Capnography Waveform Interpretation

OVERVIEW

• Capnography waveform interpretation can be used for diagnosis and ventilator-trouble shooting
• The CO2 waveform can be analyzed for 5 characteristics:–Height–Frequency–Rhythm–Baseline–Shape

NORMAL CAPNOGRAM

4 phases

• Phase I (inspiratory baseline) reflects inspired gas, which is normally devoid of carbon dioxide.
• Phase II (expiratory upstroke) is the transition between VD_ana, which does not participate in gas exchange, and alveolar gas from the respiratory bronchioles and alveoli.
• Phase III is the alveolar plateau. Traditionally, PCO₂ of the last alveolar gas sampled at the airway opening is called the PETCO₂.
• Phase 0  is the inspiratory downstroke, the beginning of the next inspiration

Other features:

• Normal end-tidal PCO2 is approximately: 38 mmHg or 5%
• the alpha angle is the transition from Phase II to Phase III
• the beta angle is the transition from Phase III to Phase I (the start of inspiration)
• an additional phase IV (terminal upstroke before phase 0) may be seen in pregnancy

ETCO2 only represents alveolar CO2 when a relatively horizontal plateau phase (phase III) is seen.

FLAT ETCO2 TRACE

• Ventilator disconnection
• Airway misplaced – extubation, oesophageal intubation
• Capnograph not connected to circuit
• Respiratory/Cardiac arrest
• Apnoea test in brain death dead patient
• Capnongraphy obstruction

INCREASED ETCO2

CO2 Production

• Fever
• Sodium bicarbonate
• Tourniquet release
• Venous CO2 embolism
• Overfeeding

Pulmonary perfusion

• Increased cardiac output
• Increased blood pressure

Alveolar ventilation

• Hypoventilation
• Bronchial intubation
• Partial airway obstruction
• Rebreathing

Apparatus malfunctioning

• Exhausted CO2 absorber
• Inadequate fresh gas flows
• Leaks in ventilator tubing
• Ventilator malfunctioning

DECREASED ETCO2

CO2 production

• Hypothermia

Pulmonary perfusion

• Reduced cardiac output
• Hypotension
• Hypovolaemia
• Pulmonary embolism
• Cardiac arrest

Alveolar ventilation

• Hyperventilation
• Apnoea
• Total airway obstruction (note high airway pressures)
• Extubation

Apparatus malfunctioning

• Circuit disconnection (note low airway pressures)
• Leaks in sampling tube
• Ventilator malfunctioning

SUDDEN DROP IN ETCO2 TO ZERO

• Kinked ET tube
• CO2 analyzer defective
• Total disconnection
• Ventilator defective

SUDDEN CHANGE IN BASELINE (NOT TO ZERO)

• Calibration error
• CO2 absorber saturated (check capnograph with room air)
• Water drops in analyzer or condensation in airway adapter

SUDDEN INCREASE IN ETCO2

• ROSC during cardiac arrest
• correction of ET tube obstruction

ELEVATED INSPIRATORY BASELINE

• CO2 rebreathing (e.g. soda lime exhaustion)
• Contamination of CO2 monitor (sudden elevation of base line and top line)
•  Inspiratory valve malfunction (elevation of the base line, prolongation of down stroke, prolongation of phase III)

IMPORTANT PATTERNS TO RECOGNISE

See Figure 2 of Kodali 2013 and Interpret your capnogram at Capnography.com

• esophageal intubation (<6 waveforms of decreasing height)
• right main bronchus intubation (ETCO2 can increase, decrease or stay the same, can also cause a bifid capnogram)
• curare cleft (partially paralysed patient on mechanical ventilation)
• cardiogenic oscillations
• camel hump (seen in patients in lateral position)
• Rebreathing capnogram of Mapleson D circuit
• phase IV in pregnancy
• Dilution of expiratory gases by the forward flow of fresh gases during the later part of expiration when expiratory flow rate decreases below the forward gas flow rate
• sometimes see reverse phase 3 slope seen in patients with emphysema (alveolar destruction leads to rapid delivery of CO2 to airways)
• Sticking inspiratory valve
• expiratory valve malfunction
• mandatory versus spontaneously triggered breaths
• dual capnogram in lung transplants
• air leak
• malignant hyperthermia
• air / oxygen dilution during mask sampling of spontaneously breathing patients

References and Links

LITFL

• Capnography and CO2 Detectors

Journal articles

• Blanch L, Romero PV, Lucangelo U. Volumetric capnography in the mechanically ventilated patient. Minerva Anestesiol. 2006 Jun;72(6):577-85. PMID: 16682932.
• Kodali BS. Capnography outside the operating rooms. Anesthesiology. 2013 Jan;118(1):192-201. PMID: 23221867.
• Thompson JE, Jaffe MB. Capnographic waveforms in the mechanically ventilated patient. Respir Care. 2005 Jan;50(1):100-8; discussion 108-9.PMID: 15636648.

FOAM and web resources

• Capnography.com (the neon e-bible of capnography!)
• KI Doc — So much hot gas – ETCO2 for non-anaesthetists (2013)
• Smalhout B. A Quick Guide to Capnography (2004)