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Canadian Journal of Anesthesia 50:930-932 (2003)
© Canadian Anesthesiologists' Society, 2003
Obstetrical and Pediatric Anesthesia

Nitrous oxide does not affect automated air tonometry in children

[Le protoxyde d’azote n’agit pas sur la tonométrie à l’air, automatisée, chez les enfants]

Markus Weiss, MD, Andreas Gerber, MD and Alexander Dullenkopf, MD

1 From the Department of Anaesthesia, University Children’s Hospital, Zurich, Switzerland.

Address correspondence to: Dr. Markus Weiss, Department of Anaesthesia, University Children’s Hospital, Steinwiesstrasse 75, 8032 Zurich, Switzerland. Phone: +41-1-266-71-11; Fax: +41-1-266-79-94; E-mail: markus.weiss{at}kispi.unizh.ch


   Abstract
TOP
 Abstract
Introduction
 Methods
 Results
 Discussion
 References
 
Purpose: To evaluate the effects of nitrous oxide on automated air tonometry in the clinical setting.

Material and methods: With approval of the Hospital Ethical Committee and after obtaining informed parental consent, an 8-F tonometry catheter was inserted orogastrically in ten children aged one to three years scheduled for elective surgery with combined regional and general anesthesia. A standardized general anesthesia technique with tracheal intubation was used in all patients and consisted of sevoflurane in oxygen/nitrous oxide (30%/70%; n = 5 patients) or in oxygen/air (FIO2 0.3; n = 5 patients). After obtaining steady state gastric CO2 values (PrCO2), fresh gas mixtures were rapidly changed from oxygen/nitrous oxide to oxygen/air (A) or vice versa (B). In addition, balloon pressures were recorded using a pressure transducer. Measurements were performed at intervals of ten minutes with recording of balloon pressures, end-tidal CO2 (PETCO2) and PrCO2 values. Pr-ETCO2-gap were calculated to eliminate influences of changes in PaCO2.

Results: Changing the fresh gas mixture from N2O/O2 to O2/air resulted in a decrease of balloon pressure of -10.4% (113.4 ± 14.7 mmHg to 101.6 ± 25.0 mmHg). Changing the fresh gas mixture from O2/air to N2O/O2 resulted in an increase of balloon pressures of 6.4% (107.6 ± 19.3 mmHg to 114.0 ± 20.3 mmHg). During both fresh gas exchange experiments no significant changes (> 0.2 kPa) in calculated Pr-ETCO2-gaps were observed.

Conclusions: Based on our in vivo data, nitrous oxide during general anesthesia can be used with automated air tonometry and does not affect air tonometric PrCO2 reading in clinical practice.


   Introduction
TOP
 Abstract
 Introduction
Methods
 Results
 Discussion
 References
 
MONITORING partial pressure of CO2 in the gastric lumen (regional PCO2 = PrCO2) represents a non-invasive method for assessing adequacy of gut mucosal perfusion.1 The introduction of automated air tonometry has facilitated measurement of gastric mucosal PrCO2 in the clinical setting.2,3 The automated air tonometer (Tonocap®, Datex-Engstrom, Tonometrics Division, Helsinki, Finland) allows periodic assessment of gastric or sigmoid PrCO2 with the established tonometry catheters. The automated air tonometer fills the tonometer balloon with air. Following an equilibration period, the air is automatically sampled and analyzed by means of an infrared-sensor, separately analyzing CO2, N2O and N2. In order to avoid depletion of gastric luminal CO2 and to reduce equilibration times, the aspirated gas mixture is re-filled into the sampling balloon.4

The tonometry catheter sampling balloon inserted into the stomach represents a closed air space within the body and therefore may be affected by the use of N2O during anesthesia.5 Recently, we reported on the marked and transient in vitro effects of N2O on balloon pressures and PrCO2 when using air tonometery.6

In the present study, our purpose was to examine the effects of N2O on automated air tonometry in the clinical setting.


   Methods
TOP
 Abstract
 Introduction
 Methods
Results
 Discussion
 References
 
With approval of the Hospital Ethical Committee and after obtaining informed parental consent, ten children aged one to three years scheduled for elective surgery with combined regional and general anesthesia were included. A standardized general anesthesia technique with tracheal intubation was used in all patients and consisted of sevoflurane in O2/N2O (30%/70%; Group A: n = 5 patients) or in O2/air (FIO2 0.3; Group B: n = 5 patients). Fresh gas flow was set at 3 L•min-1 in both groups.

After endotracheal intubation, an 8-F tonometry catheter (TonometricsTM Catheter, Datex Ohmeda Division, Helsinki, Finland) was inserted orally into the stomach. Correct gastric placement of the catheter tip was confirmed by auscultation and aspiration of gastric fluid. Balloon pressures were recorded using a pressure transducer interposed between the air tonometer and the tonometry catheter. After obtaining steady state PrCO2 values, the fresh gas mixture was rapidly changed from O2/N2O to O2/air (A) or vice versa (B). Measurements were performed at intervals of ten minutes with recording of balloon pressures, PETCO2 and PrCO2 values. Pr-ETCO2-gaps were calculated to eliminate influences from changes in PaCO2 during the recording period.


   Results
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 Abstract
 Introduction
 Methods
 Results
Discussion
 References
 
Mean age and weight of the patients was 1.5 ± 1.1 yr and 12.8 ± 1.9 kg for Group A and 1.6 ± 0.6 yr and 11.1 ± 1.1 kg for Group B. Changing the fresh gas mixture from N2O/O2 to O2/air resulted in a decrease of balloon pressure during two cycles with a maximum of -10.4% in the second cycle (113.4 ± 14.7 mmHg to 101.6 ± 25.0 mmHg). After changing the fresh gas mixture from O2/air to N2O/O2, balloon pressures increased by 6.4% (107.6 ± 19.3 mmHg to 114.0 ± 20.3 mmHg) compared to a normal fall of 6.9% due to gas-efflux from the pressurized tonometry balloon. This increase was only observed during the first cycle. Changes in ETN20 from 0 to 65% or 70% to 5% were completed within three minutes. During both fresh gas exchange experiments no significant (> 0.2 kPa) changes in the calculated Pr-ETCO2-gaps were observed (FigureGo).



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  		FIGURE Balloon pressures, air tonometric measured gastric PrCO2 and calculated Pr-etCO2-gaps are plotted against time during rapid exchange of inspiratory fresh gas mixture from O2/N2O to O2/air (top - Group A) and vice versa from O2/air (70%/30%) to N2O/O2 (bottom - Group B). Measurements were performed at intervals of ten minutes. Values are mean ± SD (n each = 5 patients).

 

   Discussion
TOP
 Abstract
 Introduction
 Methods
 Results
 Discussion
References
 
We investigated the effects of N2O on air-filled tonometry balloons and air-tonometric assessment of PrCO2. The main finding was that rapidly adding or removing N2O in the fresh gas mixture did not markedly affect in vivo balloon pressures and was without effects on PrCO2 and on the Pr-ETCO2-gap.

This is in contrast to earlier results from in vitro investigations where changes of N2O concentration in the fresh gas mixture surrounding the balloon markedly affected balloon pressures (increase 60.7 mmHg; decrease 84. 5 mmHg) and PrCO2 measurements (dilution -0.57 kPa; concentration 0.4 kPa) after the first cycle.6 However, these effects were only transient and were absent after four measuring cycles during steady state conditions.

Although changes in end-tidal N2O from 65% to < 5% N2O and vice versa from 0% to > 65% in our clinical trial were fast, the effects of adding or removing N2O on balloon pressure were small and delayed. PrCO2 and the Pr-ETCO2 gap remained almost unaffected. The weak in vivo effects of N2O on air tonometry (compared to in vitro experiments) can be explained by the much lower diffusion rate of N2O from the lung to blood to the gastric lumen to the balloon lumen within the human body compared to laboratory conditions. The remaining small effects of N2O on the tonometry balloon are outweighed in particular by the refilling mechanism of the air tonometer, but also by gas efflux and by CO2 redistribution through the tonometer balloon membrane.

Gastric tonometry represents a minimally invasive monitor for assessing adequacy of gut mucosal perfusion. It is valuable as a prognostic tool and to detect hypovolemia before it can be identified by global hemodynamic variables.2,7 Until now, the experience with gastric tonometry in children is limited.8,9 However, due to its minimal invasiveness and ease of use, gastric air tonometry may become an interesting monitor in a group of patients in whom central venous and pulmonary artery catheterization is not always easy or feasible. Further research is required to define its role during pediatric anesthesia. Although the role of N2O for general anesthesia is questioned and N2O has been abandoned in many hospitals, it is still used routinely during pediatric anesthesia.10 Thus, it is conceivable that N2O will be administered for general anesthesia in infants and children in whom automated air tonometry is used for research purposes.

We investigated the effects of N2O on air tonometry in small children who achieve rapid equilibration of gas mixtures due to their high alveolar ventilation rate and small functional residual capacity. In older children and adult patients, equilibration is slower and the effects of N2O on tonometer balloon pressure should, therefore, be less or even absent.

In summary, based on our in vivo data, N2O during general anesthesia can be used with automated air tonometry and does not affect air tonometric PrCO2 readings in clinical practice.


   Footnotes
 
The study was supported by a grant from the Hartmann-Mueller Foundation, Zurich, Switzerland.

Revision received August 11, 2003. Accepted for publication May 12, 2003.


   References
TOP
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 
1 Kolkman JJ, Otte JA, Groeneveld AB. Gastrointestinal luminal PCO2 tonometry: an upate on physiology, methodology and clinical applications. Br J Anaesth 2000; 84: 74–86.[Free Full Text]

2 Bennett-Guerrero E, Panah MH, Bodian CA, et al. Automated detection of gastric luminal partial pressure of carbon dioxide during cardiovascular surgery using the Tonocap. Anesthesiology 2000; 92: 38–45.[Medline]

3 Mäkinen MT, Heinonen PO, Klemola UM, Yli-Hankala A. Gastric air tonometry during laparoscopic cholecystectomy: a comparison of two PaCO2 levels. Can J Anesth 2001; 48: 121–8.[Abstract/Free Full Text]

4 Dullenkopf A, Cornelius A, Gerber AC, Weiss M. Factors affecting performance of air tonometry using the TONOCAP®. Anaesth Intensive Care 2002; 30: 794–9.[Medline]

5 Munson ES. Nitrous oxide and body air spaces. Middle East J Anesthesiol 1983; 7: 193–201.[Medline]

6 Weiss M, Hug MI, Fischer J, Gerber A, Baenziger O. The effect of nitrous oxide on air tonometry. Eur J Anaesthesiol 2001; 18(Suppl 21): 30 (abstract).

7 Heard SO. Gastric tonometry. The hemodynamic monitor of choice (pro). Chest 2003; 123: 469S–74S.[Abstract/Free Full Text]

8 Reinoso-Barbero F, Calvo C, Ruza F, Lopez-Herce J, Bueno M, Garcia S. Reference values of gastric intramucosal pH in children. Paediatr Anaesth 1998; 8: 135–8.[Medline]

9 Bichel T, Kalangos A, Rouge JC. Can gastric intramucosal pH (pHi) predict outcome of paediatric cardiac surgery? Paediatr Anaesth 1999; 9: 129–34.[Medline]

10 Stenqvist O, Husum B, Dale O. Nitrous oxide: an aging gentleman (Editorial). Acta Anaesthesiol Scand 2001; 45: 135–7.[Medline]





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