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Arquivos de Neuro-Psiquiatria

Print version ISSN 0004-282X

Arq. Neuro-Psiquiatr. vol.72 no.11 São Paulo Nov. 2014 


Analysis of mean transcutaneous capnography in consecutive patients undergoing polysomnography

Análise da capnografia transcutânea média em indivíduos consecutivamente submetidos à polissonografia

Giulio Cesare Pinnola

Patrícia Souza Bastos

1Neurofisiologia Clínica, Hospital Sarah, Brasília DF, Brazil


Transcutaneous capnography is a noninvasive method useful for analysis of the behavioral tendency of transcutaneous CO2 pressure (PtcCO2) in patients undergoing polysomnography, to evaluate respiratory sleep disorders.


Determine normative PtcCO2 values in normal patients undergoing polysomnography.


One hundred seventy-nine patients who underwent polysomnography with simultaneous PtcCO2 measurement were assessed by means of a transcutaneous capnograph (TCM4 series from Radiomiter).


The group classified as normal (N=53) presented a apnea/hypopnea index (AHI) <5 events/per hour of sleep and their age groups varied between 7 and 76 years of age.


Global mean values of PtcCO2 in the normal group had a Gaussian distribution that varied between 33.1 and 50.0 mmHg (SD 4,363). Such findings allowed the establishment of normative PtcCO2 values for normal individuals.

Key words: polysomonography; transcutaneous capnography; CO2 pressure


A capnografia transcutânea é um método não invasivo útil para análise da tendência comportamental da pressão de CO2 transcutânea (PtcCO2)1,2,3 em pacientes submetidos à polissonografia para análise de transtornos respiratórios do sono.


Determinar valores normativos da PtcCO2 em pacientes normais submetidos à polissonografia.


Foram analisados 179 pacientes submetidos à polissonografia com medida concomitante da PtcCO2 através de um capnógrafo transcutâneo.


O grupo classificado como normal (N=53) apresentou índice de apnéia/hipopnéia (AIH) <5 eventos/hora de sono e faixa etária variando de 7 a 76 anos.


Os valores da média global da PtcCO2 no grupo normal apresentaram distribuição gaussiana variando de 33,1 a 50,0 mmHg (DP 4.363). Tais achados permitiram a criação de valores normativos da PtcCO2 para indivíduos normais.

Palavras-Chave: polissonografia; capnografia transcutânea; pressão de CO2

Transcutaneous capnography is a noninvasive method for analysis of blood gases. It is a useful measuring instrument when it is necessary to know the behavioral tendency of arterial gases1,2,3. During the polysomnographic (PSG) examination, carbon dioxide tension (PCO2) and oxygen saturation (SatO2) are important measurements for characterization of respiratory events related to sleep4. Hypoventilating patients tend to retain carbon dioxide (CO2) and noninvasive devices that are able to detect alterations during continuous monitoring constitute tools of clinical interest5. In order for this measuring method to have credibility, good accuracy, reliable concordance limits regarding arterial carbon dioxide tension (PaCO2) and linear variation tendency are necessary2,6,7.

Although the measurement of PaCO2 is considered the most reliable direct measurement parameter, it is an invasive method with punctual analysis of the mentioned parameter, which is not sensitive to behavioral tendencies of gas exchanges over time3,4.

Nowadays devices for transcutaneous analysis of CO2 present greater stability when registering longer periods2,4,7. The differences between the values registered by each device for PaCO2 and PtcCO2 are already known and values up to ±7.5 mmHg are acceptable for radiometer devices. Such differences tend to remain constant during data capture8,9. In addition, cutaneous arterialization processes were found to be safe at temperatures up to 44°C, allowing the electrode to remain in contact with the patient for a longer period of time without damages5,10. These improvements have contributed to increase reliability and safety of the method1.

The objective of this study was to establish normative values for PtcCO2 during the polysomnographic recording in patients whose age group varied from 7 to 76 years old, defined as normal.


The study was accepted by the research ethics institutional committee at Sarah Network Hospital. One hundred seventy-nine patients underwent polysomnography with simultaneous recording of transcutaneous capnography, in the period between January 2010 and August 2012, consecutively. The normal group (N=53) was the object of our study. The patients classified as normal presented an apnea/hypopnea index (AHI) <5 events/per hour of sleep. The patients underwent PSG with the most varied objectives. The American Sleep Academy Manual of 2007 was used for sleep staging and associated events11. By means of assessment of digital clinical records it was possible to exclude those patients with respiratory co-morbidities, positive history of smoking, presence of neuromuscular diseases and ischemic or compressive lesions of the brain stem.

Abnormally high values due to calibration problems of the device or abnormally low due to poor fixation of the electrode, presence of air bubbles and a wide range of oscillation of PtcCO2 values caused by motion artifacts were reasons for not including some of the patients in the normal group.

The TCM4 series device from Radiometer was used to analyse patient’s transcutaneous capinography. The PtcCO2 electrode was placed in the second right intercostal space during the entire recording and a 42°C temperature was maintained. Biocalibration of the device was done while the patient was still awake and not repeated during the night, except if abrupt alterations of recorded values were noticed.

Capillary blood gasometry was carried out with the objective of validating the transcutaneous capnography method. Analysis of the capillary blood sample was the method of choice for validation of transcutaneous capnography because it is a less painful method than arterial gasometry and technically easier to perform. Sample collection occurred before the beginning of PSG, when the patient was at rest and awake. Blood sample was obtained from the ear lobe, as it is a region with more arterialization when compared to the digital pulp.


The mean transcutaneous capnography CO2 rate of each patient was analyzed during the polysomnographic recording. Subsequently the mean CO2 rate (called global mean) was assessed using the already calculated n (the sum of transcutaneous capnography from each patient). Such values were evaluated by means of the Shapiro-Wilk test with Lilliefors significance correction (sig.0.379) for analysis of data normality (Table and Figure 1).

Table Mean and standard deviation of age and mean transcutaneous carbon dioxide pressure (PtcCO2) in the normal group (N=53). 

Statistics Standard error
Age Mean 43.13 2.261
95% Confidence interval for mean Lower bound 38.59
Upper bound 47.67
5% Trimmed mean 43.49
Median 46.00
Variance 270.963
Standard deviation 16.461
Minimum 7
Maximum 76
Range 69
Interquartile range 21
Skewness -.472 .327
Kurtosis -.464 .644
Global Mean PtcCO2 (mmHg) Global mean 41.325 .5993
95% Confidence interval for mean Lower bound 40.122
Upper bound 42.527
5% Trimmed mean 41.339
Median 41.400
Variance 19.036
Standard deviation 4.3630
Minimum 32.4
Maximum 49.5
Range 17.1
Interquartile range 7.0
Skewness -.044 .327
Kurtosis -.872 .644

Figure 1 Normal distribution values for global mean transcutaneous carbon dioxide pressure (PtcCO2) in the normal group. Standard deviation of ±2σ. 

The behavioral tendency of PtcCO2 mean for each individual was observed overnight with the objective of excluding discrepant physiological PCO2 values. Therefore, the individual PtCO2 mean rate would tend to better reflect variations of gas exchanges at the lung level.

The transcutaneous PtcCO2 of each patient was compared to the values obtained from capillary gasometry, which were invalidated and discarded in the PtcCO2 study if the difference between the values were ±7.5 mmHg8,9.

In the normal group (N=53), the age group varied from 7 to 76 years (mean 43.13 and standard deviation (SD) 16.46), 32 subjects were female and 21 were male. Global mean value of mean PtcCO2 was 41,325 (confidence interval 95% per mean, varying from 40,122 to 42,527). Standard deviation was 4,363 and 2σ was used for calculation of standard deviation, considering as normality interval values >33.0 and ≤50.0 mmHg (Table and Figure 2).

Figure 2 Global mean transcutaneous carbon dioxide pressure (PtcCO2)distribution in the normal group. Values ranging from 33.1 to 50.0 mmHg. 

Global mean values of mean PtcCO2 of normal patients presented a behavior with Gaussian distribution. Variability of PtcCO2 during the exam presented a linear pattern, allowing an estimation of the behavioral tendency of transcutaneous CO2 during PSG in normal patients. Based on this tendency, it was possible to establish normative values of PtcCO2 for the normal patients group.

Due to technical difficulties, the capillary blood sample was carried out in only 23 patients; however, the variation curve of PtcCO2 values was similar to that of patients with a validated transcutaneous technique. Such similarity of behavior was confirmed through ANOVA (sigma between groups 0.615 and F 0.256).


The study did not attempt to assess or even correct the existence of “drift”, since the focus of analysis was the mean PtcCO2 and not PtcCO2 in punctual analysis during the night.

These results allow us to conclude that transcutaneous capnography is a noninvasive and totally reliable method. Zavorsky and Yildizdas et all demonstrated that there is no significant variation regarding capillary and arterial PaCO2, when the capillary blood sample is properly arterialized12. The behavioral similarity between capillary gasometry and transcutaneous capnography in normal individuals leads us to discard the need to perform capillary gasometry as validation method of transcutaneous capnography.

Further studies in patient with respiratory sleep disorders such as apnea and hypoventilation (obesity/hypoventilation/neuromuscular diseases syndrome) may be carried out with the objective of identifying mean PtcCO2 abnormality patterns during polysomnographic recording13,14.


The authors are indebted to Sandro Barbosa de Oliveira for your untiring assistance in the preparation of this manuscript as a statistician.


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Received: January 06, 2014; Revised: July 08, 2014; Accepted: July 28, 2014

Correspondence: Giulio Cesare Pinnola; Sarah, Neurofisiologia Clínica, SMHS, Qd 301 / bloco A; 70335-901 Brasília DF, Brasil; E-mail:

Conflict of interest: There is no conflict of interest to declare.

Creative Commons License This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License, which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.