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Print version ISSN 0034-7094
Rev. Bras. Anestesiol. vol.54 no.2 Campinas Mar./Apr. 2004
Airflow resistance of shortened tracheal tubes*
Resistencia al flujo de gases en cánulas de intubación traqueal con extensión modelo diminuido
Fernando José Gonçalves do Prado, M.D.I; Joaquim Edson Vieira, TSA, M.D.II; Fábio Ely Martins Benseñor, M.D.III
do CET/SBA do HC da FMUSP
IIAssistente da Divisão de Anestesiologia; Professor Colaborador da Disciplina de Clínica Geral da FMUSP
IIIAssistente da Divisão de Anestesiologia, Supervisor, Unidade de Apoio Cirúrgico (UAC), HC - FMUSP
BACKGROUND AND OBJECTIVES:
Tracheal tube length may be shortened just after the cuff probably without noxious
influence on airflow resistance. This study aimed at determining the effects
of such shortening under different inspiratory flows.
METHODS: Flow resistance was measured in tubes with internal diameters of 7; 7.5; 8; 8.5; 9 and 9.5 millimeters. Measurements were undertaken on standard tubes and on those shortened just after the cuff level. Flows were 0.07 liters per second (L.s-1), 0.1; 0.2; 0.33; 0.5 and 1 L.s-1.
RESULTS: Resistances were progressively lower for larger internal diameters, but were higher for a same diameter under higher flow, both, in standard and shortened tubes. Shortened tubes showed lower or equal flow resistance as compared to standard tubes with the same diameter.
CONCLUSIONS: Resistance has been lower or comparable on shortened tubes as compared to standard tubes.
Key Words: EQUIPMENTS: tracheal tube; MEASUREMENT TECHNIQUES: tracheal tube resistance; TRACHEAL INTUBATION
JUSTIFICATIVA Y OBJETIVOS:
La extensión de la porción de la cánula de intubación traqueal
después del balón de seguridad puede ser reducida, probablemente sin
influencia deletérea en la resistencia al flujo de gases. El objetivo de
este estudio fue determinar los efectos de esa reducción sobre diferentes
regímenes de flujo inspiratorio.
MÉTODO: Fueron realizadas medidas de resistencia flujo de gases en cánulas con diámetros internos de 7, 7,5, 8, 8,5, 9 y 9,5 milímetros. Las medidas fueron realizadas en cánulas con la extensión modelo y con extensión diminuida en la altura del balón de seguridad. Los flujos fueron situados a 0,07 litros por segundo (L.s-1), 0,1; 0,2; 0,33; 0,5 y 1 L.s-1.
RESULTADOS: Las resistencias obtenidas son progresivamente menores para los diámetros internos más grandes, pero más grande para un mismo diámetro sometido a flujo más intenso, en cánulas con extensiones modelo o reducidas. Las cánulas con extensiones reducidas tienen igual resistencia al flujo testado en cánula con igual diámetro interno.
CONCLUSIONES: Las resistencias se mostraron menores o comparables en las cánulas de intubación traqueal con extensión reducida en relación al tamaño patrón (modelo).
Tracheal intubation is not an innocuous anesthetic procedure. It allows controlled ventilation for the most different objectives and may be used in general anesthesia and to treat respiratory failure. Its installation leads to ventilatory mechanics change which also affect cardiovascular, renal and central nervous systems 1-5.
Specific studies on airflow resistance and quantification during its passage through tracheal tubes have shown a direct diameter:length ratio 6-8. Inspiratory flow resistance of tracheal tubes is determined by Poiseuille's Law and factors governing it are internal diameter, length and airflow pressure 9. On the other hand, tracheal tube length after cuff may vary 6 to 8 cm in transparent tubes (Rusch Uruguay Ltda). The shortening of this portion may not negatively influence airflow resistance and may decrease the risks of selective intubation.
This study aimed at determining the effects of shortening most popular tracheal tubes after the cuff under different inspiratory flows.
This study was performed in the Biophysics Laboratory, Discipline of Anesthesiology, Hospital das Clinicas, Universidade de São Paulo (HC-FMUSP). Airflow resistance was measured in tracheal tubes with internal diameter of 7, 7.5, 8, 8.5, 9 and 9.5 mm (Rusch Uruguay Ltda, Montevideo, Uruguay). Measurements were undertaken in standard and shortened tubes. Shortened tubes were cut perpendicularly to their largest axis immediately below distal cuff insertion point. So, standard tubes remained with their lengths, measured in centimeters (cm) between 32.5 (7 and 7.5), 34.5 (8 and 8.5) and 33 (9 and 9.5), while shortened tubes measured 30.5, 32.5 and 31.5 cm.
Flows were generated and maintained by Linea anesthesia machine (Intermed, SP, Brazil). Resistive pressure generated in the tubes was checked by a variable hole pneumotacograph (Bicore CP 100 Respiratory Monitor, Allied Healthcare, CA, USA) with a sensor (Var-Flex® Flow Transducer, Allied Healthcare, CA, USA) connected to the anesthesia machine circle system between the "Y" piece and the tracheal tube. Tracheal tube resistance was measured by the connection of the proximal portion of the tube to the "Y" piece of the ventilator and its distal portion was left opened to the environment, according to previously proposed method described below 7.
Flows were previously gauged (Timeter RT200, Allied Healthcare, CA, USA) and established at 0.07 liters per second (L.s-1), 0.1, 0.2, 0.33, 0.5 and 1 L.s-1 (respectively 4, 6, 12, 20, 30 and 60 liters per minute). Resistive pressure and flow signals were recorded during one minute. Analog data generated by the pneumotacograph were converted to digital in a frequency of 200 Hz and stored in a PC. Tube resistances were obtained by dividing the measurements of five randomly selected pressures by corresponding airflows.
Standard and shortened tubes with the same internal diameter were studied during the passage of different airflows and resistances were compared. Data were submitted to Student's t test considering significant p < 0.05.
Tracheal tube resistances are progressively lower for larger internal diameters, but higher for the same diameter tube submitted to higher flows (Table I). This trend was only not observed in tubes with diameters between 8 and 8.5 mm and flows of 0.2 and 0.07 L.s-1, respectively.
Tracheal tubes resistance is also progressively lower for larger internal diameters, but this resistance was higher for the same diameter submitted to higher flows, without exceptions (Table II).
These trends are best observed when comparing both groups and their resistances. Resistances tend to decrease when tubes are shortened, in spite of non-significant results in some compared measures (Table III). In general, shortened tubes have lower or equal resistance to flow tested in tubes with equal internal diameters.
In our study, resistances were lower or the same in shortened tubes as compared to standard tubes.
Expiratory flow resistance may result in intrinsic-positive end-expiratory pressure (PEEPi), with alveolar distension and ventilatory mechanics worsening 5. Inspiratory flow resistance is different from expiratory flow resistance mostly due to turbulent airflow generated at the junction of the distal portion of the tube and airways during expiration 6. Inspiratory flow is laminar during is travel inside the tube where it is measured, and becomes turbulent only after leaving the tube 8. Experimentally, inspiratory flow is more reproducible than expiratory flow, reason why it was chosen for this study. In addition, for being laminar inside the tracheal tube, inspiratory flow is governed by Poiseuille's Law 9. Fresh airflow and tracheal tubes were selected taking into consideration their applicability in anesthesia.
In a previous Brazilian study, mean tidal volume was 600 ml with respiratory rate of 10 incursions per minute and fresh airflow of 2 L.min-1. A mode of inspiratory: expiratory time ratio of 1:2 was also observed 3. With these data, one may say that the theoretical inspiratory flow used in anesthesia machines may be close to 38 L.min-1 (0.63 L.s-1). For the tested tubes, if this inspiratory pattern was maintained close to 0.5 L.s-1 it would generate increasing resistive patterns with narrower internal diameters 7. Our results with 0.5 L.s-1 flows have shown resistance decrease for all comparisons between standard and shortened tubes.
The use of fresh air without controlled humidity may result in different resistances than those observed during clinical use, which may be higher as a consequence of humidity build-up in tubes lumen 7. This factor was not considered in our experiment due to the interest in determining resistances caused exclusively by standard and shortened tubes, showing the differences caused by simply decreasing their length. Even not finding significant differences between resistances for all comparisons, there is a clear trend for decreased resistances when tubes are shortened for the vast majority of tested airflows.
Tracheal anatomic length may be determined by height. In general, height in centimeters corrected by the factor 0.0846 less 2.32 may estimate tracheal length, from the glottis region to carina bifurcation, with regression value of r = 0.87 (trachea = height x 0.0846 - 2.321 10. This simple calculation suggests a tracheal length of 10.5 to 13 cm for adults between 1.50 to 1.80 m height. Height simulation with this equation has coincided with cadaver measurements performed by the Death Service of São Paulo (Pearson, r2 = 1.00, Table IV).
Tracheal tubes may reach 7.5 cm length at the cuff region. These related figures suggest a safety margin when inserting tracheal tubes which however may not be present in the clinical practice resulting in inadvertent selective intubation. The shortening of standard Rusch tubes object of our investigation, has not determined higher resistance to airflows used in anesthesia. This might be an interesting safety factor to decrease the possibility of selective and inadvertent intubation during anesthesia.
Further clinical trials with shortened tubes and patients with preserved pulmonary function or with chronic obstruction may reveal some impact in anesthetic management. Resistance decrease in ventilation circuits may be advantageous during controlled ventilation.
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Dr. Joaquim Edson Vieira
Av. Dr. Arnaldo, 455 - Sala 2354
01246-903 São Paulo, SP
Apresentado (Submitted) em 12 de
maio de 2003
Aceito (Accepted) para publicação em 30 de julho de 2003.
* Recebido do (Received from) Centro de Ensino e Treinamento em Anestesiologia da Faculdade de Medicina da Universidade de São Paulo, São Paulo, SP