Can nurses rely on colorimetric CO2 detectors alone?

Color change confirms CO2 presence but not sustained ventilation. Waveform capnography is preferred when available; colorimetric devices may be used with clinical assessment when waveform monitoring is unavailable, per resuscitation and airway guidelines.

Why can EtCO2 drop suddenly on the monitor?

Common causes include circuit disconnect, sampling line blockage or water, accidental extubation, airway obstruction, or loss of pulmonary blood flow during cardiac arrest. Always assess the patient and circuit before assuming monitor error.

Does EtCO2 equal arterial PaCO2?

No. The gap between end-tidal and arterial CO2 widens with increased dead space, V/Q mismatch, and unstable hemodynamics. Use EtCO2 for trends and ventilation monitoring; obtain arterial blood gas when precise PaCO2 is required.

Can capnography help verify nasogastric tube placement?

Some pathways use capnography or CO2 detection at the tube port as an adjunct when approved locally. Never start enteral feeding until your unit's full verification algorithm is complete and documented.

Capnography Monitoring: EtCO₂ Waveforms & Nursing Guide | NurseOnShift

📈 Respiratory monitoring & ventilation

Capnography Monitoring: EtCO₂ Waveforms & Nursing Procedure Guide

Continuous end-tidal CO₂ (EtCO₂) tells you whether exhaled gas is reaching the monitor—pair waveform review with pulse oximetry, mechanical ventilation monitoring, and hypoxia symptoms so disconnects and hypoventilation are caught before SpO₂ alone lags.

13 min read

Updated May 23, 2026

Medically Reviewed

Written by

Sid A. Abdala Balal

Registered Nurse

Medically Reviewed by

Dr. Adam Sayedi

MD — Medical Doctor

Quick Facts

Core output

EtCO₂ + waveform (Phases I–IV)

Sampling

Mainstream or sidestream per device

Urgent pattern

Flat trace + unstable patient

Also known as

Capnography · ETCO₂ · end-tidal CO₂

Key Takeaway

A normal-looking SpO₂ cannot prove the ventilator circuit is connected or that CO₂ is leaving the lungs. Waveform capnography is your fastest bedside check that ventilation and sampling are real—treat a sudden loss of waveform as an airway emergency until you prove otherwise.

Quick procedure summary

Item	Detail
Procedure name	Capnography monitoring (capnography / end-tidal CO₂ monitoring)
Also known as	Capnography; EtCO₂; PETCO₂; end-tidal carbon dioxide monitoring
Category	Respiratory monitoring — exhaled gas analysis
Clinical purpose	Confirm ventilation at the sampling site, track CO₂ elimination trends, detect circuit or airway events, and support resuscitation and ventilator assessment
Who performs	Registered nurses, respiratory therapists, paramedics, and physicians per scope; nurses apply monitors, respond to alarms, document trends, and escalate
Estimated time	About 5–15 minutes for setup and baseline check; then continuous surveillance with periodic waveform review
Clinical settings	ICU, operating theatre, emergency department, anaesthesia recovery, transport teams, resuscitation bays, and monitored wards using capnography-enabled devices

What is capnography monitoring?

Capnography is continuous measurement and display of carbon dioxide in exhaled gas. Capnometry is the numeric value alone (often reported as end-tidal CO₂, EtCO₂ or PETCO₂). In practice, nurses use waveform capnography—a time-based capnogram—because the shape confirms that each breath is reaching the sensor and because abrupt changes flag disconnects, obstruction, or perfusion loss faster than a single number on screen.

On mechanically ventilated patients, sampling is usually at the ventilator circuit adapter on an endotracheal tube or mask interface. During basic life support (adult), quantitative waveform capnography supports airway placement confirmation and compression quality when local protocols provide the device.

Reading the capnogram: phases I–IV

Before chasing alarm limits, confirm the trace looks like a breathing patient at your sampling site.

Dead space gas (near zero CO₂)

Mixing — upslope begins

III

Alveolar plateau — EtCO₂ read here

Inspiratory downstroke (fresh gas)

Pattern cues nurses act on

Absent or flat waveform with a ventilated patient — search for disconnect, extubation, obstruction, or cardiac arrest physiology until ruled out.
Shark-fin upslope — consider bronchospasm or prolonged expiratory flow; align with wheezing and respiratory assessment.
Gradual rise in plateau — hypoventilation, increased CO₂ production, or reduced elimination; correlate with ventilator settings and sedation.
Sudden drop in EtCO₂ — embolism, hypotension, circuit leak, or sampling failure; never chart “fine” without looking at the patient.

Mainstream vs sidestream sampling

🔵 Mainstream

Sensor at the airway adapter

Fast response at the patient connection; common on ventilator circuits and some monitors.
Adds dead space at the adapter—minimise extra volume in neonates and small children per manufacturer guidance.
Weight of adapters may increase extubation risk in very small patients—secure the circuit.

🟢 Sidestream

Gas drawn through a sampling line to a remote analyser

Water or secretions in the line blunt the waveform—check filters and condensate traps.
High sample flows may affect trigger sensitivity or tidal delivery in small patients; institutional protocols may vary.
Line length and occlusion are frequent causes of false-low EtCO₂.

Colorimetric CO₂ detectors are a simplified mainstream method (color change only). They can confirm CO₂ presence but do not replace waveform monitoring for ongoing surveillance.

Overview

Capnography sits between oxygenation and ventilation assessment: pulse oximetry shows haemoglobin saturation, while EtCO₂ reflects alveolar ventilation and, when perfusion is stable, pulmonary blood flow. In many patients without major lung disease, rising CO₂ may appear on capnography before hypoxaemia triggers an SpO₂ alarm—so paired monitoring is standard in critical care and anaesthesia.

Nurses apply the monitor, verify waveform quality after every handoff or circuit change, set alarms per protocol, and escalate when trends diverge from the plan of care—including rising work of breathing with shortness of breath or failed ventilator weaning attempts. This guide aligns with the AARC capnography during mechanical ventilation clinical practice guideline and patient–ventilator assessment resources linked below. It summarises principles for education; use your licensed Royal Marsden Manual of Clinical Nursing Procedures hub and unit competency packs for verbatim institutional steps.

Clinical nursing focus

Each round: (1) waveform present and cyclical; (2) EtCO₂ trend versus baseline and arterial blood gas when available; (3) circuit integrity with mechanical ventilation monitoring; (4) patient perfusion and sedation context.

Indications

Scenario	Nursing rationale
Mechanical ventilation via ETT or mask	Confirm ongoing exhaled CO₂ at the circuit; detect disconnects and apnea during sedation.
Post-intubation and after repositioning	Adjunct to clinical assessment to reduce unrecognised oesophageal or displaced tube risk.
Cardiopulmonary resuscitation	Monitor compression-generated EtCO₂ and abrupt rises suggesting return of spontaneous circulation.
Transport of ventilated patients	Maintain waveform surveillance through transfers per transport policy.
Procedural sedation or analgesia	Early apnea detection when ordered for moderate sedation pathways.
NG tube placement verification (when approved locally)	Some algorithms use CO₂ at the tube port as an adjunct—follow NG tube placement verification exactly; never feed on capnography alone.

Limitations and cautions

There are no absolute contraindications to capnography in ventilated patients when devices are used as labelled. Interpretation must account for physiology and equipment limits.

EtCO₂ is not a substitute for PaCO₂—dead space and V/Q mismatch widen the gap.
Low cardiac output or prolonged arrest may yield false-low readings despite tracheal placement.
Leaks at cuff, mask, or circuit reduce measured EtCO₂.
Filters, water, and secretions in sampling lines distort waveforms.
High FiO₂ or nitrous oxide may require device-specific calibration corrections.
Sodium bicarbonate during CPR can transiently raise EtCO₂—do not confuse with improved compressions alone.

Equipment checklist

✓Capnograph module or standalone monitor with waveform display

✓Correct airway adapter (ETT, tracheostomy, mask, or filter port) per route

✓Sidestream sampling line, filter, and water trap when applicable

✓Calibration gas or zeroing tools per manufacturer IFU

✓Alarm profile aligned with unit policy (high/low EtCO₂, apnea)

✓Documentation access for baseline and handoff values

Pre-monitoring checks

Verify patient and indication

Confirm identifiers, airway type, ventilation mode, and whether capnography is ordered for surveillance, procedure, or resuscitation.

Inspect sampling path

Adapter seated, sampling line without kinks, water removed from traps, and filter orientation correct. After airway suctioning, confirm the waveform returns.

Calibrate per IFU

Zero or calibrate before relying on numeric targets; institutional protocols may vary for room-air checks versus circuit attach.

Establish baseline with full assessment

Record starting EtCO₂, waveform quality, SpO₂, respiratory rate, and ventilator pressures. Auscultate and review sedation scores in ventilated patients with ARDS or pneumonia.

Step-by-step monitoring procedure

Continuous surveillance

Apply and secure the sampling interface

Attach mainstream adapter or sidestream port at the patient connection; minimise unintentional disconnects during turns.

Confirm cyclical waveform

Phases I–IV should repeat with ventilator breaths or spontaneous respiration. No waveform after connection demands immediate patient and circuit assessment.

Set and test alarms

Use unit limits for high/low EtCO₂ and apnea; avoid alarm fatigue by fixing false triggers from water or loose lines.

Trend with ventilation changes

After PEEP, rate, or sedation adjustments, expect EtCO₂ to shift—notify when changes exceed protocol thresholds or clinical expectation.

Reassess after handoffs and transport

Re-verify waveform at bedside after ICU admission, imaging trips, or OR transfer—misplacement risk peaks at interfaces.

Document and communicate

Chart numeric EtCO₂, waveform quality, alarms silenced, troubleshooting performed, and clinician notifications.

EtCO₂ trends nurses interpret at the bedside

Trend	Often suggests	Nurse action
Sudden drop toward zero	Disconnect, extubation, obstruction, sampling failure, or arrest physiology	Look at patient first; check circuit and airway; activate emergency pathway if apnoeic or arrest suspected.
Gradual rise in plateau	Hypoventilation, increased CO₂ production, reduced elimination	Review ventilator settings, sedation, temperature, and perfusion; notify per protocol.
Gradual fall with stable ventilation	Improved ventilation, reduced production, or reduced pulmonary blood flow	Correlate with blood pressure and perfusion; consider embolism or shock pathways when unexplained.
Abrupt rise during CPR	Possible return of spontaneous circulation	Pause briefly for rhythm check per resuscitation algorithm; prepare team for post-ROSC care.
Low EtCO₂ during compressions	Inadequate compressions or poor airway placement	Optimise compression depth and rate; confirm airway and capnograph connection per BLS/ALS policy.

Swipe sideways on small screens to read all columns.

Waveform troubleshooting without delaying patient care

Flat line — patient unstable

Assume airway or perfusion emergency: stimulate/ventilate per scope, call for help, and inspect from patient to ventilator while another clinician manages ABCs.

Flat line — patient stable on ventilator

Check sampling line, water trap, adapter orientation, monitor cable, and whether the ventilator capnography module is enabled.

✓Replace occluded sidestream filters before blaming the lungs.

✓After nebuliser or inline medication, confirm waveform recovers.

✓Compare EtCO₂ with chest rise, ventilator volumes, and SpO₂ trend.

✓Silence alarms only after addressing the trigger and documenting why.

When to escalate

Finding	Escalate when
No waveform with ventilated patient	Immediate airway team / emergency response per local policy.
EtCO₂ falling with hypotension or mottled perfusion	Urgent medical review—consider shock, embolism, or arrest.
Rising EtCO₂ despite increased minute ventilation	Notify for refractory hypercapnia, equipment failure, or increased CO₂ production (e.g. sepsis, fever).
Suspected oesophageal intubation pattern	Stop enteral/oral positive-pressure routes; emergency airway reassessment.
Persistent apnea alarms on sedation	Notify prescriber; prepare airway support per procedural sedation policy.

Documentation

Example narrative

"Continuous waveform capnography via sidestream port on ETT adapter. Baseline EtCO₂ 38 mmHg with stable phases I–IV. After position change EtCO₂ 34 mmHg, waveform cyclical, no apnea alarms. SpO₂ 96% on FiO₂ 0.40. Ventilator unchanged. RT notified of downward trend per protocol."

Device type, sampling site, and waveform quality
EtCO₂ values with time and clinical context (ventilation change, sedation, fever)
Alarm adjustments and troubleshooting steps
Correlation with SpO₂, ABG, and ventilator data when available
Notifications and response

Clinical pearls for nurses

✓Glance at the waveform every time you glance at the ventilator—numbers without shape mislead.

✓After every circuit change, ask: “Do I see CO₂ leaving the patient?”

✓During CPR, low EtCO₂ often means compressions need improvement—not that monitoring is broken.

✓Pair capnography with oxygen therapy administration orders—SpO₂ and EtCO₂ answer different questions.

Clinical Judgment Practice

NCLEX-style clinical judgment practice — When the capnograph flatlines but the ventilator still cycles, practise waveform-first judgment for capnography monitoring, including a priority action, select-all-that-apply cue recognition, trend interpretation after intervention, and matrix escalation matching (recognise cues → analyse → prioritise → act → evaluate outcomes).

Unfolding case — ICU. Ms. Okonkwo, 62, is day 2 of invasive ventilation for pneumonia and hypoxaemic respiratory failure. Oral ETT with sidestream capnography on the ventilator circuit. Baseline EtCO₂ 36–40 mmHg with normal phases I–IV. On your entry she is pale, SpO₂ 88% on FiO₂ 0.55, and the capnograph shows a flat line near zero while the ventilator still cycles.

Question 1 — Priority action

Which action should the nurse initiate first to reduce patient risk?

Document “equipment fault” and continue routine charting at the desk. Increase FiO₂ to 1.0 and wait two hours for the next routine ABG. Assess the patient immediately (consciousness, chest movement, SpO₂), verify ETT position and circuit connections, troubleshoot sampling line, and activate emergency airway support per protocol while calling for help. Deflate the ETT cuff to see whether EtCO₂ returns.

Question 2 — Select all that apply

Select all that apply — which cues are relevant when capnography suddenly shows a flat trace on a ventilated patient?

Possible circuit disconnect, extubation, or obstruction until ruled out Need to confirm cyclical waveform and sampling-line patency Visitor meal tray delivered on time Correlation with SpO₂, perfusion, and ventilator pressures Risk of low cardiac output or arrest physiology producing very low EtCO₂ despite tracheal tube

Question 3 — Trend interpretation

After reconnecting a loose circuit adapter and clearing water from the sidestream trap:

Trend snapshot
EtCO₂: flat → 32 mmHg with restored phases I–IV
SpO₂: 88% → 94% on same FiO₂
Breath sounds: bilateral coarse crackles; no unilateral loss
Blood pressure: 98/56 mmHg, HR 108
Ventilator: no high-pressure alarms; set minute ventilation unchanged

Select all that apply — which nursing actions are appropriate now?

Document waveform restoration, EtCO₂ value, troubleshooting steps, and notify team per protocol Remove capnography because one acceptable reading appeared Continue continuous waveform monitoring and trend EtCO₂ with ventilation and SpO₂ Notify clinician if EtCO₂ remains low with hypotension, rising work of breathing, or new unilateral findings Evaluate outcomes on the next assessment: stable cyclical waveform, EtCO₂ within patient baseline, and improving SpO₂

Question 4 — Matrix judgment

For each capnography situation, select the best nursing action category (one per row).

Situation	Continue routine monitoring / supportive care	Notify clinician / urgent same-day pathway	Activate rapid response / emergency escalation
Stable cyclical waveform; EtCO₂ within baseline; SpO₂ at goal on current settings
Gradual EtCO₂ rise over 4 h with unchanged ventilator settings and increasing sedation needs
Sudden flat capnograph, absent chest rise, SpO₂ 82%, unresponsive patient
During CPR, EtCO₂ jumps from 8 to 28 mmHg with palpable central pulse

On a small screen, swipe or scroll sideways to see the full table.

Answer key & rationale

Next step Timed NCLEX Next Gen–style scenarios on NurseOnShift

Frequently asked questions

FAQ

What is the difference between capnography and capnometry?

Capnometry is the numeric EtCO₂ alone. Capnography adds the waveform, which confirms ventilation at the sensor and helps detect disconnects and obstruction patterns.

Can nurses rely on colorimetric CO₂ detectors alone?

They confirm CO₂ presence but not sustained ventilation. Use waveform capnography when available; colorimetric checks may supplement clinical assessment when waveform devices are unavailable, per resuscitation and airway guidance.

Why can EtCO₂ drop suddenly on the monitor?

Common causes include circuit disconnect, sampling-line blockage, extubation, airway obstruction, or low pulmonary blood flow during arrest. Assess the patient and circuit before assuming equipment failure.

Does EtCO₂ equal arterial PaCO₂?

No—the gap widens with dead space and V/Q mismatch. Trend EtCO₂ at the bedside; use arterial blood gas when precise PaCO₂ is required.

Is capnography used during CPR?

Yes. Quantitative waveform capnography supports airway placement confirmation, compression quality feedback, and detection of return of spontaneous circulation when EtCO₂ rises abruptly with perfusion.

Can capnography verify nasogastric tube placement?

Only as part of your unit's approved algorithm—never as a single test before feeding. Follow NG tube placement verification in full.

References

Walsh BK, Crotwell DN, Restrepo RD. AARC Clinical Practice Guideline: Capnography/Capnometry During Mechanical Ventilation (2011).
https://www.aarc.org/wp-content/uploads/2014/08/04.11.0503.pdf
American Association for Respiratory Care. Clinical Practice Guideline: Patient-Ventilator Assessment (2024).
https://www.aarc.org/wp-content/uploads/2024/10/patient-ventilator-assessment-aarc-cpg.pdf
American Heart Association. CPR and ECC Guidelines — resuscitation science hub (airway confirmation and CPR capnography).
https://cpr.heart.org/en/resuscitation-science/cpr-and-ecc-guidelines
Royal Marsden Manual of Clinical Nursing Procedures — Procedures (RMM Online).
https://www.rmmonline.co.uk/contents/procedures
OpenStax. Clinical Nursing Skills.
https://openstax.org/details/books/clinical-nursing-skills

Editorial standards & medical review

About the author: Sid A. Abdala Balal, RN, writes evidence-based nursing education focused on practical bedside skills, patient safety, and clinical decision support for nurses.

Medical review: This guide is reviewed by Dr. Adam Sayedi, MD, for clinical accuracy, clarity, and alignment with current capnography and mechanical ventilation nursing standards.

Policies: Medical Review Process · Editorial Policy · Correction Policy