Capnography
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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.