Time Delays

A capnogram is produced by the sequential analysis of inspired and expired gas flowing by the mainstream window, which is usually positioned just proximal to the endotracheal tube or breathing mask, or sampled from a sidestream sampling site connected to the breathing circuit. In the case of sidestream sampling, a definite time delay is introduced by the length and volume of the tubing carrying the sampled gas to the detector. This delay caused by the transfer of the sampled gas into the reading cell can be minimized by using high flow rates for the sampling flow and narrow, short tubing assemblies compatible with the position of the equipment with respect to the patient breathing circuit. After the sampled gas reaches the sensing cell, an additional delay is introduced by the rise time of each instrument. The rise time is a characteristic delay induced by the exponential response to a square front of changing gas concentration, in which CO₂ concentration changes instantaneously from zero to a new steady-state level (CO_2ss ). This can be described by the following exponential equation:

S = CO_2ss · (1 − e^−t/τ ) (24)

In Equation 24, S is the signal at time t, e is the base of natural logarithms (≅2.718), and τ (tau) is the time constant. When t = τ (i.e., one time constant), it is possible to calculate that S = 0.63 · CO_2ss (or 63%). This approach is used to measure the speed of response of the circuitry. It is often presented in clinical instruments as a different fraction of the response, such as the 10% to 90% response time (≅2.2 τ).

If we assume a square-wave capnogram for argument's sake as the most difficult for an instrument to follow and assume a time constant of 100 msec (a frequent value for a clinical instrument), it is theoretically possible to follow respiratory profiles of CO₂ within 5% of the real value (accuracy better than 5%) up to respiratory rates of 100 breaths/min. In practice, most clinical capnographs display this accuracy up to 60 breaths/min.^[169] A shared mass spectrometer, with the usually longer sampling line used in the share configuration, may display significant inaccuracy at respiratory rates of about 40 breaths/min.^[171] The relative duration of inspiration and expiration (I/E ratio) can also affect the accuracy of the recording instrument. Standardized respiratory cycle profiles have been used to compare the most commonly employed clinical instruments.^[172]

Sidestream capnographs are susceptible to errors related to gas diffusion. Radial diffusion of CO₂ out of the tubing can cause artificially low values.^[173] This error is related to the tubing material and the length of exposure of the sample inside the tubing (i.e., related to the sample flow rate and tubing length). Polyethylene and Teflon

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