Quick Answer: EtCO₂ (end-tidal carbon dioxide) measures the concentration of CO₂ in exhaled breath at the end of expiration. Normal EtCO₂ is 35–45 mmHg. Clinicians monitor EtCO₂ to assess ventilation status in real time, confirm endotracheal tube placement, evaluate CPR quality, and detect respiratory depression early — often before SpO₂ sensors show any change.
Walk into any ICU, operating room, or post-anesthesia care unit and you will likely see a number on the patient monitor labeled "EtCO₂" alongside a slowly scrolling waveform. For many nurses — especially those new to critical care — this parameter can feel less familiar than heart rate or SpO₂. Yet experienced clinicians often call EtCO₂ the most underappreciated vital sign on the screen.
This guide explains what EtCO₂ is, what the numbers and waveform tell you, when it is used, and what monitoring accessories keep it running accurately. It is part of our Hospital Monitor Reading & Accessories Guide series.
What Does EtCO₂ Mean?
EtCO₂ stands for end-tidal carbon dioxide — the partial pressure of CO₂ measured at the very end of an exhaled breath. Because the last portion of exhaled gas comes directly from the alveoli, EtCO₂ closely reflects how well the lungs are eliminating CO₂, making it a real-time window into ventilation, perfusion, and metabolism.
The technology that makes this possible is called capnography. A capnography device displays both a numerical EtCO₂ value (capnometry) and a graphical waveform (capnogram). Together, they give clinicians two layers of information: the number tells you how much CO₂ is present; the waveform shape tells you how ventilation is occurring.
Think of it this way: pulse oximetry tells you about oxygenation — is enough oxygen reaching the blood? EtCO₂ tells you about ventilation — is the patient actually moving air in and out of the lungs effectively? These are two distinct physiological processes, and monitoring both gives a far more complete respiratory picture.
Normal EtCO₂ Values and What They Mean
A healthy patient with normal lung function will produce an EtCO₂ of 35–45 mmHg (approximately 4.6–5.9 vol%). This range is consistent regardless of age, sex, or body size. The table below breaks down key clinical ranges.
| Clinical Situation | EtCO₂ Range | Interpretation |
|---|---|---|
| Normal ventilation | 35–45 mmHg | Adequate gas exchange |
| Hyperventilation | <35 mmHg | CO₂ being "blown off" too fast — anxiety, pain, metabolic acidosis |
| Hypoventilation | >45 mmHg | CO₂ building up — sedation, opioid depression, airway obstruction |
| Effective CPR | 10–20 mmHg | Confirms cardiac output generated by compressions |
| Poor CPR / prolonged arrest | <10 mmHg | Compressions may be inadequate or patient cannot be resuscitated |
| ROSC indicator | Sudden rise >40 mmHg | Return of spontaneous circulation — check for pulse |
Important note: EtCO₂ is an estimate of arterial PaCO₂, not an exact match. In patients with healthy lungs, the PaCO₂–EtCO₂ gradient is normally only 2–5 mmHg. In patients with significant ventilation-perfusion mismatch (e.g., COPD, pulmonary embolism), the gap widens and the two values should not be used interchangeably. For a deeper comparison, see our guide on understanding SaO₂, PaO₂, and related respiratory values.
EtCO₂ vs SpO₂: Why You Need Both
| Feature | SpO₂ (Pulse Oximetry) | EtCO₂ (Capnography) |
|---|---|---|
| Measures | Oxygenation | Ventilation |
| Detection speed | Lags — may delay 30–60 sec, especially on supplemental O₂ | Real-time — changes visible within a single breath |
| Detects apnea? | Delayed — oxygen reserves mask the event | Immediate — waveform disappears at first missed breath |
| Sensor type | SpO₂ sensors (finger clip, adhesive wrap) | CO₂ sensor + sampling line or airway adapter |
Here is a practical scenario many experienced ICU nurses describe: a post-operative patient on supplemental oxygen via nasal cannula slowly develops opioid-induced respiratory depression. The SpO₂ reads 98% because the supplemental oxygen is keeping saturations high. Meanwhile, EtCO₂ has climbed from 40 to 55 mmHg and the waveform shows progressively slower breaths. Without capnography, the first sign of trouble might not appear until SpO₂ finally drops — by which point the patient may already be in significant distress.
This is exactly why organizations like the American Society of Anesthesiologists (ASA) and China's 2017 & 2023 Clinical Anesthesia Monitoring Guidelines have made EtCO₂ monitoring a required standard for all patients under general anesthesia or deep sedation. A 2011 UK audit estimated that roughly half of emergency department deaths from airway complications could have been prevented with correct capnography use.
When Is EtCO₂ Monitoring Used?
EtCO₂ monitoring applies across a surprisingly broad range of clinical settings. Below are the most common use cases, along with the clinical rationale for each.
1. Confirmation of Endotracheal Tube Placement
Capnography is widely regarded as the "gold standard" for verifying that an endotracheal tube is in the trachea and not the esophagus. A normal rectangular waveform with consistent EtCO₂ readings confirms correct placement. An absent or rapidly diminishing waveform signals esophageal intubation — a critical finding that demands immediate action.
2. CPR Quality Assessment
During cardiopulmonary resuscitation, EtCO₂ reflects how effectively chest compressions are generating cardiac output. An EtCO₂ above 10 mmHg during CPR suggests adequate compressions. If it drops below 10 mmHg, the compressor may be fatigued and needs to be replaced. A sudden spike above 40 mmHg often signals return of spontaneous circulation (ROSC) — sometimes before a pulse is even palpable.
3. Procedural Sedation Monitoring
Patients receiving sedation for endoscopy, pain procedures, or emergency department interventions are at risk for respiratory depression. EtCO₂ monitoring detects hypoventilation and apnea in real time, providing a safety net that pulse oximetry alone cannot offer.
4. Ventilation Management in Intubated Patients
In the ICU and operating room, continuous EtCO₂ helps clinicians titrate ventilator settings, detect circuit disconnections, and identify conditions like bronchospasm or circuit leaks immediately. For patients with elevated intracranial pressure, maintaining a specific PaCO₂ target (guided by EtCO₂ trends) is a critical neuroprotective strategy.
5. Post-Anesthesia Recovery (PACU)
After surgery, patients transitioning from general anesthesia are vulnerable to airway obstruction, residual neuromuscular blockade, and opioid-induced respiratory depression. EtCO₂ monitoring in the PACU catches these complications early. As noted in the PRODIGY trial, post-operative respiratory depression is more common and clinically meaningful than many clinicians realize.
Understanding the EtCO₂ Waveform (Capnogram)
The waveform is just as important as the number. A normal capnogram has a characteristic rectangular shape with four distinct phases:
Phase I (Inspiratory Baseline): CO₂ near zero — dead space gas from the airway.
Phase II (Expiratory Upstroke): Rapid rise as alveolar gas reaches the sensor.
Phase III (Alveolar Plateau): CO₂ concentration levels off. The peak at the end of this plateau is the EtCO₂ value shown on the monitor.
Phase 0 (Inspiratory Downstroke): Sharp drop as fresh gas is inhaled.
When the waveform shape changes, it tells you something specific is happening:
| Waveform Pattern | Appearance | Possible Cause |
|---|---|---|
| "Shark fin" shape | Slanted upstroke, no clear plateau | Bronchospasm, COPD, asthma |
| Elevated baseline | CO₂ does not return to zero | CO₂ rebreathing, exhausted CO₂ absorber, or contaminated sensor |
| Absent waveform | Flat line | Esophageal intubation, apnea, disconnection, or equipment failure |
| Curare cleft | Notch in the plateau | Patient attempting spontaneous breath during mechanical ventilation |
| Gradually rising EtCO₂ | Normal shape but values climb over time | Hypoventilation, increased metabolism, fever, or malignant hyperthermia |
Clinical pearl: Many experienced nurses recommend making it a habit to glance at the EtCO₂ waveform every time you assess vitals. The waveform alone — even without reading the number — can tell you instantly whether the patient is breathing and whether airflow is obstructed. As one seasoned ICU nurse put it in an online clinical forum: "I've caught more deteriorating patients by watching the capnography trend than any other single parameter."
Mainstream vs Sidestream Capnography: Which Does Your Unit Use?
Capnography devices come in two main configurations, and the choice between them affects which monitoring accessories you need.
| Feature | Mainstream | Sidestream / Microstream |
|---|---|---|
| Sensor location | Directly at the airway (between ET tube and circuit) | Inside the monitor; gas drawn via sampling line |
| Response time | Real-time, no delay | 2–5 second delay (transit time through tubing) |
| Patient types | Intubated patients only | Both intubated and non-intubated (via nasal cannula) |
| Typical sample rate | N/A — sensor is in-line | 50–200 mL/min (microstream as low as 50–60 mL/min) |
| Moisture management | Not an issue (no sampling line) | Requires water trap or Nafion™ drying tube |
| Key accessories | Airway adapter, EtCO₂ sensor | Sampling lines, water traps, nasal cannula adapters |
Sidestream and microstream systems are by far the most common across hospital settings because they work with both intubated and spontaneously breathing patients. However, they require regular replacement of disposable sampling lines and water traps to prevent moisture-related occlusion and maintain measurement accuracy. When moisture blocks the sampling line, the waveform degrades and alarms can become unreliable — a common frustration that staff in many facilities deal with daily.
EtCO₂ Monitoring Equipment and Accessories
Reliable EtCO₂ monitoring depends on properly functioning accessories. Here is what a typical sidestream/microstream setup requires:
| Accessory | Function | Replacement Frequency |
|---|---|---|
| Sampling line (nasal/oral cannula) | Delivers exhaled gas from patient to sensor | Every 24 hours (single-use) |
| Water trap | Removes moisture before gas reaches the sensor | Per manufacturer spec (typically every 24–72 hours) |
| EtCO₂ sensor / module | Infrared CO₂ detection | Durable — based on manufacturer lifecycle |
| Adapter cable | Connects sensor/module to patient monitor | 2–3 years with proper care |
💡 Practical tip — water trap vs disposable sampling line cost:
Traditional reusable water traps require daily draining, cleaning, and sterilization — a process that can cost $35+ per cycle when you factor in staff time and sterilization resources. Many hospitals are now switching to disposable, single-use sampling lines with integrated drying technology (such as Nafion™ membrane tubing) that eliminate the water trap entirely. At roughly $20 per disposable line lasting up to 24 hours, the per-patient cost is often lower than the cleaning burden of a reusable water trap — with zero cross-contamination risk. MedLinket offers both compatible water traps and disposable sampling lines for Philips, Drager, and other mainstream monitors.
EtCO₂ Alarm Settings: What to Know
Default EtCO₂ alarm limits on most monitors are set at 10 mmHg (low) and 60 mmHg (high). Respiratory rate alarm limits typically default to 9 (low) and 30 (high) breaths per minute. These defaults may need adjustment for specific patient populations — for example, a known CO₂ retainer (such as a patient with severe COPD) may require a higher upper alarm threshold to avoid nuisance alarms.
When an EtCO₂ alarm sounds, the recommended response sequence mirrors good general alarm management practice: first assess the patient for signs of respiratory distress, check the airway, and then troubleshoot the equipment (look for kinked or disconnected tubing, blocked sampling lines, or loose connections). For a more comprehensive alarm management approach, see our guide on reading hospital monitors and interpreting key parameters.
Why MedLinket for EtCO₂ and Patient Monitoring Accessories
MedLinket (Shenzhen Med-Link Electronics Tech Co., Ltd) has been manufacturing medical cables, medical sensors, and monitoring consumables since 2004. As a publicly listed company on China's National Equities Exchange (stock code: 833505), MedLinket operates three factories — in Shenzhen, Shaoguan, and Indonesia — with full in-house capabilities from R&D and mold production through final testing and delivery.
Our EtCO₂ accessories — including compatible water traps for Philips and GE Healthcare monitors, mainstream EtCO₂ sensors, and disposable sampling lines — are backed by FDA 510(k) clearance, CE marking (MDR), and an ISO 13485:2016 quality management system. Every unit is 100% tested before shipment and currently serves 2,000+ hospitals across 120+ countries.
Need help identifying the correct EtCO₂ accessories for your patient monitors? Contact us at shopify@medlinket.com or WhatsApp +86-189-2972-7044 for free compatibility verification.
Frequently Asked Questions About EtCO₂ Monitoring
Q: What does a low EtCO₂ mean?
A: EtCO₂ below 35 mmHg (hypocapnia) usually indicates the patient is hyperventilating — blowing off CO₂ faster than the body produces it. Common causes include anxiety, pain, metabolic acidosis (compensatory hyperventilation), or over-ventilation on a mechanical ventilator. During cardiac arrest, a low EtCO₂ may indicate poor CPR quality or prolonged downtime.
Q: What does a high EtCO₂ mean?
A: EtCO₂ above 45 mmHg (hypercapnia) means CO₂ is accumulating. This commonly results from hypoventilation (e.g., opioid-induced respiratory depression, sedation, or airway obstruction), increased CO₂ production (fever, sepsis, malignant hyperthermia), or inadequate ventilator settings. If EtCO₂ rises steadily during general anesthesia, malignant hyperthermia must be considered and ruled out.
Q: Can EtCO₂ be used on non-intubated patients?
A: Yes. Sidestream and microstream capnography devices use nasal or oral-nasal cannulas to sample exhaled gas from spontaneously breathing patients. This is routinely used during procedural sedation, in the emergency department, and in PACU settings. The waveform quality may be somewhat lower than in intubated patients, but the clinical value is well-established.
Q: How is EtCO₂ different from PaCO₂?
A: SpO₂ measures oxygen saturation non-invasively; PaCO₂ measures the partial pressure of CO₂ in arterial blood (requires a blood draw); and EtCO₂ estimates PaCO₂ non-invasively by measuring exhaled CO₂. In healthy lungs, EtCO₂ is typically 2–5 mmHg lower than PaCO₂. In patients with significant V/Q mismatch, the gap can be much larger.
Q: What accessories are needed for EtCO₂ monitoring?
A: This depends on your setup. For sidestream systems, you need a sampling line (nasal cannula or airway adapter), a water trap (unless using Nafion™ drying technology), and a compatible adapter cable. For mainstream systems, you need an EtCO₂ sensor and an airway adapter. All accessories must match your monitor brand and model — contact us for compatibility verification.
📚 This article is part of our Hospital Monitor Reading & Accessories Guide series. Explore related topics:
About the Author
This article was written by MedLinket's clinical applications team, drawing on over 20 years of experience in patient monitoring accessories manufacturing and direct collaboration with clinical engineering departments at 2,000+ hospitals worldwide. Technical specifications and clinical references were reviewed against current ASA guidelines, AHA ACLS standards, and China's 2023 Clinical Anesthesia Monitoring Guidelines. MedLinket is an ISO 13485-certified and FDA-registered medical device manufacturer headquartered in Shenzhen, China (Est. 2004 | Stock Code: 833505).
Last updated: February 2026 | Reviewed for clinical accuracy
