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Print version ISSN 0034-7094
Rev. Bras. Anestesiol. vol.52 no.3 Campinas May/June 2002
Analysis of heat loss using inhalation agents in rats subjected to laparotomy and increased intra-abdominal pressure, using digital infrared thermal image *
Análisis de la redistribución de calor con agentes inhalatorios, en ratones sometidos a laparotomia y pneumoperitonio, a través de termografia infraroja
Daniel Colman, M.D.I; Maria Célia Barbosa Fabrício de Melo, TSA, M.D. II; Marcos Leal Brioschi, M.D.III; Fábio Silveira, M.D.IV ; Mário Cimbalista Júnior, M.D.V
IME do CET da Santa Casa de Misericórdia
de Curitiba, PR
IIDoutora em Clínica Cirúrgica pela Universidade Federal do Paraná. Co-responsável pelo CET/SBA da Santa Casa de Misericórdia de Curitiba; Professora auxiliar da Disciplina de Anestesiologia da Pontifícia Universidade Católica do Paraná
IIIMestre em Princípios da Cirurgia - Faculdade Evangélica de Medicina do Paraná; Professor auxiliar da Disciplina de Anatomia Médica da Pontifícia Universidade Católica do Paraná e Universidade Federal do Paraná; Presidente da Sociedade Internacional de Termografia (UKTA/ITA)
IVDoutorando em Medicina na Pontifícia Universidade Católica do Paraná
VEngenheiro Eletrecista, Diretor Técnico Thermotronics ST Ltda, PR. Membro do IEEE, Engineering in Medicine an Biology Society
BACKGROUND AND OBJECTIVES: Anesthesiology
involves the handling of situations inherent to anesthetic and surgical procedures
which lead to patients thermal homeostasis unbalance, with noxious physiological
effects. This study aimed at qualifying and quantifying thermal redistribution
in rats subjected to inhalation anesthesia, during induction and in surgical
situations of laparotomy and increased intra-abdominal pressure.
METHODS: The study involved 90 rats, submitted to inhalation anesthesia, which were distributed in three groups: halothane; isoflurane; sevoflurane. Each group was divided in subgroups: I - control; II - median laparotomy with bowel exposure; III - 15 mmHg Increase in intra-abdominal pressure. Heat loss was measured by an esophageal probe and infrared thermal image.
RESULTS: There were no significant differences among inhalation anesthetics regarding heat loss between groups I and II. In group III, there was a difference between isoflurane and sevoflurane and isoflurane was responsible for the highest heat loss.
CONCLUSIONS: Sevoflurane was the inhalation agent determining the lowest heat loss in the presence of increased intra-abdominal pressure, as compared to isoflurane and halothane.
Key words: ANESTHETICS, Volatile: halothane, isoflurane, sevoflurane; ANIMAL: rat; HYPOTHERMIA; MEASUREMENT TECHNIQUES: digital infrared thermal image
JUSTIFICATIVA Y OBJETIVOS: La Anestesiología
envuelve el manoseo de situaciones inherentes al acto anestésico y operatorio
que cursan con el desequilibrio de la homeóstasis térmica del paciente,
ocasionando efectos fisiológicos deletéreos. El presente estudio objetiva
calificar y cuantificar los fenómenos de redistribución térmica
en ratones sometidos a anestesia inhalatoria, durante la inducción, y en
situaciones quirúrgicas de laparotomia y pneumoperitonio.
MÉTODO: Fueron utilizados 90 ratones, sometidos a anestesia inhalatoria, distribuidos en tres grupos, en que fueron utilizados: halotano, isoflurano y sevoflurano. En cada grupo hubo división en otros tres sub-grupos: I - control, II - laparotomia mediana con exposición de alzas intestinales; III - pneumoperitonio de 15 mmHg. La análisis termodinámica se realizó de dos formas: a través de la temperatura central esofágica y de la imagen digital térmica infrarroja.
RESULTADOS: No hubo diferencia significativa en relación a los anestésicos inhalatorios entre los grupos I y II en relación a la pérdida de calor. En relación al grupo III, hubo diferencia entre el isoflurano y el sevoflurano, siendo el isoflurano el anestésico responsable por la mayor pérdida de temperatura en el animal.
CONCLUSIONES: El sevoflurano fue el agente anestésico inhalatorio que determinó menor pérdida de calor frente al pneumoperitonio, en relación al isoflurano y halotano.
Surgical environment, anesthetic and surgical techniques may, isolated or in combination, determine severe changes in patients temperature 1. Anesthesiologists must understand heat production and thermal homeostasis maintenance processes because, when unbalanced, homeostasis may determine severe physiological changes 2.
Body temperature is in general maintained within a narrow limit in a non-anesthetized body 3. For the metabolism to remain in a constant baseline rate, body has compensation mechanisms to support the thermal gradient between internal and external environment: vasomotor tone, thermogenic shivering, non-thermogenic shivering in neonates and infants 4, sweating and behavioral response, such as wearing warm clothing. It is important to highlight that during anesthetic induction there are changes in such mechanisms, known as heat loss, with central heat flowing to periphery eventually leading to body heat loss 4.
Accidental hypothermia is directly related to coagulopathy and metabolic acidosis 5. Thermogenic shivering increases oxygen consumption rates and may lead to cardiopulmonary overload 5. Increased catecholamines induce vasoconstriction and may cause heart overload by afterload increase. Vasoconstriction prevents nutrients to reach fibroblasts and also impair macrophages action 6. As a consequence, healing is delayed with a higher predisposition for postoperative wound infection. Such factors are directly related to higher postoperative morbidity and mortality.
Inhalation anesthetic may interfere in thermal homeostasis because their effects depend on several factors: direct myocardial depression, autonomic tone inhibition, beta-adrenergic receptors blockade, adrenal inhibition (catecholamine release), baroreceptors sensitization, decreased response to cold threshold.
This study aimed at qualifying and quantifying heat loss in rats submitted to inhalation anesthesia with sevoflurane, halothane and isoflurane, during induction and in surgical situations of laparotomy and increased intra-abdominal pressure.
The experimental protocol used in this study was approved by the Biological and Health Sciences Center, Pontifícia Universidade Católica do Paraná (CCBS-PUCPR) and was performed in compliance with the ethical principles of the Brazilian College of Animal Experiments.
The study involved 90 male Wistar (Rattus norvergicus albinos, Rodentia mammalia) rats, aged between 120 and 153 days (mean 135.9 days). Animals received standardized feed and water ad libitum until 12 hours before anesthesia.
The experiment was performed in the Spinal Cord Injury and Experimental Trauma Laboratory, PUC/PR. A minimum thermal variation was admitted, with room temperature maintained in 20 ºC and relative air humditiy in 75%. Measurements were checked with a dry and wet bulb thermo-hygrometer (Incotherm, Br). Convective heat losses were minimized, keeping door and windows closed and minimum movement around animals. Air flow was controlled with a digital anemometer with rotating blades model HHF 300 A (Omega Engineering, Inc) placed 10 cm apart from the animal and with an air flow speed lower than 0.2 m.s-1. This is the transition value between free and forced convective heat loss 7.
Anesthesia was induced and maintained with the volatile agent to be analyzed, administered through facial cone and 100% oxygen using a universal vaporizer with 1 L.min-1 gases admission flow.
Anesthesia was maintained at level III 8. Monitoring consisted of observing the presence of reflexes, respiratory rate and mucosal color 9.
Animals were induced according to the volatile anesthetics to be investigated: A - Halothane; B - Isoflurane; C - Sevoflurane, as shown in table I.
Thermal behavior was studied for 10 minutes after anesthetic induction. Then, animals were divided in three sub-groups, as shown in table I.
· Group I - Control (n=30): animals were maintained in anesthetic depth level III for additional 20 minutes;
· Group II - Exploratory laparotomy (n=30): xyphopubic laparotomy with maximum bowel exposure, followed by thermal behavior observation for additional 20 minutes;
· Group III - Increased intra-abdominal pressure (n=30). Increased intra-abdominal pressure was induced by the Eleftheriadis method 10 after 10 initial minutes. To increase intra-abdominal pressure to 15 mmHg, peritoneal cavity was punctured with an 18G teflon catheter connected to a previously gaged anerometer system (Welch Allyn, Tycos®, Arden, North Caroline, USA). After intra-abdominal pressure increase induction, thermal behavior was studied for additional 20 minutes.
Central temperature was measured with a high precision YSI44004, Bead I (2.250 Ohms resistance at 25 ºC) thermistor (semiconductor sensors) with an intrinsic error of 0.05 ºC and working range of -80 ºC to 120 ºC (Precision Thermistor, interchangeability ± 0.2 ºC, USA), which was placed in the lower third of the esophagus at 5 cm of the incisive tooth.
Animals were weighed with an electronic scale (Marte AS500, Br) with 0.01 g precision.
Total heat flow, Q, was measured considering rats specific mass and heat (3.8 kJ/kg ºC) divided by total experiment time 11:
Q - total heat flow [W];
m - animals mass (kg);
c - specific animals heat [J/(kg ºC)];
DT - difference between initial and final temperature [ºC];
Dt - time interval for each experiment [s].
Infrared digital thermal image was obtained with an Agema® 550 (Flir Co, USA) with a resolution of 320 x 240 pixels, allowing for the digital thermal infrared mapping of the exposed surface of the animal.
Infrared radiation spectrum is 8-12 µm and is naturally emitted from body surface. Such radiation is translated into an electrical signal by a liquid nitrogen detector with a spectrum range of 2.5 to 5.5 µm. Images are displayed in colors on a video monitor and thermograms thus obtained are processed by a specific computer program (Flir Research 2000 Boston, USA).
Univariate analysis ANOVA 12 and Tukeys parametric test 13 were used for statistical analysis.
Null hypothesis rejection level was established as 0.05 or 5% (p < 0.05) for all tests and significant values are highlighted with a star (*) 14.
Groups were homogeneous regarding weight (Table II).
There have been no significant differences in thermal profile (initial temperature; final temperature) and heat loss (Watts) among the three halogenate agents during the first 10 minutes (anesthetic induction) (Table III).
Control group did not show statistical difference in thermal profile as compared to other groups (Table IV)
Central temperature analysis in group II, submitted to laparotomy and maximum bowel exposure, has not shown significant differences among the three halogenate groups (Table V).
In group III, with 15 mmHg increase in intra-abdominal pressure, there has been a similar behavior between group A (halothane) and group B (isoflurane). Group C (sevoflurane) had a significantly lower heat loss (Table IV).
Thermographic images representing animals external temperature have confirmed data obtained with central temperature.
In group I (control), thermographic images have shown homogeneous skin heat loss during the whole experiment in all halogenate groups, as shown in figure 1.
In group II, thermographic images have shown loss of central temperature in the peritoneal exposure area (marked area), as shown in figure 2.
The marked area indicates a region with higher temperature (corresponding to temperature graduation to the right), because it represents the exposure to environment of the animals internal temperature.
Group III thermogram (increased intra-abdominal pressure), shows a more predominant heat loss in the thoracic region, caudally (Figure 3).
Heat loss is determined by physics (evaporation, conduction, radiation, convection) and not by differences among species 15. So, one may assume that heat loss in surgical patients would be similar to that observed in rats, both in magnitude and distribution. However, it is necessary to correct findings with animals weight, basal metabolism and fur.
It has been shown that there have been no differences among the three halogenate agents during laparotomy because heat loss is predominantly caused by the exposure of wet peritoneal surface to room air 16,17. Water has a high vaporization enthalpy (or latent vaporization heat). Each gram of evaporated water corresponds to an energy loss of approximately 2400 J or 576 calories, when the process occurs at a temperature close to the temperature of homothermal animals body (36.5 ºC). This has been shown through the infrared thermal image mapping of the peritoneal exposure area.
There have been differences in increased intra-abdominal pressure between the isoflurane group and the sevoflurane group, which had the lowest heat loss.
According to the modeling performed by the authors 18, abdominal cavity during the hypertension process may be considered a sphere which will be inflated by a fluid and which is represented by 3 compartments, as shown in figure 4.
Intra-abdominal pressure (IAP) increase may be responsible for a decrease in arterial flow caused by compression of vascular structures which, together with catecholamine-triggered vasconstriction in metabolic response to trauma, cool lower limbs 19. This results from less heated blood transportation from central body regions to the limbs, in addition to hemodynamic changes in muscles, which are also involved in thermogenesis. Venous compression, especially of inferior vena cava, leads to a venous stasis and vasodilation with a consequent lower limb heat loss.
This modeling was confirmed by the thermographic image where it can be seen a more severe heat loss from the abdominal region caudally after increasing intra-abdominal pressure. Since isoflurane is considered the most potent vasodilating agent, promoting hypotension by massive peripheral vascular resistance decrease and capacitance territory increase, with a consequent venous return decrease 20,21, more heat loss is justifiable.
The conclusion was that sevoflurane was the inhalation agentwith less heat loss during increased intra-abdominal pressure.
The same authors are finishing other studies which will allow, via thermodynamic modeling, to simulate human or animal thermal response in increased intra-abdominal pressure procedures, analyzing different physical and surgical variables involved in laparoscopic surgeries, such as: intra-abdominal pressure, maximum abdominal distention radius, temperature of injected fluids and room air, animals exposed surface and heat exchange coefficient.
The autors acknowledge the dedication of the veterinarian Indalécio Sutil, from Biotério Central, Pontifícia Universidade Católica, Paraná and Elizabeth M. Tambara, TSA, M.D., who kindly helped in reviewing the final text.
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Submitted for publication August 15, 2001
Accepted for publication October 20, 2001
* Received from Grupo de Pesquisa em Biotermodinâmica (CNPq). Pontifícia Universidade Católica do Paraná. Laboratório de Lesões Medulares e Trauma Experimental de Curitiba, PR; Trabalho vencedor do Prêmio Carlos Parsloe, Abbot/SBA, 2001