility. During transport, SpO2 and EtCO2 should both be monitored continuously, with waveform display confirmed prior to leaving the originating unit. Intubated patients are at particular risk during transport for inadvertent extubation, and a flat EtCO2 waveform on a transport monitor is the earliest detectable sign. Document both values and waveform quality on the transfer handoff record.
When using bag-valve-mask ventilation during transport, capnography confirms each ventilation is reaching the lungs. For BVM technique and circuit management, see bag-valve-mask nursing.
Using SpO2 and EtCO2 together
Pulse oximetry and capnography monitor two independent physiological processes. SpO2 measures oxygenation — whether hemoglobin is carrying oxygen. EtCO2 measures ventilation — whether CO2 is being eliminated. Each has blind spots that the other covers.
What SpO2 detects that capnography misses: Hypoxemia in patients with adequate ventilation — hypoxia from diffusion impairment, V/Q mismatch, or low FiO2 in a patient breathing adequately. Capnography can be completely normal while the patient is severely hypoxemic.
What capnography detects that SpO2 misses: Hypoventilation and apnea in patients on supplemental oxygen, rising CO2 (hypercarbia) before it causes SpO2 change, esophageal intubation, circuit disconnection, ROSC during CPR, inadequate chest compressions. A patient who is apneic for 90 seconds and well pre-oxygenated may still show SpO2 of 97–98%; their capnogram will be flat.
Scenario-based interpretation
Post-operative patient on 3 L/min nasal cannula, SpO2 97%, EtCO2 rising from 40 to 55 mmHg, respiratory rate dropping from 14 to 8 breaths/min: This patient is hypoventilating. SpO2 is reassuringly normal and will remain so for another minute or more — the supplemental O2 is masking the problem. Capnography has caught it. Stimulate the patient, reduce opioid if ordered, notify provider, prepare for reversal agent if respiratory depression is opioid-induced.
Sedated patient undergoing colonoscopy, SpO2 95%, EtCO2 flat waveform for 30 seconds: The patient is apneic. SpO2 has not yet dropped. Capnography has detected the apnea. Stimulate the patient verbally and physically, apply jaw thrust, prepare for BVM ventilation if no response, alert the proceduralist.
Intubated ICU patient, SpO2 88%, EtCO2 48 mmHg: Both oxygenation and ventilation are impaired. SpO2 below target indicates hypoxemia (increase FiO2 or PEEP); elevated EtCO2 indicates CO2 retention (increase respiratory rate or tidal volume per order). This is a combined oxygenation and ventilation problem — both parameters must be addressed. ABG ordered to confirm and guide adjustments.
CPR in progress, SpO2 unobtainable (no perfusion), EtCO2 rises from 8 to 42 mmHg: ROSC. Check for pulse and blood pressure immediately. Do not delay pulse check waiting for SpO2 to read — there will not be a SpO2 value until perfusion is restored.
Nursing monitoring protocol
Documentation requirements
Document the following at every monitoring interval:
- SpO2 value and probe location
- Supplemental oxygen in use (device type, flow rate or FiO2)
- EtCO2 value and waveform quality (if capnography in use)
- Respiratory rate
- Patient’s subjective respiratory effort and comfort
- Any alarms triggered and nursing response
For capnography, document whether the waveform is present, morphology (normal rectangular vs abnormal pattern), and any trend changes since the previous documentation point.
Monitoring frequency
Minimum documentation frequencies by context:
- Procedural sedation: Continuous monitoring throughout procedure; document q5 minutes and at baseline, during procedure, and post-procedure recovery intervals per facility policy
- Post-operative (PACU): Continuous monitoring; document per Aldrete/PADSS score intervals (typically q15 min until discharge-ready)
- ICU / mechanically ventilated: Continuous monitoring; document q1h or per unit protocol
- Ward patient on supplemental O2: Minimum q4h; q1–2h if unstable
- Spot-check for stable ward patient: With routine vital signs (typically q4–8h)
Provider notification thresholds
Contact the provider or activate rapid response for:
- SpO2 <90% (or below facility-specified target) that does not correct with position change and supplemental O2 adjustment
- EtCO2 <30 mmHg (hyperventilation, possible alkalosis or reduced perfusion) or >50 mmHg (significant hypoventilation or CO2 retention) that is a new finding
- Sudden waveform change — particularly absent waveform in a previously intubated patient
- Any upward trend in EtCO2 with concurrent decrease in respiratory rate (evolving hypoventilation)
- SpO2 drop that is occurring rapidly rather than gradually, regardless of absolute value
Facility protocols may specify tighter thresholds for specific patient populations (post-cardiac surgery, neurological monitoring). Always follow the more conservative threshold — facility protocol or clinical judgment — whichever requires earlier escalation.
Patient and family education
Explaining the monitor
Patients and families frequently become anxious about monitoring equipment, particularly when alarms sound. A brief, clear explanation at the start of monitoring reduces anxiety and improves cooperation.
For the pulse oximeter: “This small clip on your finger measures the oxygen level in your blood. It shines a light through your skin — it doesn’t hurt and doesn’t draw blood. We’ll see a number on the screen showing your oxygen percentage. If the number drops too low or the signal is lost, an alarm sounds so we can check on you.”
For capnography via nasal cannula: “This is a special nasal cannula that both delivers oxygen and samples the air you breathe out. It measures the carbon dioxide you exhale, which tells us how well you’re breathing. It doesn’t go into your airway — it just sits at your nose.”
Managing alarm anxiety
Reassure patients that alarms are precautionary — most are triggered by movement, probe position, or a brief interruption and do not indicate a clinical problem. However, explicitly tell the patient: “If the alarm sounds and it bothers you, press your call button — do not remove the probe yourself, because we need to see the signal to know everything is fine.”
For families: explain that a monitoring alarm does not automatically mean something is wrong. Staff will assess and determine whether it represents a true clinical change or a technical artifact. Encourage them to notify the nurse rather than attempt to adjust or silence equipment themselves.
NCLEX review: 20 high-yield tips
| Tip # | Clinical pearl |
|---|---|
| 1 | SpO2 <90% = hypoxemia requiring intervention. This is the universal action threshold — below 90%, the oxyhemoglobin dissociation curve is on its steep portion and small further drops in PaO2 cause large drops in saturation. |
| 2 | Normal SpO2 is ≥95% in a healthy adult. Values of 92–94% are borderline; values of 90–91% are at the critical inflection point of the oxyhemoglobin dissociation curve. |
| 3 | COPD target SpO2 is 88–92%, not ≥95%. Over-oxygenation in chronic CO2 retainers suppresses hypoxic ventilatory drive and worsens hypercarbia. |
| 4 | Dark nail polish — especially blue, black, or green — causes falsely low SpO2. Remove polish with acetone or move the probe to the earlobe. |
| 5 | Carbon monoxide poisoning: standard pulse oximetry reads carboxyhemoglobin (COHb) as oxyhemoglobin — SpO2 appears falsely normal (often 98–100%). Order co-oximetry (ABG with co-oximetry) to measure COHb directly. |
| 6 | Methemoglobinemia: SpO2 reads approximately 85% regardless of true saturation when MetHb is elevated. Suspect with exposure to dapsone, benzocaine, or other oxidizing agents. Confirm with co-oximetry. |
| 7 | SpO2 does not detect hypercarbia (elevated CO2). A patient can have SpO2 of 98% and a PaCO2 of 80 mmHg simultaneously — SpO2 measures oxygenation, not ventilation. |
| 8 | Supplemental oxygen masks hypoventilation. SpO2 remains normal for 60–90 seconds after apnea in a well-oxygenated patient. Use capnography to detect apnea before SpO2 falls. |
| 9 | Normal EtCO2 range is 35–45 mmHg. Values outside this range indicate hypoventilation (>45 mmHg, CO2 retention) or hyperventilation (<35 mmHg, excessive CO2 washout). |
| 10 | Capnography is the gold standard for confirming ETT tracheal placement. A normal rectangular waveform confirms tracheal placement; a flat waveform (EtCO2 = 0) after attempted intubation indicates esophageal intubation until proven otherwise. |
| 11 | Capnography detects apnea 60+ seconds before SpO2 drops in a patient receiving supplemental oxygen — making it the more sensitive early warning tool during procedural sedation. |
| 12 | EtCO2 during CPR: values persistently <10 mmHg indicate inadequate chest compressions (insufficient cardiac output). Reassess compression depth and rate immediately. |
| 13 | ROSC during CPR: a sudden rise in EtCO2 from <10 mmHg to ≥35 mmHg is the most reliable non-invasive ROSC indicator — it often precedes a palpable pulse. Stop compressions and check for pulse when you see this. |
| 14 | Shark fin capnogram = bronchospasm. The slow, sloping waveform with no defined plateau is caused by uneven airway obstruction that delays alveolar gas reaching the sensor. Administer bronchodilator per order. |
| 15 | EtCO2 is typically 2–5 mmHg lower than PaCO2 in patients with normal lungs and cardiac output. The gradient widens with increased dead space (pulmonary embolism, low cardiac output, severe COPD). |
| 16 | A rising baseline on the capnogram (Phase I does not return to zero) indicates CO2 rebreathing — check CO2 absorber in anesthesia circuits or increase fresh gas flow. |
| 17 | Earlobe SpO2 responds faster than finger SpO2 (20–30 sec vs 60+ sec). Use the earlobe in low-perfusion states or when you need the most current oxygenation status. |
| 18 | Severe anemia can produce a falsely reassuring SpO2. If hemoglobin is very low, SpO2 may read 98% because the small amount of hemoglobin present is fully saturated — but total oxygen content (CaO2) is critically reduced. Check Hgb/Hct alongside SpO2. |
| 19 | Poor pleth waveform = unreliable SpO2. If the waveform on the monitor is flat or irregular, the numeric value cannot be trusted. Reposition the probe, warm the extremity, or move to an alternative site before documenting or acting on the reading. |
| 20 | SpO2 and EtCO2 monitor different physiological processes and each has blind spots the other covers. SpO2 detects oxygenation failure; EtCO2 detects ventilation failure. In any high-acuity patient, both together provide a more complete respiratory picture than either alone. |
Summary
Pulse oximetry and capnography are complementary monitoring modalities that provide real-time assessment of two separate and essential respiratory functions. SpO2 is the nurse’s window into oxygenation — whether hemoglobin is carrying sufficient oxygen to meet metabolic demand. EtCO2 is the window into ventilation — whether CO2 is being eliminated effectively and the airway is patent.
Competent use of these tools requires more than reading the numeric values. It requires understanding where each modality can mislead — from nail polish and poor perfusion affecting SpO2, to the EtCO2-PaCO2 gradient widening in disease states — and recognizing the waveform patterns that carry the most clinical urgency: the absent waveform of esophageal intubation, the shark fin of bronchospasm, the rising EtCO2 that heralds ROSC. Used together and interpreted in context, they form one of the most powerful surveillance systems available to the bedside nurse.