A novel application of capnography during controlled human exposure to air pollution

The primary objective of current and future air pollution studies is to further understand the mechanisms for epidemiologic data on increased morbidity and mortality due to exposure to a ambient pollutants, e.g. fine particulate air pollution, i.e. particles of aerodynamic diameter < 2.5 μm (PM_2.5) [1–5]. In addition, the next objective is to link the data from animal studies [6] with clinical evidence related to adverse cardio-pulmonary and vascular events; in particular, coronary and cerebro-vascular events related with such exposure [7–10]. To this end, environmental air pollution studies aim to reveal the physiologic changes and molecular mechanisms that follow inhalation of gaseous and particulate pollution and lead to a systemic inflammatory response. Moreover, they aim to provide plausible explanations of the association between air pollution exposure and morbidity/mortality from cardio-pulmonary and vascular etiology. Understanding these outcomes is of paramount importance in susceptible populations such as children, the elderly, asthmatics, patients suffering from chronic obstructive pulmonary disease (COPD), cardiovascular diseases and diabetes.

Different health outcomes and indices have been assessed during controlled human exposures to gaseous and particulate air pollution: ECG and lung function changes [11, 12], heart rate variability [13–15], blood and sputum markers of inflammation – cellular variability and mediators including interleukin 6 (IL-6), leukotriene B4 (LTB₄), tumor necrosis factor alpha (TNFα,) interleukin 8 (IL-8), endothelins and fibrinogen, [13, 16].

Acute PM_2.5 exposure can promote systemic inflammation by activating the capillary endothelium in the pulmonary circulation [17, 18]. Therefore, we redirected our interests in detecting acute effects of PM_2.5 exposure from lung mechanics to gas exchange surface of the alveolar-capillary membrane. Our fine concentrated ambient particles (CAP) exposure facility can concentrate particles sized 0.15 – 2.5 μm in aerodynamic diameter. Due to geometry of the airways, fine particles (PM_2.5) can be inhaled deep into the small conducting bronchioles and the alveoli. If adverse effects of PM_2.5 are to be measured, a major site that could be examined is the alveolar-capillary membrane. Therefore, we considered capnography as the most suitable procedure that can provide real-time information about alveolar ventilation, pulmonary perfusion, respiratory pattern, and CO₂ elimination/production. Clinically, capnography has been used in concert with pulse oxymetry to detect adverse respiratory events during anesthesia [19, 20]. In addition, capnography has been used in emergency rooms and during emergency transportation [21–24]. However, although widely used in clinical and pre-hospital settings, an extensive literature search confirmed that capnography has not been used as a procedure or outcome measure in settings of controlled human air pollution studies.

In our previous studies [17, 25], we could not detect any statistically significant changes in standard pulmonary function measures, such as forced expiratory volume in the 1^st second (FEV₁) or lung diffusing capacity for carbon monoxide (D_LCO). However, we have reported a constriction of the brachial artery immediately following a 2-hour CAP + O₃ exposure [18] and an acute increase in diastolic blood pressure in healthy adults during controlled exposure to CAP ± O₃ [26]. We hypothesize that "real-time" capnography would be a more sensitive or even specific marker of the inflammation induced by pollutants (CAP ± O₃) at the alveolar-capillary membrane, as well as pollutant-induced airway constriction, rather than more gross, effort dependent functional tests such as spirometry and lung diffusion. Since events at the alveolar-capillary membrane are an instantaneous reflection of ventilatory/cardiovascular function, any compromise of the ventilation-perfusion relationship, which may result after breathing CAP ± O₃, could have some detectable consequences. Therefore, capnography as a non-invasive, instantaneous physiological measurement, which could provide insight into alveolar ventilation, physiological dead space changes, pulmonary perfusion, ventilation-perfusion mismatch, changes of cardiac output and respiratory patterns, seems to be a potentially valuable and needed procedure in air pollution research.

In this study, we tested acquisition of the capnographic wave signal for stability and repeatability through customized hardware & software components interfaced with the human exposure facility of The Gage Occupational and Environmental Health Unit, St. Michael's Hospital and University of Toronto. The HEF design and exposure characterization have been described in details elsewhere [17, 25]. See Figure 1 showing HEF with the door open.

Summary data for the ventilatory parameters, V_TE and RR comparing EXP#1 and EXP#2 for the four subjects are shown in Table 3. The coefficients of variation for these two variables were above 10%, 21.87% for the CV of RR for the first measurement and 33.25% for CV of V_TE for the second measurement. In addition, none of the mean values for these parameters was significantly different comparing the open versus sealed and operational HEF.

Descriptive statistics for EtCO₂ signal variables measured within the open and sealed HEF (EXP#1 and EXP#2) are presented in Table 4. The coefficient of variation for RMS is smaller than the CV for MEAN, for both exposures, while the EtCO₂ signal zero (MINIMUM) has the highest CV of all variables. The paired t-test for means showed no significant statistical difference between measurements within open and sealed HEF for any of these variables.

We added customized hardware and software components to interface with the HEF in order to utilize the advantages of capnography over the standard tests of lung function. We chose to use capnography in the HEF as we expect that capnography will be a useful tool for the detection of changes in ventilation during human exposure studies. Capnography, as a non-invasive, real-time physiologic measurement of adequacy of ventilation and perfusion, has found its role in a variety of medical disciplines including anesthesiology, critical care, emergency medicine, cardiology, pediatrics, neurology and respiratory care. Current capnographic hardware and software is designed for these aforementioned environments. As such, we did not find a commercially available, ready-to-go solution for the implementation of capnography in the experimental environment of human air pollution exposure studies. Our goal was to use relatively inexpensive and commercially available hardware components to acquire EtCO₂ wave signals within sealed and operational HEF, but with acceptable signal quality. Our first choice for the capnograph was the Microcap^® CO₂ portable bedside monitor, with nominal performance features adequate for our study's setup. However, due to the absence of an analog output, this monitor needed an additional digital-to-analog converter. We used one supplied as an option by the original manufacturer, designed to be used with this particular model of capnograph. Further processing of the four analog signals (EtCO₂ wave, RR, EtCO₂ wave signal validity and ground) utilized the standard solution (terminal board, expansion cable and MF 5500 AD card). Although the signal was less than 1.0 V, due to the low level of noise, we found that a single ended connection was adequate. We performed zero CO₂ calibration of the capnograph with 100% O₂, in order to measure the system signal noise for the overall hardware and software setup. The average noise level was 0.0052 ± 0.016 V and RMS was 0.0168 V. Signal-to-noise ratio (SNR) over the full scale of 100 mmHg was 34.59 dB. We found the WinView data acquisition software easy to install and setup using the MF 5500 AD board. The EtCO₂ wave signal was saved as an ASCII file and imported into WinDaq Waveform Browser. This software was quite adequate for EtCO₂ wave signal analysis.

An additional concern, related to the EtCO₂ sampling line required a customized solution. We used two original sampling lines to make one custom line, which attaches on both sides of the custom-made needle port screwed into the plexiglas HEF window. Accordingly, we did not observe additional delay added to frequency response of the monitor. We considered the possibility that sampling through a longer line than originally designed and recommended by the manufacturer could also affect the performance of the capnography monitor. However, we did not observe that sampling in this way affected the reading of EtCO2 and RR on the capnograph itself. Mason et al. [36] showed that an experimental sampling line as long as 9.0 m was equally reliable as the standard 3.0 m long one, which agrees with our observation.

Comparisons of the differences between capnographic measurements obtained under two different conditions, relevant to our design of human exposure studies (open and sealed HEF), did not yield any statistical significance. Yet, some differences existed in absolute values, but allocation of these differences among observed measures was to be expected. In the group of EtCO₂ wave parameters, the variable with the lowest CV was the zero-corrected EtCO₂, 3.89% and 3.62%, followed closely by EtCO₂, 4.14% and 4.48%. Verschuren et al. [43] considered CV of the EtCO₂ less than 5% as an indicator of steady-state status for their patients The EtCO₂ is the main output value of the capnograph and correction of this value with the minimum-recorded value, for a particular wave, increased the accuracy of true EtCO₂ reading. Among seven out of eight tests, we found that the range of recorded zero values for EtCO₂ was between 0 and 0.55664 mmHg (1.42% of maximum-recorded EtCO₂ of 39.167 mmHg). Surprisingly, we found that one subject's exposure (EXP#2 for that subject, with 15 acceptable analyzed waves) had recorded a zero range between 1.4274 to 2.8547 mmHg (7.29% of the maximum-recorded EtCO₂ of 39.167 mmHg). Reviewing the lab log for this test, we found that 5 min after the enclosure was sealed, the recorded CO₂ within the HEF was approximately 0.38%. This could have happened, because the particle delivery/CO2 removal mask either was not in place or correctly positioned, or in the case of the CO₂ removal pump system malfunction. Thus, the maximum recoded zero for EtCO₂ was approximately the background concentration of CO₂ in the sealed HEF (749 mmHg × 0.0038 = 2.846 mmHg). Later, we were able to simulate this pattern of zero shift with the particle delivery/CO₂ removal mask incorrectly positioned and this explained the unexpected CV of the MINIMUM variable for the EtCO₂ signal. See Figure 4 for the EtCO₂ tracing of that original test in trend format.

CV for the slopes of phase II and III, for both measurements, showed that these two variables are also the ones with the largest physiologic variability even in the steady state. Thus, they have the potential to be considered as variables capable of detecting a deviation from the normal pattern as well. Changes in RR, V_TE, and ventilation maldistribution expressed as ventilation/perfusion (V/Q) mismatch, can result in changes of phase II and III of the EtCO₂ wave [42]. For both, EXP#1 and EXP#2, RR and V_TE were virtually identical, allowing us to have slopes of phase II and III clearly comparable between the two measurements. The CV for RR and V_TE (range 21.87 to 33.25) were somewhat higher than reported by Verschuren at al. [43] for their patients. We believe that this might be possibly due to the psychological effect of the limited space in the HEF, which might affect the variability of the breathing pattern.

In the group of EtCO₂ signal variables, we found that the CV for RMS was smaller than for the MEAN, because RMS has been calculated as the true RMS and better represents the "average" value of expired CO₂ than the MEAN value obtained by the standard equation. The true RMS, in this case, better approximates mixed-expired CO₂ (P_ECO₂) per one breath or over the time that the EtCO₂ signal was "averaged". The P_ECO₂ and arterial partial pressure for CO₂ (P_aCO₂) are used to calculate dead space to tidal volume ratio (V_D/V_T).

We have described a customized, inexpensive and feasible hardware and software implementation of standard capnography, as an adjunct to our human exposure facility, which is used in air pollution studies. Our hardware and software setup for acquisition and analysis of EtCO₂ wave signal was demonstrated as reliable and stable. We would have preferred that the selected capnograph monitor have an analog output, so we could avoid digital to analog signal conversion. The instrument was meant to be used in standard pre-hospital or hospital environments, therefore the manufacturer had not considered that option. However, extending the use of capnography into research labs like ours, where we carry out air pollution human exposure studies, has been long overlooked. Using capnography in the way we described, we may be able to detect and better characterize the effect of exposure to air pollution on cardiopulmonary system, particularly on lung ventilation and perfusion. We plan to use this technique in our on-going and subsequent controlled human exposure studies as a physiologic outcome measure.