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Revista Brasileira de Anestesiologia

Print version ISSN 0034-7094

Rev. Bras. Anestesiol. vol.54 no.2 Campinas Mar./Apr. 2004

http://dx.doi.org/10.1590/S0034-70942004000200009 

SCIENTIFIC ARTICLE

 

Macroscopic infrared analysis of inhaled nitrous oxide diffusion to abdominal cavity in rats submitted to pneumoperitoneum*

 

Análisis macroscópica infrarroja de la difusión del óxido nitroso vía inhalatoria para la cavidad abdominal, en ratones sometidos a pneumoperitoneo

 

 

Daniel Colman, M.D.I; Marcos Leal Brioschi, M.D.II; Mário Cimbalista Júnior, M.D.III; Elizabeth Mila Tambara, TSA, M.D.IV; Maria Célia Barbosa Fabrício de Melo, TSA, M.D.V; Leonardo Pimpão Blume, M.D.VI

IMédico Anestesiologista do Hospital de Clínicas da UFPR. Mestrando em Clínica Cirúrgica pela UFPR
IIMestre em Princípios da Cirurgia - Faculdade Evangélica de Medicina do Paraná. Doutorando em Clínica Cirúrgica pela UFPR. Professor da disciplina de Anatomia Médica da PUC/PR e UFPR. Presidente da Sociedade Internacional de Termografia (UKTA/ITA)
IIIEngenheiro Eletricista, Diretor Técnico Thermotronics ST Ltda. Membro do IEEE - Engineering in Medicine na Biology Society
IVDoutora em Clínica Cirúrgica pela UFPR. Responsável pelo CET/SBA do HC da UFPR. Professora Titular da disciplina de Anestesiologia da PUC/PR. Professora Adjunta de Anestesiologia da UFPR
VDoutora em Clínica Cirúrgica pela UFPR. Responsável pelo CET/SBA da Santa Casa de Misericórdia de Curitiba. Professora Adjunta da Disciplina de Anestesiologia da PUC/PR
VIGraduando em Medicina pela PUC/PR

Correspondence

 

 


SUMMARY

BACKGROUND AND OBJECTIVES: Nitrous oxide (N2O), for its tri-atomic asymmetric structure, has high energy emission and absorption characteristics within the infrared spectrum, with maximum absorption at 4.5 µm, what makes it visible at short infrared, when contrasted with a heat emission source (hot support). Many authors have described N2O diffusion to closed cavities by chromatography methods and gas analyzers, which do not allow a detailed macroscopic study of the gas. This study aimed at macroscopically filming in the infrared spectrum inhaled N2O diffusion to the peritoneal cavity of rats submitted to 20 mmHg room air pneumoperitoneum.
METHODS: Animals where divided in three groups according to the anesthetic drug: I - Intravenous control: intraperitoneal thiopental; II - inhaled control: 1.2% isoflurane in 100% O2; III - nitrous oxide: 66% N2O in oxygen and 0.6% isoflurane. Thermal images of abdominal decompression where captured by an AGEMA 550 radiometer filmed at 7 frames per second.
RESULTS: N2O was visible to infrared. At abdominal decompression, groups I and II have not shown visible gas traces at infrared thermographs, while group III had visible infrared traces.
CONCLUSIONS: Our conclusion was that 66% inhaled nitrous oxide has diffused to peritoneal cavity of rats submitted to 20 mmHg room air pneumoperitoneum, with no intra-abdominal pressure increase.

Key Words: ANESTHETICS, Gaseous: nitrous oxide ANIMAL: rat; MEASUREMENT TECHNIQUES: infrared image


RESUMEN

JUSTIFICATIVA Y OBJETIVOS: El óxido nitroso (N2O), por ser una estructura tri-atómica asimétrica, toma características de alta emisión y absorción de energía en el espectro infrarrojo, con un pico característico de absorción en 4,5 µm, lo que lo hace visible al infrarrojo corto, cuando contrastado con una fuente emisora de calor (resguardo caliente). Diversos autores han descrito la difusión del N2O para cavidades cerradas por métodos como cromatografia gaseosa y analizador de gases, que no permiten un estudio macroscópico detallado del gas. El presente estudio tubo como objetivo la filmación macroscópica en el espectro infrarrojo de la difusión de N2O, utilizado en anestesia inhalatoria, para la cavidad peritoneal de ratones sometidos a pneumoperitoneo de 20 mmHg con aire ambiente.
MÉTODO: Los animales fueron divididos en tres grupos, de acuerdo con el anestésico utilizado: I- Control venoso: tiopental intra-peritoneal; II- Control inhalatorio: isoflurano a 1,2% en O2 100%; III- Óxido Nitroso: N2O 66% en oxígeno e isoflurano a 0,6%. Los termogramas provenientes de la descompresión abdominal fueron obtenidos, por medio de un radiómetro AGEMA 550 filmados a 7 cuadros por segundo.
RESULTADOS: El N2O se demostró visible al infrarrojo. En el momento de la descompresión abdominal, no hubo en los grupos I y II termogramas con rastros de gases visibles al infrarrojo. Hubo, todavía, rastros de gases visibles al infrarrojo en el grupo III.
CONCLUSIONES: Se concluye que el óxido nitroso inhalatorio a 66% se difundió para la cavidad peritoneal de ratones sometidos a pneumoperitoneo de 20 mmHg con aire ambiente, sin aumento de la presión intra-abdominal.


 

 

INTRODUCTION

Nitrous oxide (N2O) is the oldest inhaled agent and is widely used worldwide 1. Its properties, especially its low solubility 2, give it special and desirable pharmacokinetic properties for an inhaled agent. Uptake, distribution and elimination processes are very fast. It has mild side-effects, and minor cardiovascular and respiratory repercussions should be emphasized 3,4. In addition, it is poorly metabolized.

Structurally, N2O is a tri-atomic asymmetric molecule with characteristics of major infrared irradiation absorption and emission 5 with maximum absorption at 4.5 µm. For being used in high concentrations and widely used in pediatric anesthesia 6, it has been chosen as a model for macroscopic studies of gases involved in anesthesiologists' occupational exposure. This way, it adds a new dimension to the study of anesthetic gases dispersion in the operating center and to the evaluation of OR's air drainage systems 7. It also helps understanding the correct facial mask adaptation to prevent leakage and the development of educational material, acting also as an indicator of gases and vapors lost in the environment, both quantitatively and qualitatively 8.

There are some controversies about the use of N2O in anesthesiology: thanks to its higher solubility as compared to nitrogen 9, it is now playing the role previously plaid by nitrogen in body cavities filled with gases (such as middle ear) 10-12, increasing both in volume and pressure when inside the cavity. Although its potential usefulness for laparoscopy as inhaled agent or in the pneumoperitoneum itself 13, N2O may contribute for intestinal loops distension 14, although this effect has been questioned by more recent studies. Spivak et al. have not observed significant differences between the use of N2O or atmospheric air with regards to surgical field conditions in laparoscopic surgeries 15.

This study aimed at macroscopically evaluating nitrous oxide diffusion to the peritoneal cavity by filming in the infrared spectrum rats submitted to pneumoperitoneum with 20 mmHg room air pressure.

 

METHODS

This experimental protocol has been approved by the Biological and Health Sciences Center, Pontifícia Universidade Católica, Paraná (CCBS-PUC/PR), was carried out according to Colégio Brasileiro de Experimentação Animal (COBEA) ethical principles and was filed with the Infrared Images Research Group of the National Research Council - PUC/PR.

The experiment was performed with 15 male Wistar rats (Rattus norvegicus albinus, Rodentia mammalia), aged 120 to 153 days (mean 135.9 days), which received standardized feed and water ad libitum up to 12 hours before anesthesia.

The experiment was performed in the Medullary Injury and Experimental Trauma Lab, PUC/PR. Minor thermal variation was accepted, room temperature was maintained in 20 ºC and relative air humidity in 75%, both checked by dry and wet bulb thermo-hygrometer (Incotherm, Br). Heat losses by convection were minimized by maintaining windows and doors closed and minimal movement around animals. Airflow was controlled with a digital rotation blade anemometer model HHF 300 A (Omega Engineering, Inc.) placed 10 cm away from animals and maintaining airflow velocity below 0.2 m.s-1, which is the transition value between heat loss by free and forced convection 16.

Anesthesia was induced and maintained with the studied agents: sodium thiopental, isoflurane and isoflurane associated to 66% N2O, being inhaled anesthetic agents administered through facial cone with oxygen and universal vaporizer with 3 L.min-1 gases fresh flow.

Animals were maintained in Guedel's anesthetic plane degree III. Monitoring consisted of checking for the presence of reflexes, respiratory rate, mucosal color 17 and intra-abdominal pressure changes. Animals were divided in 3 groups:

  • Group I - intravenous control: anesthetized with peritoneal thiopental and facial cone with 3 L.min-1 oxygen;
  • Group II - inhaled control: anesthetized with 1.2% isoflurane and facial cone with 3 L.min-1 oxygen;
  • Group III - nitrous oxide: anesthetized with 0.6% isoflurane and facial cone with 1 L.min-1 oxygen associated to 2 L.min-1 N2O (66.6%).

A "T" shaped plastic tube was coupled to universal gases vaporizer outlet to bypass the flow to two directions: one to the facial cone in contact with the animal for anesthetic induction and maintenance, and the other to control anesthetic gases and filming.

To obtain pneumoperitoneum 18 with 20 mmHg intra-abdominal pressure, peritoneal cavity was punctured with 22G teflon catheter connected to a previously gaged anerometer system (Welch Allyn, Tycos®, Arden, USA). System remained coupled to abdominal cavity allowing continuous intra-abdominal pressure monitoring. Once the desired pressure was reached, inflation was interrupted. Forty-five minutes after pneumoperitoneum was deflated by a new abdominal puncture with 16G teflon catheter and gas leaving the abdominal cavity was filmed within the infrared spectrum between 3.5 to 5 µm. This second puncture aimed at obtaining a faster emptying. During abdominal decompression, anesthetic gases administration was withdrawn to prevent N2O contamination during filming.

Thermovision AGEMA 550 (FLIR System™, Sweden) radiometer was used to capture spectral electromagnetic waves range between 3.5 and 5 µm, that is, short infrared spectrum waves. Maximum spatial resolution obtained was 0.1 to 0.2 mm.

Naturally emitted infrared irradiation by objects in the environment is captured and converted into electric signal by a PtSi detector cooled by liquid nitrogen (steerling cycle), This signal is processed in a numeric spreadsheet with 76 thousand absolute temperature points and gaged by frame, instantaneously represented by thermal image with resolution of 320 x 240 pixels and thermal sensitivity above 0.1 ºC. Tri-molecular gases, including N2O, have the physical property of energy absorption and emission in the infrared range 4,19, which makes them visible at infrared when placed between a heat-emitting source and radiometer.

An acrylic box with 40 ºC water was used in our study as the heat emitting source for nitrous oxide visualization at infrared.

Radiometer was assembled in a vertical support 1 m away from the acrylic box and directly focused on animals' ventral surface forming a 60º angle with the teflon catheter during filming of abdominal decompression. For control purposes, gases being administered via facial cone were also filmed through the "T" outlet of the anesthesia system, as shown in figure 1.

Images were processed by a 750 MHz Pentium III computer coupled to a PCMCIA board. Using a specific program, ThermaCAM™ Researcher 2001, FLIR Systems (Sweden), captured images were recorded at 7 frames per second throughout abdominal decompression, in the same above-described environmental conditions. All images were represented by infrared thermographs in a video monitor and recorded in CD-Rom for further analysis by the program.

Images were plotted using thermal range between 50 ºC and 20 ºC, level temperature of 42 ºC and continuous "RAINBOW900" color palette, in which white, red, yellow, green, blue and black represented a decreasing temperature gradient, equally distributed throughout the scale from hottest to coldest and maintained until experiment completion. For black and white printing purposes, images were treated with Adobe Photoshop 5.0 software through which colors were filtered in the red palette followed by the application of grey shades for a better contrast among researched elements.

For semi-quantitative analysis of infrared-visible gases, temperature differential between what animals were inhaling (item B, Figure 1) and what left abdominal cavity was calculated (item D, Figure 1).

 

RESULTS

Nitrous oxide (66%) is visible to infrared through the method described and recorded in thermographs, where yellowish colors represent heat emission by the acrylic support filled with warm water, and greenish colors (colder) represent the energy absorbed by N2O forming a visible contrast at infrared filming, as shown in figure 2.

There were no infrared-visible gas traces in thermographs obtained for Group I during macroscopic filming of gases from the facial cone (100% O2), as shown in figure 3. O2 was invisible at infrared. There were also no gas traces images in thermographs obtained during macroscopic filming of intra-abdominal cavity gases during decompression, as shown in figure 4.

There were no infrared-visible gas traces in thermographs obtained for Group II during macroscopic filming of gases from the facial cone (100% O2 carrying 1.2% isoflurane). In our study, with the method and spectral range used, isoflurane was invisible to infrared. There were also no gas traces images in thermographs obtained during macroscopic filming of intra-abdominal cavity gases during decompression. Thermograph patterns in Group II follow figure 3 and figure 4 patterns.

There have been infrared-visible gas traces in thermographs obtained for Group III during macroscopic filming of gases from the facial cone (33.4% O2 associated to 66.6% N2O and 0.6% isoflurane). This clearly shows that N2O is visible in thermographs obtained in the infrared range as shown in figure 2. There have been also gases trace images in thermographs obtained during macroscopic filming of intra-abdominal cavity gases during decompression, as shown in figure 5 and figure 6. Temperature differential between B in figure 2 and D in figure 5 and figure 6 was close to zero, suggesting that N2O concentration leaving the cavity was close to the inspired fraction, that is, 66%.

This shows the diffusion of nitrous oxide used in inhaled anesthesia to the abdominal cavity of rats submitted to pneumoperitoneum with 20 mmHg room air.

Intra-abdominal pressure was maintained stable in 20 mmHg throughout the study for all Groups.

 

DISCUSSION

Most gases and vapors with bipolar molecular structure absorb infrared energy. If the gas is placed between an infrared-emitting object and a radiometer, gas will absorb infrared irradiation and this absorbed energy will be translated into temperature decrease, being visible at thermograph analysis 19. This method allows for the visualization of gases invisible to human eye and has been used for the study of most popular inhaled agents pollution levels. It allows for the study of pollutant gases and vapors dispersion during our professional activity. A study has evaluated pollution levels in pediatric service rooms and has shown the importance of accurately handling facial masks to prevent pollutant anesthetic gases losses to the environment. It has concluded that this method, in addition to allowing macroscopic gas diffusion analysis, also provides a semi-quantitative analysis correlated to data obtained from the gases analyzer 19. The same author has concluded that pollutant gases are poorly eliminated from the OR during ventilation. In our study, in addition to macroscopically analyzing inhaled N2O diffusion to pneumoperitoneum, we could conclude that its concentration was close to the inspired fraction through images subtraction analysis.

Atmosphere has some energy emission and absorption properties. Gas and vapor molecules vibrate in certain frequencies 5 which, when reached, absorb an energy photon, allowing the gas to be contrasted when passing between emitting source and radiometer.

Monoatomic gases, such as neon, have only electronic transitions, while biatomic gases, such as O2 and N2, have no electric dipole moment, thus not absorbing or emitting significant energy in environmental conditions and being considered invisible at infrared 5. This has been observed in our study because there has been no image whatsoever of infrared-visible gas when O2 was used in Group I.

Pneumoperitoneum was achieved with room air which is knowingly invisible to infrared.

Asymetric diatomic molecules, such as NO and CO emit and absorb infrared energy, but in a very weak manner in environmental conditions. Triatomic molecules, however, such as N2O, SO2, H2O and CO2, have strong energy emission and absorption trends. These gases may impair the interpretation of image components because the radiometer used in our study has an observation window in the range of 3.5 to 5 µm, while such molecules have windows with absorption peaks in the following ranges 5: CO2 (2,0; 2,7, 4,3; 15 µM); H2O (1,4; 1,9; 2,7; 6,3 20 µM) and N2O (4,5 µM). This phenomenon could be controlled in our study by using room air for pneumoperitoneum, which is knowingly invisible to infrared.

Knowing that O2 is invisible to infrared, images obtained with isoflurane in Group II (as control of what was being vaporized), have shown that this agent was also invisible in the conditions of our study. Since there has been no infrared-visible gas traces at abdominal decompression of Group II animals, it is unlikely that water vapor from the peritoneal serosa in the abdominal cavity would influence results found by this study.

As to Group III receiving inhaled N2O with isoflurane and O2, the only visible element at infrared was the subtraction image caused by N2O, since both O2 and isoflurane had already shown to be invisible (Groups I and II). The presence of infrared-visible image at abdominal decompression in Group III has clearly shown that this was due to N2O, because if there was water vapor or other gases influence, this would had been shown during abdominal decompression of Groups I and II.

This study has clearly shown N2O diffusion to closed cavities in 45 minutes, in rats submitted to 20 mmHg pneumoperitoneum. An author 7 has concluded in an animal model with swine, that N2O used in general anesthesia would build up in the pneumoperitoneum, and that in less than 2 hours, swine intra-abdominal cavity had been filled with 29% N2O, enough to undergo combustion. The same author has preconized continuous gas escape with replacement with fresh CO2 in a rate of 4 to 8 times/hour the initial CO2 injected volume to inflate cavity at 12 mmHg 29. The author has collected gas from swine peritoneal cavity at 10-minute intervals detected by gas chromatography. This method implies gases collection in propylene syringes and has limitations and interpretation difficulties, since gases are diffused to the environment and may affect final results 21.

Infrared filming method was a landmark in the study of occupational exposure to inhaled agents and may be used in laparoscopic surgery research or in any other Anesthesiology research involving macroscopic gases analysis and semi-quantification. It may be continuously performed with no need for intra-peritoneal gases sample collection. Filming is instantaneous and data are recorded for further analysis, not depending on laboratories or on time, which could interfere with final results.

Although its potential use in laparoscopy as inhaled agent, or even in the pneumoperitoneum itself 13, some critical and questionable issues still remain as to its use in anesthesia, especially its contribution in distending intestinal loops, although this defect has been contested by more recent studies 22. There has been no intra-abdominal pressure increase in our study, in spite of N2O diffusion to the intra-peritoneal cavity. Another concern would be the potential formation of carburetant blends 21,23,24, but this discussion is restricted to surgeries where there is intestinal loop injury with gases leakage.

N2O has been preconized for laparoscopic surgeries by some authors, especially for cholecystectomies 13 and surgeries where there is no intestinal gases leakage to the cavity. N2O as intracavitary gas for laparoscopic surgeries has shown a lower incidence of hemodynamic and ventilatory changes 25 as compared to CO2 and has not promoted arhythmias 26-29, surgical field difficulties or an increase in the incidence of postoperative nausea and vomiting, in addition to allowing lower consumption of opioids 30.

From the data obtained in our study, one may conclude that 66% inhaled nitrous oxide administered for 45 minutes has diffused to the abdominal cavity of rats submitted to pneumoperitoneum with 20 mmHg room air, since it has been macroscopically detected by infrared filming without intra-abdominal pressure increase in all studied groups.

 

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Correspondence to
Dr. Daniel Colman
Rua Desembargador Westphalen 824/704-B Rebouças
80220-030 Curitiba, PR

Apresentado (Submitted) em 30 de novembro de 2002
Aceito (Accepted) para publicação em 25 de julho de 2003

 

 

* Recebido do (Received from) Grupo de Pesquisas em Imagem Infravermelha, CNPq/PUC/PR. Realizado no Laboratório de Lesões Medulares e Trauma Experimental da Pontifícia Universidade Católica do Paraná (PUC/PR). Artigo vencedor do prêmio Renato Ângelo Saraiva de 2002