Physics of Capnography
Slow analyzers distort CO2 wave forms
|2008-07-29Response time (Components)||What is it?||Affect on CO2|
|Transit time||Time to move from point of sampling to point of measurement||Underestimation due to dispersion of gases at longer transit times|
|Rise time (T70/T90)||Time to change in response to a step changes in CO2.||Abnormal wave forms, reduce slope of phase II at longer rise times|
Response time can be reduced by
Use of more powerful amplifiers
Minimizing the volume of the sampling chamber and tubes
Use of relatively high sampling flow rates (150 ml-min-l)
For accurate measurements of CO2, a capnograph needs a rapid response time. There are two components of a response time. The transit time and the rise time.1 The transit time is the time required for the sample to move from the point of sampling to the detector cell. Prolonged transit time delays the appearance of the waveform at the detector, which causes a phase shift, but no distortion. However, a bolus of gas is subjected to dispersal caused by convection and diffusion during transit down the catheter. Such a dispersal converts a square wave front into a sigmoid shape, with loss of the highest and lowest gas concentration peaks. This results in an underestimation of PETCO2, particularly in children. The extent of the error increases with increased length and width of the sampling tube, reduced sample flow rate (50 ml.min-l) and higher breathing frequency of the patient (more than 31 breaths min-l ).2,3 The rise time (T90)is the time taken by the output from the capnometer to change from 10% of the final value to 90% of the final value in response to a step change in PC02. Alternatively, rise time may be specified as T70 which is the time taken to change from 10% to 70% of the final value.4 The rise time is dependent upon the size of the sample chamber and the gas flow.1 Slower flow rates increase the time required to flush the infra-red sample cell, which can increase the rise time. The rise times of capnographs for clinical use range from 50-600 ms. Carbon dioxide waveform is a function of rise time of the capnometer. Prolonged rise time can reduce the slope of phase II resulting in an underestimation of anatomical dead space.3,5 The rise time of commercially available CO2 analyzers is fast enough to measure PETCO2 in adults, with 5% accuracy (less than 30 breaths min-l). However, when the ventilatory rate is high, as in children, faster analyzers with rise time (T70) of 80 ms are necessary to measure PETCO2 values with 5% accuracy (at 100 breaths min-l and l:E ratios less than 2:1).4 The response time of the C02 monitors has been considerably reduced in newer units by (i) the use of more powerful amplifiers, (ii) minimising the volume of the sampling chamber and tubes and (iii) the use of relatively high sampling flow rates (150 ml-min-l). In order to achieve predictable PETCO2 values and C02 waveforms it is recommended that the response time of the analyzer be less than the respiratory cycle time of the patient.6
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