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Canadian Journal of Anesthesia 50:137-142 (2003)
© Canadian Anesthesiologists' Society, 2003
General Anesthesia

Both the OxyArmTM and Capnoxygen mask provide clinically useful capnographic monitoring capability in volunteers

[L’OxyArm et le masque Capnoxygen permettent une surveillance capnographique chez des volontaires]

James Paul, BSc MD MSc FRCPC*, Elizabeth Ling, BSc MD MSc FRCPC*, Julius Hajgato, CET{dagger} and Lee McDonald, RN{dagger}

* From the Department of Anesthesia McMaster University Southmedic Inc.
{dagger} Barrie, Ontario, Canada.

Address correspondence to: Dr. James Paul, Assistant Clinical Professor, McMaster University, Department of Anesthesia, Hamilton Health Sciences, Hamilton General Site, 237 Barton Street East, Hamilton, Ontario L8L 2X2, Canada. Phone: 905-527-4322-46698; Fax: 905-577-8023; E-mail: paulj{at}quickclic.net


   Abstract
TOP
 Abstract
Introduction
 Methods
 Results
 Discussion
 References
 
Purpose: To compare the capnography monitoring performance of the new OxyArmTM (OA) with the Capnoxygen mask (CM), a conventional oxygen mask with a carbon dioxide sampling port.

Methods: Eleven healthy volunteer adult subjects underwent capnographic monitoring (in a non-randomized, un-blinded crossover study) at baseline and while receiving oxygen at seven different flow rates (0.5, 1.0, 2.0, 4.0, 6.0, 8.0, and 10 L•min-1), applied first with the CM and then with the OA.

Results: Both the OA and CM produced acceptable capnographs with consistent waveforms. The measured end-tidal (ET) CO2 was equivalent for the two devices at all seven oxygen flow rates. On average, the ETCO2 measured with the OA was about 2 mmHg greater than that of the CM. Regression analysis showed an inverse relationship between oxygen therapy flow rate and measured ETCO2 whereby the measured value of CO2 decreased as the oxygen flow rate was increased (P < 0.001). Both the CM and OA produced consistent measurements of ETCO2 as illustrated by their reliability coefficients, 0.95 and 0.86 respectively. The biggest source of variation in measured CO2 for both devices was inter-subject differences, followed by variable oxygen flow rates.

Conclusions: This study suggests that the OA and CM can prove useful for respiratory monitoring and oxygen delivery in spontaneously breathing volunteers, and the OA could potentially be used as an alternative to the conventional methods of oxygen delivery and CO2 sampling in patients.


   Introduction
TOP
 Abstract
 Introduction
Methods
 Results
 Discussion
 References
 
THE measurement of end-tidal CO2 (ETCO2) in expired gases is one of the standard monitors used in the operating room for patients under general anesthesia.1 In this setting ETCO2 is useful as a confirmation of endotracheal intubation, an estimate of PaCO2 for guiding ventilator settings, and as a monitor for malignant hyperthermia, bronchospasm and venous air embolism.2 ETCO2 monitoring also has practical applications in spontaneously breathing, non-intubated, patients. It can be used to detect airway obstruction or apnea, and also to identify the rate and pattern of respiration.3 For this purpose, divided nasal cannulae, specialized facemasks, and several make shift devices that utilize various combinations of angio-catheters and facemasks have been developed.3–5

The OxyArmTM (OA; Southmedic Inc., Barrie, Ontario, Canada) is a new minimal contact oxygen delivery device that was modeled after the headsets used for hands-free telecommunication devices. There are currently two versions of the device available, one for oxygen therapy alone and another that can be used for both oxygen therapy and capnographic measurement. The device consists of a headset that traverses across the top of the head, O2 supply and CO2 sampling (in the capnographic version) lines attached to an adjustable boom, and a diffuser consisting of a pin shaped like a mushroom with a hole in the centre in the original version and with a pin shaped like a funnel (or inverted cone) in the capnographic version.6 It was necessary to create a new diffuser in the capnographic version to prevent the delivered oxygen plume and surrounding ambient gases from contaminating the gas sampled at the CO2 sampling port. The OA received FDA approval in February 2001 and has been available commercially since June 2001 (Southmedic Inc., Barrie, Ontario, Canada).

The Capnoxygen mask (CM) is an example of a modified oxygen mask that has a carbon dioxide sampling port for capnography. This device was approved by the FDA in September 1997 and has been available commercially since the spring of 1998 (Capnoxygen LLC, Lebanon, Tennessee).

The purpose of this study was to compare the capnography monitoring performance of the capnographic OA with that of the CM.


   Methods
TOP
 Abstract
 Introduction
 Methods
Results
 Discussion
 References
 
Eleven volunteer adult subjects (five female and six male) were recruited for the study after informed consent. All subjects were healthy, ASA physical status I or II. Each subject was monitored at baseline and while receiving oxygen therapy at seven different flow rates (0.5, 1.0, 2.0, 4.0, 6.0, 8.0 and 10.0 L•min-1) applied with the CM first and then with the OA. Each monitoring period involved five repeated measurements of the ETCO2 (% of exhaled gases) during rest by a single un-blinded investigator. Test subjects were not given any specific instructions regarding their breathing. Five readings, at one-minute intervals, were captured at each oxygen flow rate. The ETCO2 measurements were made with a side-stream AS/3 Datex-Ohmeda monitor (Datex-Ohmeda, Helsinki, Finland), which sampled gases at 200 mL•min-1. Data was not collected for subject ten because of technical problems with the CO2 sampling by the Datex-Ohmeda monitor at that time.

Analysis
To examine the differences between the measured values of ETCO2 obtained by the two devices a paired sample t test was used. The mean ETCO2% was calculated for each subject for both devices for each oxygen flow rate and this value was used to calculate the differences (OA ETCO2% – CM ETCO2%). The degree of agreement between the two measurements was further explored graphically by using the method of Bland and Altman, where the difference between the methods was plotted against their mean.7

It was observed that the measured ETCO2 decreased as the rate of oxygen flow was increased. To investigate the relationship between ETCO2 and oxygen flow rate a univariate regression analysis was done with ETCO2 set as the dependent variable, oxygen flow rate set as the independent variable, and an interaction term defined by mask type and ETCO2. The resulting regression lines for the OA and CM were compared to assess if they had a common intercept and slope using a t test.

To examine the reliability of ETCO2 measurement of the two devices the "variance components procedure" in the statistical software SPSS was employed [SPSS Graduate Pack (Statistical Software) – Version 10.0. SPSS Inc. of Chicago, Illinois. 1999]. For the procedure, ETCO2% was set as the dependent variable, and measurement number (numbered 1–5 for each oxygen flow rate), subject number and oxygen flow were set as random factors. A reliability coefficient was constructed from the resulting variance components as an estimate of the repeatability of the two methods of ETCO2 measurements.8 The ‘classic’ reliability was calculated by the proportion of the variance that was due to differences between subjects.

For the paired sample t test that was used to compare the mean differences in ETCO2 between the two devices a Bonferroni adjustment (P value = 0.05/number of tests) was made for significance testing; for the remaining tests significant differences were assumed with P values # 0.05.


   Results
TOP
 Abstract
 Introduction
 Methods
 Results
Discussion
 References
 
The subjects included in the analysis included five females and five males with a mean age of 39 yr. Photographs of the devices used in the study are shown in Figure 1Go and examples of the capnographs produced by the devices are shown in Figure 2Go. Capnography worked well throughout the study period for both devices, producing consistent waveforms.



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  		FIGURE 1 a) Origial OxyArmTM with head band, boom, mushroom pin diffuser, and oxygen tubing; b) Capnographic OxyArmTM with head band, boom, funnel pin diffuser, oxygen tubing and carbon dioxide sampling tubing; c) Capnoxygen mask with headstrap, face mask, oxygen tubing and a carbon dioxide sampling port. The Capnographic OxyArmTM and Canoxygen mask were the devices compared in this study.

 


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  		FIGURE 2 Example capnogaphs with ten consecutive waveforms obtained from (a) the Capnoxygen mask and (b) the Capnographic OxyArmTM. Both examples were obtained from subject one while receiving oxygen at 1.0 L•min-1.

 
The analyses of the overall differences in ETCO2 at each oxygen therapy flow rate are shown in Figure 3Go. The mean difference in ETCO2% (OA minus CM) was greater than zero at all oxygen flow rates and there was a trend for this difference to increase with increasing oxygen flow rates. A paired sample t test showed no significant differences in ETCO2% between the two devices at all oxygen flow rates. Figure 4Go displays a plot of the mean differences in ETCO2% (OA minus CM) against the average ETCO2% obtained by the two devices. The mean difference in ETCO2% between the two devices was 0.24 with a 95% confidence interval of -0.81 to 1.29, illustrating that the OA measurements were on average greater than the CM measurements. The difference translates to approximately a 2-mmHg difference in pCO2 at sea level.



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  		FIGURE 3 Mean end-tidal CO2% (ETCO2%) difference (OxyArmTM CO2% minus Capnoxygen mask%) with 95% confidence intervals. The mean ETCO2% was calculated for each subject for each device at every oxygen flow rate and this value was used to calculate the mean differences. The ETCO2% obtained with each device was compared at each oxygen flow rate using a paired sample t test. For all comparisons the P values were not significant.

 


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  		FIGURE 4 Difference against the mean for end-tidal CO2% (ETCO2%) data. Mean values for ETCO2% were calculated for each subject at each oxygen flow rate and these values were used to calculate the difference. The average ETCO2% was calculated using all repeated measurements for both devices (ten measurements at each oxygen flow rate).

 
The regression analysis revealed that there was a significant inverse linear relationship between ETCO2 and oxygen therapy flow rate (P < 0.001). The comparison between the two regression equations showed no significant difference in the intercept (projected baseline ETCO2), but the slope of the CM regression line was significantly greater than that of the OA line (Figure 5Go). The greater slope for the CM is a reflection of a stronger influence of oxygen flow rate on ETCO2 measurements.



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  		FIGURE 5 Regression analysis results of ETCO2 and oxygen flow rate. The regression line equations show the estimates for the intercepts and slopes with the 95% confidence interval. The asterisks (*) denote a significant difference between the groups.

 
The sources of variance in ETCO2 measurements were explored with an ANOVA. For both the CM and the OA the main source of variance in ETCO2 measurements was the subjects, indicating that most of the variability in measurements was due to systematic differences between subjects. The second largest source of measurement variance was oxygen flow rate, and repeated measurements at a given oxygen flow rate did not contribute significantly to the variance. The reliability coefficient for ETCO2 measurement for both the OA and CM is shown in the TableGo. The classical reliability coefficient, which describes the proportion of the total variance in measurements that was due to differences between subjects, was greater than 0.85 for both devices. Generally, a reliability coefficient of 0.85 is considered acceptable for repeatability in a measurement.8


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  		TABLE Reliability coefficient1 for the Capnoxygen mask (CM) and OxyArmTM (OA)
 
An a priori sample size calculation was not done, but a post hoc power calculation showed that the sample size was sufficient to detect a difference in ETCO2% of less than one with 80% power. Hence this study had the power to detect approximately a 2-mmHg difference in pCO2 at sea level. This small difference in ETCO2 is less than what would be considered a minimal clinically important difference.


   Discussion
TOP
 Abstract
 Introduction
 Methods
 Results
 Discussion
References
 
Comparing the OA device to the CM showed that the OA could measure ETCO2 as well as the CM over a range of oxygen flow rates. On average, the ETCO2 measured with the OA was about 2 mmHg greater than that of the CM. Although there was an inverse relationship between the measured ETCO2 and oxygen flow rate for both devices, this effect is more pronounced with the CM. Presumably the cause of this inverse relationship between measured ETCO2 and delivered oxygen is increased contamination of expired respiratory gases at the sampling port with increasing oxygen flow rates. The main source of variability in ETCO2 for both devices was inter-subject differences, followed by variable oxygen flow rates.

No attempt was made in this study to test the validity of the measured ETCO2 values. That is, how they compared to arterial pCO2 values. Given the main purpose of measuring ETCO2 in conscious, spontaneously breathing, un-intubated patients is to monitor for respiratory depression and apnea, it is not necessary to know the precise value of the arterial pCO2 in this setting. Avoiding arterial blood gas analysis made the study much less invasive for the volunteer subjects.

This study was a non-randomized, un-blinded crossover trial. Blinding was considered unnecessary because of the objective nature of reading a digital display of ETCO2 values. Some crossover trials randomize subjects to the order in which they receive the alternative treatments; randomization was also considered unnecessary for this trial. The purpose of the randomization is to control for any potential period effect or treatment-by-period interaction.9 A period effect can happen when the patient’s condition changes over the time frame of the trial, getting better or worse. Since this study used healthy subjects one would not expect their clinical/physiological status to change over the short duration of the study period, making a period effect unlikely. A treatment-by-period interaction can occur when there is a residual carryover effect from the first treatment that affects the outcome of the second treatment. The only intervention in this study was oxygen therapy and this would not be expected to affect the ETCO2 values in healthy subjects, making a residual carryover effect unlikely.

The OA is an alternative to the conventional oxygen mask and nasal cannula. Besides delivering oxygen, it can also be used in a variety of healthcare settings where it is necessary to monitor for respiratory depression and apnea. The most obvious applications would be to monitor sedated patients in the emergency room, operating theatre and postanesthetic care unit. Apnea monitoring would also be useful in the intensive care unit for newly extubated patients. The OA has the potential (although unproven) advantages of being more comfortable than nasal cannula or oxygen masks and it may not hinder verbal communication and eating like the oxygen mask does.

At this point further studies are necessary to assess the clinical utility of the capnographic capabilities of the OA, the relative comfort of the OA in comparison with nasal cannulae and the oxygen mask and its acceptance amongst patients in a clinical setting.


   Acknowledgments
 
This study was supported by Southmedic Inc. of Barrie, Ontario, Canada.


   Footnotes
 
The Capnographic OxyArmTM development was carried out at Southmedic Inc. and the study was carried out at Dr. S. McDonald’s office, 50 Alliance Blvd, Barrie, Ontario, Canada. Supported by a grant from Southmedic, Inc. of Barrie, Ontario, Canada.

Revision received November 6, 2002. Accepted for publication February 11, 2002.


   References
TOP
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 
1 Canadian Anesthesiologists’ Society. Guidelines to the practice of anesthesia. Revised edition 2000. Can J Anesth 1999; (Suppl) 46: 9.

2 Moon R, Camporesi E. Respiratory monitoring. In: Miller RD (Ed.). Anesthesiology, fourth edition, volume 1. New York: Churchill Livingstone Inc.; 1994: 1253–91.

3 Egleston CV, Aslam HB, Lambert MA. Capnography for monitoring non-intubated spontaneously breathing patients in an emergency room setting. J Accid Emerg Med 1997; 14: 222–4.[Abstract]

4 Loughnan TE, Monagle J, Copland JM, Ranjan P, Chen MF. A comparison of carbon dioxide monitoring and oxygenation between facemask and divided nasal cannula. Anaesth Intensive Care 2000; 28: 151–4.[Medline]

5 Cheng KI, Tang CS, Tsai EM, Wu CH, Lee JN. Correlation of arterial and end-tidal carbon dioxide in spontaneously breathing patients during ambulatory gynecologic laparoscopy. J Formos Med Assoc 1999; 98: 814–9.[Medline]

6 Ling E, McDonald L, Dinesen TR, DuVall D. The OxyArmTM– a new minimal contact oxygen delivery system for mouth or nose breathing. Can J Anesth 2002; 49: 297–301.[Abstract/Free Full Text]

7 Bland JM, Altman DG. Statistical methods for assessing agreement between two methods of clinical measurement. Lancet 1986; I: 307–10.

8 Streiner DL, Norman GR. Health Measurement Scales. A Practical Guide to Their Development and Use. Second Edition. New York: Oxford University Press; 1995.

9 Hills M, Armitage P. The two-period cross-over clinical trial. Br J Pharmac 1979; 8: 7–20.




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