Carbon Dioxide Monitoring — Capnography Principles

a) Scientific principles and sampling (30%)

Measurement principle

• Capnography is the continuous graphical display of exhaled CO₂ concentration or partial pressure against time.
• Clinical capnometers usually use non-dispersive infrared absorption.
• CO₂ absorbs infrared light strongly at approximately 4.26 μm.
• The measurement follows the Beer–Lambert law: absorbance is proportional to the concentration of the absorbing gas and the optical path length.
• An infrared source passes light through the gas sample to a detector. Increasing CO₂ causes greater absorption and therefore less transmitted light; the analyser converts this into CO₂ concentration / partial pressure.
• Because water vapour, pressure, temperature and other gases can affect measurement, modern systems use narrow-band filters, calibration and compensation algorithms.

Sampling techniques

Important measurement errors

• Water/secretions: contaminate sampling lines or optical windows and may distort the signal.
• Leaks / entrained room air: dilute sampled CO₂ and lower the measured value.
• Sampling-line obstruction: attenuates or abolishes the waveform.
• Response time / excessive sampling tubing: delays and smooths the waveform, especially at high respiratory rates.
• Gas interference and pressure effects: can alter infrared absorbance; modern analysers compensate for common interferents.

b) Normal capnogram waveform and features (20%)

Time capnogram: CO₂ partial pressure/concentration on the y-axis versus time on the x-axis.

• ETCO₂ is measured at the end of phase III, immediately before inspiration.
• In healthy lungs ETCO₂ is usually slightly lower than PaCO₂, commonly by about 2–5 mmHg; the gradient widens when alveolar dead space or V/Q mismatch increases.
• Alpha angle: angle between phases II and III; increases when expiratory time constants become heterogeneous, classically with airflow obstruction.
• Beta angle: angle between phase III and the inspiratory downstroke; an increased beta angle / failure to return to zero suggests CO₂ rebreathing.

c) Ventilation and perfusion information derived from the waveform (50%)

Ventilation

Perfusion

• ETCO₂ approximates alveolar and therefore arterial PCO₂ when V/Q matching is relatively normal.
• With ventilation and metabolic CO₂ production held approximately constant, the amount of CO₂ delivered to alveoli depends on pulmonary blood flow / cardiac output.
• Therefore a fall in cardiac output reduces CO₂ delivery to the lungs and causes a rapid fall in ETCO₂; examples include severe hypotension, cardiac arrest and massive pulmonary embolism.
• Conversely, an abrupt sustained rise in ETCO₂ during CPR can indicate increased pulmonary blood flow and may suggest return of spontaneous circulation.
• The PaCO₂ − ETCO₂ gradient widens as alveolar dead space / high-V/Q lung increases because poorly perfused alveoli contribute low-CO₂ gas to the exhaled mixture.
• Thus a widening gradient may occur with pulmonary embolism, low cardiac output, excessive PEEP or other causes of reduced pulmonary perfusion.

Integrated V/Q information

• The waveform combines information about ventilation, perfusion and metabolism; ETCO₂ must therefore be interpreted in clinical context rather than as a direct substitute for PaCO₂.
• Phase III slope reflects heterogeneity of alveolar time constants and V/Q ratios: a steeper plateau suggests uneven emptying, particularly in obstructive lung disease.
• An increased PaCO₂ − ETCO₂ gradient indicates increased physiological dead space / V/Q mismatch.
• Capnography is excellent for trending rapid physiological change, but a single waveform abnormality is usually not disease-specific.

Exam focus

• Part (a): state Beer–Lambert + infrared absorption + mainstream versus sidestream.
• Part (b): draw the waveform accurately and label phase 0, phases I–III, ETCO₂, alpha angle and beta angle.
• Part (c): split your answer into ventilation and perfusion, and explain the mechanism behind every inference rather than merely listing “cardiac output”, “respiratory rate” or “V/Q mismatch”.