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,§
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Departments of
*Child Health,
Anesthesiology,
Surgery, and the Divisions of
§Pediatric Critical Care/Pediatric Anesthesiology and
||Pediatric Cardiothoracic Surgery, The University of Missouri, Columbia, Missouri
Address correspondence and reprint requests to Joseph D. Tobias, MD, Department of Child Health, The University of Missouri, M658 Health Sciences Center, One Hospital Dr., Columbia, MO 65212. Address e-mail to Joseph_Tobias{at}muccmail.missouri.edu
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Abstract |
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5 mm Hg) during the first postoperative hour underwent placement of the TC electrode (30 of 33 patients). If the TC-CO2 to arterial difference was 5 mm Hg, the TC-CO2 electrode was recalibrated and reapplied on another site. If the discrepancy was still 5 mm Hg, the case was considered a clinical failure and no further data were collected (3 of 30 patients). If the arterial to TC gradient was <5 mm Hg, the patient was included in the data collection (27 of 30 patients). One to five sample sets (TC and arterial CO2) were collected from these patients. Statistical analysis included linear regression analysis and Bland-Altman analysis. The cohort for the study included 27 patients ranging in age from 2 days to 9 yr and in weight from 3.2 to 25 kg. A total of 101 sample sets were analyzed. The mean ± SD absolute difference between the TC-CO2 and arterial CO2 was 1.7 ± 1.4 mm Hg (range 0–9 mm Hg). The TC-CO2 to arterial CO2 difference was 0–2 mm Hg in 82 of 101 values (81%), 3–5 mm Hg in 18 of 101 values (18%), and >6 mm Hg in 1 of 101 values (1%). Linear regression analysis revealed a slope of 0.90, an r value of 0.9410, and an r2 value of 0.8854 (P < 0.0001). Bland-Altman analysis revealed a bias of 0.58 mm Hg with a precision of ±2.1 mm Hg when comparing the TC-CO2 with the arterial CO2. Implications: We conclude that, with certain caveats in mind, including the need to correlate the transcutaneous CO2 with an initial arterial CO2 value, transcutaneous CO2 monitoring can be used to estimate arterial CO2 in most neonates and children after cardiothoracic surgery.
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Introduction |
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Transcutaneous CO2 (TC-CO2) monitoring is accurate in toddlers and infants with respiratory failure (8), but it has not been studied in patients with alterations in cardiovascular function or those requiring pharmacologic support of cardiovascular function. We therefore prospectively investigated the efficacy and accuracy of TC-CO2 monitoring in infants and children after cardiothoracic surgery.
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Methods |
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The study population included consecutive patients who had undergone cardiothoracic surgical procedures. ETCO2 was determined by infrared spectroscopy with a sidestream aspirator using a flow rate of 150 mL/min. If the ETCO2 and arterial CO2 values did not correlate (gradient 5 mm Hg) during the first postoperative hour, a TC-CO2 electrode was placed. If the TC-CO2 to arterial difference was 5 mm Hg, the TC-CO2 electrode was recalibrated and reapplied on another site. If the discrepancy was still 5 mm Hg, the case was considered a clinical failure and no further data were collected. If the arterial to TC-CO2 gradient was <5 mm Hg, the patient’s data were included.
TC-CO2 was measured using a standard TC-CO2/O2 device (Radiometer, Copenhagen, Denmark). The protocol for TC-CO2 monitoring included placement and maintenance of the monitor by the respiratory therapy staff. Before placement, the electrode membrane was cleaned and calibrated. The monitoring site was changed every 3–4 h to avoid thermal injury, and the electrode was recalibrated before placement at a new site. The working temperature of the electrode was kept at 43–43.5°C.
When clinically indicated, arterial blood gas tensions were measured at 37°C and corrected to the patient’s temperature. TC and arterial CO2 values were recorded, with one to five sample sets (arterial CO2 and TC-CO2) collected from each patient. To avoid biasing the data, no more than five sample sets were collected from any single patient. The absolute difference between the TC and arterial CO2 was calculated. No negative numbers were used, because this could have artificially lowered the mathematical mean of the differences between the TC-CO2 and the arterial CO2. For example, if the TC reading was 7 mm Hg higher or 7 mm Hg lower than arterial CO2, 7 was used, not -7 or +7. Using the raw data (positive and negative numbers), a linear regression analysis and a Bland-Altman (10) analysis were performed to compare the TC and arterial CO2 values. The Bland-Altman analysis included a bias, defined as the mean difference between values and precision, defined as the standard deviation of the bias. All data are expressed as the mean ± SD; P < 0.05 was considered significant.
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Results |
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5 mm Hg, and the TC-CO2 electrode was placed. In 3 of these 30 patients, the TC-CO2 to arterial CO2 gradient was 5 mm Hg, despite recalibrating the TC-CO2 monitoring and placing the probe on another body location. These three patients all exhibited cardiovascular instability and received dopamine 20 µg · kg-1 · min-1 and epinephrine 0.3–0.5 µg · kg-1 · min-1. The remaining 27 patients formed the cohort for the study. The data presented were collected from these 27 patients.
The 27 patients included 16 boys and 11 girls ranging in age from 2 days to 9 yr (1.7 ± 2.4 yr) and in weight from 3.2 to 25 kg (8.9 ± 4.4 kg). There were 6 neonates (
1 mo of age), 15 infants (1 mo to 2 yr of age), and 6 children (2 yr of age). The cardiothoracic procedures included 3 thoracotomies for mediastinal masses, 4 closed heart procedures (Blalock-Taussig shunt, repair of coarctation of the aorta, and coarctation repair with pulmonary artery banding), and 20 open heart procedures with cardiopulmonary bypass. The open heart procedures included repair of arterial septal defect (n = 6), repair of ventricular septal defect (n = 3), repair of tetralogy of Fallot (n = 3), and repair of atrial septal defect with patch angioplasty of the pulmonary artery (n = 1). They also included repair of atrial septal defect and partial anomalous pulmonary venous return (n = 1), aortic valvotomy (n = 1), pulmonary valvotomy (n = 1), mitral valve annuloplasty and repair of left ventricular outflow tract obstruction (n = 1), repair of anomalous left coronary artery (n = 1), Norwood procedure (n = 1), and arterial switch (n = 1). Sixteen patients received infusions of vasoactive medications, including dopamine (0–5 µg · kg-1 · min-1 [n = 7], 6–10 µg · kg-1 · min-1 [n = 3], 11 µg · kg-1 · min-1 [n = 1]); dobutamine (0–5 µg · kg-1 · min-1 [n = 8], 6–10 µg · kg-1 · min-1 [n = 5], 11 µg · kg-1 · min-1 [n = 1]); milrinone 0.3–1 µg · kg-1 · min-1 (n = 6); nicardipine (n = 1); and nitroglycerin (n = 1).
A total of 101 sample pairs of TC and arterial CO2 data were analyzed. The mean absolute difference (see Methods) between the TC-CO2 and arterial CO2 was 1.7 ± 1.4 mm Hg (range 0–9 mm Hg). The TC to arterial CO2 difference was 0–2 mm Hg in 82 of 101 values (81%), 3–5 mm Hg in 18 of 101 values (18%), and >6 mm Hg in 1 of 101 values (1%). Linear regression analysis revealed a slope of 0.90, an r value of 0.9410, and an r2 value of 0.8854 (P < 0.0001) (Figure 1). Bland-Altman analysis revealed a bias of 0.58 mm Hg with a precision of ±2.1 mm Hg when comparing the TC-CO2 and the arterial CO2 (Figure 2).
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Discussion |
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Although previous studies have validated the accuracy of TC-CO2 monitoring in the neonatal population (9), its application outside of the neonatal population has been limited (13). The current study included only 16 of 27 patients older than 6 mo of age, and additional studies are needed to determine its accuracy in older children and adolescents. Because previous studies have demonstrated that TC-O2 monitoring in this age group is not reliable (14), we did not attempt to use it in the current study.
The technique for TC-CO2 monitoring has been described previously (9). Proper use and accuracy of this technique is dependent on appropriate training and expertise of personnel. Several factors may affect the accuracy of TC-CO2 monitoring. Technical variables, such as trapped air bubbles, improper placement technique, damaged membranes, and inappropriate calibration, can be avoided by careful use of the equipment and training of the respiratory therapy staff. Aside from technical problems, patient problems may affect the accuracy of TC-CO2 monitoring. The latter may include variations in skin thickness, the presence of edema, tissue hypoperfusion, or the administration of vasoconstricting drugs. All of these factors affect the ability of CO2 to diffuse from the capillary bed to the membrane of the monitor. As no technique can be expected to be 100% reliable, periodic calibration with arterial values is recommended. Although the current study included only tracheally intubated and mechanically ventilated patients, TC-CO2 monitoring can be used in spontaneously breathing patients without an artificial airway.
Inadequate tissue perfusion from cardiovascular depression or vasoconstriction from vasoactive drugs can be a problem immediately after cardiovascular surgery. Although some investigators have suggested that shock does not affect TC-O2 and -CO2 values (15), others have confirmed the clinical impression that the gradient between TC-CO2 and arterial CO2 increases as tissue perfusion decreases.12 We found a poor correlation of the TC and arterial CO2 in the three patients with cardiovascular instability requiring dopamine 20 µg · kg-1 · min-1 and epinephrine 0.3–0.5 µg · kg-1 · min-1. Using these vasoactive drugs at these doses can lead to peripheral and subcutaneous vasoconstriction. These factors, combined with diminished cardiac output, lead to decreased tissue perfusion and most likely explain the inaccuracies of TC-CO2 monitoring in these patients. However, in other patients, even with the use of moderate doses of vasoactive drugs including dobutamine (20 µg · kg-1 · min-1), dopamine (10 µg · kg-1 · min-1), and milrinone (0.3–1 µg · kg-1 · min-1), there was a clinically acceptable correlation of TC and arterial CO2.
Martin et al. (16) also demonstrated that the TC to arterial CO2 gradient may widen as the arterial CO2 increases because of an imbalance between local tissue CO2 production and removal. We noted no such inaccuracy in our patients with hypercarbia; however, only 7 of the 101 arterial CO2 values were 50 mm Hg.
A consideration of the limitations of the current study must include the number of sample sets (n = 101) and the limited patient population (n = 27 patients). Based on the number of patients, no statistical analysis to evaluate the effect of patient age, weight, type of surgical procedure, or type of cardiac lesion (left to right versus right to left) on the accuracy of ETCO2 monitoring was possible. Additionally, because the arterial blood gas analyses were performed at various times after application of the TC electrode, we cannot comment on variations of its accuracy over time. However, the overall accuracy of the TC-CO2 monitor in the current and previous studies suggests that there is not a significant decrease in accuracy over time provided the monitoring site is changed every 4 h. We did note a poor correlation in patients with cardiovascular instability who required pharmacologic support with epinephrine or dopamine in doses >10 µg · kg-1 · min-1.
In summary, in the current study, we demonstrated that TC-CO2 measurement provides a clinically acceptable estimate of arterial CO2 in infants and children after cardiothoracic surgery. The technique may be inaccurate in patients with cardiovascular dysfunction requiring vasoactive medications, especially at large doses.
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Footnotes |
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2 Kashyap S, Stefanski M, Schulze K, James LS. Effects of changes in circulation on transcutaneous PCO2 [abstract]. Pediatr Res 1981;15:666.
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This article has been cited by other articles:
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  E J Molloy and K Deakins Are carbon dioxide detectors useful in neonates? Arch. Dis. Child. Fetal Neonatal Ed., July 1, 2006; 91(4): F295 - F298. [Abstract] [Full Text] [PDF]
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 J. Wilson, P. Russo, J. Russo, and J. D. Tobias Noninvasive Monitoring of Carbon Dioxide in Infants and Children With Congenital Heart Disease: End-Tidal Versus Transcutaneous Techniques J Intensive Care Med, September 1, 2005; 20(5): 291 - 295. [Abstract] [PDF]
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 J. W. Berkenbosch, J. Lam, R. S. Burd, and J. D. Tobias Noninvasive Monitoring of Carbon Dioxide During Mechanical Ventilation in Older Children: End-Tidal Versus Transcutaneous Techniques Anesth. Analg., June 1, 2001; 92(6): 1427 - 1431. [Abstract] [Full Text] [PDF]
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