Since the development of minimally invasive surgery, the laparoscopic operation has gained increasingly popular in pediatric surgeries due to its advantages of less trauma, a short hospital stay, less postoperative wound pain, and fewer complications. CO2 is the most common gas utilized to create a pneumoperitoneum and provide a good operating view for the surgeon. However, the diffusion capacity of CO2 is strong, and the absorption of CO2 is sufficient in children due to factors such as the small volume of the abdominal cavity, the proximity of the capillaries to the peritoneum, and the larger abdominal surface area related to weight compared to adults [10]. A risk of hypercapnia is associated with prolonged artificial pneumoperitoneum. Increased CO2 alters the body’s acid–base balance and stimulates sympathetic nerves, thus increasing catecholamine and cortisol release and leading to hemodynamic fluctuations [10, 11]. Close monitoring of the CO2 level during laparoscopic surgery and timely adjustment of the ventilator parameters is essential to avoid the disruption of physiological functions. PaCO2 levels are stabilized after 60 min of pneumoperitoneum [12]; hence, a pneumoperitoneum time of at least 90 min was appropriate to observe the variables in this investigation.
PetCO2 is a routine measurement during the perioperative period and one of the primary indicators used to adjust ventilator parameters. However, factors that affect lung ventilation/perfusion may interfere with the accuracy of PetCO2 measurements, and thus, the use of PetCO2 in non-tracheal intubated patients is restricted. The increased abdominal pressure during laparoscopic surgery results in a diaphragmatic rise and an increase in thoracic pressure; subsequently, airway resistance and airway pressure also rise, with pulmonary vasoconstriction and reduced pulmonary blood flow. Pediatric patients are vulnerable to pneumoperitoneal pressure effects. This study revealed that during the entire monitoring process, a good correlation was established between PetCO2 and PaCO2, r = 0.83 (P < 0.01). Nevertheless, as the pneumoperitoneum time was prolonged, the correlation between PetCO2 and PaCO2 decreased gradually, which was consistent with previous findings [6, 9].
Several clinical studies have focused on the application of PtcCO2 monitoring in different types of surgery and patients [13,14,15,16,17] under non-tracheal intubation monitoring anesthesia [18, 19]. These studies confirmed the effectiveness of PtcCO2 monitoring. The current results showed a close correlation between PtcCO2 and PaCO2, r = 0.84 (P < 0.01), and although the correlation was decreased with prolonged pneumoperitoneum time, it was not very significant compared to PetCO2. According to Bland–Altman analysis, a lesser mean difference was detected between PetCO2 and PaCO2 than between PetCO2 and PaCO2. Therefore, PtcCO2 performed better than PetCO2 in estimating PaCO2, which is in agreement with the previous results [6, 7, 9]. In our experiment, we can combine PetCO2, PaCO2, and PtcCO2 to regulate the patient's acid–base, so there is no accumulation of CO2 during the whole operation. However, due to the limitations of PetCO2 monitoring, especially in the case of prolonged pneumoperitoneum, relying solely on PetCO2 to regulate the patient's respiratory parameters cannot guarantee that the patient is in acid–base balance, particularly for young children. And PtcCO2 can more accurately estimate PaCO2, so its application can reduce the risk of CO2 accumulation. Conway et al. conducted a meta-analysis on the effectiveness of PtcCO2 monitoring [20] and demonstrated that is challenging to achieve a uniform standard due to the involvement of various clinical aspects, including the monitoring site, electrode heating temperature, and application population; thus, it is critical to monitor the PtcCO2 trend throughout the monitoring process.
The CO2 level measured by PtcCO2 monitoring consists of two parts: one derived from the blood (arterial, capillary, and venous), and the other from the metabolism of the tissue cells [21, 22]. The warming effect of the electrode increases the skin blood flow and enhances the contribution of arterial blood to CO2 by opening the precapillary sphincter [23]. A rise in the local skin temperature increases the metabolism of tissue cells, producing excessive CO2. Therefore, the PtcCO2 monitoring value is theoretically higher than that of PaCO2. PtcCO2 monitors used in clinical practice correct the initial measured value based on the selected heating temperature to reduce the deviation from PaCO2 [21]. In the present study, PtcCO2 monitoring values were less than PaCO2 in 51/128 data sets; hence, the correction method for PtcCO2 monitors needs to be investigated further.
Several factors affect PtcCO2 monitoring, including the temperature of the electrodes, the monitoring location of the sensor, and the patient’s clinical state. Nishiyama et al. demonstrated that when the anterior chest (between the clavicle and nipple) was chosen as the monitoring site, PtcCO2 correlated best with PaCO2 at 43 °C (R2 = 0.7568) among the different electrode-heating temperatures (37, 40, 42, 43, and 44 °C) in its setting, and the monitoring required less time to stabilize at higher temperatures as blood CO2 levels change, but required > 150 s [24]. According to a study on the optimal electrode temperature for monitoring PtcCO2 in preterm infants, the mean difference between PtcCO2 and PaCO2 was the smallest at 42 °C [25]. A higher temperature may result in skin damage in pediatric patients due to thin skin, but previous studies have not reported any skin injuries in children or infants. In this study, we chose 42 °C as the electrode temperature for PtcCO2 monitoring; no adverse events were observed.
Nishiyama et al. reported that PtcCO2 was correlated with PaCO2 when the monitoring sensor was located on the chest (R2 = 0.76) but not when it was located on the upper arm and forearm (R2 < 0.5) [26]. When the anterior chest is chosen as a monitoring site in pediatric patients, the area of surgical disinfection might be affected, especially in younger kids. Anesthesiologists were usually positioned on the cephalic side of the patient, such that the forehead was selected as the site in this study, facilitating the administration of the probe. In the current study, PetCO2 showed a close correlation with PaCO2 than PtcCO2 before pneumoperitoneum; however, the mean difference between PtcCO2 and PaCO2 was smaller than the mean difference between PetCO2 and PaCO2. However, whether PetCO2 correlates better with PaCO2 than PtcCO2 in pediatric patients under non-pneumoperitoneal conditions with the forehead as the monitoring site needs to be studied further with a large sample size.
In the event that patients’ peripheral tissues and organs are not supplied adequately with blood, such as in shock, the CO2 produced by tissue metabolism cannot be carried away quickly, and PtcCO2 monitoring values increase gradually [22]. Thus, PtcCO2 can be utilized as one of the indicators for assessing a patient’s microcirculatory status, which is useful in guiding the treatment [27]. However, the study on PtcCO2 monitoring in surgical patients with circulatory failure has been rarely reported, and the correlation between the PtcCO2 gradient changes and skin tissue perfusion status requires further clinical investigation. Other factors, such as poor skin contact with the fixed connection loop and insufficient gel, may allow contact between the probe and air, thus interfering with the monitoring results. CO2 permeability films that have not been replaced for a long time or are damaged or air bubbles under the film can also affect the accuracy of PtcCO2 monitoring.
Since PtcCO2 monitoring is a continuous and noninvasive method that can be used to assess PaCO2 to some extent, its perioperative application is promoted in the different types of surgery and populations. Endotracheal intubation is not required for gastrointestinal endoscopy or other operations that can be performed using nerve blocks. The use of intravenous anesthetic medications intraoperatively can improve operating conditions and increase patient comfort during these procedures. When the nerve block is unsatisfactory, or when specific operations call for an enhanced level of sedation, supplemental narcotic medicines are required. Understanding the CO2 level of patients allows us to more precisely regulate the intravenous anesthetic medicine dosage. However, it is often difficult to accurately monitor the CO2 levels of patients during these operations. In this situation, PtcCO2 monitoring is a good choice. It has been shown [18, 19, 28] that PtcCO2 monitoring is an effective way to detect hypoventilation in patients, which reduces the incidence, extent, and duration of hypercapnia, improving the safety of patients under sedation. High-frequency ventilation is often used to maintain oxygenation in some airway procedures performed with a rigid bronchoscope. However, evaluating the ventilatory status of patients with PetCO2 in the open ventilation mode of high-frequency ventilation is challenging. As a result, we can adjust the parameters of high-frequency ventilation to avoid the accumulation of CO2 according to PtcCO2 [29].
Usually, patients undergoing thoracic surgery have chronic lung diseases and require one-lung breathing in the lateral decubitus position during operation. These factors can affect lung ventilation/perfusion, leading to the increase of the difference between PetCO2 and PaCO2, and patients are more likely to develop respiratory acidosis. Oshibuchi M's study [30] showed that PtcCO2 can more accurately predict PaCO2 compared with PetCO2 in both two-lung ventilation and one-lung ventilation. It has been shown [31] that PtcCO2 monitoring remains highly accurate even when one-lung ventilation is prolonged (more than 2 h) and permissive hypercapnia is present.
Most patients need to recover in anesthesia recovery room after surgery. Due to the presence of residual opioids and muscle relaxants, patients are at potential risk of developing respiratory depression, especially in elderly and obese patients. PtcCO2 monitoring effectively reflects PaCO2 levels and is more suitable for observing changes in CO2 fluctuations over time so that we can take appropriate treatment measures [32]. For pediatric patients, the anesthesia has certain particularity. Children are often unable to cooperate with us for some examination operations. They must be under sedation and analgesia conditions prior to nerve block or spinal anesthesia. Children must maintain a certain level of sedation throughout the whole operation, but the use of anesthetic drugs will always have an impact on their breathing more or less. By using PtcCO2 monitoring, anesthesiologists are able to determine in time whether CO2 accumulation in patients is occurring so that the appropriate treatment can be administered.
PtcCO2 monitoring also has some limitations. The monitoring site should be cleaned in advance to remove the hair and grease; also, the PtcCO2 monitor requires a calibration time of approximately 15 min before use and needs to be recalibrated either after the patient is removed from the monitoring site or after prolonged monitoring. When CO2 in the blood changes, PtcCO2 monitoring takes about 2 min to reflect PaCO2 with a degree of delay [33]. These factors limit its use in surgery patients. Therefore, the PtcCO2 monitor needs further improvement to facilitate its use during the perioperative period. Some studies have reported that PtcCO2 monitoring techniques are not based on electrochemical principles [34,35,36]. Because of the different monitoring mechanisms, the local heating on the skin is avoided, and the time required for calibration and stabilization is short, rendering the monitors convenient to use. However, the application is still not mature in clinical practice. Although PetCO2 is susceptible to various factors, it plays an essential role in determining the position of the tracheal tube, tube folding, and accidental decannulation [37]. Additionally, PetCO2 provides information about the patient’s pulmonary blood flow status and circulatory function [38], and the patient’s airway status can also be determined from the PetCO2 waveform, thereby deeming that PtcCO2 is not a substitute for PetCO2.
In conclusion, PtcCO2 shows a close correlation with PaCO2 when the forehead is chosen as a monitoring site in children undergoing laparoscopic surgery. Compared to PetCO2, PtcCO2 can accurately estimate PaCO2 and could be used as an auxiliary monitoring indicator to optimize anesthesia management for laparoscopic surgery in children; however, it is not a substitute for PetCO2.