Breath Rate Distortion
The effect of rise time distortion to the gas curve becomes apparent when the breathing rate
increases so that the time for a full inspiratory or expiratory event gets shorter. In those
situations, due to the effect of the rise time, the gas curve does not reach the true end‐tidal (or
first inspired value) and the end‐tidal gas value may then be underestimated. Correspondingly,
the first inspired value may be overestimated. Below is an exaggerated illustration of the effect.
The breath rate limit for accurately resolved end‐tidal gas values (at an I:E ratio of 1:1) may be
found in Appendix A. The effect of other I:E ratios may be calculated by determining the length of
the shortest inspiratory/expiratory event that can be resolved accurately:
The difference in these results when compared to the rise time's specification is that rise time's
only tests 10‐90% performance. This specification is for (0 + accuracy) to (100 ‐ accuracy) % and is
thus much tougher. The ability to properly resolve end‐tidal values can be measured by using the
set‐up described in ISO 80601‐2‐ 55:2011 figure 201.101. In short, the method consists of
sampling gas from two different sources connected to an electrically controlled pneumatic valve
to permit rapid switching between the two sources. During the test, the valve is set to switch gas
source at a number of frequencies (simulating the range of specified breath rates) and for each
frequency the end‐tidal value presented by the gas analyzer is noted. From a diagram of end‐tidal
value over frequency, the frequency at which the gas analyzer is no longer able to resolve end‐
tidal values according to specification is identified. This ability to properly resolve end‐tidal values
is listed in the specification.
Expression MR400 Instructions for Use
Select the CO2 VS box.
The CO2 menu appears. Current settings are displayed.
Select Zero Cal.
Calibration will begin and Performing CO2 Zero will be displayed.
When complete, the message will be removed.
- - at gas sampling site
— as presented by gas analyzer
= 60 / (2 x BR
(I:E) = 60 / ((I + E) x t
Monitoring CO2 (LoFlo Option) 7‐3