Capnography

Principles of capnography:

By the Beer-Lambert law:

$$\text{Absorbance} = \Delta L \cdot \epsilon^{\text{wavelength}}_{gas} \cdot P_{gas}$$

IR light is shone through 2 chambers - the sample chamber and reference chamber (contains no CO2, for calibration); the absorbance is used to calculate the CO2 partial pressure.

Sample can be side-stream

• sample taken from thin tube connected to ventilator circuit
• increases delay to ~3 seconds
• sampler can be placed e.g. beneath a hudson mask

...or main-stream

• sample taken directly from circuit
• adds dead space

A capnograph waveform has 4 phases:

• Phase 1: Inspiration and the first part of expiration, where anatomical dead space gas is expired. No CO2.
• Phase 2: mixed dead space gas and alveolar gas
• Phase 3: pure alveolar gas.
• Phase 0: inspiration washes out CO2 from monitor

Sources of error:

• Blockage by secretions / condensation
• Ambient IR light
• N2O absorbance
• Collision broadening from high FiO2 or FiN2O

Limitations

• False positive CO2 from gastrict gas post BVM ventilation
• Increased dead space will widen PaCO2-EtCO2 gap

Ventilation and perfusion info obtained from trace:

• Height (EtCO2): composed of
• PaCO2 (\uparrow\) with \(\downarrow V_A\) or \(\uparrow\) CO2 generation e.g. malignant hyperthermia.
• PaCO2 - EtCO2 gap which \(\propto\) dead space or high V/Q lung units. Usually <5mmHg. Gap widened with \(\uparrow\) anatomical/instrumental dead space, PE, fall in cardiac output.
• Frequency = respiratory rate
• Rhythm = presence of abormal breathing patterns, e.g. dyssynchrony or Cheyne-Stokes
• Baseline = Presence of rebreathing (closed ventilator circuit \(\to\) exhausted lime, open ventilator circuit \(\to\ \ V_D > V_T\))
• Shape
• Decreased phase II slope, increased phase II slope, "shark fin" appearance \(\to\) prolonged mixing of alveolar and dead space gas \(\to\) time constant heterogeneity (e.g. obstructive lung disease)
• Curate clefts \(\to\) dyssynchrony
• Irregular waveform \(\to\) intermittent blockage by secretions, leakage