Transcutaneous CO2 (TcCO2) more accurately reflects blood levels of the gas than end-tidal CO2(EtCO2) in infants, according to investigators at Stony Brook University, New York.
Kseniya Khmara, MD, a clinical anesthesia resident at Stony Brook University School of Medicine, explained that the inaccuracies often observed using EtCO2 because of the small tidal volumes inhaled by neonates motivated her team to investigate the efficacy of TcCO2 as an alternative.
Dr. Khmara and her colleagues recruited 31 infants undergoing elective surgery under general endotracheal anesthesia, of whom 22 were included in the final analysis due to difficulties collecting the final sample. Twelve patients were recruited from the neonatal ICU (NICU) and 19 from the pediatric ICU (PICU), ambulatory unit or floor. The median age was 10.6 weeks (1 day-51 weeks), and median weight on the day of surgery was 4 kg (1-9.8 kg). Fourteen patients were American Society of Anesthesiologists physical status I/II, and 17 were III/IV.
Dr. Khmara, presenting the study at the 2017 meeting of the Society for Pediatric Anesthesia/American Academy of Pediatrics Section on Anesthesiology and Pain Medicine (abstract RRA-4), said, in order to measure TcCO2, the team used the SenTec Digital Monitoring System (SDMS, SenTec AG), an FDA-approved noninvasive monitor of TcCO2, SPO2 and pulse rate that takes measurements using sensors placed on the skin (Figure 1). Instead of using arterial CO2 as the metric for comparing TcCO2 with EtCO2, the investigators used partial pressure of carbon dioxide in venous blood (PvCO2), as they thought arterial sampling to be too invasive. Samples were obtained at the beginning and end of the study. The investigators calculated the mean difference of ETCO2 and TcCO2, and used independent t-tests to compare the mean differences between NICU and non-NICU patients.
The investigators found that TcCO2 closely approximated PvCO2 in both the NICU and non-NICU patients. They also found that although the change in EtCO2was significantly greater in NICU than in non-NICU patients (27.7±10.5 vs. 9.8±8.2 mm Hg; P<0.001), there was no significant difference in the change in TcCO2 between the two groups (2.3±12.0 vs. 3.0±9.8 mm Hg).
Moreover, statistical analysis by SPSS found that the two delta measures were significantly different within each group (P<0.005). “[Regarding] the transcutaneous CO2 delta, if you do an unpaired t-test comparing the two subsets of our sample, the NICU and the non-NICU patients are not significantly different, meaning that the transcutaneous CO2 monitor does not depend on what subset of the patient population you’re monitoring—you’ll still get a close approximation of venous CO2 values,” Dr. Khmara said. “However, if you compare the mean difference of the end-tidal CO2 in NICU patients and older babies, they are significantly different. You get a greater undershoot of venous CO2 values when you use end-tidal CO2 in NICU patients compared to the older kids”.
Dr. Khmara said she and her colleagues concluded that TcCO2 is a more accurate measure of PvCO2 levels in infants, and particularly in NICU patients,and that its use should be considered in the operating suite.
This method is a promising solution to a persistent issue, according to Constance Houck, MD, FAAP, senior associate in perioperative anesthesia at Boston Children’s Hospital and associate professor in anesthesia at Harvard Medical School, in Boston. “We’ve always had issues with end-tidal CO2monitoring in very young children, especially when it really matters. It’s nice to know that there’s a better monitor for carbon dioxide monitoring.”
She said, “This will probably be important for settings like laparoscopic surgeries, where you really are dependent on the fact that you know you’re going to have an increased CO2 absorption, and you know you’re going to have issues with maintaining your metabolic balance. We do so much more laparoscopically now in younger children, and we really don’t have a good sense of how to monitor their CO2 during surgery. So this might prove to be a much better alternative.”