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Capnography Waveforms
TEXTMonitoring I · 10 min read
Four phases, one waveform — capnography is the fastest, most specific monitor for tube placement, disconnect, embolism, and ventilation problems.
After this lesson you can
4 min read8 sections- Identify the four phases of the normal capnogram.
- Diagnose six common abnormal capnogram patterns.
- Explain why capnography is the gold standard for ETT confirmation.
- Recall the ETCO₂-to-PaCO₂ gradient and what widens it.
Why capnography is the most important monitor
- ESOPHAGEAL INTUBATION (confirms tracheal placement within 1-2 breaths — gold standard)
- DISCONNECT (drops to zero immediately)
- AIRWAY OBSTRUCTION (waveform changes)
- HYPOVENTILATION (rising baseline + level)
- CARDIAC ARREST (abrupt drop reflecting loss of pulmonary blood flow)
- AIR EMBOLISM (abrupt drop with hemodynamic change)
ASA Standard II requires continuous ETCO2 + waveform during all general anesthesia and during moderate sedation.
NAP4 + closed-claims data show absence of capnography contributes to majority of catastrophic airway outcomes — never tape over the monitor or ignore the alarm.
Normal 4-phase capnogram
PHASE II (early exhalation) — rapid upstroke as anatomic dead space gas (no CO₂) is washed out by alveolar gas reaching the sample line.
PHASE III (alveolar plateau) — slight upslope from late-emptying alveoli with higher CO₂; well-matched lungs give nearly flat plateau.
PHASE IV (start of next inspiration) — sharp drop back to baseline.
END-TIDAL CO₂ (ETCO₂) = highest point of phase III ≈ alveolar PCO₂ ≈ arterial PCO₂ in healthy lungs.
PaCO₂ − ETCO₂ GRADIENT typically 2-5 mmHg in healthy patients; widens to 10+ mmHg in dead-space disease (PE, ARDS, hypovolemia, COPD).
Shark fin pattern — bronchospasm + obstruction
Cause: heterogeneous alveolar emptying — alveoli with high time-constant (obstructed bronchi) empty late and with progressively higher CO₂, sloping up the waveform.
- bronchospasm (most common intraop — also rising peak airway pressure, wheeze)
- COPD (chronic shark fin)
- kinked ETT (mechanical obstruction)
- mucus plug in airway
- ETT bevel against tracheal wall
- bronchial intubation with one-lung obstruction
Severity correlates with slope steepness.
TREATMENT for intraop bronchospasm: deepen volatile anesthetic (sevoflurane is a bronchodilator), inhaled albuterol via circuit, IV magnesium 1-2 g, IV epinephrine 10-100 mcg, IV hydrocortisone if presumed inflammatory/anaphylactic, ensure adequate exhalation time (longer I:E ratio).
Abrupt ETCO₂ drop — the crisis waveform
Differential by capnogram morphology: CARDIAC ARREST ETCO₂ drops over a few breaths to very low values; chest compressions if effective produce small irregular waves (good CPR generates ETCO₂ ≥10-20 mmHg — used as resuscitation marker).
PULMONARY EMBOLISM (thrombotic, air, fat, amniotic, CO₂ for laparoscopic) abrupt drop with hemodynamic instability + hypoxia.
CIRCUIT DISCONNECT drops immediately to flat zero (no respiratory waves).
EXTUBATION flat zero.
SEVERE HYPOVOLEMIA drop reflects decreased cardiac output.
ANAPHYLAXIS drop from cardiovascular collapse + bronchospasm.
Distinguishing: visualize the circuit + tube + chest movement before assuming arrest.
Elevated baseline (rebreathing)
Causes: EXHAUSTED CO₂ ABSORBENT (color change in soda lime — but color change can fade; verify with capnogram), INCOMPETENT UNIDIRECTIONAL VALVE (inspiratory valve allowing exhaled gas to flow backward, or expiratory valve not closing properly), INADEQUATE FGF in a Mapleson system (FGF must exceed minute ventilation for non-rebreathing), VERY LOW FGF in circle system with marginal absorber.
- temporarily high FGF (8-10 L/min) to flush rebreathed gas while diagnosing
- replace exhausted absorbent
- inspect + replace failed unidirectional valves
Failing valves classically produce a phase I that no longer reaches zero AND a phase II with delayed upstroke.
Curare cleft + spontaneous breathing patterns
Indicates returning motor function — often the first sign that NMB is wearing off.
Use as a timing cue for TOF assessment + reversal dose.
Different from sustained spontaneous effort ('riding the vent' with assist or pressure-support mode) — that produces wider rhythmic waveform variations.
Spontaneous breathing during volatile maintenance is normal at light anesthesia and obvious on capnogram.
Apnea during emergence on volatile maintenance: deepen, support ventilation, troubleshoot opioid excess.
Phase IV slope changes + leak detection
SLOW phase IV downslope = leak around ETT cuff or LMA exhaled gas continues escaping around the tube during inspiration, extending the appearance of phase IV.
- audible leak from the mouth
- cuff pressure check by manometer
- tidal volume mismatch (delivered ≫ exhaled measured)
- peak airway pressure low for set tidal volume
- cuff under-inflated (use cuff manometer to target 20-30 cmH₂O)
- ETT too small for the airway
- cuff herniated through the cords
- LMA seal failed (reposition or upsize)
A slow phase IV is a quiet sign that something is wrong with the patient-circuit seal — easy to miss but high-yield to recognize.
Specific clinical scenarios
This is the GOLD STANDARD test, more reliable than auscultation + chest rise.
PATIENT-VENTILATOR DYS-SYNCHRONY: irregular waveforms, often correctable by adjusting trigger sensitivity or mode.
Capnography during transport — mandatory if intubated patient is moving (ICU transfer, scans, OR-to-PACU).
Procedural sedation outside the OR: capnography catches hypoventilation 30+ seconds before SpO₂ drop (especially with supplemental O₂ masking desaturation).
⚠ Common pitfalls
- Calling sudden ETCO₂ drop 'disconnect' without checking pulse — could be cardiac arrest.
- Ignoring an elevated baseline — that's rebreathing (exhausted absorbent or stuck valve), not just calibration drift.
- Reading any ETCO₂ as success on intubation — a brief CO₂ wave can come from gastric CO₂ if the patient drank carbonated beverages.
- Assuming ETCO₂ = PaCO₂ — gradient widens with deadspace (PE, low CO, severe COPD).
💎 Clinical pearls
- Shark-fin waveform = bronchospasm or COPD — treat the airflow obstruction.
- Curare cleft on phase III = paralytic wearing off — give reversal or another dose, not just more volatile.
- Sudden ETCO₂ drop + sudden BP drop = think PE, fat embolism, or air embolism.
- ETCO₂ as CPR quality metric: ≥10 mmHg during compressions correlates with ROSC; aim for ≥20.
Recap
- Shark-fin waveform = bronchospasm or COPD — treat the airflow obstruction.
- Curare cleft on phase III = paralytic wearing off — give reversal or another dose, not just more volatile.
- Sudden ETCO₂ drop + sudden BP drop = think PE, fat embolism, or air embolism.
- ETCO₂ as CPR quality metric: ≥10 mmHg during compressions correlates with ROSC; aim for ≥20.
Mark each section done to complete the module.