Print version ISSN 0034-7094
Rev. Bras. Anestesiol. vol.54 no.3 Campinas May/June 2004
Volume and pressure of tracheal tube cuffs filled with air or nitrous oxide*
Determinación de volúmenes y presiones de balones de tubos traqueales insuflados con aire ambiente u óxido nitroso
Ernesto Leonardo Cárpio Peña, M.D.I; Waldemar Montoya de Gregori, M.D.II; Luiz Piccinini Filho, M.D.III; Joaquim Edson Vieira, TSA, M.D.IV; Lígia Andrade da Silva Telles Mathias, TSA, M.D.V
IME3 (2002) do CET/SBA
IIAssistente do Serviço de Anestesiologia da ISCMSP
IIIChefe do Serviço de Anestesia, Hospital Santa Isabel, São Paulo
IVCoordenador do Centro para Desenvolvimento da Educação Médica, CEDEM da FMUSP
VDiretora do Serviço e Disciplina de Anestesiologia da ISCMSP e Faculdade de Ciências Médicas da Santa Casa de São Paulo; Responsável pelo CET/SBA da ISCMSP
BACKGROUND AND OBJECTIVES: Tracheal tube
cuff pressure against tracheal wall should to prevent air leaked or pulmonary
aspiration, allowing adequate capillary blood flow. This study aimed at determining
pressure variations of tracheal tube cuffs filled with air or 100% nitrous oxide.
METHODS: Participated in this study 30 patients submitted to balanced general anesthesia with tracheal intubation. Cuff was inflated according to clinical criteria. The intracuff pressure measurements were undertaken after 15 minutes of anesthesia by means of an aneroid manometer gaged in cm of H2O and have provided initial pressure and volume values. The patients was allocated in two groups: cuff inflated with air, group A, and with nitrous oxide, group B. Measurements were recorded during one hour and results were compared. Groups were homogeneous in gender and age.
RESULTS: Both groups have shown baseline pressure values close to 40 cmH2O with 8 ml of volume. Group filled with air had pressures increased up to 36 cmH2O in one hour. Group filled with nitrous oxide had pressures decreased below 20 cmH2O after 20 to 30 minutes of anesthesia.
CONCLUSIONS: 100% nitrous oxide to fill tracheal tube cuffs is not a safe method resulting in progressive air leakage. The cuff air promotes volume and pressure increase, with higher chances for tracheal mucosa injury.
Key Words: ANESTHETICS, Gaseous: nitrous oxide; EQUIPMENTS: tracheal tube; MEASUREMENT TECHNIQUES: cuff pressure; TRACHEAL INTUBATION
JUSTIFICATIVA Y OBJETIVOS: La presión
ejercida por el balón del tubo traqueal contra la pared de la traquea debe
permitir flujo capilar adecuado y prevenir escapes de aire o aspiración
pulmonar. Esta pesquisa buscó determinar las variaciones de presión
del balón insuflado con aire ambiente o con óxido nitroso a 100%.
MÉTODO: Treinta pacientes fueron seleccionados para recibir anestesia general balanceada con entubación orotraqueal. El balón fue insuflado conforme criterios clínicos. Las medidas de base fueron realizadas después de 15 minutos del inicio de la anestesia con un manómetro aneróide calibrado en cm de H2O y dieron los valores iniciales de presión y volumen. Los pacientes fueron divididos en dos grupos: balón reinsuflado con aire ambiente, grupo A, o con óxido nitroso, grupo B. Las medidas de presión fueron obtenidas en intervalos hasta la primera hora y los resultados comparados.
RESULTADOS: Los grupos se mostraron comparables para edad y sexo. En ambos grupos los valores basales medios para presión fueron próximos de 40 cmH2O con 8 ml de volumen. En el grupo con aire ambiente, las presiones aumentaron hasta 36 cmH2O en una hora. En el grupo de balón insuflado con N2O, las presiones cayeron abajo de 20 cmH2O entre 20 y 30 minutos de anestesia.
CONCLUSIONES: El uso de N2O a 100% para insuflación del balón de sonda traqueal no constituye método seguro, acarreando progresiva pérdida de la capacidad de vedación. El uso de aire ambiente promueve aumento de volumen y de presión en el balón, aumentando la posibilidad de lesión de la mucosa traqueal.
General anesthesia presupposes airway control and very often tracheal intubation is the most effective method. Tracheal tubes may or may not have cuffs, which are more frequently used in adults. Specific cuff function is to seal trachea, helping positive pressure ventilation and preventing gastric content aspiration 1.
Tracheal intubation-associated morbidity is attributed to factors such as tube size, tracheal mucosa injury, tube movement and accidental extubation. Tracheal mucosa injury directly related to the cuff is influenced by intubation duration and lateral tracheal wall pressure, being the latter the most important factor 2-4. Tracheal mucosa perfusion pressure is 30 to 40 mmHg 5,6 and higher cuff induced pressures increase the incidence of tracheal injury. Lower pressures minimize tracheal damage by decreasing lateral tracheal wall pressure, but may result in aspiration, leakage and tube displacement. So, tracheal tube cuff pressure against lateral tracheal wall should be low enough to allow adequate capillary blood flow, and high enough to prevent air leakage and gastric content aspiration.
It is recommended that cuff pressure remain between 25 and 34 cmH2O. Pressures above 25 cmH2O safely prevent gastric content aspiration and 20 cmH2O is the lowest limit below which there may be broncho-aspiration. Pressures above 34 cmH2O would impair tracheal mucosa perfusion 5-8. Several factors affect cuff pressure, among them tracheal tube type and size, cuff volume and the mixture of gases administered to patients. N2O in the inhaled mixture leads to its diffusion to inside the cuff (especially when it is inflated with air), causing cuff pressure increase which is directly proportional to mixture administration time and inversely proportional to cuff thickness 2,7.
This study aimed at evaluating cuff pressure variations in different surgical times, when it is inflated with air or 100% nitrous oxide.
After the Medical Ethics and Research Committee, Hospital Central, Irmandade da Santa Casa de Misericórdia, São Paulo approval, participated in this study 30 patients of both genders, aged 20 to 50 years, physical status ASA I and II, to be submitted to different elective surgeries after adequate fast and with body mass index not exceeding 25.
All patients were submitted to balanced general anesthesia (O2 and N2O 50%:50%, isoflurane, pancuronium and fentanyl) with tracheal intubation (adequate tracheal tube with high volume and low pressure cuff). Tracheal tube was chosen according to subjective criteria after glottis visualization by direct laryngoscopy. Cuffs were inflated by the anesthesiologist with randomized volumes according to clinical criteria to seal airways. Measurements were undertaken after 15 minutes of anesthesia. First, oral cavity, pharynx and stomach secretions were sucked and then a 3-way tap was connected to pilot tube cuff, to a 10 ml glass syringe and to an aneroid manometer (Marshalltown MPN Inc, Iowa, USA) gagged in cmH2O.
A first baseline measurement (PB) of the air-inflated cuff was undertaken and, smoothly sucking with the glass syringe, the necessary volume (VB) to reach this measurement (PB) was determined. This first measurement has provided pressure and volume values obtained by inflation according to clinical parameters. As from this moment, patients were divided in two groups.
In Group A, cuff was re-inflated with enough air to reach approximately 30 cmH2O (V0, P0) and moment zero (T0) was determined. Then cuff pressure was measured at 5, 10, 20, 30, 40, 50 and 60 minutes (M5, M10, M20, M30, M40, M50, M60,). In group B, cuff was re-inflated with enough nitrous oxide to reach 30 cmH2O and pressures were measured in the same moments.
Nitrous oxide was directly collected from an old KT 675 anesthesia machine without safety valve for low O2 concentrations administration and no longer used for clinical anesthesia. Respiratory system was washed with 100% nitrous oxide for 2 minutes to then allow nitrous oxide aspiration in a 10 ml syringe through a tap series system, which prevented gas leakage and at the same time allowed cuffs inflation.
In both groups, pressures could not exceed 35 cmH2O or be lower than 20 cmH2O. For such, cuffs should be deflated or re-inflated as needed to maintain their sealing ability. At this intervention, the experiment was interrupted.
Non-Paired t test and Analysis of Variance (ANOVA) were used for statistical analysis considering significant p < 0.05.
Demographics data were similar for both groups (Table I). For both groups 7.5 - 8 mm (internal diameter) tracheal tubes were used for females and 8 - 9 mm tubes were used for males.
First inflation (baseline) volumes (VB) and pressures (PB) were similar between groups, however some pressures have reached 50 cmH2O (Table II). Cuff volumes needed to reach 30 ± 1 cmH2O baseline pressure (T0) are shown in table III for the air inflated cuff group, and in table IV for nitrous oxide inflated cuff group. Pressures measured during one hour can also be observed.
In Group A (Air), there have been significant cuff pressure differences (p < 0.0001, ANOVA) with progressive cuff pressure increase (Table III). In Group B (N2O) there has been progressive cuff pressure decrease in all cases, reaching values close to 20 cmH2O, especially in moments between 20 and 30 minutes. As established by the protocol, as soon as pressures reached values below 20 cmH2O, the experiment was interrupted. ANOVA has shown significant differences among different times (p < 0.0001), with progressive cuff pressure decrease during the study period.
Results suggest that 100% N2O for tracheal tube cuff inflation may determine loss of sealing ability; otherwise, the air promotes cuff pressure and volume increase.
Most severe tracheal intubation (TI) complications include ulceration, tracheomalacia or tracheal stenosis, all related to prolonged intubation and high cuff pressures. Tracheal tubes with pressure in the range of 26 to 35 cmH2O allow tracheal perfusion in addition to effective sealing airways, thus decreasing the incidence of such complications 7,8. In addition, pressure should remain above 20 cmH2O to decrease aspiration risks 5-8.
Our study has found initial high pressures and volumes (Table II). This could be explained by anesthesiologists' clinical practice in handling cuffs and possibly by the lack of devices able to measure cuff pressures. This practice has promoted cuff inflation according to clinical criteria, trying to assure airway sealing. However, pressure values could remain above safe tracheal mucosa perfusion limits.
Cuff inflation with mean 6.1 ml air has been enough to reach the desired pressure of up to 30 cmH2O. However with time and N2O diffusion in the inhaled mixture, pressure has increased and has reached maximum limit in approximately 40 or 50 minutes, time that could be recommended as optimal for partial cuff deflation. The removal of approximately 1 ml may decrease pressure to baseline values 3,5. Cuff inflation with air has shown to be a risk practice, in addition to forcing the anesthesiologist to continuously measure cuff pressure during anesthesia. Deflation may be not so accurate, but on the other hand the maintenance of high levels may lead to complications, especially tracheal mucosa injury.
Nitrous oxide diffusion to inside the cuff has already been documented 9. Pressure control as a consequence of this diffusion is suggested as a factor to decrease high pressure risks and, as a consequence, tracheal injury 10. Low pressure cuffs, such as Lanz' or Brandt's tubes, which have devices for intra-cuff volume and pressure auto-controlled, seem to be safe for prolonged intubation 11-13.
Increased cuff volume during general anesthesia may be minimized by using the same gases mixture administered to patients to fill cuff volume 14. Clinical nitrous oxide use, when established in 67% of the inhaled mixture, may have its diffusion to the cuff decreased when it is filled with 40% to 50% mixture in air 15,16. In addition to the risks of decreasing tracheal mucosa perfusion pressure, cuff pressure may favour the development of angina. Saline solution may eliminate mixture gas diffusion, thus decreasing the incidence of such complication 17. On the other hand, lidocaine as solution to fill the cuff may be reported as possible factor for lower incidence of angina 18.
Cuff inflation with 100% nitrous oxide showed a decreasing behavior for initial pressures when general anesthesia was maintained with 50% mixture of oxygen and nitrous oxide. There has been fast decrease in approximately 10 minutes, reaching values even lower than 20 cmH2O in the period 20 to 30 minutes. Nitrous oxide to inflate cuffs seems to be still a controversial subject and its use at 100% may not be a practical method for routine use in anesthesia because in our practice, a special source is needed for 100% nitrous oxide supply.
Our results suggest that 100% N2O for tracheal tube cuff inflation is not a safe method due to its progressive loss of sealing ability. Air, on the other hand, promotes cuff volume and pressure increase, increasing the chances for low tracheal mucosa perfusion. Both results were obtained for anesthesia using 50% oxygen and nitrous oxide.
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Profª. Dra. Lígia Andrade da Silva Telles Mathias
Alameda Campinas, 139/41
01404-000 São Paulo, SP
Submitted for publication May 12, 2003
Accepted for publication August 18, 2003
* Received from CET/SBA do Serviço de Anestesiologia da Irmandade da Santa Casa de Misericórdia de São Paulo (ISCMSP)