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Ann Thorac Surg 2002;74:1191-1194
© 2002 The Society of Thoracic Surgeons
a Department of General/Cardiothoracic Surgery, Conemaugh Memorial Medical Center, Johnstown, Pennsylvania, USA
b Department of Pharmacy, Conemaugh Memorial Medical Center, Johnstown, Pennsylvania, USA
Accepted for publication June 7, 2002.
* Address reprint requests to Dr Collier, Department of General/Cardiothoracic Surgery, Conemaugh Memorial Medical Center, 1086 Franklin St, Johnstown, PA 15905-4398 USA
e-mail: chiltoncollier{at}pol.net
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Abstract |
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 Top Abstract IntroductionMaterials and methodsResultsCommentReferences |
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Methods. We prospectively studied 84 elective coronary artery bypass grafted patients and randomized treatment groups to a PEEP of 5 or 10 cm H2O. Forty-four patients were assigned a PEEP of 5 cm H2O and 40 patients received a PEEP of 10 cm H2O.
Results. Preoperative, intraoperative, and postoperative demographics were similar between groups. There was no statistically significant difference between the 5 cm H2O PEEP group and the 10 cm H2O PEEP group with regard to chest-tube output at 6 hours, at 24 hours, or in total output. There was no statistical difference in hemoglobin levels immediately postoperatively, at 8 hours, or at 36 hours.
Conclusions. This study demonstrates that the use of postoperative PEEP levels of 10 cm H2O, although safe, does not reduce chest-tube output or transfusion requirements.
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Introduction |
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One strategy is to use positive end-expiratory pressure (PEEP) postoperatively with patients on positive-pressure ventilation [9]. PEEP increases end-expiratory airway pressure to more than atmospheric pressure. Two indications for PEEP are refractory hypoxemia and decreased functional residual capacity. PEEP levels as high as 15 cm of H2O have been used to control mediastinal drainage [10]. Postoperative chest-tube output has been reported to be a risk factor for transfusion after open-heart surgery [11, 12]. A conventional PEEP level of 5 cm H2O is applied to our patients postoperatively while on mechanical ventilation. When patients experience increased chest-tube output (theoretically representing blood loss from the mediastinum and left or right internal mammary artery beds), PEEP is increased to 7.5 to 15 cm H2O. This presumably works by increasing mediastinal pressure, creating a mechanical tamponade. The goal is to prevent red-cell transfusions and mediastinal explorations [10]. There is little literature on the prophylactic application of PEEP.
To elucidate the benefits and risks associated with prophylactic PEEP, we conducted a randomized, controlled, clinical trial in patients undergoing coronary artery bypass graft (CABG) surgery to determine whether a PEEP of 10 cm of H2O versus a PEEP of 5 cm of H2O applied postoperatively reduces mediastinal chest-tube output.
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Materials and methods |
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Consecutive patients were randomized to either a PEEP of 10 cm of H2O (experimental group) or a PEEP of 5 cm of H2O (control group). Sealed envelopes arranged in a computer-generated random order were opened sequentially to determine the patients’ treatment. After surgery, patients immediately were taken to the coronary-care unit. Each patient was placed on synchronized intermittent mandatory ventilation. Patients received a tidal volume of 10 mL/kg of ideal body weight and a respiratory rate of 12/min. These measurments were changed according to the CO2 (goal of 40 mm Hg) and pH (goal of 7.40). Patients also received a pressure support of 10 cm H2O and either a PEEP of 5 or 10 cm H2O.
Routine postoperative care with continuous Swan-Ganz catheterization and intraarterial blood-pressure monitoring was performed as part of cardiac-surgical intensive care. The patients were weaned and extubated from the ventilator over the next several hours in the usual manner. Patients randomized to a PEEP of 10 cm of H2O had their PEEP decreased to 5 cm of H2O before extubation. Patients were followed until discharge (or up to 30 days). The attending physician determined the need for transfusion. No specific transfusion protocol was utilized for the study.
Demographic, diagnostic, and therapeutic information was obtained for patients. Standard intraoperative and postoperative values were noted to facilitate comparisons between groups. Cumulative chest-tube output was recorded at 6 and 24 hours postoperatively and during chest-tube removal. Hemoglobin concentrations were recorded immediately after surgery, within the next 8 hours, and 36 hours postoperatively. Red-cell transfusions and intravenous fluids were noted.
The primary outcome measure was the average chest-tube output between the 5 cm H2O group and the 10 cm H2O group. In addition, the safety and tolerability of PEEP were investigated for prophylactic use in postoperative CABG patients. This strategy presumably would reduce the use of blood products. Secondary outcome measures included cumulative transfusions postoperatively and the crystalloid or colloid administered. The frequency and severity of adverse events associated with PEEP also were monitored. This study was stopped by the researchers because of the higher volume requirements in the 10 cm H2O to maintain MAP.
Variables were analyzed with SPSS 10.0 (Chicago, IL). Dichotomous variables were analyzed using a 
2 test, whereas continuous variables were assessed by Student’s t-test and Mann-Whitney U test. For all statistical tests, a p value less than 0.05 was considered significant.
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Results |
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For postoperative hemoglobin concentration, there were no statistical differences immediately postoperatively (11.7 ± 1.3 g/dL versus 11.8 ± 1.7 g/dL, p = 0.24), at 8 hours postoperatively (10.4 ± 1.4 g/dL versus 10.2 ± 1.5 g/dL, p = 0.50), or at 36 hours postoperatively (10.0 ± 1.4 g/dL versus 10.1 ± 1.5 g/dL, p = 0.74).
Postoperatively, patients were administered crystalloid (lactated ringers or normal saline) and colloid fluids via albumin or packed red blood cells. The types of fluid and the amounts were administered to resuscitate patients to a MAP between 65 and 85 mm Hg, a CI greater than 2.0, and a urine output of 0.5 mL · kg-1 · h-1. Clinical judgment rendered the choice of crystalloid or colloid. The PEEP of 10 cm H2O group had a slightly higher transfusion rate of packed red blood cells (0.8 ± 1.4 U versus 1.1 ± 1.6 U, p = 0.50), required more ICU crystalloid (2.0 ± 1.0 L versus 2.6 ± 2.2 L, p = 0.17) and albumin (59.2 ± 37.0 g versus 73.7 ± 31.5 g, p = 0.05). In addition, the PEEP of 10 cm H2O group had slightly longer hospital stays (5.2 ± 1.7 days versus 5.7 ± 2.5 days, p = 0.34). However, the differences between the groups were not statistically significant.
Morbidity/mortality
Of the 84 patients, only one required reoperation for bleeding. The patient originally was entered in the 10 cm H2O PEEP group and had no other adverse events. There also was one death in each group. Neither death was related to PEEP use, and there were no signs of increased chest-tube output or evidence of pericardial effusion or tamponade. None of the patients demonstrated new onsets of renal insufficiency, myocardial infarction, or cerebrovascular accident.
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Comment |
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Positive end-expiratory pressure has been used for decades as an adjunct to ventilator therapy. Its main purpose is to increase arterial oxygen and to improve oxygenation of peripheral tissues [13]. Proposed mechanisms include recruitment of lung units and increasing alveolar-surface area during expiration [14]. Major PEEP complications include alveolar over-distention and decreased cardiac output by decreasing preload/venous return and coronary blood flow [15, 16].
Several studies have evaluated PEEP use following open-heart surgery. One was a prospective study that examined the effect of PEEP (0, 5, and 10 cm of H2O) on the severity of impaired oxygen transfer and roentgenographic evidence of atelectasis after CABG. This study concluded routine PEEP does not alter arterial hypoxemia or roentgenographic evidence of atelectasis [17].
Positive end-expiratory pressure also has been reported to manage nonsurgical bleeding in CABG patients. Though a small series, Ilabaca and coworkers [10] demonstrated a decrease in chest-tube output with PEEP use (10 or greater cm H2O) in patients whose drainage was at least 200 mL/h. In addition, upward of 73% of patients may have avoided reoperation.
When evaluating chest-tube output, we did not find a statistical difference between those who had PEEP levels of 5 cm H2O compared with those who had PEEP levels of 10 cm H2O. Chest-tube drainages were similar at 6 hours and 24 hours postoperatively. In addition, total chest-tube output was unaffected by the increased PEEP. Furthermore, no statistical difference was found in hemoglobin levels postoperatively between the 5 cm H2O PEEP group and the 10 cm H2O PEEP group.
Additionally, no statistical difference was identified between the groups in relation to crystalloid fluids or packed red blood cells, but their use was slightly higher in the 10 cm H2O PEEP group. As increased levels of PEEP decreases venous return and CI, more fluid resuscitation may be needed.
In addition to crystalloid resuscitation, our institution uses albumin in the postoperative period. Five percent or 25% albumin was given. When comparing the two groups, the 10 cm H2O PEEP group received more grams of albumin than the 5 cm H2O PEEP group. The increased intrathoracic pressure induced by higher PEEP may have necessitated additional albumin administration.
It is important to note that this study suffered from one important limitation. The study’s population (n = 84) could have led to type-two error because of statistical power. Future research with a larger sample could examine whether this occurred. However, as noted earlier, this trial was stopped early because it was clear to the researchers that the intervention was not enhancing care and could have been causing detrimental effects.
No definitive evidence exists whether increased amounts of crystalloid or colloid administered to postoperative CABG patients affects outcome, but we tended to provide more fluids to the 10 cm H2O PEEP group. These patients tended to have longer hospital stays, although these differences were not statistically significant. However, this does not demonstrate poorer outcomes. In fact, there was no difference in morbidity or mortality between the groups. But it does suggest more money is spent on patients with a PEEP of 10 cm H2O applied prophylactically to CABG patients for as little as 6 to 7 hours (Table 3).
In conclusion, this study demonstrates prophylactic administration of postoperative PEEP levels of 10 cm H2O, although safe, does not seem to decrease chest-tube output or transfusion requirements. The use of higher PEEP levels may increase the fluids required to sustain cardiac measurements, possibly extending hospital stay.
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This article has been cited by other articles:
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  The Society of Thoracic Surgeons Blood Conservatio, V. A. Ferraris, S. P. Ferraris, S. P. Saha, E. A. Hessel II, C. K. Haan, B. D. Royston, C. R. Bridges, R. S.D. Higgins, G. Despotis, et al. Perioperative Blood Transfusion and Blood Conservation in Cardiac Surgery: The Society of Thoracic Surgeons and The Society of Cardiovascular Anesthesiologists Clinical Practice Guideline Ann. Thorac. Surg., May 1, 2007; 83(5_Supplement): S27 - S86. [Abstract] [Full Text] [PDF]
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