Print version ISSN 0034-7094
Rev. Bras. Anestesiol. vol.51 no.6 Campinas Dec. 2001
Effects of low laryngeal mask cuff pressure on the laryngopharyngeal mucosa of dogs*
Efectos de bajas presiones en el balón de la máscara laríngea en la mucosa faringolaríngea del can
Regina Helena Garcia Martins, M.D.I; José Reinaldo Cerqueira Braz, M.D.II; José Marcos Pechula Moura, M.D.III; Graziela de Araújo Costa, M.D.III; Lídia Raquel de Carvalho, M.D.IV
da Disciplina de Otorrinolaringologia do Departamento de Oftalmologia, Otorrinolaringologia
e Cirurgia de Cabeça e Pescoço da FMB UNESP
IIProfessor Titular do CET/SBA do Departamento de Anestesiologia da FMB - UNESP
IIIDoutorando do Curso de Medicina da FMB UNESP. Bolsista de Iniciação Científica da FAPESP
IVProfessora Doutora do Departamento de Bioestatística do Instituto de Biociências de Botucatu, UNESP
BACKGROUND AND OBJECTIVES:
Injuries to laryngopharyngeal tissues and artery and nerve compression
have been reported and attributed to high laryngeal mask (LM) cuff pressure.
This study aimed at evaluating the laryngopharyngeal mucosa of dogs when in
contact with LM cuff under low pressures and studying ventilatory conditions
METHODS: This study involved 8 mixed-breed dogs anesthetized with pentobarbital and maintained on mechanical ventilation after insertion of a number 4 LM. Pulse rate (PR), mean blood pressure (MBP), inspiratory pressure (IP), end tidal CO2 (PETCO2) and oxygen saturation (SpO2) were evaluated at 0 (control), 30, 60, 90 and 120 minutes after LM insertion. After euthanasia, laryngopharyngeal mucosa regions in contact with LM were biopsed and examined under light microscopy and scanning electron microscopy (SEM).
RESULTS: The attributes studied have shown no significant changes during the experiment but a minor MBP and PETCO2 increase towards the end of the experiment. At light microscopy, laryngopharyngeal epithelium was normal in most examined areas, but in some areas a minor inflammatory reaction with infiltration of polinuclear neutrophils and mild congestion of the subepithelial layer was seen, without significant differences among areas (p < 0.05). At SEM no significant differences in the laryngopharyngeal mucosa was observed.
CONCLUSION: In dogs, low LM cuff pressure (60 cmH2O) is safe for airway patency maintenance and does not affect laryngopharyngeal mucosa.
Key Words: ANIMAL: dog; EQUIPMENTS: laryngeal mask airway
JUSTIFICATIVA Y OBJETIVOS:
Lesiones de la mucosa faringolaríngea y compresiones de vasos y de nervios
han sido relatadas y atribuidas a las altas presiones del balón de la máscara
laríngea (ML). El objetivo de este trabajo fue estudiar en canes la mucosa
faringolaríngea en contacto con el balón de la ML bajo bajas presiones
y evaluar las condiciones ventilatorias durante la anestesia.
MÉTODO: En 8 canes bajo anestesia con pentobarbital fue introducida ML de número 4, manteniéndose la presión en el balón en 60 cmH2O. Los atributos: frecuencia de pulso (FP), presión arterial media (PAM), presión inspiratoria (PI), presión expiratoria final de CO2 (PETCO2) y saturación de pulso de O2 (SpO2) fueron estudiados en 0 (control), 30, 60, 90 y 120 minutos después de la introducción de la ML. Después de eutanasia, se realizaron biopsias en las áreas de contacto de la mucosa faringolaríngea con la ML para examen al microscopio óptico (MO) y electrónico de barredura (MEV).
RESULTADOS: Los atributos estudiados se mantuvieron sin alteraciones significativas durante el experimento, ocurriendo apenas pequeño aumento de los valores de la PAM y de la PETCO2 en los tiempos finales del experimento. Al MO, el epitelio de la mucosa faringolaríngea se presentó sin alteraciones en la grande mayoría de las áreas examinadas, mas, en algunas áreas hubo pequeña infiltración inflamatoria de polimorfonucleares neutrófilos y leve congestión en la camada subepitelial, sin diferencia significativa entre las áreas (p < 0,05). El estudio del MEV también mostró el epitelio de la mucosa laringofaríngea sin alteraciones significativas.
CONCLUSIONES: En los canes, la utilización de presión de 60 cmH2O en el balón de la ML asegura perfecta manutención de la permeabilidad de las vías aéreas y no provoca alteraciones en la mucosa faringolaríngea.
Some studies have highlighted laryngeal mask-induced morbidity, such as vessels1 and nerve2-4 compression and pharyngeal and laryngeal mucosa injuries5, which may determine postoperative odynophagia, dysphagia and dysphonia. Injuries have been attributed to high LM cuff pressures during inflation5 or to nitrous oxide diffusion into the LM cuff6,7.
Ideal air volumes for LM cuffs are not yet totally established, but it is known that previously preconized air volumes8 (for example, 30 ml of air for number 4 LM) are no longer recommended for generating too high pressures; it is prudent, then, to keep cuff pressures around 60 cmH2O. On the other hand, very low pressures may not prevent air leakages or gastric content aspiration during mechanical ventilation10.
The effectiveness of the LM in maintaining airway patency in dogs has been recently shown11.
This study aimed at observing the effects of 60 cmH2O cuff pressure on the laryngopharyngeal mucosa of dogs using light microscopy and scanning electronic microscopy, in addition to evaluating ventilatory conditions during the experiment.
The study was approved by the Animal Research Ethics Committee of our institution and involved 8 adult dogs, 5 males and 3 females, weighing from 14 to 20 kg constituting a single experimental group (Group G1). A previous study has shown that number 4 LM is the best option for animals of this size for effectively adapting to the larynx and promoting a true "seal", without air leakage during mechanical inspiration with inspiratory pressure around 10 15 cmH2O11.
Controlled ventilation was established with a semi-closed circle system using fresh gas flow made up of a mixture of compressed air (1 L.min-1) and O2 (1 L.min-1) in an Ohmeda's (USA) Exel 210 SE anesthesia machine. Tidal volume was always 20 ml.kg-1 and respiratory rate of 10 to 15 movements/minute, trying to maintain end tidal CO2 (PETCO2) between 30 and 35 mmHg.
After a 12-hour fasting, a 20G catheter was inserted in the cephalic vein of the dog. Animals received an intravenous sodium pentobarbital bolus followed by a continuous infusion (3 mg.kg-1. h-1) with an infusion pump (Anne, Abbott, USA). The same professional using a technique adapted from the human technique8 inserted the LM with deflated cuff always. Dogs were placed in the supine position with extended head and tongue totally exposed by external traction of its free portion. LM was inserted with its dorsal portion firmly supported against the hard palate until resistance to its progression was felt. Cuff was inflated with a digital Mallinckrodt's P-V (USA) pressure gage. LM was fixed to animals' mouth. If cuff pressure decreased during the experiment, air was injected in the cuff to reestablish initial pressure (60 cmH2O).
A 20G catheter was introduced in the right femoral artery for invasive blood pressure monitoring. An 18G catheter was inserted in the right femoral vein for lactated Ringer's infusion (5 ml.kg-1) and injection of alcuronium chloride in an initial dose of 0.2 mg.kg-1 and intermittent 0.06 mg.kg-1 doses at every 45 minutes. Pulse rate and oxygen saturation (SpO2) were determined by a forceps-shape sensor placed on the animals' tongue. PETCO2, respiratory rate, tidal volume, inspiratory pressure and O2 inspired fractions were determined with a respiratory module with sample collection between the LM tube and the Y connection of the ventilation system. SpO2, invasive blood pressure and respiratory modules were connected to a Datex-Engstrom's AS 3 (Finland) biomonitor.
Hemodynamic, respiratory and oxygenation parameters were recorded immediately, 30, 60, 90 and 120 minutes after LM insertion.
At the end of the experiment, animals were euthanized with an excessive anesthetic dose and biopsies were performed in 10 predetermined areas in contact with LM (Figures 1 and 2). For light microscopy, specimens were fixed in 2.5% glutaraldehyde and maintained in pH 7.3 by sodium sulphate solution (0.1 M) for at least 24 hours. Then, the material was post-fixed in 2% osmium tetroxide in the same buffer in a dark chamber for one hour. After this period, material was dehydrated in incremental graduations of ethanol. Specimens were dried in a critical point device using liquid carbon dioxide. Specimens were mounted on a metal base with silver glue and golden coated (5 nm thick). For specimen's exam and photographs a Philips' scanning electronic microscope 515 (The Netherlands) was used. Remaining fragments were submitted to a sequence of preparations for light microscopy exams: biopsies of the same areas described for SEM, fixation in 10% formalin for at least 48 hours, inclusion in paraffin, sections by conventional methods and hematoxylineosin dying.
The epithelium (erosion and presence of polimorphonuclear neutrophils PMN) and the subepithelial layer (congestion, hemorrhage and presence of PMN) were analyzed by light microscopy: Histological attributes, except epithelial erosion, were submitted to semi-quantitative analysis using a score from 0 to 3 depending on the involvement of the specimen (absent, mild, moderate or severe). The extension of epithelial erosion was analyzed (0 no erosion, 1 30% erosion, 2 31% to 60% erosion, 3 61% to 100% erosion). Laryngopharyngeal segments of three dogs not submitted to LM insertion were euthanized with excessive sodium pentobarbital and were used as controls (G0).
The same researcher, blind to the analyzed samples, performed light microscopy and SEM evaluations.
Analysis of variance (ANOVA) for repeated measures was used for pulse rate, followed by Tukey's test to compare among moments. Non-parametric Friedman's method was used for remaining variables, followed by Sudent-Newman- Keuls test for multiple comparisons among moments. The comparison among biopsy areas within the group was made using Friedman's non-parametric method for dependent samples. SEM results were studied by a descriptive analysis.
Statistics were considered significant when p < 0.05. Data are shown in mean ± standard deviation of median, with indication of percentiles in 25% and 75%. Scores are presented in medians, with indication of maximum and minimum values.
LM was always introduced in the first attempt obtaining efficient airway in all dogs, with adequate ventilatory parameters confirmed by PETCO2 and inspiratory pressure values (Table I); there has been no air leakage during inspiration or tidal volume or oxygenation decrease, as confirmed by SpO2.
During anesthesia there were no LM displacement, laryngospasm, cyanosis, regurgitation or lung aspiration. After LM removal no blood was seen on the cuff.
Mean blood pressure increased and remained high (approximately 15%) 30 minutes after LM insertion (p < 0.05), without significant changes in pulse rate (Table I).
No epithelial changes were seen at light microscopy, such as erosion or inflammatory infiltration, and the morphological pattern was normal (Figure 3). In some sub-epithelial layers a mild congestion and inflammatory cell infiltrate of polimorphonuclear neutrophils was observed (Table II and Figure 4) without significant differences among areas (p > 0.10). No hemorrhage was detected.
The maintenance of adequate ventilatory and oxygenation parameters throughout the experiment is of great value for evaluating LM ability to maintain an efficient "seal" around dogs larynx, even with low cuff pressures (60 cmH2O) and controlled ventilation. We had already shown in a previous study the efficacy of LM to maintain dog's airway patency during spontaneous ventilation11.
In humans, LM12,13 minimally changes cardiovascular dynamics. In our study, the minor mean blood pressure increase along the experiment may be related to lower values soon after anesthetic induction because a temporary blood pressure decrease is common after anesthetic induction with pentobarbital in dogs, as a consequence of the negative inotropic effect of the drug on the myocardium14.
LM cuff pressure values were kept constant throughout the experiment (60 cmH2O) through continuous monitoring. There were virtually no effects of cuff pressure on the pharyngeal mucosa at light microscopy and SEM.
In a human study, microsensors were placed in different LM cuff sites to determine the pressure on the laryngopharyngeal mucosa during cuff inflation with increasing volumes of air until reaching 40 ml15. Mucosal pressure increased with the increase both of the cuff pressure and its volume, however always maintaining values below 40 cmH2O, which are considered safe for the laryngopharyngeal mucosa, except at the cuff's proximal portion where pressures of 69 cmH2O were recorded with the introduction of 20 ml of air in the LM cuff. Other studies have confirmed the weak relation between cuff volume increase and pressure on the pharyngeal mucosa16,17.
A human study has shown that LM position is not always ideal, even when there is a good function16. This may compress the pharyngeal tissue against rigid structures, such as the hyoid bone and the cervical spine and may explain the compression of the lingual artery and the paralysis of lingual and hypoglossal nerves, diagnosed after LM during anesthesia with nitrous oxide1-4. Brain, more recently, has recommended LM cuff inflation with a volume enough to maintain pressure in 60 cmH2O to minimize complications8. For this author, air leakages during lung inflation are caused by poor LM positioning or inadequate size, which should be as large as possible.
Several studies have investigated whether gas removal of the LM cuff would change the incidence of postoperative laryngopharyngeal complaints18-21. In most studies, LM cuff gas decrease to a minimum to maintain "seal" effective pressure (60 cmH2O) has significantly decreased postoperative odynophagia18-20. In contrast, a different study has shown that different LM cuff pressures did not influence the incidence and intensity of laryngopharyngeal complaints21.
Our study has shown the relative inocuity of LM contact with the laryngopharyngeal mucosa with the use of low pressures. Even with higher pressures, by cuff hyperinflation22 or by nitrous oxide diffusion inside the cuff due to its high diffusion capacity7, no important laryngopharyngeal changes were observed in dogs with just an increase in epithelial desquamation.
It must be highlighted that all factors potentially changing laryngopharyngeal mucosa histology were apparently excluded. The same LM was always used for all dogs and was always inserted by the same researcher. Lubricants were not used to facilitate LM insertion. Anesthetic depth during insertion was adequate for all animals and assured an easy, safe and accurate insertion. The perfect LM positioning in dogs is easier than in humans due to a smaller angle between the mouth and the oropharyngeal cavity and better animal mouth opening.
Laryngopharyngeal histological structure in dogs is very similar to humans23. The epithelium is pavimentous and pluristratified, composed of several cell layers in constant renewal and proliferation. This epithelium is far more resistant to injuries than the ciliated pseudostratified respiratory epithelium covering the trachea.
We concluded that, in dogs, a 60 cmH2O LM cuff pressure provides an adequate airway patency without important effects on the laryngopharyngeal mucosa in contact with the cuff.
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Correspondence to Submitted for publication February 19, 2001 * Received
from Laboratório de Anestesiologia Experimental do Departamento de Anestesiologia
do CET/SBA da Faculdade de Medicina de Botucatu (FMB) da UNESP
Dr. José Reinaldo Cerqueira Braz
Address: Departº de Anestesiologia da FMB - UNESP
ZIP: 18618-900 City: Botucatu, Brazil
Accepted for publication April 27, 2001
Submitted for publication February 19, 2001
* Received from Laboratório de Anestesiologia Experimental do Departamento de Anestesiologia do CET/SBA da Faculdade de Medicina de Botucatu (FMB) da UNESP