# Capnography > Capnography for Anaesthesia Technology: infrared CO2 measurement, waveform phases, ETCO2 to PaCO2 gradient, abnormal traces, air embolism and exam answers. - Canonical URL: https://prepelephant.com/topics/allied/anaesthesia-technology/capnography-basics-at - Exam / course: Allied Health · Subject: Anaesthesia Technology - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Capnography", PrepElephant, https://prepelephant.com/topics/allied/anaesthesia-technology/capnography-basics-at ## Direct answer Carbon dioxide absorbs infrared light at 4.3 micrometres, and measuring that absorption breath by breath produces the capnogram — the single most information-dense monitor in theatre. A normal trace has four phases: a flat inspiratory baseline (phase I), a sharp expiratory upstroke (II), an alveolar plateau whose end is the ETCO2 (III), and the inspiratory downstroke (IV). End-tidal CO2 normally reads 35-45 mmHg and sits 2-5 mmHg below arterial PaCO2, because expired gas mixes alveolar air with dead-space gas. The trace is simultaneously a confirmation of airway placement, a report on ventilation, a window on cardiac output, and — in its sudden rises — an early alarm for malignant hyperthermia. ## What you must remember - Terminology: capnometry is the number, capnography the waveform; mainstream devices put the sensor in the airway (fast, bulky), sidestream devices aspirate 50-200 mL/min through a sampling line to a remote sensor (needs a water trap). - A raised baseline (phase I above zero) means rebreathing — exhausted sodalime, a faulty circle valve, or inadequate fresh gas flow in a Mapleson system. - Rising ETCO2: hypoventilation, laparoscopic CO2 absorption, sepsis, sodium bicarbonate, and — fastest and most sinister — malignant hyperthermia in an unexplained, tachycardic, rigid patient. - Falling ETCO2: hyperventilation, falling cardiac output, pulmonary embolism, and sudden large drops with venous air embolism; a lost trace means disconnection, obstruction or oesophageal intubation. - Zero CO2 after intubation means oesophageal placement until proven otherwise; a trace sustained over six or more breaths confirms tracheal placement — the gold standard, since condensation and auscultation both mislead. - A sloped, shark-fin upstroke merging into the plateau signals airway obstruction: asthma, COPD, kinked tube, secretions, or a partially obstructed filter. - Notches carved into the plateau (the "curare cleft") reveal spontaneous respiratory effort despite paralysis — a sign to treat, not to ignore. - In cardiac arrest, ETCO2 tracks pulmonary blood flow and hence CPR quality: persistently below 10 mmHg predicts poor outcome, and a sudden jump announces return of spontaneous circulation. ## A sudden fall during craniotomy A posterior fossa craniotomy runs in the sitting position; forty minutes in, the ETCO2 falls from 35 to 17 mmHg over less than a minute while the SpO2 still reads 99%. That dissociation is the signature of venous air embolism: air entrained through non-collapsible dural veins reaches the pulmonary circulation, creates dead space, and the end-tidal CO2 collapses while oxygenation has not yet suffered. The sequence that follows is a checklist. Tell the surgeon immediately — the field is flooded and bone edges waxed. Stop nitrous oxide at once (it expands the embolised bubbles). Position head down and left lateral if feasible. Aspirate from a correctly placed central line. Support the circulation with fluids and vasopressors. The capnograph, asked nothing about air, made the diagnosis earlier than any other monitor; precordial Doppler would corroborate. Contrast the gradual pattern: the same case, twenty minutes of pneumoperitoneum during laparoscopy, and the ETCO2 climbs steadily from 38 to 48. Peritoneal CO2 is being absorbed — expected physiology, managed by increasing minute ventilation by roughly a fifth to a third, not a crisis but a number to chase deliberately. ## Where students slip The gradient direction trips candidates: ETCO2 sits below PaCO2, never above it in health, because alveolar gas is diluted by CO2-free dead-space gas; the gradient widens with dead-space disease and with a failing cardiac output. Second, the flat-line traces get lumped together: a flat capnogram with the ventilator still cycling means no CO2 is reaching the sensor — disconnection, total obstruction or oesophageal tube — whereas a low-but-present plateau with a preserved shape usually means circulation or over-ventilation. Third, relying on tube condensation or bilateral auscultation for intubation confirmation; the current standard answer, asked in every exam, is sustained waveform capnography over six breaths. ## Frequently asked questions ### What is the normal ETCO2 to PaCO2 gradient? Two to five mmHg, ETCO2 being lower; widening suggests dead-space disease or falling cardiac output. ### Why is capnography the gold standard for confirming tracheal intubation? Only exhaled metabolic CO2 comes from the lungs; a sustained waveform over six breaths excludes oesophageal intubation, which may transiently show CO2 for the first few breaths. ### Which capnographic pattern suggests malignant hyperthermia? An unexplained, rapidly rising ETCO2 in a tachycardic, hyperthermic or rigid patient — the earliest reliable sign. ### What does an elevated inspiratory baseline indicate? Rebreathing of CO2: exhausted sodalime, a stuck circle-system valve, or inadequate fresh gas flow. ### Why does ETCO2 rise during laparoscopic surgery? Peritoneal carbon dioxide is absorbed across the peritoneum; manage by increasing minute ventilation. ### What happens to ETCO2 in venous air embolism? A sudden fall, often with a rise in end-tidal nitrogen, out of proportion to any change in ventilation — an immediate operative emergency.