The first point of admission for patients experiencing acute respiratory distress is usually the emergency department (ED). Establishing a safe and protected setting for individuals experiencing respiratory distress is paramount. Furthermore, it is imperative to efficiently detect and diagnose any underlying factors contributing to their shortness of breath. For this reason, measuring devices that evaluate respiratory status with noninvasive, easily applicable, and reliable methods in busy ED environments are of interest. End-tidal carbon dioxide (EtCO2) measurement strengthens its place in clinical practice day by day as a handy and valuable method for evaluating patient ventilation in emergency settings.1–6

The use of EtCO2 monitoring is becoming increasingly common in a variety of clinical settings, both prehospital and hospital.7 In addition to being considered the most reliable confirmation method of the correct placement of the endotracheal tube, it also helps the physician in life support efforts, such as determining the quality of cardiopulmonary resuscitation and detecting the return of spontaneous circulation.8,9 Capnography is used in various situations, including procedural sedation and analgesia, patient transfer, pulmonary embolism, and acute follow-up of chronic obstructive pulmonary disease (COPD).10–14

EtCO2 is the level of carbon dioxide released at the end of expiration. For noninvasive EtCO2 measurement, capnometry, which provides only numerical values, or capnography, which provides both graphical and numerical results, is used. Measurement is made as a mainstream or a sidestream. In sidestream, the CO2 sensor is located inside the monitor, and a small portion of the exhaled air reaches the breathing circuit through a cable with a delay. In the mainstream type, the CO2 sensor is directly connected to the breathing circuit and displays the result without delay. While sidestream can be used in intubated and non-intubated patients, mainstream is primarily used in intubated patients.2,6,12,15 While increased dead space and clogging with secretions are substantial disadvantages in the sidestream type, these are no longer a problem in the mainstream type, and the results are more reliable.2,6

Partial pressure of arterial carbon dioxide (PaCO2) is the gold standard for diagnosing and treating patients presenting to the ED with respiratory distress. However, in busy EDs, there is a need for fast, practical, cost-effective, and noninvasive methods that will help us comprehend the severity of patients while they are still in the triage phase. In many of the studies in the literature on non-intubated patients, correlation studies were conducted between EtCO2 measurements obtained through sidestream detectors and PaCO2.1,16–18

The main aim of this study is to determine whether the EtCO2 value obtained using the mainstream detector at the first presentation to the ED in non-intubated patients can be safely used instead of the patient's PaCO2 value. The reason for using the mainstream detector, which is used in intubated patients, instead of the sidestream detector in non-intubated patients is that the previously described disadvantages that may alter the measurement results, such as dead space and obstruction by secretions, are less likely to be encountered. The secondary aim of the study was to determine whether partial pressure of venous carbon dioxide (PvCO2) measurement would be an alternative to partial pressure of arterial carbon dioxide (PaCO2) measurement, especially in patients who are not suitable for EtCO2 (confused or intubated). The tertiary aim of the study was to determine whether EtCO2 measurement was associated with hospitalizations.

Of the 623 patients admitted to the ED with respiratory distress, 512 agreed to participate in the study (%82). Of these, 415 were excluded from the study for reasons explained in the method. The study was planned with 97 patients (Fig. 1). The average age of the patients was 70.9 ± 9.6 years, and 60.8% (n = 59) were male. 43.3% (n = 42) of the patients who admitted were ex-smokers. The most common comorbidities were hypertension (HT) with 76% (n = 74) and COPD with 58% (n = 57). 34% of the patients were hypercarbic at first presentation (0th minute, PaCO2 at admission >45 mmHg). Descriptive data of the patients are presented in Table 1.

The results of EtCO2 values between hospitalized and discharged patients, including measurements at admission, 60th and 120th minutes, are shown in Table 2. EtCO2 value was statistically significantly higher in all measurements in hospitalized patients than in discharged patients.

Table 3 shows the results of the correlations between EtCO2 and PaCO2 and PvCO2 measurements at the time of admission, the sixtieth and the 120th minute, in detail. When the EtCO2 value was above 45 mmHg, a strong positive (r = 0.844 and r = 0.803) and significant relationship was found between EtCO2 and PaCO2 and PvCO2 measurements (p < 0.001 for both).

When the EtCO2 value was above 45 mmHg, a strong positive and significant relationship was found between EtCO2 and PaCO2 and PvCO2 measurements (r = 0.730 and r = 0.702; p < 0.001 for both).

A strong positive and significant correlation was found between PaCO2 and PvCO2 measurement when the EtCO2 value was below 35 mmHg (r = 0.858; p < 0.001). A strong positive and significant relationship was seen when PaCO2 and PvCO2 EtCO2 values were between 35−45 mmHg (r = 0.883; p < 0.001). There was a strong positive and significant relationship between PaCO2 and PvCO2 measurement when the EtCO2 value was above 45 (r = 0.891; p < 0.001).

In the last measurements at the 120th minute, a moderately positive and significant relationship was found between EtCO2, PaCO2 and PvCO2 values (r = 0.677 and r = 0.609; p < 0.001 for both). When the EtCO2 value was above 45 mmHg, a moderate positive and significant relationship was observed between EtCO2 and PaCO2 and PvCO2 measurements (r = 0.667 and r = 0.563; p < 0.001 for both). When the EtCO2 value was below 35 mmHg, a moderately positive and significant correlation was observed between PaCO2 and PvCO2 measurement (r = 0.697; p < 0.001). A strong positive and significant (r = 0.791; p < 0.001) relationship was detected between PaCO2 and PvCO2 measurements when the EtCO2 value was between 35−45 mmHg. There was a strong positive and significant relationship between PaCO2 and PvCO2 measurement when the EtCO2 value was above 45 mmHg (r = 0.870; p < 0.001).

We found a strong positive and significant relationship between EtCO2 and PaCO2 measurements at admission (r = 0.820; p < 0.001). The variance explained by the variables on each other is 67% (Fig. 2). We found a strong positive and significant relationship between 60th-minute EtCO2 and PaCO2 measurement (r = 0.729; p < 0.001). The variance explained by the variables on each other is 53% (Fig. 3). A strong positive and significant relationship existed between PaCO2 and PvCO2 measurement at the 60th minute (r = 0.937; p < 0.001). The variance explained by the variables on each other is 87% (Fig. 4). A moderately negative and significant relationship existed between EtCO2 and SpO2measurement at admission (r = −0.516; p < 0.001). There was no significant relationship between EtCO2 and SpO2 measurement at the 60th minute and 120th minute (p = 0.402 and p = 0.771).

Figure 2.

Linear correlation between end-tidal carbon dioxide (EtCO2) and PaCO2 levels at the admission.

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Figure 3.

Linear correlation between end-tidal carbon dioxide (EtCO2) and PaCO2 levels at the time of 60th min.

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Figure 4.

Linear correlation between PaCO2 and PvCO2 levels at the time of 60th min.

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Our study found a high correlation between EtCO2 at admission (zero minutes) and PaCO2 and PvCO2 measurements (r = 0.820 and r = 0.772, respectively). We also found a high correlation between PaCO2 and PvCO2 (r = 0.891; p < 0.001). We found a moderately positive and significant relationship between EtCO2 and PaCO2 and PvCO2 measurements when the EtCO2 value was between 35−45 mmHg (r = 0.621 and r = 0.657, respectively; p < 0.001 for both). When the EtCO2 value was above 45 mmHg, a strong positive and significant relationship was found between EtCO2 and PaCO2 and PvCO2 measurements (r = 0.844 and r = 0.803, respectively; p < 0.001 for both). Apart from this, a strong correlation was observed between PaCO2 and PvCO2 for all measurements. These results showed that in triage patients presenting to the emergency department with respiratory distress, if the EtCO2 value measured with a single blow is above 45 mmHg, the patient is considered hypercapnic, and treatment can be started early. Another important conclusion is that venous blood gas measurement is a strong alternative to arterial blood gas measurement, which is an excruciating procedure.

Arterial blood gas sampling in a patient presenting with respiratory distress is often challenging for both the patient and the physician. It may need to be repeated from time to time. Many clinical studies have been designed to increase patient comfort and identify new reliable methods. One of the alternative methods is EtCO2 measurement. Healey et al. demonstrated a high correlation between EtCO2 and PaCO2 measured before and after withdrawal of assisted-controlled mechanical ventilation.21 Plewa et al. found that the patients' EtCO2 value obtained by the forced expiration model showed a high correlation with the PaCO2 value. They also reported the negative predictive value of capnographic hypercapnia (EtCO2 > 45 mmHg) in detecting arterial hypercapnia (PaCO2 > 45 mmHg) as 95%.22 Other studies conducted with non-intubated patients have also reported a high correlation.1,18

Measuring EtCO2 in non-intubated patients has difficulties. Patient cooperation comes first among these difficulties. Although the initial panic of outpatients with respiratory distress is another difficulty for measuring EtCO2 in the ED, it is easier than arterial blood gas sampling. Suzuki et al. their study of non-intubated patients followed up due to respiratory diseases found a high positive correlation between EtCO2 and PaCO2 (r = 0.88; p < 0.0001).23 Joe et al. in their study, it was stated that there was a significant correlation between EtCO2 measurement and PvCO2 in patients with chronic pulmonary disease and using oxygen (r = 0.63) and that the PvCO2 level of the patients could be predicted with capnography, which is a noninvasive method.24 These studies support our results. EtCO2 is a more comfortable, faster, and easily reproducible alternative to arterial blood gas sampling when starting the initial treatment of patients presenting to the ED with respiratory distress.

Venous blood gas sampling is easy. Although it is an invasive procedure, it is not as difficult for the patient and physician to repeat as arterial blood gas sampling. Therefore, understanding the relationship between PaCO2, EtCO2 and PvCO2 was investigated.25,26 However, studies in the literature contain a partial consensus. In a systematic review, only 22.5% of included studies identified a strong correlation between arterial and venous parameters.25 Predictably, different results may occur depending on patient characteristics. In their methodological study with 20 patients with various diagnoses and complaints, Lumholdt et al. investigated the mathematical adaptability of peripheral venous blood gas values to arterial blood gas values. They stated that there was a mathematically predictable relationship between PaCO2 and PvCO2.27 A study in non-intubated patients followed up for respiratory diseases indicated a positive correlation between PaCO2 and PvCO2 (r = 0.81; p < 0.001).23 Our study showed a high correlation between venous and arterial blood gas in terms of partial carbon dioxide pressure (pCO2) in patients with respiratory distress from admission until the second hour of treatment. In light of all these findings, end-tidal carbon dioxide measurement seems to be an excellent alternative to arterial carbon dioxide measurement. In addition, venous carbon dioxide measurement is a good alternative in non-cooperative patients unsuitable for non-invasive end-tidal carbon dioxide measurement. The fact that it is less painful and can be taken from the vein simultaneously with many other blood tests makes venous carbon dioxide measurement more important.

Conflicting results have been revealed in the literature regarding whether capnography can replace PaCO2 as a reliable tool in patients with acute respiratory distress.2,3,28,29 In our study, EtCO2 values of patients admitted with acute respiratory distress at admission and in the 1st and 2nd hours after treatment were associated with hospitalization. EtCO2 measurements were higher in hospitalized patients compared to discharged patients.

The main limitation of this study is that it was conducted in a single center with a study group consisting of patients who presented to the ED of a tertiary university hospital. The low number of cases in the city where the study was conducted caused the number of patients included in this study to be limited. Second, we did not correct pCO2 based on body temperature in any patient. Phan et al. noted that EtCO2 correlated better with pCO2 after temperature correction.17 Third, the patient's respiratory rate was not recorded. When the respiratory rate increases, the dead space ratio increases, and the EtCO2 level may decrease. Additionally, the capnometer may not detect EtCO2 changes in case of a high respiratory rate.24 The fourth limitation is that end tidal carbon dioxide measurement can only be performed in cooperative patients. Finally, conditions such as age, existing lung disease, and smoking may cause EtCO2 values to change. Prediction ability decreases in the elderly and those with lung disease.22 We did not perform subgroup analysis on the patients. Large-scale and multi-center studies in which subgroups are studied separately and necessary corrections are made according to the patient's vital values, such as respiratory rate and temperature, are needed to confirm this alternative diagnostic method.

The EtCO2 value obtained from adding an insufflation device we produced to the capnograph and spontaneously inhaling patients with respiratory distress is a powerful alternative to arterial blood gas in the ED triage of patients and the first two hours of acute treatment. It has also been shown that venous blood gas may be a good alternative. In patients with acute respiratory distress (patients who can cooperate and whose hemodynamics have not yet deteriorated), this simple, noninvasive EtCO2 measurement is also helpful in the early prediction of hospitalization decisions.

KS, MCD, MB contributed to conception; KS, MCD, MB contributed to design; MCD, EŞ, MB contributed to supervision; KS, HG, AKFK, MT, EE contributed to data collection and processing; KS, MCD contributed to analysis and interpretation; KS, MCD, EŞ, MB contributed to literature review; KS, MCD, EŞ, MB contributed to writing; MCD, MB contributed to critical review.

The author(s) received no financial support for the research, authorship, and/or publication of this article.

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Submitted work is original and has not been published elsewhere in any language. Raw data are available for the editor on request.

Ethics Committee approval was obtained from the local ethics committee (Date: March 20, 2023, Decision No: 2023/42).

Written consent was obtained from all patients.

Authors declare that human rights were respected according to the Declaration of Helsinki.

Düzce University, School of Medicine, Düzce, Türkiye.