Anaes · Measurement & monitoring physics

Capnography and anaesthetic gas analysis

Also known as Capnography · Waveform capnography · End-tidal CO2 · Infrared gas analysis · Sidestream capnograph · Mainstream capnograph

Capnography is the continuous, breath-by-breath display of carbon-dioxide concentration against time, alongside analysis of the sampled breathing-system gas. Non-invasive CO2 measurement uses an infrared analyser; the end-tidal value is the closest breath-by-breath estimate of arterial PaCO2 but cannot replace it. The framework rests on seven exam-critical ideas. First, INFRARED ANALYSIS: exhaled CO2 is measured with an infrared analyser, giving breath-by-breath estimates of respiratory gas exchange used to verify intubation and monitor anaesthesia and ventilation. Second, the analysers come in two architectures: MAINSTREAM, a sensor in an airway adapter sampling directly from the airway, and SIDESTREAM, which pulls exhaled gas through a small tube to a remote detector (about 50 mL per minute sufficient). Third, the NORMAL TIME CAPNOGRAM divides into an inspiratory segment (phase 0) and an expiratory segment (phases I, II and III, occasionally IV): phase I is CO2-free dead-space gas, phase II the S-shaped alveolar upswing, phase III the alveolar plateau — with the ALPHA angle between phases II and III and the BETA angle between phase III and the descending limb of phase 0. Fourth, the ABNORMAL TRACE is pattern recognition: oesophageal intubation shows low, falling CO2 trending to near-zero within about eight ventilations with morphology distinct from tracheal traces; a rising end-tidal CO2 despite increased minute ventilation is an early diagnostic clue to malignant hyperthermia. Fifth, the PA-PETCO2 DIFFERENCE is typically 2 to 5 mmHg and increases with lung disease and ventilation-perfusion mismatch, when end-tidal monitoring can mislead and only blood gas gives the true PaCO2. Sixth, in CARDIOPULMONARY RESUSCITATION end-tidal CO2 reflects perfusion: higher mean values accompany return of spontaneous circulation, resuscitated patients average around 15 mmHg versus around 7 in those not resuscitated, a sudden continuous rise to 40 mmHg or above indicates return of circulation, and paediatric guidance sets an ETCO2 target above 20 mmHg for haemodynamic-directed CPR. Seventh, the circle system's CARBON-DIOXIDE ABSORBENT — calcium hydroxide plus strong bases since 1924 — degrades sevoflurane to compound A and desflurane to carbon monoxide via its strong-base additives; eliminating potassium hydroxide and keeping sodium hydroxide below 2 percent removes the concern, and modern low- or no-NaOH formulations are available. Built on the NAP4 anaesthesia complications report (Cook 2011), the time-capnography terminology and phase reviews (Bhavani-Shankar 1995; Bhavani-Shankar 2000), the mechanically-ventilated capnogram review (Thompson 2005), the CO2-measurement review with full text read (Umeda 2021), the CO2-absorption review (Feldman 2021), the malignant-hyperthermia review (Rosenberg 2015), the oesophageal-intubation waveform study (Groeneveld 2026), the ICU-CPR EtCO2 study (Singh 2026), the ILCOR paediatric update summary (Sankar 2026), the Integrated Pulmonary Index cohort (Ozden Sertcelik 2026), the paediatric NIRS outcomes study (Gabriel 2026), the exhaled-breath mass-spectrometry study (Wang 2026), the overnight breath-hydrogen study (Okumura 2026), and the VOCORDER breath-analysis study protocol (Kontopidou 2026).