Interactive Cardiovascular and Thoracic Surgery 2:156-159(2003)
© 2003 European Association of Cardio-Thoracic Surgery
Institutional review - Cardiopulmonary bypass |
Beneficial effects of bilevel positive airway pressure after surgery under cardiopulmonary bypass
Yoshiyuki Takami* and
Hiroshi Ina
Division of Cardiovascular Surgery, Kasugai Municipal Hospital, 1-1-1 Takagi-cho, Kasugai City, 486-8510 Japan
* Corresponding author. Tel.: +81-568-57-0057; fax: +81-568-57-0067
cvs{at}hospital.kasugai.aichi.jp
Received July 20, 2002;
received in revised form December 19, 2002;
accepted January 7, 2003
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Abstract
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To support injured lungs, we have been applying bilevel positive airway pressure for adult patients undergoing surgery with cardiopulmonary bypass. Among 120 consecutive patients, 31 patients whose PaO2/FiO2 decreased to less than 180 after extubation assigned to the intermittent 15min bilevel positive airway pressure (7.3±3.6 times per patient). Bilevel positive airway pressure improved oxygenation (PaO2/FiO2: 128±43 vs. 198±62,  ) and allowed the patients with poor oxygenation after extubation to maintain PaO2/FiO2 levels similar to those of the patients without bilevel positive airway pressure. In conclusion, the bilevel positive airway pressure therapy after extubation was effective to improve lung oxygenation non-invasively in adult patients undergoing more invasive surgery with prolonged cardiopulmonary bypass.
Key Words: Cardiopulmonary bypass; Bilevel positive airway pressure; Oxygenation; Postoperative management; Ventilatory support
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1. Introduction
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The lung is a common site for injury induced by cardiopulmonary bypass (CPB) [1]. The first mechanism of lung injury is atelectasis due to lung collapse and pleural disruption during CPB. Several pre-existing conditions also predispose to atelectasis, including chronic obstructive pulmonary disease, smoking, and obesity. The second mechanism is acute lung injury referred to as ‘pump lung’, which may be induced by microemboli, complement activation, and inflammatory responses. Poor tissue oxygenation due to impaired lung function after surgery may result in delay in postoperative recovery and surgical wound infection [2]. Therefore, perioperative respiratory management is crucial in patients undergoing surgery with CPB.
To support injured lungs after CPB, we have been applying a bilevel positive airway pressure (BiPAP) therapy after weaning from mechanical ventilation and extubation, which is a new mode of non-invasive pressure support ventilation without intubation [3]. In the present study, our clinical experience with BiPAP was analyzed retrospectively for its effect on oxygenation after extubation and characteristics of the patients who needed it.
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2. Materials and methods
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The subject of this study included 120 consecutive patients, being 77 men and 43 women with a mean age of 65.5±8.8 years, undergoing surgery with CPB. The operations performed were coronary artery bypass grafting alone  , valve surgery with and without coronary surgery ( , mitral in 18, aortic in 15, and both in two), thoracic aortic surgery ( , ascending in 11, arch in two, and descending in two), and others (atrial septal defect closure in six and left atrial tumor resection in two), including 28 emergency operations. Except for aortic surgery, for which deep hypothermic circulatory arrest with a minimal rectal temperature of 23.1±1.6 °C were applied, all the operations were performed under moderate hypothermic CPB with a minimal rectal temperature of 33.7±1.6 °C.
On arrival in the intensive care unit (ICU) after surgery, the patients were managed with a mechanical ventilator: T Bird AVSIII (Bird Products Corporation, Palm Springs, CA) or Servo 300A (Siemens Elema, Uppsala, Sweden). The initial ventilator setting was in the assist-control mode with a tidal volume of 10ml/kg of body weight, a respiratory rate of 12–15 breaths/min, a positive end-expiratory pressure (PEEP) of 3cm H2O, and an FiO2 of 1.0. The FiO2 was reduced down to 0.5 or less based upon monitored oxygen saturation by a pulse-oxymetry (SpO2). Intravenous propofol of 1mg/kg/h was given for sedation during recovery from the general anesthesia. When spontaneous breathing reappeared, the ventilator settings were changed to an intermittent mandatory mode with a pressure support (PS) of 10cm H2O. Patients were weaned from the ventilator in a continuous positive airway pressure mode by reducing the level of PS to 5cm H2O as tolerated. Then the patients underwent extubation if they maintained respiratory rate lower than 30 breaths/min and a PaO2/FiO2 greater than 200.
Following extubation, the patients received oxygen through a face mask with a reservoir, adjusting FiO2, which was chosen at the oxygen blender, to keep SpO2over 94%. Those patients whose PaO2/FiO2 decreased to less than 180 received the BiPAP therapy. A special full-face mask with an inflatable soft-cushion seal was connected with a BiPAP ventilator (BiPAP Vision, Respironics Inc., Murrysville, PA), even though with a nasogastric tube. It incorporates oxygen blender that allows for more precise titration of inspired oxygen concentration without dilution by high flow. The settings were an FiO2 of 0.7, an inspiratory positive airway pressure (IPAP) of 18cm H2O, and an expiratory positive airway pressure (EPAP) of 8cm H2O, responding to the patient's respiratory cycle and alternating between a higher flow rate during inhalation and a lower flow rate during exhalation [4]. The BiPAP ventilation was applied for up to 15min as tolerated every 3–4h during daytime. Additional BiPAP was applied when the SpO2 was less than 94%. We discontinued the BiPAP when the patient's SpO2 kept to be 95% or more with a nasal O2 of 3l/min during eating in ICU.
During the stay in the ICU, blood gas analysis and lactate measurement of both samples were performed in a blood gas analyzer and an enzyme-electrode analyzer, respectively (ABL510 and EML105, Radiometer, Copenhagen, Denmark). The oxygenation index (PaO2/FiO2) were recorded on arrival in ICU, before extubation, before and after the third BiPAP therapy. The perioperative factors were retrospectively compared between the patients who underwent the BiPAP therapy and those who did not. All data are expressed as means±standard deviations. Comparison of data between two groups was performed using the Student t-test. Categorical data were compared against a chi-square distribution. A P value of less than 0.05 was considered to be statistically significant.
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3. Results
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Thirty one of 120 patients received the BiPAP therapy. All of them were normocapnic (PaCO2=35.4±5.7mmHg). The intermittent 15min BiPAP were repeated 7.3±3.6 times per patient in the ICU. Comparison between the patients who needed the BiPAP therapy ( , group B) and those who did not ( , group N) revealed the following differences in the perioperative factors. The women outnumbered the men in group B. The patients were of smaller constitution and had lower preoperative total protein values in group B (Table 1). More cases were emergency and using circulatory arrest in group B (Table 2). In group B, CPB duration was longer with lower rectal temperature and hemoglobin values. Immediately after surgery, cardiac index was lower and systemic vascular resistance was higher in group B (Table 3). The initial PaO2/FiO2 in ICU was lower in group B. Intubation period was longer and PaO2/FiO2 before extubation was lower in these patients. Postoperative maximal value of creatine phosphokinase was also greater in group B, indicating that those patients underwent more invasive surgery. In both groups, reintubation was necessary in 3% of the patients, principally due to hemodynamic instability.
The PaO2/FiO2 of the patients receiving BiPAP changed as demonstrated in Fig. 1. The PaO2/FiO2 before extubation was significantly lower than that on arrival in the ICU (330±105 vs. 265±79,  ). The PaO2/FiO2 before the BiPAP therapy decreased still significantly (265±79 vs. 128±43,  ). After three times of the intermittent BiPAP therapy, the PaO2/FiO2 improved from the value before the therapy (128±43 vs. 198±62, ). PaO2/FiO2 values upon ICU discharge were not significantly different between the patients who received BiPAP and those who did not (Table 3). All patients were well tolerated to BiPAP and there were no complications associated with BiPAP.
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4. Discussion
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BiPAP is a mode of non-invasive ventilatory support, which achieves the combination of pressure-controlled ventilation and spontaneous breathing. BiPAP creates two different positive airway pressure levels (IPAP and EPAP), responding to the patient's respiratory cycle [4]. Theoretically, the EPAP provides the physiological advantages similar to those of PEEP in mechanical ventilation. The IPAP creates further PS effects to permit passive ventilation. These two pressure levels alternating at preset time intervals improve lung mechanics by recruiting atelectatic alveoli, increasing pulmonary compliance, and reducing the work of breathing [5]. The BiPAP ventilation has been demonstrated to be an effective means of improving gas exchange in patients with many types of acute respiratory failure [6] and cardiogenic pulmonary edema [7], and in postoperative high-risk patients [8]. In our study, oxygenation also improves by BiPAP in adult patients after surgery under CPB. The BiPAP administration increased PaO2/FiO2 in the patients with poor oxygenation on arrival in ICU immediately after surgery and before extubation. The BiPAP therapy allowed those patients after extubation to maintain PaO2/FiO2 levels similar to the levels in the patients without BiPAP.
There are several potential advantages of BiPAP for these patients. First, BiPAP allows a better alveolar recruitment during both inhalation and exhalation [9]. Second, continuous positive airway pressure increases oxygen uptake in the setting of impaired oxygen diffusion, which results from acute lung injury by CPB [7]. Third, improved oxygenation may result in increased tissue partial pressure of oxygen and may promote wound healing and avoid surgical wound infection [2]. Fourth, BiPAP, especially IPAP, unloads the work of breathing, which may increase myocardial oxygen demand. Fifth, BiPAP may avoid the morbidities of endotracheal intubation for mechanical ventilation, including ventilator-associated pneumonia [10], patient discomfort, absence of oral intake and speech, and requirement of deep sedation. Sixth, BiPAP may also improve hemodynamics by reducing afterload [11], transmural pressure, sympathetic nerve activity, and afterload, resulting in enhanced ventricular performance [12]. In our study, the BiPAP bestowed benefits especially on female patients with lower total protein and small body mass index, who underwent more invasive emergency operation with prolonged CPB with lower temperature and hemoglobin levels. These advantages of BiPAP may give more beneficial effects to high-risk patients with poor nutrition undergoing more invasive surgery.
Of course, the BiPAP therapy is not appropriate for all patients after surgery with CPB. The major premise for applying BiPAP is stable hemodynamic condition. In our study, reintubation for full ventilatory support was necessary in one patient who received BiPAP because of hemodynamic instability. For patients who are extremely agitated or uncooperative, BiPAP is not likely to be well tolerated. In our patients, all patients adapted themselves well to BiPAP with careful coaching of the nurses and gradual titration of positive pressure. Patients with excessive airway secretion are also poor candidates for BiPAP because there is no direct access to remove secretions. In general, complications associated with BiPAP are uncommon [13]. Gastric insufflation is rare at pressures of less than 30cm H2O. Aspiration of gastric contents has a very low incidence of 5% or less when the airway reflex is intact. Local complications such as skin abrasion, sinus complaints, and conjunctivitis are rather more common. In our patients, there were no complications associated with BiPAP. Since the BiPAP therapy produces less demerits and more merits, earlier institution of BiPAP may be better in postoperative managements of patients undergoing CPB.
doi:10.1016/S1569-9293(03)00005-7
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Top
Abstract
1. Introduction
. The PaO2/FiO2 before the BiPAP therapy decreased still significantly 
. After three times of the intermittent BiPAP therapy, the PaO2/FiO2 improved significantly from the value before the therapy 
. Data are expressed as means±standard deviations.