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On-line version ISSN 1806-907X
Rev. Bras. Anestesiol. vol.57 no.5 Campinas Sept./Oct. 2007
Alveolar recruitment maneuver to reverse hypoxemia in the immediate postoperative period of cardiac surgery*
Maniobra de reclutamiento alveolar en la reversión de la hipoxemia en el postoperatorio inmediato en cirugía cardiaca
José Otávio Costa Auler Junior, TSAI; Emília NozawaII; Eliane Kobayashi TomaIII; Karin Lika DegakiIII; Maria Ignêz Zanetti FeltrimIV; Luiz Marcelo Sá Malbouisson, TSAV
Titular da FM/USP; Diretor do Serviço de Anestesiologia e Unidade de
Terapia Intensiva Cirúrgica do InCor
IIFisioterapeuta Chefe da Unidade de Terapia Intensiva Cirúrgica do InCor; Doutora em Ciências FM/USP
IIIFisioterapeuta da Unidade de Terapia Intensiva Cirúrgica do InCor; Especialização em Fisioterapia Cardiorrespiratória
IVFisioterapeuta, Diretora do Serviço de Fisioterapia do InCor; Doutora em Reabilitação pela UNIFESP
VMédico Assistente do Serviço de Anestesiologia e Unidade de Terapia Intensiva Cirúrgica do InCor; Doutor em Ciências pela FM/USP
BACKGROUND AND OBJECTIVES: To evaluate the effects of the alveolar
recruitment maneuver on oxygenation and exhaled tidal volume, in patients with
hypoxemia, in the immediate postoperative period of cardiac surgery.
METHODS: This is a prospective, consecutive study. Among the 469 cardiac surgeries performed from February to April 2006, 40 patients (8.5%) who, at the time of admission to the surgical intensive care unit, presented PaO2/FIO2 < 200, were included in the protocol. A standard prospective protocol of alveolar recruitment maneuvers with pressure of 20 cmH2O in the upper airways in the presence of the ratio PaO2/FIO2 < 200, 30 cmH2O with PaO2/FIO2 < 150, and 40 cmH2O when Pa2O2/FIO2 remained below 150 after recruitment maneuver with pressure of 30 cmH2O, was applied to this group of patients. Continuous positive pressure was applied to the airways with a mechanical ventilator, 3 times, for approximately 30 seconds each. Parameters of oxygenation and exhaled tidal volume were compared before and immediately after the recruitment maneuvers.
RESULTS: Of the 40 patients in the study, 30 showed good responses to recruitment maneuvers with 20 cmH2O, and 10 cases required 30 cmH2O. It was not necessary to apply pressure of 40 cmH2O. There was a significant improvement in oxygenation after the recruitment maneuvers, demonstrated by an increase in PaO2/FIO2 (p = 0.001), peripheral oxygen saturation (p = 0.004), and exhaled tidal volume (p = 0.038).
CONCLUSIONS: Alveolar recruitment maneuvers were successful on correcting hypoxemia and increasing the exhaled tidal volume in patients on mechanical ventilation in the immediate postoperative period of cardiac surgery.
Key Words: COMPLICATIONS: hypoxemia; SURGERY, Cardiac; VENTILATION: mechanic, positive pressure, alveolar recruitment maneuver.
JUSTIFICATIVA Y OBJETIVOS: Evaluar los efectos de la maniobra de reclutamiento
alveolar en la oxigenación y volumen corriente exhalado en pacientes
con hipoxemia en el postoperatorio inmediato de intervención quirúrgica
MÉTODO: Estudio prospectivo, consecutivo. Entre 469 intervenciones quirúrgicas cardíacas entre febrero a abril de 2006, fueron incluidos en el protocolo 40 pacientes (8,5%), que en la primera evaluación en la admisión de la unidad de terapia intensiva quirúrgica presentaban relación PaO2/FiO2 < 200. En ese grupo de pacientes se aplicó prospectivamente protocolo estandarizado de maniobras de reclutamiento alveolar con presión en las vías aéreas de 20 cmH2O en la vigencia de relación PaO2/FiO2 < 200, 30 cmH2O en la relación PaO2/FiO2 < 150, y 40 cmH2O cuando la relación PaO2/FiO2 se mantuviese inferior a 150 después de la maniobra de reclutamiento con presión n la vías aéreas de 30 cmH2O. La presión aplicada por medio del ventilador mecánico fue en la modalidad de presión positiva continua en las vías aéreas, por tres veces, con una duración promedio de 30 segundos cada una. Los parámetros de oxigenación y volumen corriente exhalado fueron comparados antes e inmediatamente después de las maniobras de reclutamiento.
RESULTADOS: De los 40 pacientes estudiados, 30 respondieron favorablemente a la maniobra de reclutamiento con presión de 20 cmH2O y en 10 hubo necesidad de 30 cmH2O. Ninguna vez fue necesario aplicar maniobra de reclutamiento con presión de 40 cmH2O. Después de la aplicación de la maniobra de reclutamiento hubo una mejoría significativa de la oxigenación caracterizada por aumento de la relación PaO2/FiO2 (p = 0,001), saturación periférica de oxígeno (p = 0,004); y del volumen corriente exhalado (p = 0,038).
CONCLUSIONES: Las maniobras de reclutamiento alveolar fueron efectivas en la corrección de la hipoxemia y aumento del volumen corriente exhalado en pacientes bajo ventilación mecánica en el postoperatorio inmediato de intervención quirúrgica cardiaca.
Alveolar recruitment maneuver (ARM), isolated or associated with positive end-expiratory pressure (PEEP), to correct hypoxemia secondary to alveolar collapse remains controversial 1.
Several factors contribute to the development of alveolar collapse, or atelectasis, in the intraoperative period, a common cause of postoperative hypoxemia in surgeries of the upper abdomen and thoracic cavity. Among them we should mention the cephalad dislocation of the diaphragm, caused by anesthetics and neuromuscular blockers, and compression of the lungs by mediastinal structures 2. In the case of cardiac surgeries, the inactivity of the lungs during extracorporeal circulation, surgical handling of the pleural cavity, and inflammatory reactions caused by the extracorporeal circulation, also contribute to the formation of atelectasis 3,4. The presence of collapsed regions in the lungs has been associated with an increased risk of pulmonary infections in the postoperative period, as well as hypoxemia due to the increase in shunt 4. The re-expansion of those areas is necessary to improve oxygenation, reduce the risk of pneumonia, making it possible to extubate the patient. Positive end-expiratory pressure (PEEP) has been increasingly used to expand the alveoli 5, by itself or associated with alveolar recruitment maneuver (ARM) 6.
Alveolar recruitment maneuver have been used routinely in the acute respiratory distress syndrome (ARDS) with hypoxemia refractory to PEEP 7, although some studies have demonstrated that, in the early stages of ARDS, oxygenation is not sustained after ARM 8. On the other hand, even though its importance might have been emphasized, the use of ARM during or at the end of general anesthesia has not been a routine. In this situation, the aim of ARM would be to reverse atelectasis and to increase the alveolar surface available for gas exchange 9. The current polemic is how and when to use it, which values of intra-alveolar positive pressure are necessary to open collapsed areas, and how to measure its efficacy. Since ARM causes large intra-alveolar pressurization, it has inherent risks, such as alveolar rupture, leading to pneumothorax, and hypotension, secondary to the reduction of venous return during its use. Although ARM related issues have been extensively studied, in situations such as hypoxemia and ARDS 10, its use is still a controversial issue in the immediate postoperative period of cardiac surgeries 6,11.
The objective of this study was to evaluate the efficacy of ARM on reverting hypoxemia in patients in the immediate postoperative period of cardiac surgeries, as well as to propose a standardization of the method.
After approval by the Institutional Ethics Committee, 40 (8.5%) patients, from a total of 469 patients undergoing cardiac surgeries and admitted to the surgical ICU, 29 males and 11 females, from February to April 2006, participated in this study. All patients included in this study presented hypoxemia, characterized as a PaO2/FIO2 below 200. During this period, other patients with a PaO2/FIO2 below 200 with concurrent subcutaneous emphysema, bronchopleural fistula, low cardiac output, and pulmonary hypertension, were not included in the study.
All patients were classified as ASA II or III and received oral midazolam, ranging from 5 to 10 mg, depending on age and physical status, one hour before surgery. The anesthetic technique was similar for all patients, standardized by the institution. Depending on the preoperative ejection fraction (EF), etomidate (EF below or equal to 0.5) or propofol was used for induction, associated with appropriated doses of fentanyl citrate or sufentanil citrate. The neuromuscular blocker used was pacuronium or, in the presence of renal failure, atracurium. Maintenance was accomplished with isoflurane and additional doses of the opioid, as deemed necessary by the attending physician.
Monitoring included a 5-lead ECG, invasive blood pressure, central venous pressure, naso-pharyngeal temperature, and pulse oximetry (Siemens monitor); when indicated, a pulmonary artery catheter was used. Capnography, and inspired and expired concentrations of oxygen and Isoflurane, were obtained directly from the panel of the anesthesia equipment (Cícero Drager Luebeck Germany).
In the operating room, respirator settings included volume-controlled (tidal volume of 8 mL.kg-1), inspired oxygen concentration (FIO2) of 0.5 (air-oxygen mixture), and PEEP of 5 cmH2O (Cícero Drager Luebek Germany). Alveolar recruitment maneuver were not done in the operating room. When necessary, FIO2 was adjusted to increase blood oxygenation, maintaining pulse oximetry equal or greater than 95%.
Cardiopulmonary bypass (CPB) and membrane oxygenator system adult (OXI Master Century Braile Biomédica, São José do Rio Preto) were used in every patient in this series. Mild hypothermia, with a minimum of 32°C, and normovolemic hemodilution with Ringer's lactate were also used. Packed red blood cells were, as a rule, transfused when the hematocrit was below 28%. During CPB, the mechanical respirator was turned off, and lungs remained depressurized. At the end of CPB, mechanical respiration was re-started using the same settings, without ARM.
Patients were admitted to the Intensive Care Unit (ICU) still under anesthesia, maintaining volume-controlled settings (Ventilador Veolar Hamilton Rahzuns, Switzerland). Initially, parameters included tidal volume of 8 mL.kg-1, respiratory rate (f) of 12 bpm, FIO2 of 0.6, and PEEP of 5 cmH2O, to keep peripheral oxygen saturation (SpO2) equal or greater than 95%. Fifteen minutes after adjusting ventilator settings, arterial blood was collected for determination of arterial blood gases (Radiometer, model ABL 715, Copenhagen). If it demonstrated the presence of hypoxemia (PaO2/FIO2 < 200) and if the patient fulfilled the criteria, the patient was included in the protocol.
Three mechanical ventilators, of the same mark and model, whose flow and pressure were calibrated with a calibrator analyzer (Respical-Timeter, mod. T300, Allied Healthcare, USA), with international and national (Inmetro) certifications, were selected for this study; the procedure was done by an electronics technician specialized on the equipment. Calibration was done to assure the accuracy of the exhaled volume and pressures used to pressurize the airways. Special care was taken to detect any leaks that might hinder the determination of the exhaled volume and to maintain airways pressure.
In this study, alveolar recruitment maneuver were similar to, but with small modifications, the method described in the literature 12. Alveolar recruitment maneuver were performed after determining whether the patient was still anesthetized; it consisted of three sustained insufflations with 20, 30, or 40 cmH2O of pressure in the airways, lasting 30 seconds each (Figure 1). For this maneuver, ventilator settings were adjusted to continuous positive airways pressure (CPAP) activating PEEP control, and FIO2 of 0.6. Insufflation of the airways was continuous, starting with a PEEP of 5 cmH2O and increasing up to 20, 30, or 40. Between each 30-second maneuver, ventilator settings returned to pressure controlled with PEEP of 5 cmH2O, FIO2 of 0.6, and f of 12 bpm during five cycles. Pressure of airways insufflation of 20, 30, or 40 was selected according to the PaO2/FIO2. Pressure of 20 cmH2O was used when PaO2/FIO2 was below 200 and greater than 150, 30 cmH2O when it was below 150, and 40 cmH2O when PaO2/FIO2 remained below 150 after application of ARM with 30 cmH2O. Exhaled volume was obtained directly from the panel of the ventilator before and immediately after ARM in the volume-controlled setting. Peripheral oxygen saturation was determined by pulse oximetry (Oxyplenth, mod. DX2405 Dixtal, São Paulo), before and after ARM.
If oxygenation was restored after ARM, weaning off mechanical ventilation followed the standard protocol of the ICU, starting with pressure-controlled ventilation, and synchronized intermittent mandatory ventilation (SIMV) associated with pressure support (VPS). After ARM, PEEP was readjusted randomly between 6 and 8 cmH2O. During ARM, blood pressure and heart rate were observed and, if hemodynamic instability was present, the maneuvers were discontinued. To decide whether to continue or not weaning off mechanical ventilation and extubate the patient, it was necessary the presence of: hemodynamic and electrolyte stability, absence of cardiac arrhythmias, urine output, bleeding through chest tubes, and body temperature > 36°C.
During the process of weaning off mechanical ventilation, patients were evaluated at 5- to 15-minute intervals. The criteria to reduce respiratory frequency of the ventilator included level of consciousness; presence of spontaneous breathing intercalated with those provided by the ventilator; and pulse oximetry guided the reduction of FIO2. Reduction of the respiratory frequency of the mechanical ventilator followed always the same sequence: from 12 to 10 bpm, then to 8 bpm, 5 bpm, and finally 2 bpm. At the same time, the level of response and respirations of the patient were observed until s/he presented the parameters necessary for extubation. Weaning off mechanical ventilation was gradual, until the following parameters were achieved: pressure-controlled ventilation with synchronized intermittent mandatory ventilation (SIMV) associated with pressure support (VPS); f: 2 bpm; FIO2: 0.4; PEEP: 5 cmH2O; VPS: 10 cmH2O; exhaled TV > 5 mL.kg-1; PPI < 25; and f/TV (Tobin index) £ 100. The patient was extubated when he achieved these parameters.
All parameters are expressed as mean and standard deviation. Since parameters were within normal limits, we decided to use the parametric test. Parameters evaluated before and after alveolar recruitment maneuvers were analyzed by the Student t test. A p < 0.05 was considered statistically significant.
Forty (8.5%) of 469 patients undergoing cardiac surgery, from February to April 2006, were included in the protocol. Tables I and II show the demographic and surgical data. Table III describes the values of airways pressure (AP) and the number of ARMs applied, according to the number of cases. Twenty-nine patients needed ARM with 20 cmH2O, while 10 needed 30 cmH2O. One of the patients scheduled for ARM with 30 cmH2O developed hemodynamic instability and was re-classified to the AP 20 group, for a total of 20 patients. Patients did not have any evidence of pneumothorax before and after this study. Two out of the 40 patients in the study presented air bubbles in the chest tube after ARM. The pressure of 40 cmH2O was not necessary in this group.
There was a significant increase in SpO2 values (p = 0.004), exhaled tidal volume (p = 0.038), and PaO2/FIO2 (p = 0.001) after ARM, which can be observed in table IV. Figures 2, 3, and 4 and table IV show a comparison between ARM of 20 and 30 cmH2O regarding parameters of oxygenation and exhaled tidal volume.
The mean duration of intubation in patients with ARM of 20 cmH2O was 438 minutes, or 7.3 hours. In patients who needed ARM of 30 cmH2O, it was 474 minutes, or 7.9 hours. This difference was not statistically significant (p > 0.05).
All patients had a good clinical evolution, and were extubated and discharged from the ICU within 48 hours.
The results of this study demonstrate that the random use of PEEP of 5 cmH2O in the protocol of the immediate postoperative period is not enough to reverse hypoxemia. The data demonstrated a significant improvement in oxygenation after ARM in groups exposed to airways pressures of 20 and 30 cmH2O. Some studies on the use of ARM in the postoperative period of cardiac surgeries consider arterial oxygenation an important parameter 5,13,14. Similar to the studies mentioned, in this study ARM increased arterial oxygenation in hypoxic patients, demonstrated by an increase in PaO2/FIO2 and peripheral saturation of oxygen. This indicates that the pressure of alveolar recruitment resulted in the opening of collapsed broncho-alveolar areas, improving arterial oxygenation. This increased in oxygenation after ARM also suggests an improvement in the ratio between ventilation and pulmonary perfusion. There are controversies regarding the degree of effectiveness of ARM used by itself, associated with PEEP, or PEEP alone to correct hypoxemia after cardiac and pulmonary surgeries 5,11,15,16. A prior study, also with patients in the postoperative period of cardiac surgery, demonstrated that the progressive use of PEEP until 15 cmH2O increased oxygenation with minimal negative hemodynamic effects 5. Although it was not the scope of the present study, increasing oxygenation and reopening of collapsed alveolar units, with different methodologies, has been the greatest paradigm in patients with ARDS 17-22. Lachman was one of the pioneers who disseminated the concept and strategy to open collapsed alveolar units and to keep them opened 17. As mentioned, the strategy of using PEEP and ARM to revert areas of alveolar collapse has shown a huge scientific growth in the past few years, and almost exclusively in patients with ARDS. Although there is still no consensus about the values of ARM and PEEP, many of those studies, based on imaging techniques, showed the importance of ARM in improving oxygenation 18-20. Even though the physiopathology of ARDS is different, the strategy to treat hypoxemia in these patients, which includes ARM 18-22, as well as specific studies about cardiac surgery using the same technique 6, confirmed and motivated the conduction of the present study. There are several studies with smaller numbers of patients, when compared with those with ARDS patients, on the physiopathology and treatment of atelectasis in the intraoperative period. According to the classic study of Rothen et al. 9, the speed of atelectasis formation is three times greater with FIO2 of 1.0, than with 0.3, during general anesthesia. Tusman et al. 16 demonstrated that, during general anesthesia for thoracic surgery, ARM promoted an improvement in gas exchange and lung ventilation in such a way, that it increased oxygenation and decreased dead space. Patients undergoing cardiac surgeries are especially prone to develop atelectasis, and its physiopathology is multifactorial 3-5. The main factors that contribute to the formation of atelectasis are: inflammatory reaction caused by the extracorporeal circulation 3,23, hypothermia, sternotomy handling of the pleural cavities, use of neuromuscular blockers, and ventilation with high fraction of inspired oxygen in the intraoperative period. According to a study by Malbuisson et al., the weight of the heart is also important in the genesis of atelectasis 24.
In the present study, we observed that oxygenation increased simultaneously with the increase in exhaled tidal volume, demonstrating that the recruitment of collapsed alveoli contributed for an improvement in gas exchange. Similar results were reported in patients in the postoperative period of cardiac surgery with the use of alveolar recruitment with CPAP with 45 cmH2O of airways pressure associated with PEEP of 12 cm H2O 11. Claxton et al. 15 studied a similar population and period of follow-up, but with PEEP of 15 cmH2O, allowing for a peak inspiratory pressure of 40 cmH2O. There was a significant improvement in oxygenation, demonstrated by the Pa2O/FIO2, in the group undergoing alveolar recruitment, 30 minutes and 1 hour after the maneuvers when compared to the group without PEEP and with PEEP of 5 cmH2O.
Valta et al. 13 and Michalopoulos et al. 14, studying the effects of PEEP on oxygenation after cardiac surgery, demonstrated that values below 10 cmH2O are not effective in opening collapsed alveolar units. The use of different levels of PEEP (5, 10, and 15 cmH2O) in patients in the immediate postoperative period of cardiac surgery resulted in a reduction in the resistance to the airflow and of complacency, reflected on the improvement of respiratory mechanics, as well as oxygen transportation 5.
As can be observed in figures 2 to 4, the technique of ARM used in the present study improved oxygenation. After the intervention, arterial oxygenation increased immediately after ARM, and remained elevated, allowing weaning the patient off mechanical ventilation. These results indicate that more elevated pressures are necessary to open collapsed alveolar units in the postoperative period, when compared with conventional values of PEEP, around 5 cmH2O. But the values of ARM after general anesthesia are still controversial. Rothen et al. 9 reported that 40 cmH2O for 7 to 15 seconds are necessary to recruit collapsed alveoli after 20 minutes of general anesthesia in healthy individuals. Tusman et al. reported similar values of pressure in the airways in the improvement of oxygenation after anesthesia for general surgery. There are very few reports on the use of ARM in cardiac surgery 11,12. Dyhr et al. 6 defined ARM as the application of four sustained insufflations with airways pressure of 45 cmH2O, for 10 seconds each, with the ventilator on CPAP. During the interval between insufflations, of 20 seconds, patients were ventilated on volume-controlled with PEEP (0 or 12 cmH2O). The authors verified that, in the postoperative period of cardiac surgery, ARM combined with maintenance of PEEP, resulted in increased exhaled lung volume and improved oxygenation. Regarding hemodynamics, Villagrá et al. 21, in patients with ARDS, and Claxton et al. 20, in patients undergoing cardiac surgery, did not see important changes in circulatory parameters during the application of ARM. In the present study, we did not observe any intercurrence, except for the two patients who presented air bubbles in the chest tube, considered to have been accumulated prior to the use of technique, which did not interfere in weaning off mechanical ventilation.
In conclusion, ARM can be considered a method that is fast and easy to use, being effective in the correction of hypoxemia and restoration of the exhaled tidal volume and, thus, facilitating weaning patients off mechanical ventilation after cardiac surgery. There is still no consensus in the literature on the ideal values of AP and the standardization of alveolar recruitment maneuvers.
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Submitted em 11 de agosto de 2006
Accepted para publicação em 12 de junho de 2007
* Received from Unidade de Terapia Intensiva Cirúrgica, Instituto do Coração (InCor), Hospital das Clínicas da Faculdade de Medicina da Universidade de São Paulo (HC-FM/USP) São Paulo, SP