Reijane Oliveira LimaI; Daniel Lago BorgesI; Marina de Albuquerque Gonçalves CostaI; Thiago Eduardo Pereira BaldezI; Mayara Gabrielle Barbosa e SilvaI; Felipe André Silva SousaI; Milena de Oliveira SoaresI; Jivago Gentil Moreira PintoII
BMI: Body mass index
CABG: Coronary artery bypass grafting
FiO2: Inspired oxygen fraction
FiO2N: Necessary inspired oxygen fraction
ICU: Intensive Care Unit
IMV: Invasive mechanical ventilation
NIV: Noninvasive ventilation
PaO2: Arterial oxygen partial pressure
PaO2I: Ideal arterial oxygen partial pressure
PEEP: Positive end-expiratory pressure
PSV: Pressure support ventilation
SaO2: Arterial oxygen saturation
SBT: Spontaneous breathing trial
Coronary artery bypass grafting (CABG) is a therapeutic modality widely used to treat coronary artery disease, minimize symptoms, improve cardiac function, and improve survival[1,2].
Intraoperative conditions, such as general anesthesia, manual compression of the left lower lung lobe during exposure of the posterior heart surface, manual compression of the right lung during cannulation of the inferior vena cava, manual compression of lungs during dissection of the internal mammary artery and apnea during cardiopulmonary bypass (CPB) may impair pulmonary function. Thus, pulmonary complications occur in up to 60% patients undergoing CABG.
Invasive mechanical ventilation (IMV) is essential during the first few hours after CABG to allow recovery from anesthesia and reestablish homeostasis. Typical restoration of hemodynamic stability occurs 5–6 h after surgery in uncomplicated CABG. This interval also correlates with regaining of consciousness and IMV weaning.
When IMV is no longer required, the most appropriate method for its discontinuation must be determined. The spontaneous breathing trial (SBT) is a simple method using pressure support ventilation (PSV) to determine whether a patient would tolerate IMV interruption. This ventilation mode consists of a pressure support of 7 cm H2O (the minimum level to overcome circuit resistance), positive end-expiratory pressure (PEEP) of 5-8 cm H2O (nearest to physiological values), and inspired oxygen fraction (FiO2) < 40%. This trial lasts 30-120 min and is helpful in identifying patients who are able to maintain spontaneous breathing.
Following endotracheal tube and artificial ventilation removal, respiratory support should be provided with oxygen to ensure arterial oxygen saturation (SaO2) close to physiological levels (95%). Oxygen therapy can be offered using a nasal catheter, nebulization mask, or Venturi system.
In this study, we investigated the effects of different PEEP levels applied during SBT on oxygenation indices in patients undergoing CABG.
We performed a randomized clinical trial with 78 patients undergoing CABG between August 2013 and March 2014 who were admitted to the Cardiovascular Intensive Care Unit (ICU) at Hospital Universitário da Universidade Federal do Maranhão, in São Luís, Maranhão, Brazil. We excluded patients with chronic obstructive pulmonary disease and those undergoing emergency, off-pump, or combined surgeries. We excluded patients who required surgical reintervention or noninvasive ventilation during the first 6 h after extubation.
Before surgery, patients received explanations and information about the research. After surgery, data were collected from physiotherapy evaluation forms and medical records. All data were registered in a form that captured preoperative, intraoperative, and postoperative periods. All included patients underwent general anesthesia and median sternotomy.
After ICU admission, mechanical ventilation was applied using an Evita 2 Dura (Dräger Medical, Lübeck, Germany). Patients were ventilated in volume-controlled mode, according to the routine protocol, with the following settings: a tidal volume of 6–8 mL/kg, respiratory rate of 14 bpm, inspiratory flow of 8-10 times the minute volume, inspiratory time of 1 s, and inspired oxygen fraction of 40%.
During the preoperative period, patients were randomized into groups by simple draw, and this information was shared with the ICU care providers. SBT was initiated once the following clinical conditions were met: hemodynamic stability, absence of bleeding or minimal bleeding, absence of vasopressor use or low and stable doses of vasopressors, Glasgow Coma Scale = 10 and strong respiratory drive.
The spontaneous breathing trial was administered using pressure support ventilation (support pressure 7 cm H2O and FiO2 30%). The sample was divided into three groups: Group A, PEEP = 5 cm H2O; Group B, PEEP = 8 cm H2O; and Group C, PEEP = 10 cm H2O. Extubation was performed after 30–120 min with no destabilization signs. Following extubation, all patients received additional oxygen support by Venturi mask (Galemed Corporation, Wu-Jia, Taiwan) with an FiO2 of 31% to ensure arterial oxygen saturation close to physiological levels (around 95%).
Arterial blood samples were collected before extubation and at 1, 3, and 6 h after mechanical ventilation withdrawal. Samples were processed by an ABL 800 FLEX blood gas analyzer (Radiometer, Bronshoj, Denmark), according to the routine protocol. We then identified the arterial oxygen partial pressure (PaO2) and PaO2/FiO2 ratio.
Following the first arterial blood gas analysis after extubation, oxygen support was adjusted according to the necessary inspired oxygen fraction (FiO2N). To estimate the ideal arterial oxygen partial pressure (PaO2I) for each patient, we used the following equation to account for age and supine position: PaO2I = 109 - (0.43 × age).
The inspired oxygen fraction provided following extubation was calculated according to the following formula: FiO2N = FiO2K × PaO2I/PaO2K, in which FiO2N = the inspired oxygen fraction necessary after extubation, FiO2K = the inspired oxygen fraction applied at the moment of arterial blood sample collection, PaO2I = the ideal arterial oxygen partial pressure, and PaO2K = the arterial oxygen partial pressure as measured by the last arterial blood gas. Oxygen was administered by Venturi mask using the following criteria:
FiO2N <21%: room air;
FiO2N = 21%–24%: blue connector, FiO2 24%, O2 flow 4 lpm;
FiO2N = 24.1%–28%: yellow connector, FiO2 28%, O2 flow 4 lpm;
FiO2N = 28.1%–31%: white connector, FiO2 31%, O2 flow 4 lpm;
FiO2N 31.1%–35%: green connector, FiO2 35%, O2 flow 6 lpm;
FiO2N 35.1%–40%: red connector, FiO2 40%, O2 flow 8 lpm;
FiO2N >40%: orange connector, FiO2 50%, O2 flow 12 lpm.
When noninvasive ventilation (NIV) was required following extubation, it was applied, as per the routine protocol, according to the individual's needs. It is noteworthy that patients who used NIV during the first 6 h after extubation were excluded.
Ethical approval was obtained from the local Ethics Committee (protocol No. 327.798), as required by Resolution 466/12 of the National Health Council. All patients provided written informed consent.
Data were evaluated using the Stata/SE statistical software version 11.1 (StataCorp, College Station, TX, USA). To test normality, we used the Shapiro–Wilk test. Quantitative variables were described as means and standard deviations, and differences were determined using the Student's t, ANOVA, or Kruskal–Wallis test, depending on normality. Qualitative variables were expressed as proportions and tested by G-test and William's correction. Results were considered statistically significant when P value was <0.05.
Ninety patients were randomized and underwent CABG during the study period. Of these, twelve (four of each group) were excluded because of postoperative surgical reintervention (6) and noninvasive ventilation use during the first 6 h after extubation (6) (Figure 1). Therefore, the final sample included 78 patients, who were predominantly male (69.3%) and from the countryside (53.8%), with a mean age of 61.7±8.6 years and body mass index of 26.1±3.7 kg/m2. Groups did not differ significantly with regard to demographic, clinical, or surgical variables, as seen in Tables 1 and 2.
The mean mechanical ventilation duration was 12.8±6.9 h. Patients in Group A (PEEP 5 cm H2O) were ventilated for 13.6±8.1 h, whereas those in Group B (PEEP 8 cm H2O) were ventilated for 11.7±6 h and those in Group C (PEEP 10 cm H2O) were ventilated for 13.2±4.8 h (P=0.69). There were no differences in mean gas exchange values (PaO2/FiO2) between groups at 1, 3, and 6 h after extubation (Table 3).
Gas exchange impairment is a significant complication during the CABG postoperative period. In thoracic surgeries, these changes may be related to intraoperative procedures, such as mechanical ventilation with low volumes and PEEP, pain, and thoracotomy (which alters chest wall compliance)[11,12]. Therefore, we chose to evaluate oxygenation indices after extubation, because they properly reflect changes in pulmonary function following on-pump surgery.
To reopen collapsed lung units and improve arterial oxygenation following thoracic surgery, different PEEP levels have been proposed. Dongelmans et al., who compared high versus physiological PEEP (10 vs. 5 cm H2O) after CABG, showed that the highest PEEP levels improve oxygenation and lung compliance but are associated with increased mechanical ventilation duration. In their randomized clinical trial of 136 patients undergoing CABG who were mechanically ventilated at 5, 8, or 10 cm H2O of PEEP, Borges et al. showed that the highest PEEP levels may increase respiratory mechanics and provide better oxygenation indices in the immediate postoperative period.
Our hypothesis that application of higher PEEP levels throughout SBT would improve oxygenation after extubation was not supported by our measurements during the first 6 h after extubation. The results were consistent with those measured in the randomized clinical trial by Marvel et al., in which patients undergoing CABG and ventilation with PEEP of 0, 5, or 10 cm H2O did not experience a sustained arterial oxygenation benefit from higher PEEP levels.
A question that arose during our research was what PEEP level would be considered physiological to avoid alveolar collapse while performing SBT, given that the "expiratory delay function" of the glottis (which serves as an organic PEEP mechanism to prevent or minimize alveolar collapse) is removed during artificial ventilation? During mechanical ventilation of adult patients, PEEP is generally set to 3–5 cm H2O, as this is considered physiological. However, our study provided some evidence that levels between 5 and 8 cm H2O, possibly up to 10 cm H2O, may more closely mimic normal respiratory physiology for such patients.
The knowledge of physical therapy was found to be generally applied across the entire treatment process. Physical therapists play an important role in conducting patient-screening protocols for mechanical ventilation weaning[21,22]. Our research emphasizes identification of optimal variables during weaning as fundamental to this process so as to minimize patient complications.
In this sample of patients undergoing CABG, the use of different PEEP levels before extubation did not affect gas exchange or oxygen therapy utilization in the first 6 h after endotracheal tube removal.
1. Cutlip D, Levin T, Aroesty J. Bypass surgery versus percutaneous intervention in the management of stable angina pectoris: clinical trials [Cited 2015 Jul 23]. Available from: http://www.uptodate.com/contents/bypass-surgery-versus-percutaneous-intervention-in-the-management-of-stable-angina-pectoris-clinical-studies
2. Rocha LA, Maia TF, Silva LF. Diagnóstico de enfermagem em pacientes submetidos à cirurgia cardíaca. Rev Bras Enferm. 2006:59(3);321-6. [MedLine]
3. Zocrato LBR, Machado MGR. Fisioterapia respiratória no pré e pós-operatório de cirurgia cardíaca. In: Machado MGR. Bases da fisioterapia respiratória: terapia intensiva e reabilitação. Rio de Janeiro: Guanabara Koogan; 2008. p.338-62.
4. Nardi C, Otranto CPM, Piaia IM, Forti EMP, Fantini B. Avaliação da força muscular, capacidades pulmonares e função pulmonar respiratória de pacientes submetidos à cirurgia cardíaca. In: 4ª Mostra Acadêmica e Congresso de Pesquisa da UNIMEP [on line]: 2006. Out, 24-26. Piracicaba. Anais eletrônicos [Cited 2015 Jul 23]. Available from: http//www.unimep.br/phpg/mostraacademica/anais/4mostra/pdfs/171pdf
5. Lourenço IS, Franco AM, Bassetto S, Rodrigues AJ. Pressure support-ventilation versus spontaneous breathing with "T-Tube" for interrupting the ventilation after cardiac operations. Rev Bras Cir Cardiovasc. 2013;28(4):455-61. [MedLine]
6. Umeda IIK. Manual de fisioterapia na cirurgia cardíaca: guia prático. Barueri: Manole; 2004.
7. Piotto RF, Maia LN, Machado MN, Orrico SP. Effects of the use of mechanical ventilation weaning protocol in the Coronary Care Unit: randomized study. Rev Bras Cir Cardiovasc. 2011;26(2):213-21. [MedLine] View article
8. Goldwasser R, Farias A, Freitas EE, Saddy F, Amado V, Okamoto V. III Consenso Brasileiro de Ventilação Mecânica. Desmame e interrupção da ventilação mecânica. J Bras Pneum. 2007;33 (Suppl 2S):S128-S136.
9. Diniz GCLM, Machado MGR. Oxigenoterapia. In: Machado MGR. Bases da fisioterapia respiratória: terapia intensiva e reabilitação. Rio de Janeiro: Guanabara Koogan; 2008. p.182-97.
10. Singh NP, Vargas FS, Cukier A, Terra-Filho M, Teixeira LR, Light RW. Arterial blood gases after coronary artery bypass surgery. Chest. 1992:102(5);1337-41. [MedLine]
11. Ambrozin ARP, Cataneo AJM. Pulmonary function aspects after myocardial revascularization related to preoperative risk. Rev Bras Cir Cardiovasc. 2005;20(4):408-15. View article
12. Park DJ, Jeong JH, Lee HO. The effects of a self-training physiotherapy program on pulmonary functions, postoperative pulmonary complications and post-thoracotomy pain after lobectomy of patients with lung cancer. J Phys Ther Sci. 2013;25(3):253-5.
13. Cui H, Zhang M, Xiao F, Li Y, Wang J, Chen H. Comparison and correlative analysis of pulmonary function markers after extracorporeal circulation. Beijing Da Xue Xue Bao. 2013;35(2):195-9.
14. Dyhr T, Nygård E, Laursen N, Larsson A. Both lung recruitment maneuver and PEEP are needed to increase oxygenation and lung volume after cardiac surgery. Acta Anaesthesiol Scand. 2004;48(2):187-97. [MedLine]
15. Dongelmans DA, Hemmes SN, Kudoga AC, Veelo DP, Binnekade JM, Schultz MJ. Positive end-expiratory pressure following coronary artery bypass grafting. Minerva Anestesiol. 2012;78(7):790-800. [MedLine]
16. Borges DL, Nina VJS, Costa MAG, Baldez TEP, Santos NP, Lima IM, et al. Effects of different PEEP levels on respiratory mechanics and oxygenation after coronary artery bypass grafting. Rev Bras Cir Cardiovasc. 2013;28(3):380-5. [MedLine] View article
17. Marvel SL, Elliott CG, Tocino I, Greenway LW, Metcalf SM, Chapman RH. Positive end-expiratory pressure following coronary artery bypass grafting. Chest. 1986;90(4):537-41. [MedLine]
18. Annest SJ, Gottlieb M, Paloski WH, Stratton H, Newell JC, Dutton R, et al. Detrimental effects of removing end-expiratory pressure prior to endotracheal extubation. Ann Surg. 1980;191(5):539-45. [MedLine]
19. David CM. Ventilação Mecânica: Da fisiologia à prática clínica. 2ª ed. Rio de Janeiro: Revinter; 2011.
20. Ryu YU, Park J. Medical and narrative use of physical therapy knowledge in clinical reasoning by Korean physical therapists. J Phys Ther Sci. 2011;23(2):251-4.
21. Ely EW, Meade MO, Haponik EF, Kollef MH, Cook DJ, Guyatt GH, et al. Mechanical ventilator weaning protocols driven by nonphysician health-care professionals: evidence-based clinical practice guidelines. Chest. 2001;120(6 Suppl):454S-63S. [MedLine]
22. Plani N, Becker P, van Aswegen H. The use of a weaning and extubation protocol to facilitate effective weaning and extubation from mechanical ventilation in patients suffering from traumatic injuries: a non-randomized experimental trial comparing a prospective to retrospective cohort. Physiother Theory Pract. 2013;29(3):211-21 [MedLine]
No financial support.
Authors' roles & responsibilities
ROL: Analysis and/or interpretation of data; study design; implementation of projects and/or experiments; manuscript writing or critical review of its content
DLB: Analysis and/or interpretation of data; statistical analysis; final approval of the manuscript; study design; implementation of projects and/or experiments; manuscript writing or critical review of its content
MAGC: Conduct of operations and/or experiments
TEPB: Conduct of operations and/or experiments
MGBS: Conduct of operations and/or experiments
FASS: Conduct of operations and/or experiments
MOS: Conduct of operations and/or experiments
JGMP: Analysis and/or interpretation of data; manuscript writing or critical review of its content
Article receive on Sunday, February 15, 2015