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From the Departments of *Anesthesiology and Intensive Care, Cardiovascular Surgery, Istanbul University Cardiology Institute; Department of Anesthesiology and Intensive Care, Yeditepe University Hospital; and Department of Anesthesiology and Intensive Care, Istanbul Medical Faculty, Istanbul University, Istanbul, Turkey.
Address correspondence and reprint requests to Özge Köner, Gedikli sok. Vehbi Aytan ap., 31/3 34724, Kiziltoprak, Istanbul, Turkey. Address e-mail to ozgekoner{at}superonline.com.
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Abstract |
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 Top Abstract IntroductionMETHODSRESULTSDISCUSSIONREFERENCES |
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METHODS: Sixty patients undergoing coronary artery bypass surgery were randomized into three groups after operation: recruitment maneuver with continuous positive airway pressure (CPAP) (CPAP-40 group, n = 20), recruitment by positive end-expiratory pressure (PEEP) (PEEP-20 group, n = 20), and 5 cm H2O PEEP (PEEP-5 group, n = 20). In the CPAP-40 group, 40 cm H2O peak inspiratory pressure was applied for 30 s, then PEEP was reduced to 20 cm H2O and ventilation was continued with baseline variables with PEEP decreased until the best Pao2 was achieved. In the PEEP-20 group, 20 cm H2O PEEP was set for 2 min, tidal volume was adjusted to achieve a peak inspiratory airway pressure of 40 cm H2O during the maneuver, then PEEP was decreased until the best Pao2 had been achieved. In the PEEP-5 group, 5 cm H2O PEEP was applied postoperatively.
RESULTS: The mean arterial blood pressure of the CPAP-40 group was lower than that of the PEEP-20 (P < 0.01) and PEEP-5 groups (P < 0.01) during the interventions. Oxygenation was higher in both recruitment groups than in the PEEP-5 group during the mechanical ventilation period. There was no significant difference among the groups beyond that period. The atelectasis score of the PEEP-5 group (1.3 ± 0.9) on postoperative day 1 was higher than that of the CPAP-40 (0.65 ± 0.6; P = 0.01) and PEEP-20 (0.65 ± 0.5; P = 0.01) groups.
CONCLUSIONS: The recruitment techniques with postmaneuver PEEP increased oxygenation and decreased atelectasis equally, whereas PEEP-20 provided more stable hemodynamic conditions than the CPAP maneuver.
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Introduction |
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TopAbstractIntroductionMETHODSRESULTSDISCUSSIONREFERENCES |
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In patients with acute respiratory distress syndrome, sustained inflation up to 45 cm H2O for 20 s to achieve alveolar recruitment has been shown to be effective in improving oxygenation (5). In adults with healthy lungs, inflation up to 40 cm H2O for 7–8 s can expand all the collapsed lung tissue (6). Alveolar recruitment maneuvers (RM) have also been used to improve oxygenation during open heart surgery (7,8). Predetermined positive end-expiratory pressure (PEEP) levels after the maneuvers were preferred in these studies.
Various RM techniques have been used in different clinical studies. One of those is recruitment by means of continuous positive airway pressure (CPAP) (5). "Extended sigh" and recruitment with pressure-controlled ventilation on high PEEP levels are other RM techniques (9,10). Recruitment by means of CPAP has recently been shown to significantly compromise left ventricle function after cardiac surgery (11).
In this study we evaluated the hemodynamic and pulmonary effects of RM by CPAP and RM with 20 cm H2O PEEP on volume-controlled ventilation. We hypothetized that RM with 20 cm H2O PEEP on volume-controlled ventilation would provide better hemodynamic stability than RM by means of CPAP.
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METHODS |
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TopAbstractIntroductionMETHODSRESULTSDISCUSSIONREFERENCES |
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Total IV anesthesia was used in all patients. Mechanical ventilation was continued with a 40% oxygen–air mixture. All patients were tracheally intubated with a cuffed endotracheal tube and ventilated with volume-controlled ventilation (Servo 900C; Siemens, Solna, Sweden) which consisted of a tidal volume of 7 mL/kg on zero end-expiratory pressure. The respiratory rate was adjusted to between 12 and 14 bpm to achieve a Paco2 of 35–45 mm Hg and arterial pH within the physiologic range. The inspiratory/expiratory ratio was 1:2. A 7F thermodilution catheter (Edwards Lifesciences, Irvine, CA) was introduced into the pulmonary artery. Cardiac output was measured with the thermodilution technique, averaging the results of three cold injections (10 mL 5% dextrose at 4°C), using a hemodynamic monitor (Siemens, SC 7000, Stockholm, Sweden).
The lungs were not ventilated and the endotracheal tube was kept open to the atmospheric pressure during CPB. Before discontinuation of CPB, lungs were manually inflated until visible atelectasis disappeared. Ventilation was started with a Fio2 of 0.6, then reduced 15 min later to 0.4. Red blood cell concentrates were transfused to achieve a hemoglobin level of about 9–10 g/dL after CPB. The left internal thoracic artery was used and the left pleura was routinely opened in all patients. All the patients were transferred to the intensive care unit (ICU) under deep sedation with propofol infusion, on volume-controlled mechanical ventilation. Continuous analgesia was used with IV morphine sulfate. All patients had one mediastinal and one left pleural drain. None of the patients received inotropic drugs. Nitroglycerin was infused at a range of 0.5–1.5 mcg · kg–1 · min–1 for 24 h postoperatively.
In the ICU, as soon as hemodynamic stability had been achieved (defined as systolic blood pressure >105 mm Hg, central venous pressure, CVP, equal to baseline value or cessation of systemic blood pressure fluctuations with respiration, heart rate <100), patients were randomized into three groups using a closed envelope system: 1) RM with CPAP and decremental PEEP group (CPAP-40, n = 20), 2) RM by decremental PEEP group (PEEP-20, n = 20), and 3) PEEP group, 5 cm H2O (PEEP-5, n = 20). In the CPAP-40 group, 40 cm H2O peak inspiratory pressure was applied for 30 s using CPAP at a Fio2 value of 40%, then PEEP was immediately reduced to 20 cm H2O and decreased in 1–2 cm H2O steps every 5 min until the lowest PEEP level above 5 cm H2O providing the best partial arterial oxygen pressure (Pao2) had been achieved, after which ventilation with baseline variables was reinstituted with the additional optimal PEEP (Fig. 1). In the PEEP-20 group, a PEEP value of 20 cm H2O was set while ventilating with baseline variables, and tidal volume was adjusted to achieve a peak inspiratory airway pressure of 40 cm H2O during the maneuver. This pressure was maintained for 2 min at a ventilatory frequency of 12 per minute. Then PEEP was decreased in steps of 1–2 cm H2O every 5 min until the lowest value above 5 cm H2O providing the best Pao2 had been achieved (Fig. 2). Arterial blood was drawn at each step to determine the best Pao2. A Fio2 value of 40% was used during the determination of optimal PEEP in RM groups over a 25–30-min period. About 5 cm H2O PEEP was applied in the PEEP-5 group after randomization. In both the CPAP-40 and the PEEP-20 groups, maneuvers were repeated after PEEP titration to prevent derecruitment. The oxygen concentration was set at 40% after the interventions. All patients received rocuronium before the interventions, and a closed suction system was used until extubation. Lung aspiration was performed at the same time intervals in all patients as follows: before the maneuvers, 2 h after the first suctioning, and before the extubation. The patients were not disconnected from the ventilator during the mechanical ventilation period. They were ventilated in a volume-controlled mode for the first 4 h after the maneuvers, and then the ventilation mode was switched to pressure support ventilation. After their ICU stay, all patients were transferred directly to a surgical acute care ward.
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Arterial and mixed venous blood were drawn and analyzed (EML 505, Radiometer, Copenhagen, Denmark) before the interventions, 15 min and 1, 2, 4, and 24 h after the interventions, and 30 min after extubation in the ICU to determine Pao2/Fio2 ratio.
Total PEEP, and peak, pause, and mean airway pressures were obtained from the ventilator's display and recorded before and 15 min after the interventions. The intrinsic PEEP level was determined with the end-expiratory occlusion technique. Plateau airway pressure was determined with the inspiratory hold maneuver for 5 s during volume-controlled ventilation from the pause airway pressure. None of the patients had an intrinsic PEEP level above 1 cm H2O. The total quasi-static lung compliance was calculated by dividing tidal volume by the difference between pause and end-expiratory airway pressures, and dynamic lung compliance was calculated by dividing tidal volume by the difference between peak and end-expiratory airway pressures. Total quasi-static lung compliance values were recorded before and 15 min after the interventions.
MAP, CVP, and heart rate were recorded before, during, and 15 min after the interventions, whereas cardiac index, mean pulmonary artery pressure, pulmonary capillary wedge pressure, and systemic and pulmonary vascular resistance indexes were measured before and 15 min after the interventions.
Chest radiographs were taken on postoperative day 1 and evaluated by a radiologist blinded to the groups, in order to differentiate the pleural effusion from atelectasis and to determine the amount of atelectasis. Atelectasis was graded as 0: no atelectasis; 1: partial atelectasis of the left lower lobe; 2: total atelectasis of the left lower lobe; and 3: total atelectasis of the left lower lobe and other atelectatic lung regions (12). Auscultation for the evaluation of the atelectasis was done by the intensivist, who was also blinded to the groups.
All the values are reported as mean (±sd). One-way ANOVA and post hoc Bonferonni tests were used to compare means among the groups. Repeated measures of ANOVA was used for within-group comparisons. This study was designed to have a 90% power to detect a difference of 100 between the mean Pao2/Fio2 ratios of the control and study group patients with a significance level of (
) 0.05. Atelectasis scores were compared by Mann–Whitney U-test, and P < 0.05 was considered significant. Unistat version 5.0 for Windows (Unistat, London, UK) was used for the statistical analysis.
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RESULTS |
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TopAbstractIntroductionMETHODSRESULTSDISCUSSIONREFERENCES |
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Heart rate values were not different among groups throughout the study. However, during the recruitment maneuver, MAP values of the CPAP-40 group were significantly lower than those of the PEEP-20 (P < 0.01) and PEEP-5 groups (P < 0.01) and than the baseline value (P < 0.01). In the CPAP-40 group, MAP values of five patients decreased to a level between 40 and 50 mm Hg toward the end of the maneuver, but returned to baseline as soon as the maneuver resumed (Fig. 3). The mean CVP of the CPAP-40 group was significantly higher than that of both the PEEP-20 and PEEP-5 groups during the intervention period (both P < 0.01) (Fig. 4). Systemic and pulmonary vascular resistance indexes, cardiac index, mean pulmonary artery pressure, and pulmonary capillary wedge pressure were identical among the groups during the study period (Table 2, Fig. 5).
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The Pao2/Fio2 ratios of the CPAP-40 and PEEP-20 groups were significantly higher than in the PEEP-5 group at 15 min, 1, 2, and 4 h after the recruitment maneuvers. Beyond that period, there was no statistically significant difference in Pao2/Fio2 ratio among the groups. Pao2/Fio2 ratios of the RM groups increased significantly from baseline until the extubation period, then returned to baseline values. However, the Pao2/Fio2 ratios in the PEEP-5 group increased significantly from baseline only at 1 and 2 h after 5 cm H2O PEEP application (Fig. 6).
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Total static and dynamic compliances of both RM groups were significantly higher than those of the PEEP-5 group after the interventions (P < 0.01 for both groups). Total PEEP of both RM groups after the PEEP titration was significantly higher than in the PEEP-5 group, and peak and plateau airway pressures of both RM groups were also significantly higher. Airway pressures, total respiratory system compliance (lung and chest wall), and the total PEEP values of the groups are shown in Table 3. Mixed venous oxygen saturation was not different among groups (Table 3).
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Total operative fluid balance (856 ± 624 mL in the CPAP-40 group, 882 ± 883 mL in the PEEP-20 group, and 886 ± 795 mL in the PEEP-5 group) and red blood cell concentrate transfusion (2.3 ± 0.9 U in the CPAP-40 group, 2.3 ± 1. 4 U in the PEEP-20 group, and 2 ± 0.5 U in the PEEP-5 group) were not different among groups.
The nitroglycerin infusion rate in the PEEP-5 group was significantly lower than that in both RM groups (CPAP-40 group: 1.05 ± 0.2 mcg · kg–1 · min–1; PEEP-20 group: 1.05 ± 0.3 mcg · kg–1 · min–1; PEEP-5 group: 0.8 ± 0.15 mcg · kg–1 · min–1; P = 0.02 when compared with both RM groups).
The atelectasis score of the PEEP-5 group on chest radiograph (1.3 ± 0.9) was significantly higher than the scores of the CPAP-40 (0.65 ± 0.6; P = 0.01) and PEEP-20 (0.65 ± 0.5; P = 0.01) groups. None of the patients in the RM group had a Grade 3 atelectasis on the chest radiograph (total atelectasis of the left lower lobe and other atelectatic lung regions), compared with four patients in the PEEP-5 group.
None of the patients showed evidence of pulmonary barotrauma on postoperative chest radiograph. Two patients in the CPAP-40 group were excluded from the study and the intervention was terminated immediately because of excessive hypotension and bradycardia during the maneuver. One patient in the PEEP-5 group experienced total left lung atelectasis (confirmed by thorax computed tomography, CT, on postoperative day 3) and hypoxia after extubation. As a result, this patient's stay in the ICU (4 days) and in the hospital (12 days) were prolonged compared to the mean length of stay.
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DISCUSSION |
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TopAbstractIntroductionMETHODSRESULTSDISCUSSIONREFERENCES |
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Atelectasis has been shown to be correlated with impaired gas exchange, reduced arterial Po2, and increased shunt fraction in the postoperative period (13). Increases in extravascular lung water and further collapse of the lung tissue may aggravate this shunt after cardiac surgery (14). In one study, atelectatic lungs could be re-opened using high inspiratory pressures and kept open with high PEEP (15).
It has been shown that RM followed by 5 cm H2O PEEP improves oxygenation for only 1 h when compared with zero end-expiratory pressure and 5 cm H2O PEEP without RM after heart surgery (7). These authors applied the RM during CPB with an open sternum. Manipulations after CPB, such as chest tube placement or compression of the lungs during control of bleeding, may lead to new atelectatic areas in the lung, and may be the reason for the brief improved oxygenation period. If the lungs are ventilated with 100% oxygen, atelectasis recurs within 5 min after a RM; nevertheless, 10 cm H2O PEEP application has been shown to reduce atelectasis formation (16). In the case of high Fio2, higher PEEP levels (14 ± 3 cm H2O) are required to maintain lung volumes and improve oxygenation after open heart surgery (8). The latter study improved oxygenation for a prolonged period (2.5 h) after RM. The reason for the comparable oxygenation of the RM groups is probably the equally high end-inspiratory pressures generated by both RM techniques and their recruitment effects on atelectatic alveoli, as shown previously in different studies. Peak end-inspiratory pressure levels of about 35–40 cm H2O generated with external PEEP have been shown to provide lung recruitment (17).
The lower inflection point (LIP) can be estimated from the overall pressure–volume curve of the respiratory system. It is technically difficult to measure LIP, and it is not possible to determine this point in some patients (18). This is because the LIP indicates the pressure at which lung recruitment starts, not the pressure at which the lung collapse starts. This was revealed by a study showing that derecruitment induced by PEEP obtained using a stepwise reduction is more useful than LIP-based PEEP determination (19). Furthermore, the location of the LIP is influenced both by lung and chest wall elastances (20). The latter has been shown to increase after open heart surgery (21).
PEEP pressure levels in our study were probably below the LIP when we look at the PEEP levels determined according to pressure–volume curves (8). Furthermore, it has been shown that once the lungs are fully recruited by a RM, it is possible to keep them open with PEEP levels below LIP (22). Because of the aforementioned reasons, we decided to use a decremental PEEP trial to find an optimal and individualized value.
As there were only a few reports evaluating the effects of peri-RM hemodynamic variables, RMs were generally thought to be well tolerated (10,23). However, a recent study revealed that sustained inflation markedly reduced cardiac output and left ventricular end-diastolic area in hemodynamically stable patients after cardiac surgery (11). Lung expansion at high PEEP levels has been shown to compress the heart and to increase right atrial pressure, impeding venous return (24,25). Increased right ventricular afterload and concomittantly decreased left ventricular preload due to the increased intrathoracic pressure and lung volume are probably the reason for the excessive hypotension observed during sustained inflation (11). Sustained inflation was shown to lead to systemic hypotension and decreased aortic blood flow in an experimental acute lung injury model (24). In that study, as in ours, RM performed during continuous pressure-controlled ventilation using high peak airway pressure (20 cm H2O PEEP to achieve a peak airway pressure of 40 cm H2O) provided a more stable hemodynamic condition than sustained inflation, and provided comparable oxygenation. Intermittent peak inspiratory airway pressure application in the PEEP-20 group is probably the reason for the better hemodynamic condition when compared with sustained inflation.
Although all the groups had comparable oxygenation at the end of the study, atelectasis scores of both RM groups were lower than those of the PEEP-5 group. An explanation is that a PEEP level of 5 cm H2O is not capable of opening the lung after surgery. A study evaluating the relationship of atelectasis and arterial oxygenation revealed that, despite reversing lung densities on CT scan, 10 cm H2O PEEP had no effect on arterial oxygenation. It has been suggested that PEEP altered the pulmonary shunt and atelectasis relation by distributing the blood toward the dependent atelectasis and shunt regions (3). As our patients were tracheally extubated during the chest radiograph evaluation, we suggest that the nitroglycerin infusion may be the explanation for the altered oxygenation and atelectasis relationship in both recruitment groups. Hypoxic pulmonary vasoconstriction, through diversion of blood from atelectatic regions toward the recruited lung regions, might have prevented the decline in the P/F ratio of group PEEP-5. Furthermore, perfusion of the hypoventilated lung regions, apart from atelectasis, may also lead to decreased arterial oxygenation. Extravascular lung water, which was not measured in our study, may also be the reason for the similar P/F ratios of all the groups despite the different atelectasis scores. Whether open pleura alters the atelectasis and oxygenation relationship is not clear.
Despite achieving better oxygenation with both RMs compared to the PEEP-5 group during the mechanical ventilation period, we could not prolong the effects on oxygenation beyond that period. The reason that both RM groups had more favorable atelectasis scores than the PEEP-5 group needs to be clarified with further studies.
Evaluating atelectasis by means of chest radiograph instead of CT may be a limitation of our study. However, it has been reported that it is difficult to distinguish the pleural fluid from atelectasis after cardiac surgery even with thorax CT (3). Also, the small number of patients participating in our study may have interfered with the outcome data, such as length of hospitalization and ICU stay.
In conclusion, the two RM techniques we used resulted in comparable oxygenation, lung compliance, and atelectasis scores in cardiac surgery patients, whereas the PEEP-20 trial provided better hemodynamic stability than a RM by CPAP. However, duration of mechanical ventilation, ICU stay, and the length of hospitalization were not different among the groups.
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Footnotes |
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This work was presented at the 18th annual congress of the European Society of Intensive Care Medicine, held September 25–28, 2005, in Amsterdam.
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REFERENCES |
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TopAbstractIntroductionMETHODSRESULTSDISCUSSIONREFERENCES |
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This article has been cited by other articles:
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  S. Celebi, O. Koner, F. Menda, O. Omay, I. Gunay, K. Suzer, and N. Cakar Pulmonary Effects of Noninvasive Ventilation Combined with the Recruitment Maneuver After Cardiac Surgery Anesth. Analg., August 1, 2008; 107(2): 614 - 619. [Abstract] [Full Text] [PDF]
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