What does a sudden loss of the capnography trace mean?
Clinical Measurement — Capnography and the Interpretation of the Waveform
The question
During a laparoscopic cholecystectomy in a stable patient, the end-tidal carbon dioxide falls abruptly from 4.8 kPa to zero within two breaths and the capnograph shows a flat trace at the baseline. The ventilator continues to cycle, the measured peak airway pressure has fallen, and no expired tidal volume is registered. Which is the most likely explanation?
- Massive pulmonary embolism
- Disconnection of the breathing system or displacement of the tracheal tube
- Severe bronchospasm
- A fall in cardiac output from occult haemorrhage
- Exhausted carbon dioxide absorbent in the circle system
The answer
Disconnection of the breathing system or displacement of the tracheal tube
Massive pulmonary embolism — A massive PE does cause an abrupt drop in end-tidal CO2, but expired gas still reaches the analyser. The result would be a waveform of reduced height, not a flat line. Furthermore, PE does not lower the airway pressure.
Disconnection of the breathing system or displacement of the tracheal tube — Correct. A flat trace at zero, no expired volume, and falling airway pressure indicate that no gas is reaching the sampling point. This is a mechanical problem: something in the airway or circuit. Disconnection is the most common cause.
Severe bronchospasm — Bronchospasm produces a shark-fin trace: a sloping upstroke, no proper plateau, usually accompanied by rising end-tidal values and rising airway pressures. The waveform changes shape; it does not disappear.
A fall in cardiac output from occult haemorrhage — Low cardiac output does reduce end-tidal CO2 because less CO2 reaches the lungs, but the change is gradual and the waveform retains its shape. A decline from 4.8 to zero within two breaths is not consistent with haemorrhage.
Exhausted carbon dioxide absorbent in the circle system — Exhausted absorbent causes rebreathing, so the inspiratory baseline rises instead of returning to zero between breaths. The trace elevates; it does not disappear.
Explanation
The normal capnogram consists of four phases: an inspiratory baseline at zero, a steep expiratory upstroke as alveolar gas replaces dead-space gas, an alveolar plateau sloping gently upward, and a steep inspiratory downstroke. Almost every clinically useful abnormality reduces to which of those features has changed. The single most important distinction is whether the trace has changed shape or whether it has disappeared entirely.
A flat trace at zero means no CO2 is reaching the analyser. Since any perfused patient produces CO2 continuously, the problem lies in transport, not metabolism. The differential includes disconnection of the circuit, tube displaced out of the trachea, oesophageal intubation, complete tube obstruction or kinking, ventilator failure, gas supply failure, or a fault in the sampling line. The accompanying signs indicate which cause is responsible. Falling airway pressure with no expired volume points to a leak or disconnection. Rising airway pressure with no expired volume points to obstruction. This is the reasoning behind the rule: no trace means wrong place. A sustained waveform over several breaths is the standard confirmation of tracheal placement, and its absence after intubation must be treated as oesophageal until proven otherwise.
A trace that persists but shrinks represents a different problem entirely. If gas is still being expired, a falling end-tidal value means less CO2 is reaching the alveoli. This is a circulatory or dead-space problem: falling cardiac output, hypovolaemia, PE, air embolism, or cardiac arrest during CPR. The change is gradual and the waveform retains its shape while losing height. Following the physiology in reverse explains why a sudden rise in end-tidal CO2 during CPR is a reliable early sign of return of spontaneous circulation (ROSC).
The other classic patterns are worth retaining as visual images rather than verbal descriptions. A sloping upstroke with no true plateau (shark fin) indicates obstructed expiratory flow, usually bronchospasm or a kinked tube. A raised inspiratory baseline indicates rebreathing: exhausted absorbent, faulty circle valve, or inadequate fresh gas flow. A cleft in the plateau indicates that the patient is making respiratory efforts against the ventilator, suggesting returning neuromuscular function. A slow, progressive rise in the end-tidal value over many breaths indicates hypoventilation, increased CO2 production, or absorption of insufflated CO2 during laparoscopy.
Tips for the candidate
The first question to ask is whether the trace has changed shape or whether it has disappeared. That single distinction accomplishes most of the differentiation. A flat line at zero indicates a mechanical problem in the patient, tube, or circuit. A trace that persists but is reduced in height indicates a circulatory or dead-space problem. Starting there narrows the differential to two or three options.
The other numerical values in the stem warrant careful attention. Airway pressure and expired tidal volume separate the mechanical causes: low pressure with no volume indicates a leak or disconnection, high pressure with no volume indicates obstruction, and normal pressure and volume with absent CO2 points to a sampling problem or absent circulation.
The four named waveform shapes should be learned as recognisable images rather than recalled descriptions: shark fin for obstructed expiratory flow, raised baseline for rebreathing, a plateau cleft for spontaneous respiratory efforts, and a slowly climbing end-tidal value for hypoventilation or CO2 absorption. When a stem describes a shape, identifying the pattern before reading the options is advisable.
Two resuscitation-specific rules merit particular attention. Loss of the waveform after intubation indicates oesophageal placement until proven otherwise. A sudden rise during chest compressions indicates ROSC. Both appear regularly in examinations, and both follow directly from the principle that end-tidal CO2 requires both ventilation and perfusion.