Evaluating the use of the tobin index when weaning patients from mechanical ventilation after general anesthesia

Abstracts

JUSTIFICATIVA E OBJETIVO: O índice preditivo de descontinuação da ventilação mecânica é parâmetro de prognóstico de desmame. O objetivo deste estudo foi avaliar a aplicação do índice de Tobin antes da extubação traqueal em pacientes submetidos à anestesia geral. MÉTODO: Foram analisados 80 pacientes, 40 (50%) homens e 40 (50%) mulheres submetidos a colecistectomia sob anestesia geral. A média de idade foi 57,7 ± 12,4 (33 a 82 anos) e a média do peso corpóreo foi 70,85 ± 11,07 (48 a 90 kg). Trinta e sete (46,2%) pacientes eram tabagistas. O índice de Tobin foi obtido pela relação entre freqüência respiratória e volume corrente em ciclos/litro/minuto (c.L-1.min-1). Foram estudadas as variáveis: idade, peso, tabagismo, risco anestésico-cirúrgico, duração do procedimento, volume corrente expirado e freqüência respiratória pós-operatória, tempo de permanência na recuperação pós-anestésica, ocorrências clínicas intra-operatórias e gravidade das ocorrências clínicas após extubação. Foram utilizados testes estatísticos paramétricos e não-paramétricos, e análise de regressão logística univariada e múltipla. RESULTADOS: A idade (p = 0,019), peso corpóreo (p = 0,006), tabagismo (p = 0,014), risco anestésico (p = 0,0001), volume corrente (p < 0,0001), freqüência respiratória (p < 0,0001), tempo de permanência na recuperação pós-anestésica (p < 0,0001) e ocorrências clínicas pós-extubação (p < 0,0001) influenciaram de forma significativa o índice de Tobin. As variáveis independentes com risco de insucesso do desmame ventilatório foram idade avançada e o peso corpóreo elevado (p = 0,023). CONCLUSÕES: O desmame ventilatório pós-operatório com índice de Tobin > 80 c.L-1.min-1 e < 100 c.L-1.min-1 apresentou risco elevado de ocorrências clínicas pós-extubação traqueal. Nos pacientes com índice de Tobin > 80 c.L-1.min-1, a idade avançada e o peso corpóreo elevado constituíram variáveis independentes de risco para o insucesso do desmame da ventilação mecânica.

CIRURGIA, Abdominal; COMPLICAÇÕES; VENTILAÇÃO

SCIENTIFIC ARTICLE

Evaluating the use of the Tobin index when weaning patients from mechanical ventilation after general anesthesia^*

Evaluación de la aplicación del índice de tobin en el destete de la ventilación mecánica después de la anestesia general

Nara de Cássia Mantovani^I; Lúcia Maria Martins Zuliani^II; Daniela Tiemi Sano^II; Daniel Reis Waisberg^III; Israel Ferreira da Silva, TSA^IV; Jaques Waisberg^V

^IFisioterapeuta; Encarregada do Serviço de Fisioterapia do Hospital Nossa Senhora de Fátima da Sociedade Portuguesa de Beneficência de São Caetano do Sul

^IIGraduanda, Faculdade de Medicina do ABC

^IIIGraduando, Faculdade de Medicina da Universidade de São Paulo

^IVDiretor do Serviço de Anestesiologia e Co-Responsável pelo CET-SBA do Hospital do Servidor Público Estadual de São Paulo Francisco Morato de Oliveira

^VProfessor Assistente, Disciplina de Cirurgia do Aparelho Digestivo, Faculdade de Medicina do ABC

^{Correspondence to}

SUMMARY

BACKGROUND AND OBJECTIVES: The predictive index for discontinuing mechanical ventilation is a prognostic parameter. The objective of this study was to evaluate the use of the Tobin index before tracheal extubation in patients undergoing general anesthesia.

METHODS: Eighty patients, 40 (50%) men and 40 (50%) women undergoing cholecystectomy under general anesthesia were analyzed. The mean age of the patients was 57.7 ± 12.4 (33 to 82 years) and mean body weight was 70.85 ± 11.07 (48 to 90 kg). Thirty-seven patients (46.2%) were smokers. The Tobin index was obtained through the relationship between respiratory rate and tidal volume in cycles/Liter/minute (c.L^-1.min^-1). Age, weight, smoking, anesthetic-surgical risk, duration of the procedure, expired tidal volume, and postoperative respiratory rate, length of time in the recovery room, intraoperative clinical intercurrences and severity of the clinical intercurrences after extubation were studied. Parametric and non-parametric statistical tests, and analysis of univariate and multiple logistic regression were used.

RESULTS: Age (p = 0.019), body weight (p = 0.006), smoking (p = 0.014), anesthetic risk (p = 0.0001), tidal volume (p < 0.0001), respiratory rate (p < 0.0001), length of time in the recovery room (p < 0.0001), and post-extubation clinical intercurrences (p < 0.0001) had a significant influence in the Tobin index. The independent variables with risk of failure to wean included advanced age and elevated body weight (p = 0.023).

CONCLUSIONS: Weaning patients off mechanical ventilation in the postoperative period with a Tobin Index > 80 c.L^-1.min^-1 and < 100 c.L^-1.min^-1 was associated with elevated risk of postoperative clinical intercurrences after extubation. Patients with a Tobin index > 80 c.L^-1.min, advanced age and increased body weight were independent variables of risk of failure to discontinue mechanical ventilation.

Key Words: COMPLICATIONS: tracheal extubation; VENTILATION: mechanical controlled; SURGERY, Abdominal: cholecystectomy.

RESUMEN

JUSTIFICATIVAS Y OBJETIVOS: El índice de predicción de descontinuación de la ventilación mecánica es parámetro de pronóstico de destete. El objetivo de este estudio fue evaluar la aplicación del índice de Tobin antes de la extubación traqueal en pacientes sometidos a la anestesia general.

MÉTODO: Fueron analizados 80 pacientes, 40 (50%) hombres y 40 (50%) mujeres sometidos a colecistectomía bajo anestesia general. El promedio de edad fue 57,7 ± 12,4 (33 a 82 años) y la media del peso corporal fue 70,85 ± 11,07 (48 a 90 kg). Treinta y siete (46,2%) pacientes eran tabaquistas. El índice de Tobin se obtuvo por la relación entre frecuencia respiratoria y volumen corriente en ciclos/litro/minuto (c.L^-1.min^-1). Fueron estudiadas las variables edad, peso, tabaquismo, riesgo anestésico-quirúrgico, duración del procedimiento, volumen corriente expirado y frecuencia respiratoria postoperatoria, tiempo de permanencia en la recuperación post anestésica, incidencias clínicas intraoperatorias y gravedad de las incidencias clínicas después de la extubación. Fueron utilizados pruebas estadísticas paramétricos y no paramétricos, y análisis de regresión logística variada y múltiple.

RESULTADOS: La edad (p = 0,019), peso corpóreo (p = 0,006), tabaquismo (p = 0,014), riesgo anestésico (p = 0,0001), volumen corriente (p < 0,0001), frecuencia respiratoria (p < 0,0001), tiempo de permanencia en la recuperación post anestésica (p < 0,0001) e incidencias clínicas pos extubación (p < 0,0001) influyeron significativamente en el índice de Tobin. Las variables independientes con riesgo de fracaso del destete por ventilación fueron edad avanzada y el peso corpóreo elevado (p = 0,023).

CONCLUSIONES: El destete por ventilación postoperatorio con índice de Tobin > 80 c.L^-1.min^-1 y < 100 c.L^-1.min^-1 presentó riesgo elevado de incidencias clínicas después de la extubación traqueal. En los pacientes con índice de Tobin > 80 c.L^-1.min^-1, la edad avanzada y el peso corpóreo elevado constituyeron variables independientes de riesgo para el fracaso del destete de la ventilación mecánica.

INTRODUCTION

In most patients the transition from mechanical to spontaneous ventilation can be done without important complications. However, in patients who experience breathing difficulties during the discontinuation of mechanical ventilation, this process can be challenging ^1,2.

The successful discontinuation of mechanical ventilation implies a reduction in complications caused by the reduced time patients remain under mechanical ventilation and decreases the length of stay in intensive care units: besides it avoids re-intubation difficulties due to possible complications of nasal and tracheal intubation ^3-5.

Tobin et al. ⁶ proposed a practical, effective, and non-invasive method to obtain the predictive index of success or failure of discontinuing mechanical ventilation. This index was called index of rapid and superficial respiration (IRS or Tobin index) and establishes the physiological relationship between the respiratory rate (f) and expired tidal volume (V_T) expressed in L.min^-1.

This index is based on the fact that better complacency and greater inspiratory effort associated with adequate gas exchange and reduced respiratory rate, are associated with greater probability of maintaining spontaneous ventilation indefinitely ^1,6. The same authors demonstrated that patients who failed discontinuation of mechanical ventilation presented a characteristic respiratory pattern observed immediately after disconnecting it, called rapid (f greater than 30 breaths per minute) and superficial (V_T smaller than 0.3 L) ventilation index. Values higher than 100 cycles.Liter^-1.minute^-1 (c.L^-1.min^-1) were associated with failure of discontinuing mechanical ventilation in 86% and success in 10% of the patients ^7-10.

Good reproducibility of classical weaning parameters with similar bedside techniques was demonstrated, especially when indexes that estimate the ventilatory pattern using V_T and respiratory rate, such as the Tobin index, which is deemed to be the most accurate to predict the prognosis of wean ^11-13, were evaluated. Although the accuracy of this index is better than the other extubation tests frequently used ^14-19, 20% of false positive results were observed, i.e., the relationship f/V_T predicted a successful extubation but the patient could not maintain adequate ventilatory autonomy and had to be re-intubated, or presented clinical complications related with the extubation process ^20-24.

The objective of this study was to evaluate the use of the Tobin index with values between 80 and 100 c.L^-1.min^-1 or lower than 80 c.L^-1.min^-1 before extubation in patients undergoing general anesthesia.

METHODS

This study was approved by the Ethics Committee on Research of the Hospital Nossa Senhora de Fátima da Sociedade Portuguesa de Beneficiência de São Caetano do Sul. Patients signed an informed consent after it was properly explained to them.

In this prospective study, 80 patients were analyzed, 40 (50%) males and 40 (50%) females admitted for elective conventional cholecystectomy for the treatment of uncomplicated biliary lithiasis under general anesthesia from June 2004 to January 2005. Every procedure was performed by the same surgical and anesthesiology teams. All patients underwent general balanced anesthesia preceded by the administration of 7.5 mg or 15 mg of midazolam one hour before anesthetic induction. The anesthetic technique consisted of propofol (2 mg.kg^-1), fentanyl (5 to 10 µg.kg^-1), and atracurium (0.4 mg.kg^-1), followed by tracheal intubation and administration of adequate alveolar concentrations of isoflurane, as a mixture of oxygen and nitrous oxide, with an inspired fraction of 50% to 70% of the later, for maintenance of a satisfactory anesthetic plane. Additional doses of atracurium (1/4 of the initial dose) were administrated to maintain the neuromuscular blockade, and its reversion was achieved with the administration of atropine (1 mg) and neostigmine (2 mg) at the end of the procedure. Postoperative analgesia was complemented with intravenous dypirone (30 mg.kg^-1) and ketoprofen (100 mg) in the presence of pain equal o greater than five by the Visual Analogue Scale. Procedures and agents administered during the induction, maintenance and recovery of the patient after discontinuing the anesthetics were the same for every patient in the study.

A longitudinal, median incision was used in all patients. Intraoperative cholangiography was performed in 20 (25%) patients, and was normal in all of them. Percutaneous drainage of the peritoneal cavity was not used.

The inclusion criteria were: patients undergoing elective surgeries with general anesthesia and tracheal intubation, suppression of sedation and neuromuscular blockade in the immediate postoperative period, hemodynamic stability, absence of sepsis, and axillary temperature equal or lower than 37°C. Exclusion criteria included: Tobin index below 100 c.L^-1.min^-1, presence of stroke, chronic obstructive pulmonary disease (COPD), atelectasis, or pleuropulmonary alterations in the PA and lateral chest X-rays, patients with tracheostomies, with Glasgow coma scale equal or lower than seven, and bronchospasm at the time of wean. The presence of COPD was determined by the results of the clinical exam, pulmonary function tests, chest X-rays, and arterial blood gases.

Of the patients included in this study, 43 (53.8%) were non-smokers and 37 (46.2%) were smokers. Smokers were instructed to stop smoking when the need of surgery was detected.

Patients were classified according to the operative risk as low or elevated risk. Patients with low operative risk included the ones who received a score of I or II of the American Society of Anesthesiologists (ASA) scale, whereas elevated risk included those with scores of III or IV of the ASA.

The criteria observed by the anesthesiologist just before extubation included: ability to breath spontaneously, presence of protective reflexes of the airways, ability to obey simple commands, hemodynamic stability, oxygen saturation greater than 90% with a inspired oxygen fraction of 0.21, absence of residual neuromuscular blockade determined by the peripheral nerve stimulator, maximal inspiratory pressure below -25 cm H₂O, VE below 7 mL.kg^-1,vital capacity below 10 mL.kg^-1, Tobin index below 100 c.L^-1.min^-1, ratio PaO₂/FiO₂ greater than 200, and PaCO₂ below 40 mmHg. Residual neuromuscular blockade was indicated by the presence of diplopia or dysphagia, incapacity to protrude the tongue and to maintain the hand closed or head elevated for, at least, five seconds ^25,26.

Wean was defined as the process of gradually taking the patient off ventilatory support ^18,19. At the end of the surgery, and with the patient awake when called or completely awake, the wean was initiated in the surgery room. The patient was placed in horizontal dorsal decubitus with the dorsum at a 15° elevation, and with 100% oxygen to maintain oxygen saturation (SatO₂) equal or greater than 95%. Controlled mechanical ventilation was changed to synchronized intermittent mandatory ventilation (SIMV) followed by continuous positive airways pressure (CPAP). Some patients were changed from controlled ventilation directly to CPAP, without the intermediate step of SIMV. The patient remained in those conditions until he/she achieved spontaneous ventilation at a rate of 12 to 16 breaths per minute to allow extubation. After this period, the ventilometer (Wright Respirometer, Ferrais Mark 8 model) was connected to the cannula of the patient or to the expiratory valve of the ventilator of the anesthesia device for a minute, and the measurements after significant efforts were excluded, such as those obtained during tracheal aspiration and coughing, and determining the VE, in liters, and f, in breaths per minute (bpm), were recorded as indicated by the capnograph. With those values, the formula f/VE (L.min^-1) was applied to obtain the Tobin index (f/Vt). This index was considered satisfactory to begin wean procedures when below 100 c.L^-1.min^-1. Values greater than 100 c.L^-1.min^-1 were considered an indication of extubation with unsatisfactory results.

Patients were divided in two groups: Group I, with a Tobin index below 80 c.L^-1.min^-1 (low Tobin index), and Group II, with a Tobin index equal or above 80 c.L^-1.min^-1 and below 100 c.L^-1.min^-1 (elevated Tobin index).

Besides the parameter obtained by the Tobin index, the following criteria for failed extubation were observed: respiratory rate below 35 bpm, oxygen saturation below 95%, heart rate above 140 beats per minute (bpm), systolic blood pressure above 180 mmHg or below 90 mmHg, and the presence of severe sudoresis or pain. In the presence of any one of those criteria, controlled mechanical ventilation was reinstituted.

After extubation, the patient remained in the recovery room for a variable period, where opened oxygen mask with a flow of 3 L.min^-1 was place, and the patient was discharged from the recovery room after achieving a score of 10 in the Aldrete-Kroulik scale.

During wean procedures, data regarding the diagnosis, clinical evaluation, ventilator parameters, and Tobin index were recorded on the evaluation form. With those data, the ratio f/V_T was applied, and a value below 100 c.L^-1.min^-1 was considered satisfactory.

Extubation was considered successful when the patient did not develop, in the immediate post-extubation period, clinical intercurrences, such as sudoresis, respiratory discomfort with or without changes in arterial blood gases, and cardiovascular changes with or without hemodynamic repercussions. If the patient developed any one of them, extubation was not considered successful.

The following variables were studied: age, body weight, smoking history, preoperative anesthetic-surgical risk as determined by the ASA classification, duration of the procedure, tidal volume and respiratory rate at the end of the surgery, Tobin index, length of stay in the recovery room. intraoperative clinical occurrences, and clinical occurrences and their severity after extubation.

Inexpressive respiratory disturbances were deemed mild clinical occurrences; cardiovascular changes without hemodynamic repercussions or respiratory disturbances without changes in arterial blood gases were considered moderate. Respiratory changes with severe discomfort or cardiovascular events with hemodynamic repercussions were classified as severe.

Qualitative variables are presented as relative and absolute values, and quantitative variables as central tendency and dispersion values. The chi-square test was used to determine the association between the Tobin index and the qualitative variables. For quantitative variables, the homogeneity of variances was determined by the Lavene test and the normality by the Kolmogorov-Smirnov test. The parametric test (t test) to compare study and control groups was used for the variables that demonstrated adequate homogeneity and normality. In the absence of normality and homogeneity, the non-parametric test (Mann-Whitney U test) was used. Significant variables by the univariate logistic regression were analyzed by the multiple logistic regression. In the logistic regression model, the Tobin index below 80 c.L^-1.min^-1 or equal or greater than 80 c.L^-1.min^-1 and below 100 c.L^-1.min^-1 were used as dependent variables, and, as independent variables, those that were statistically significant by the univariate logistic regression analysis. A level of 5% was considered significant.

RESULTS

The mean age was 57.7 ± 12.4 years (33 to 82 years); and the mean body weight was 70.85 ± 11.07 kg (48 to 90 kg). In Group I, the mean age was 54.50 ± 11.65 years (33 to 82) and the mean weight was 67.48 ± 11.03 (48 to 90 kg). In Group II, the mean age was 60.97 ± 12.44 years (33 to 82) and mean weight was 74.23 ± 10.15 kg (52 to 90 kg).

Group I, with a Tobin index below 80 c.L^-1.min^-1 (low Tobin index), and Group II, with a Tobin index equal or greater than 80 c.L^-1.min^-1 and below 100 c.L^-1.min^-1 (elevated Tobin index), showed significant differences regarding the following variables: age, body weight, smoking, anesthetic risk (ASA), tidal volume, respiratory rate, Tobin index, period in the recovery room, and post-extubation clinical intercurrences. Variables that showed no significant differences included duration of the procedure, intraoperative clinical intercurrences, and severity of the postoperative clinical intercurrences (Table I).

In the intraoperative period, nine (11.25%) patients, four (10%) in Group I and five (12.5%) in Group II, developed bronchospasm controlled by medication. This difference was not statistically significant (p = 0.72).

Forty-one (51.2%) patients had post-extubation clinical intercurrences divided as follows: 26 (63.4%) with tachypnea, 6 (14.6%) with respiratory failure, 5 (12.2%) with cardiac arrhythmia, and 4 (9.8%) with bradypnea. Thirty-nine patients (48.8%) did not experience those intercurrences. In Group I, 30 patients (75%) did not develop post-extubation intercurrences while 10 (25%) did so. In Group II, 31 (77.5%) patients developed post-extubation clinical intercurrences and nine (22.5%) did not. The number of post-extubation intercurrences in Group II was significantly greater than in Group I (p < 0.0001). Patients who developed clinical intercurrences did not need to be reintubated.

Thirty (37.5%) patients developed post-extubation intercurrences classified as mild, 5 (6.3%) moderate, and 6 (7.5%) severe. In Group I, 10 patients (25%) developed clinical intercurrences while in Group II 32 (80%) did so. In Group I, 10 intercurrences were classified as mild. In Group II, 20 (64.5) patients developed mild intercurrences, 5 (16.1%) moderate, and 6 (19.4%) severe. This difference showed a tendency to be significant (p = 0.08).

The mean Tobin index was 62.0 ± 26.7 c.L^-1.min^-1 (16 to 100 c.L^-1.min^-1). In Group I, the mean Tobin index was 37.66 ± 14.30 (16 to 69.3 c.L^-1.min^-1), and in Group II it was 86.31 ± 4.95 c.L^-1.min^-1 (80 to 100 c.L^-1.min^-1). Those values showed statistically significant differences (p < 0.001).

Univariate logistic regression analysis was significant and the risk of failure of wean for age, body weight, and smoking history. As for the other variables, the univariate logistic regression analysis was not significant (Table II).

Multiple logistic regression analysis of age and body weight was significant for failure of wean. Regarding the association of age, body weight, and smoking or the association of age, weight, and respiratory rate, the multiple logistic regression analysis was not significant for failure of wean (Table II).

DISCUSSION

While in the medical practice most patients undergoing surgeries under balanced general anesthesia are easily extubated, some of the patients remain in prolonged respiratory wean ^20,21.

None of the current extubation criteria is considered an absolute predictor, but directives for safe extubation, indicating the probability of success or failure ^22-24.

The respiratory pattern (frequency and tidal volume) as a guide for the evolution and prognosis of wean has called attention. The main advantages of those measurements include the ease estimation at the bedside with a simple device (ventilometer), the low operational cost, and are easily reproducible. Failure to wean is attributed to the imbalance between the capacity of the respiratory muscles and respiratory demand. The relationship between respiratory rate and tidal volume (superficial breathing index) during spontaneous breathing increases in the presence of this imbalance, and this relationship can predict the success or failure of wean ^6-9.

The Tobin index can be easily calculated by the anesthesiologist, nurse, or respiratory therapist in approximately 10 minutes, and it does not need any laboratorial evaluation. This index can be useful to reduce the incidence of premature attempts to wean from mechanical ventilation and to eliminate the delay to wean patients potentially capable of breathing on their own. More importantly, the components to calculate the Tobin index can be used to indicate to the medical team the presence of problems that should be corrected to facilitate the weaning process.

In the present study, we observed that the Tobin index, as a parameter for removal of mechanical ventilation, could be measured and the method was safe, well accepted by patients, and did not have associated complications.

Anesthetics normally influence the Tobin index due to their respiratory depressor effect, and can change the respiratory rate and tidal volume ^26-30. In our study, the anesthetic agents did not interfere with the Tobin index because, at the time of its measurement, the anesthetic drugs were present only in residual amounts, it was done after administration of an antagonist of the neuromuscular blockade, and wean was carried on at a time when the serum levels of those agents were minimal. In this setting, the effects of those drugs on respiratory rate and tidal volume were minimized. As long as there are no baseline diseases, such as primary neurological or neuromuscular disorders, structural abnormalities of the chest, hyperinsufflation ^26,27,31-33, or even metabolic changes that might interfere with the mechanics of ventilation, the Tobin index, in the conditions of the present study, was not influenced by the anesthetic agents at the time it was measured, i.e., at the end of the surgery, when patients were receiving 100% oxygen, to maintain oxygen saturation (SatO₂) equal or greater than 95%, and when the patient presented spontaneous respiration at a frequency of 12 to 16 breaths per minute.

A Tobin index below 100 c.L^-1.min^-1 is one of the parameters for tracheal extubation of patients who underwent general anesthesia. In the present study, we attempted to evaluate the post-extubation clinical intercurrences related to values of the Tobin index between 80 and 100 c.L^-1.min^-1 and below 80 c.L^-1.min^-1.

Emmerich et al. ¹⁹ observed that the efficacy of the Tobin index was lower than expected, demonstrating that it was as sensitive or more sensitive and/or not as specific than other predictive indexes. However, their study consisted of patients with acute respiratory failure on prolonged mechanical ventilation in intensive care. Thus, those patients, due to the nature of their conditions, had severe morbidities that led to respiratory failure. In comparison, the patients in the present study did not present prior respiratory failure or other severe disorders, and were on mechanical ventilation because they underwent general anesthesia for surgery of a non-complicated condition. Under those circumstances, the Tobin index was considered effective on predicting clinical complications after extubation.

In the present study, the anesthetic risk of the ASA was significantly lower in the group of patients with a Tobin index below 80 c.L^-1.min^-1 when compared with patients whose Tobin index was between 80 and 100 c.L^-1.min^-1. This result probably reflected the presence of associated comorbidities in patients with elevated Tobin index. Those disorders probably became more consistent factors in the wean process, increasing the respiratory frequency and/or decreasing the tidal volume. Besides, increased age and body weight constituted, in the present study, independent risk factors for the development of clinical intercurrences in the post-extubation period,

Regarding the duration of the anesthesia, this study indicated that, although this period had been smaller in the group with a Tobin index below 80 c.L^-1.min^-1, it was not significantly different from the group with a Tobin index between 80 and 100 c.L^-1.min^-1. This result can be explained by the lack of intraoperative technical difficulties in both groups.

The mean tidal volume was greater in patients with smaller Tobin index when compared with patients with elevated Tobin index. The significant predominance of smokers in the group with elevated Tobin index and the reduction in elasticity of the lungs caused by smoking probably influenced those results. Besides, this study indicated that the mean preoperative body weight was significantly greater in patients with Tobin index between 80 and 100 c.L^-1.min^-1, contributing to the reduction in tidal volume in this group.

The respiratory rate in patients with elevated Tobin index was greater than in patients with lower Tobin index. This result might indicate a smaller respiratory reserve in the group of patients with a Tobin index between 80 and 100 c.L^-1.min^-1. Besides, in the same group the stay in the recovery room was greater, indicating increased difficulty to return to physiological conditions.

In the current study, bronchospasm, which was controlled by medication, affecting 9 (11.25%) patients, was the only intraoperative clinical intercurrence observed. Since there was no significant difference in the incidence of bronchospasm between both groups, the development of this complication is probably related to the ventilatory condition of the patient.

On the other hand, post-extubation clinical intercurrences were significantly lower in the group with Tobin index below 80 c.L^-1.min^-1 than those with a Tobin index between 80 and 100 c.L^-1.min^-1. This result indicates that, in the present study, the Tobin index could be considered a predictor of post-extubation clinical intercurrences. Tobin et al. sustained that patients with indexes below 100 c.L^-1.min^-1, but above 80 c.L^-1.min^-1, would have lower indexes of post-extubation intercurrences. It is possible that the difference between the results of the present study and that of other authors ^7-9 would be attributed to the fact that the latter studied patients on prolonged mechanical ventilation and most of them had COPD. On the contrary, the patients in the current study did not have COPD and mechanical ventilation was not prolonged, since the mean duration was 53.4 ± 12.3 minutes.

Regarding the severity of the clinical intercurrences, patients with Tobin index between 80 and 100 c.L^-1.min^-1 (Group II) had a greater frequency of severe post-extubation complications than those with Tobin index below 80 c.L^-1.min^-1. This result might reflect the different biodemographic conditions (age and body weight) and/or habits (smoking) between both groups. This event had a tendency towards significance, and it is possible that increasing the number of patients this difference would be statistically significant.

Univariate logistic regression identified three variables that indicated risks for the development of intercurrences during wean: age, body weight, and smoking. Multiple logistic regression identified two independent risk factors: age and body weight.

The incidence of smoking was significantly greater in patients whose Tobin index was between 80 and 100 c.L^-1.min^1-1 than in patients with Tobin index of 80 c.L^-1.min^-1. It is possible that smoking decreased lung capacity due to the development of pulmonary fibrosis and, therefore, hindered the return to preoperative ventilatory conditions.

Body weight was significantly greater in patients with Tobin index between 80 and 100 c.L^-1.min^-1. Increased body weight hinders respiration due to the increased visceral fat in the abdomen, which increases the respiratory rate and reduces tidal volume and, therefore, increases the Tobin index.

Similarly, due to a reduction in respiratory rate and tidal volume secondary to an increase in pulmonary fibrosis, advanced age could have contributed to the incidence of Tobin index values between 80 and 100 c.L^-1.min^-1 in the present study.

This study allows us to suggest that a Tobin index between 80 and 100 c.L^-1.min^-1 could be predictive of post-extubation clinical intercurrences in patients undergoing general anesthesia. A Tobin index below 80 c.L^-1.min^-1 should be reached to minimize the risk of post-extubation clinical intercurrences, especially in the elderly, obese, and smokers.

The results of this study can be used as a guide to other investigations, and for professionals involved in the process of wean to be aware of the importance of the analysis of predictive indexes, such as the Tobin index, for the success of wean and the extubation of the patient.

REFERENCES

Submitted em 12 de dezembro de 2006

Accepted para publicação em 21 de agosto de 2007

• 01. Tobin MJ, Perez W, Guenter SM et al. The pattern of breathing during successful and unsuccessful trials of weaning from mechanical ventilation. Am Rev Respir Dis, 1986;134:1111-1118.
• 02. Dries DJ Weaning from mechanical ventilation. J Trauma, 1997;43:372-384.
• 03. Imbeloni LE Complicações da intubação traqueal. Rev Bras Anestesiol, 1986;36:501-508.
• 04. Nemer SM, Abreu LMM, Azeredo L et al. Índice de Nemer, um estudo preliminar como prognóstico do desmame da ventilação mecânica. Rev Bras Ter Intensiva, 1997;9:64-70.
• 05. Velho GV, Zurba S, Wiggers GA et al. Avaliação dos índices preditivos na descontinuação da ventilação mecânica. Rev Bras Ter Intensiva, 1999;11:10-14.
• 06. Tobin MJ, Jenouri G, Birch S et al. Effect of positive end-expiratory pressure on breathing patterns of normal subjects and intubated patients with respiratory failure. Crit Care Med, 1983;11:859-867.
• 07. Tobin MJ Predicting weaning outcome. Chest, 1988;94:227-228.
• 08. Tobin MJ, Yang KL Weaning from mechanical ventilation. Crit Care Clin, 1990;6:725-747.
• 09. Yang KL, Tobin MJ A prospective study of indexes predicting the outcome of trials of weaning from mechanical ventilation. N Engl J Med ,1991;324:1445-1450.
• 10. Yang KL Reproductibility of weaning parameters: a need for standardization. Chest, 1992;102:1829-1832.
• 11. Epstein SK Etiology of extubation failure and the predictive value of the rapid shallow breathing index. Am J Respir Care Med, 1995;152:545-549.
• 12. Chatila W, Jacob B, Guaglionone D et al. The unassisted respiratory rate-tidal volume ratio accurately predicts weaning outcome. Am J Med, 1996;101:61-67.
• 13. Gluck EH Predicting eventual success or failure of wean in patients receiving long-term mechanical ventilation. Chest, 1996;110:1018-1024.
• 14. Morganroth ML, Morganroth JL, Nett LM et al. Criteria for weaning from prolonged mechanical ventilation. Arch Intern Med, 1984;144:1012-1016.
• 15. Shikora SA, Bristian BR, Borlase BC et al. Work of breathing: reliable predictor of weaning and extubation. Crit Care Med, 1990;18:157-162.
• 16. Lee KH, Hui KP, Chan TB et al. Rapid shallow breathing (frequency tidal volume ratio) did not predict extubation outcome. Chest, 1994;105:540-543.
• 17. Scheinhorn DJ, Artiniam BM, Catlin JL Weaning from prolonged mechanical ventilation. Chest, 1994;105:534-539.
• 18. Crespo AS, Carvalho AF, Costa Filho RC Desmame do suporte ventilatório. Rev Bras Anestesiol, 1994;44:135-146.
• 19. Emmerich JC, Vicêncio SRL, Siqueira HR et al. Estudo comparativo entre três modalidades de desmame do suporte ventilatório: tradicional (tubo em T) versus S-IMV versus PSV. Rev Bras Ter Intensiva, 1997;9:167-174.
• 20. Mador MJ, Tobin MJ The effect of inspiratory muscle fatigue on breathing pattern and ventilatory response to CO₂ J Physiol, 1992;455:17-32.
• 21. Jubran A, Tobin MJ Pathophysiologic basis of acute respiratory distress in patients who fail a trial of weaning from mechanical ventilation. Am J Respir Crit Care Med, 1997;155:906-915.
• 22. Pierson DJ Weaning from mechanical ventilation in acute respiratory failure: concepts, indications and tecniques. Resp Care, 1983;28:646-60.
• 23. Morganroth ML, Crum CM Weaning from mechanical ventilation. J Intensive Care Med, 1988;3:109-120.
• 24. Krieger BP, Ershowsky PF, Becker DA et al. Evaluation of conventional criteria for predicting successful weaning from mechanical ventilatory support in elderly patients. Crit Care Med, 1989;17:858-861.
• 25. ASA Task Force on Postanesthetic Care Practice guideline for postanesthetic care. Anesthesiology, 2002;96:742-752.
• 26. Oliveira Filho GR Rotinas de cuidados pós-anestésicos de anestesiologistas brasileiros. Rev Bras Anestesiol, 2003;53:518-534.
• 27. Langhi F, Tobin MJ Disorders of the respiratory muscles. Am J Respir Crit Care Med,2003;168:10-48.
• 28. Epstein SK, Ciubotaro RL Independent effects of etiology of failure and time to reintubation on outcome for patients failing extubation. Am J Resp Crit Care Med, 1998;158:489-493.
• 29. Epstein SK Predicting extubation failure: is it in (on) the cards? Chest, 2001;1120:1061-1063.
• 30. MacIntyre NR Evidenced-based guidelines for weaning and discontinuous ventilatory support. Chest, 2001;120:3758-65.
• 31. Almeida MCS, Camargo DR, Linhares SF et al. Avaliação do bloqueio neuromuscular residual e da recurarização tardia na sala de recuperação pós-anestésica. Rev Bras Anestesiol, 2004;54:518-531.
• 32. Rodrigues MM, Fiore Jr JF, Benassule E et al. Variações na mensuração dos parâmetros de desmame da ventilação mecânica em hospitais da cidade de São Paulo. Rev Bras Ter Intensiva, 2005;17:28-32.
• 33. Assunção MSC, Machado FR, Rosseti HB et al. Avaliação do teste de tubo T como estratégia inicial de suspensão da ventilação mecânica. Rev Bras Ter Intensiva, 2006;18:121-125.

*

Estudo realizado no Hospital Nossa Senhora de Fátima da Sociedade Portuguesa de Beneficência de São Caetano do Sul, SP e no Departamento de Cirurgia da Faculdade de Medicina do ABC, Santo André, SP

Publication Dates

• Publication in this collection
  30 Nov 2007
• Date of issue
  Dec 2007

History

• Accepted
  21 Aug 2007
• Received
  12 Dec 2006