Analysis of mean transcutaneous capnography in consecutive patients undergoing polysomnography

Análise da capnografia transcutânea média em indivíduos consecutivamente submetidos à polissonografia

Abstracts

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.

Objective

Determine normative PtcCO₂ values in normal patients undergoing polysomnography.

Method

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

Results

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.

Conclusion

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.

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.

Objetivo

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

Método

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

Resultados

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.

Conclusão

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.

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 gases^1,2,3. 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 necessary^2,6,7.

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 time^3,4.

Nowadays devices for transcutaneous analysis of CO₂ present greater stability when registering longer periods^2,4,7. 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 capture^8,9. 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 damages^5,10. 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.

Method

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.

Results

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 mmHg^8,9.

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).

Discussion

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 recording^13,14.

Acknowledgement

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

References

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Publication Dates

• Publication in this collection
  Nov 2014

History

• Received
  06 Jan 2014
• Reviewed
  08 July 2014
• Accepted
  28 July 2014