European Journal of Cardiovascular Medicine

Non-invasive cardiopulmonary monitors are instruments that can be utilised more extensively to influence the quality of care in the evolving healthcare environment of today. One practical method for measuring carbon dioxide pressure in patients who are intubated is capnometry. EtCO2 can be measured non-invasively using a device called a capnometry.[1] Generally speaking, an elevated exhaled CO2 level indicates either increased metabolic activity or hypoventilation. Low exhaled CO2 levels, which are frequently observed in shock, may be a sign of hyperventilation, reduced cardiac output, or inadequate pulmonary perfusion.[2]

Research has demonstrated that in healthy individuals with normal lungs and uncompromised pulmonary function, the difference between PaCO2 and EtCO2 is just 1-2 mmHg. While a portable colorimetric device has been created that allows for semiquantitative EtCO2 assessment, its primary applications are in emergency care, post-anesthetic recovery, and intensive care.[3] Its accuracy and usefulness for nonintubated patients have not been thoroughly investigated.[4]

This was a cross-sectional study carried out in the AIG Hospital, Hyderabad, between 2020 to 2022 involving 100 patients presenting to the emergency department with respiratory distress with a respiratory rate greater than 30/min, use of accessory group of muscles, Spo2%< 90% or abnormal pattern of breathing. Patients with respiratory distress secondary to hypovolaemia were excluded. PaCO2 was measured using an arterial blood gas analyzer and EtCO2 was measured using capnometry. The data was compiled on a Windows XL spread sheet and analyzed using SPSS version 19.

The majority (33%) were above 60 years old, followed by 28% between 51 and 60 years old. Males constituted 74% (n = 74), while females were 26% (n = 26). (Table 1)

Out of 100 subjects, 50% (n = 50) had a respiratory cause, while the rest had a non-respiratory cause respiratory distress. COPD accounted for the majority (28%) among the respiratory causes, followed by asthma in 20%, ARDS in 16%, and pneumothorax and pleural effusion in 12%. Pneumonia and ILD formed the remaining 8% and 4%, respectively. Table 2

Among non-respiratory conditions, the majority (44%) had renal issues. DKA accounted for 22%, trauma for 12% and cardiac problems for 10%. Neurological problems, massive ascites, and septic shock were seen in 4% each. Table 3

There was a positive correlation between EtCO2 and PaCO2 among patients with respiratory conditions (correlation coefficient 0.956). Table 4 and Figure 1

There was a positive correlation between EtCO2 and PaCO2 among patients with non-respiratory conditions also. The correlation coefficient is 0.781. Table 5 and Figure 2.

It is common for patients who arrive to the emergency room to need to have their breathing, oxygenation, and acid-base status evaluated. Rapid, noninvasive measurement of these metrics would make evaluating and reevaluating ED patients much easier. Capnometry has been shown to be an effective way to monitor carbon dioxide tension in patients who are intubated in recent years.^[5] Capnometry has been utilized as an indirect measure of cardiac output and the possibility of resuscitation following cardiac arrest. It has also been used to assess the suitability of alveolar ventilation during anesthesia and to wean patients off of mechanical ventilation.^[6–8] Despite the fact that techniques for monitoring expired CO2 in nonintubated patients have been published, only one recent report^[9] raises the possibility that capnometry may be a helpful measurement in these patients.^[10] By establishing the relationship between end-tidal carbon dioxide and Paco2, we aimed to ascertain the possible application of capnometry in ED patients who are not intubated.

Finding such a link will enable us to replace arterial blood sample PaCO2 with the EtCO2 level. In fact, the findings demonstrated that, among the 100 patients experiencing respiratory distress, there was a substantial association between arterial carbon dioxide and EtCO2. Patients were split into two groups: those with respiratory disorders and those without, and it was found that there was a substantial association between the two groups (p<0.05). Various age groups were included, ranging from 20 years to 80 years. The majority of the subjects were above the age of 60 (33%). The study conducted by Pekdemir M. et al.^[11] showed a good correlation between EtCO2 and PaCO2 in patients with an average age of 60.9 years. The current study included 74% male and 26% females. There was no correlation of EtCO2 with PaCO2 in relation to the sex. Thus, we can infer that sex does not interfere with the PaCO2 and EtCO2 values.

Previous studies conducted by Pekdemir M. et al. ^[1] and Barto1 CW ^[12] inferred that measuring ETCO₂ is very useful in patients with respiratory distress, in both pediatrics and adults. But not every study of this kind has yielded the same outcomes. In the Jabre P et al. study ^[13] EtCO2 measurements poorly reflected PaCO2 values in the population of nonintubated patients with respiratory distress of various origins.

EtCO2 measured by a portable capnometer had a substantial connection with PaCO2, as shown by Takano et al. ^[14] Additionally, they showed that in patients with impaired pulmonary function, EtCO2 and PaCO2 had a strong correlation. These findings demonstrate the value of using a portable capnometer to monitor EtCO2 in order to estimate PaCO2 and identify hyper- or hypoventilation in patients who are breathing on their own. Even though our study covered patients with a variety of lung conditions, our findings supported their findings and suggested that even in individuals with impaired pulmonary function, there is not much of a difference between PaCO2 and EtCO2.

Traditionally, patients with chronic or severe respiratory failure who were intubated and on mechanical ventilation were the focus of studies examining the link between PaCO2 and EtCO2.^[15] Studies on the connection between PaCO2 and ETCO2 in patients who breathe on their own, however, have not yet been thoroughly investigated.^[16]

Using the sidestream ETCO2 device, Yosefy C. et al. ^[17] performed a correlation study between EtCO2 and PaCO2 on patients who were referred to the ED for respiratory distress. There was a significant connection (r = 0.792) between arterial PaCO2 and EtCO2. In order to ascertain if the mainstream EtCO2 measurement can reliably forecast the PaCO2 level of patients who arrive at the emergency department with acute dyspnoea, Cinar O. et al. ^[18] undertook a study. According to the study, ETCO2 and PaCO2 had a statistically significant, positive connection (r = 0.911, P <.001). The table displays the results of the current investigation as well as the findings of the two studies mentioned above.

The capnometer has been shown to be a helpful instrument, yet at greater respiratory rates, it might not be able to detect changes in end-tidal carbon dioxide.^[19] Moreover, the frequently coexisting hypotension may worsen ventilation-perfusion mismatching and alter the relationship between end-tidal carbon dioxide and PaCO2.

Endtidal carbon dioxide has a strong correlation with PaCO2 in a range of patients with various underlying medical conditions, which may indicate a broad use. It is yet unknown if capnometry is helpful as a screening tool or as a gauge of a patient's ventilatory trend (such as in the case of an asthmatic patient who is becoming tired and whose end-tidal carbon dioxide levels are rising). The gold standard for determining ventilation, oxygenation, and acid-base status is still arterial blood gas analysis. For patients who are not intubated, there is insufficient data to justify the substitution of capnometry for arterial blood gases. It is necessary to conduct additional randomised experiments to ascertain whether the non-invasive EtCO2 can fully replace PaCO2 measurement.

End-tidal carbon dioxide concentration measurements show a strong correlation with PaCO2 values in nonintubated patients who arrive at the emergency room for a range of ailments. In some cases, end-tidal carbon dioxide readings may be adequate indicators of PaCO2 and eliminate the requirement for repeated arterial blood gas analysis. More research is necessary

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