Evaluation of Capnography Sampling Line Compatibility and Accuracy when Used with a Portable Capnography Monitor

A series of four bench tests were performed to compare the accuracy and compatibility of matched and cross-paired capnography sampling lines with a portable capnography monitor. These calibrated tests measured average rise time and ETCO₂ levels across 10 independent repeat measures for each of the 16 sampling lines tested, and identified minimal variation in the results. While the tensile strength of the commercial sampling lines remained within the product specifications, the rise time differed significantly between capnography monitor matched and cross-paired sampling lines (p<0.001), and ETCO₂ accuracy as a function of respiratory rate and in the presence of supplemental O₂ was higher in capnography monitor matched sampling lines as opposed to cross-paired sampling lines. In particular, several of the cross-paired adult and pediatric sampling lines had rise times considered inaccurate at a maximum respiratory rate 150 BPM. The same sampling lines exhibited poor ETCO₂ accuracy at high respiratory rate or in the presence of supplemental oxygen.

The tensile strength test utilized a calibrated tensile testing jig to successfully measure tension across capnography sampling line components ranging from 1.33 to 26.6 kg. Although tensile strength tests are often performed on other types of medical devices^24,25, our method was unique in that it examined the tensile strength of each segment of the capnography sampling line. Therefore, in addition to determining the tensile strength of each sampling line component, it also allowed for identification of the overall weak point of the complete sampling line. The test results confirmed that nearly all of the sampling lines do meet product specifications, pre-defined as withstanding a force of 2 kg. One limitation of this testing system is the continuous, gradual increase in force applied to the sampling line, as opposed to a sudden strong force, which could be encountered in clinical settings. Importantly, as a validated instrument, the jig used to measure the tensile strength of the capnography sampling lines could be used for other applications, such as measuring the tensile strength of other sampling tubes and medical devices that have the potential to experience tension in a clinical setting.

Rise time is an important technical feature of sidestream capnography sampling lines and determines their ability to provide a precise, high resolution reading of CO₂ in exhaled breath^1,14. Due to the importance of this technical feature, we sought to measure the rise time using a validated rise time measurement device, so that the maximum respiratory rate and exhalation time could be calculated. We needed to modify the rise time measurement parameters to remove the upper time limit on the rise time jig, so that the rise time could be collected for all sampling lines before the measurement period ended. The long rise time observed for some capnography sampling lines could reflect an increased volume of dead space in these sampling lines. Importantly, as part of this method, we determined the maximum respiratory rate and exhalation time for two unique breathing patterns, defined by inhalation:exhalation ratios equal to 1:1 and 1:2. This unique aspect of the analysis allowed evaluation of the accuracy of measured CO₂ in circumstances that represent patients whose breathing pattern is uniform or whose exhalation time lasts longer than their inhalation time. In sampling lines in which the calculated maximum respiratory rate was >150 BPM, we concluded that the sampling line was accurate. Although a rapid breathing rate of 150 BPM is unlikely to be encountered clinically, we determined the accuracy of each sampling device at this high breath rate because it is considered the technical upper limit for many capnography sampling lines. While a respiratory rate of 150 BPM is non-physiologic, the bench test highlights that while some capnography sampling lines were accurate across the full technical range of respiratory rates, other sampling lines failed to achieve the same accuracy standard. Compared to the capnography monitor matched sampling lines, some of the cross-paired sampling lines, including sampling lines 2 and 7, failed to achieve accuracy at 150 BPM for the 1:1 inhalation:exhalation ratio, and sampling lines 3, 6, and 13 failed to achieve the accuracy standard at 150 BPM for both inhalation:exhalation ratios. This could be due to a larger dead space within the sampling lines, which results in a longer rise time and a mixing of breath samples.

To apply the rise time findings to a clinical setting, we performed two tests to examine ETCO₂ accuracy when sampling lines were connected to a portable capnography monitor via a manikin. For both tests, we needed to modify the default capnography monitor settings to allow the monitor to recognize cross-paired sampling lines. First, similar to a previous study, we controlled respiratory rate using a respiratory rate controller, and monitored the resulting ETCO₂ measurements for each sampling line¹⁸. A key component of this test was the use of a pre-defined set of respiratory rates ranging from 10 to 150 BPM, to determine ETCO₂ accuracy across respiratory patterns that patients could exhibit. While the expected ETCO₂ level was 34 mmHg in all circumstances, we observed many instances in which, as respiratory rate increased, sampling lines no longer reported accurate ETCO₂ readings, but instead, dropped to 0 mmHg, which is not a clinically meaningful result. In fact, only sampling lines 1, 8, 9, 10, 15, and 16 did not measure ETCO₂ values of 0 mmHg at any respiratory rate. This accuracy could be due to the design of the sampling lines, such that those with higher friction or larger dead space volume result in lower resolution breath samples at increased respiratory rate, similar to what we observed in the rise time test. While the sampling lines with high ETCO₂ readings may contain less dead space that enable them to deliver discrete breath samples, the error of ETCO₂ readings above 38 mmHg was pre-defined as ±5% of the reading + 0.08 for every 1 mmHg above 38 mmHg. This could partially explain why the ETCO₂ readings were increased above 34 mmHg during high respiratory rate in some sampling lines. In contrast, the sampling lines with low or zero ETCO₂ readings may contain more dead space, resulting in mixed breath samples that the capnography monitor does not recognize as valid breaths, and thus reports as no breath. Importantly, 3 of the cross-paired sampling lines from one manufacturer did not exhibit accurate ETCO₂ readings at any respiratory rate tested between 10 and 150 BPM, suggesting that it does not provide clinically reliable ventilatory information when cross-paired with the capnography monitor used in the test (Table of Materials). Together, these observations suggest that devices with a longer rise time have a lower maximum accurate respiration rate and exhibit low ETCO₂ accuracy at the maximum accurate respiration rate.

In the second test of ETCO₂ accuracy using a manikin, we maintained a constant respiratory rate but introduced the flow of supplemental oxygen to the system. This test mimics a common occurrence in hospital settings in which patients being monitored by sidestream capnography receive supplemental oxygen, and where ETCO₂ accuracy is key in understanding a patient’s respiratory function, as supplemental oxygen can mask ventilation challenges due to high oxygen saturation readings from pulse oximetry^30,31. Similar to the ETCO₂ accuracy test with varying respiratory rate, in this test, a key step in the protocol was to measure ETCO₂ accuracy across multiple supplemental oxygen flow rates. The main limitation of the ETCO₂ tests is that the tests are performed using a manikin and a controlled breathing system, as opposed to a human subject, in which breathing patterns vary between individuals. In a control reading without supplemental O₂, we observed that sampling lines 3, 4, and 12, all from the same manufacturer, failed to report the expected ETCO₂ value of 34 mmHg, and only sampling lines 8, 9, and 11 reported this value. In the presence of 2, 4, or 6 L/min supplemental O₂, a majority of the sampling lines exhibited reduced ETCO₂ accuracy, with the exception of the matched sampling lines 8 and 9 and the cross-paired sampling line 7. In particular, similar to our observations upon increase of the respiratory rate, the ETCO₂ readings for sampling lines 2 and 5 dropped to 0 mmHg in the presence of supplemental O₂, suggesting that their ETCO₂ accuracy when cross-paired with a capnography monitor is very low. This may be due to the design of the sampling lines, and in particular, the nasal cannula design, which is designed to both deliver oxygen to a patient and collect breath samples from a patient. If the nasal cannula contains a large amount of dead space, mixing of the supplemental oxygen and the exhaled breath can occur, resulting in low amplitude, mixed breaths that the capnography monitor does not detect as exhaled breath. In such a case, the ETCO2 measurement would drop to zero, as we observed with some of the cross-paired sampling lines tested.

Similar to previous studies examining the accuracy of capnography, we successfully identified circumstances where the ETCO₂ accuracy using a variety of sampling lines was acceptable, including cases in which there was a moderate respiratory rate or when no supplemental O₂ was used^{19,20,21,22,23,32}. Importantly, many of the sampling lines failed to maintain ETCO₂ accuracy upon an increase in respiratory rate or upon the introduction of supplemental O₂, which is consistent with previous assessments of capnography accuracy^15,18,20,23. Together, the findings are consistent with previous bench tests that successfully measure the accuracy of capnography sampling lines^15,18. Given that many of the sampling lines cross-paired to the capnography monitor exhibited reduced ETCO₂ accuracy in clinically relevant circumstances, care should be taken to ensure that any cross-paired commercial sampling lines and monitors are validated before being used to monitor patient ventilation status.