Skip to main content
Download PDF

Maintaining moderate versus lower PEEP after cardiac surgery: a propensity-scored matched analysis

BMC Anesthesiology volume 24, Article number: 55 (2024) Cite this article

Abstract

Background

Setting positive end-expiratory pressure (PEEP) at around 5 cm H2O in the early postoperative period seems a common practice for most patients. It remains unclear if the routine application of higher levels of PEEP confers any meaningful clinical benefit for cardiac surgical patients. The aim of this study was to compare moderate versus conventional lower PEEP on patient-centered outcomes in the intensive care unit (ICU).

Methods

This is a single-center retrospective study involving patients receiving cardiac surgery from June 2022 to May 2023. Propensity-score matching (PSM) was used to balance the baseline differences. Primary outcomes were the duration of mechanical ventilation and ICU length of stay. Secondary outcomes included PaO2/FiO2 ratio at 24 h and the need for prone positioning during ICU stay.

Results

A total of 334 patients were included in the study, 102 (31%) of them received moderate PEEP (≥ 7 cm H2O) for the major time in the early postoperative period (12 h). After PSM, 79 pairs of patients were matched with balanced baseline data. The results showed that there was marginal difference in the distribution of mechanical ventilation duration (p = 0.05) and the Moderate PEEP group had a higher extubation rate at the day of T-piece trial (65 [82.3%] vs 52 [65.8%], p = 0.029). Applying moderate PEEP was also associated with better oxygenation. No differences were found regarding ICU length of stay and patients requiring prone positioning between groups.

Conclusion

In selective cardiac surgical patients, using moderate PEEP compared with conventional lower PEEP in the early postoperative period correlated to better oxygenation, which may have potential for earlier liberation of mechanical ventilation.

Peer Review reports

Introduction

Cardiac surgery has a marked effect on the cardiopulmonary physiology [1]. Cardiac patients are usually at higher risk of weaning failure for mechanical ventilation and postoperative pulmonary complications including decreased lung compliance, atelectasis and infection [2, 3]. Most postoperative cardiac surgical patient are admitted to an intensive care unit (ICU) intubated and mechanically ventilated. Ventilatory support is an important component of postoperative management. Appropriate ventilator settings help to improve lung physiology, reduce postoperative pulmonary complications, and facilitate weaning from mechanical ventilation, allowing earlier ICU discharge and improved outcomes.

The effect of positive end-expiratory pressure (PEEP) setting is often underestimated in postoperative patients. Although personalized PEEP is recommended, it seems that setting PEEP at 5 cm of water (cm H2O) is a routine practice in most postoperative patients [4, 5], especially for those who show no signs of significant hypoxia on ICU admission. Some data suggest that higher initial PEEP after cardiac surgery (8 ~ 10 cm H2O vs 5 cm H2O) improve oxygenation, lung compliance and reduce atelectasis [4, 6, 7], yet it remains unclear if the routine application of higher PEEP results in any meaningful clinical benefit. Conflicting results exists for cardiac surgical patients on the impact of PEEP on mechanical ventilation days or ICU length of stay [4, 8,9,10]. In addition, postoperative hypoxemia due to atelectasis or consolidation may trigger the decision of prone positioning, which could increase clinical workload and bring some extra risks.

The aim of this retrospective study was to compare the effects of early application of moderate versus conventional lower PEEP levels in selected postoperative cardiac patients on the duration of mechanical ventilation, ICU length of stay (as primary outcomes), as well as the impact on oxygenation and the need for prone positioning during ICU stay (as secondary outcomes).

Methods

Settings and study population

This is a single-center retrospective study in the Department of Critical Care Medicine of a tertiary teaching hospital, with 30 surgical ICU beds. Patients admitted to the ICU after cardiac surgery during the period from June 1, 2022 to May 31, 2023 were reviewed. The exclusion criteria were patients (1) aged less than 18 years old; (2) not intubated on admission or extubated within 12 h; (3) non-survival or expected non-survival; (4) ICU length of stay over 7 days; (5) requiring fraction of inspired oxygen (FiO2) equal or greater than 50% on ICU admission. This study was approved by the Institutional Research and Ethics Committee of Peking Union Medical College Hospital.

Data collection and patient grouping

Patient baseline data was retrieved from the electronic medical record system. ICU-related data were extracted from the Critical Care Monitor System in Peking Union Medical College Hospital, which recorded real-time clinical data hourly with bedside equipment. The following information was collected: demographic data, including age, sex, body mass index (BMI) and Acute Physiology and Chronic Health Evaluation (APACHE) II score; comorbidities and disease severity (hypertension, diabetes, obstructive lung disease, atrial fibrillation, chronic heart failure defined as New York Heart Association [NYHA] Classification of Heart Failure class III ~ IV, left ventricular ejection fraction [LVEF] before surgery, pulmonary hypertension); surgical information (type of surgery, cardiopulmonary bypass time, dose of epinephrine and norepinephrine); Oxygenation on ICU admission and outcomes (duration of mechanical ventilation, ICU length of stay, oxygenation after 24 h of ICU admission, prone positioning during ICU stay).

PEEP was initially set and adjusted according to the preference of physician in charge. Generally, the Lower PEEP/Higher FiO2 Table [11] was the reference on the initial PEEP. To take the duration of PEEP into consideration, patients were divided into two groups based on the major PEEP levels (more than 6 h) during the first 12 h of ICU admission: the Moderate PEEP group (equal or greater than 7 cm H2O) and the Lower PEEP group (less than 7 cm H2O).

Weaning process and prone positioning criteria

In the study center, typical weaning process for patients consists of a two-step trial. A patient with eligibility for weaning (PEEP < 10 cm H2O, FiO2 < 50%, resolution of acute phase, existence of spontaneous breathing and stable hemodynamics judged by physician) would be first turned to pressure support ventilation with low-level support (PEEP 5 ~ 8 cm H2O, PS 6 ~ 10 cm H2O) for 30 min. If the patient showed no signs of respiratory distress and hemodynamic deterioration, then a T-piece trial would be followed usually for 30 min to 2 h. Patients who successfully passed the trial would be extubated and sequenced to high-flow nasal oxygen or nasal catheter. The criteria for passing the trial depended on the attending doctor and were generally stricter than existing guidelines [12, 13] to achieve a lower re-intubation rate (less than 5% in cardiac patients).

Prone positioning was advocated when the PaO2/FiO2 ratio was lower than 200 mm Hg (PEEP ≥ 8 cmH2O or FiO2 ≥ 50%) and there existed imaging evidence indicating consolidation or poor ventilation in the dorsal lungs.

Statistical analysis

Continuous variables are expressed as median (interquartile range, IQR) and compared using Mann–Whitney U test as appropriate. Categorical variables were expressed as frequency and percentage and compared using the chi-square test or Fisher's exact test. We performed a 1:1 propensity score-matched analysis (using ‘MatchIt’ Package [14] in R software) using nearest neighbor matching without replacement and a caliper value of 0.05 to compare the Moderate PEEP group and the Lower PEEP group patients. Propensity score matching was performed based on sex, age, body mass index, APACHE II score, comorbidities, LVEF before surgery, type of surgery, cardiopulmonary bypass time, dose of epinephrine and partial pressure of arterial oxygen (PaO2) on ICU admission. The quality of match was assessed by examining the standardized mean differences of variables before and after matching. R software Version 4.2.2 were used for all statistical analyses, with p value ≤ 0.05 indicating statistical significance.

Results

Patient characteristics

During the study period, a total of 443 patients were reviewed and, of these, 334 were included in the initial analysis (232 [69.5%] classified as the Lower PEEP group and 102 [31.5%] as the Moderate PEEP group; Fig. 1). The patients had a median age of 57(46–65) years, and 134 (40.1%) of them were women.

Fig. 1
figure 1

Flowchart of patient inclusion, exclusion and grouping in the study

Full size image

The Moderate PEEP group had higher body mass index (24.8 [23.5–27.3] vs 23.7 [21.5–25.7] kg/m2, p < 0.001), lower partial pressure of oxygen (PaO2; 89 [75–119] vs 106 [86–129] mm Hg, p < 0.001) and lower PaO2/FiO2 ratio (259 [209–337] vs 311 [246–384] mm Hg, p < 0.001) on ICU admission. Other baseline characteristics were comparable (Table 1). After propensity score matching in a 1:1 ratio, there were 79 patients in each group, and no significant difference was found between the two groups regarding collected baseline data (Table 1; Fig. 2).

Table 1 Demographic and clinical data before and after propensity matching
Full size table
Fig. 2
figure 2

Parameters involved and visualization of propensity-scored match

Full size image

Outcomes

After propensity matching, there was marginal difference in the distribution of duration of mechanical ventilation (p = 0.050). Patients in the Moderate PEEP group had a higher rate of extubation after the first T-piece trial (65 [82.3%] vs 52 [65.8%], p = 0.029). No difference was found between groups regarding ICU length of stay and days from ICU admission to first T-piece trial (Table 2).

Table 2 Outcomes before and after propensity matching
Full size table

The Moderate PEEP group had a higher PaO2 (102 [89–132] vs 95 [83–106] mm Hg, p = 0.039) and PaO2/FiO2 ratio (300 [253–344] vs 330 [254–427] mm Hg, p = 0.048) at 24 h after ICU admission. The recorded lowest PaO2 and PaO2/FiO2 ratio within 24 h after ICU admission were also higher in the Moderate PEEP group. The proportion of patients receiving prone positioning during their ICU stay (8 [10.1%] vs 14 [17.9%] mm Hg, p = 0.237) did not show difference between groups (Table 2).

Discussion

In this single-center retrospective study, we found that the early application of moderate PEEP in postoperative cardiac patients was associated with oxygenation improvement and potentially shorter duration of mechanical ventilation.

In our cohort, only around 30% of postoperative cardiac patients received moderate or higher PEEP strategy on ICU admission, others reporting this proportion around 25 ~ 34% [4, 5]. Setting lower PEEP levels (usually no more than 5 cm H2O) in the early postoperative period remains a common practice [15, 16], especially when the oxygenation was not or only mildly impaired, as seen in the study population. Physicians typically start considering the possibility of lung collapse and trying to increase PEEP when hypoxia was severe enough to be noticed. Atelectasis and hypoxia may contribute to weaning failure, resulting in a longer duration of mechanical ventilation. In this study, not extubating the patient after the first T-piece trial was a surrogate for weaning failure. Applying and maintaining moderate or higher PEEP levels in the early postoperative period may prevent the patients from atelectasis [17, 18].

Prone positioning may be applied to promote lung recruitment and improve oxygenation. However, turning patients prone not only increases the workload but also carries potential risks including pressure ulcers, device displacement and hemodynamic instability [19]. Additionally, surgeons may have concerns regarding the impact of prone positioning on surgical incision healing. Setting moderate or higher PEEP levels in the early postoperative period may reduce lung collapse [7], facilitate successful weaning from mechanical ventilation, and avoid the need for prone positioning as a rescue therapy. Although our data showed no statistically significant difference between groups regarding the need for prone positioning, the point estimation was in favor of the Moderate PEEP group both in the unmatched and matched cohorts.

There have been some studies with conflicting results on this topic previously, yet this study may have the following strengths: (1) The duration of PEEP was considered in patient grouping. Some studies examined the effects of intraoperative PEEP [7] or the initial PEEP at ICU [4] on patient outcomes. The time-dependent effect or the adjustment of PEEP could have been underestimated. We therefore defined that patients classified as the Moderate PEEP group should maintain at higher PEEP levels for the major time within 12 h after surgery. (2) Patient selection may be important to recognize the specific population that benefit from moderate PEEP. Patients whose primary outcomes were unlikely to be associated with their exposure PEEP levels were excluded. For example, those who did not survive or had ICU length of stay longer than 7 days were more likely to be affected by low cardiac output syndrome or the need for a second surgery rather than PEEP levels. (3) Propensity-score match with a relative strict caliper value was used to balance baseline characteristics.

There are several limitations in the present study. While the cutoff PEEP value for patient grouping may seem arbitrary and take the risk of selection bias, we originally intended to increase the PEEP differences between groups as much as possible. The effect of 1 cmH2O could be too little to be detected. Clinical adjustment of PEEP usually favors 2 ~ 3 cmH2O per step, as could be seen in the ARDSnet PEEP/FiO2 table and PEEP titration protocols in some randomized trials. Cutoff PEEP value of 6 or 8 cmH2O for grouping were also analyzed (see in Supplementary Material). The results on P/F ratio improvement favored the higher PEEP levels, while statistical significance was not found for the duration of mechanical ventilation. Lower PEEP difference between groups might mask the benefit of moderate PEEP. Second, reasons for failing to extubating patients after the first T-piece trial could be multifactorial, with the impact of PEEP may only account for a limited, yet significant part. Cardiac function could be a major potential confounding factor for weaning outcome [20]. Sometimes it is not easy to distinguish cardiac factor from lung dysfunction for weaning failure. The dose of epinephrine was recorded and balanced as an alternative parameter reflecting cardiac function. Third, measurements of static respiratory mechanics and lung volume were unavailable due to its retrospective nature of the study. Fourth, concerns on hemodynamics may be raised when using moderate or higher PEEP, but using 10 cm H2O PEEP was usually safe for hemodynamics [8]. In this study, dose of norepinephrine was not higher in the Moderate PEEP group. Last, propensity-score matching provides a way for narrowing basal differences between study groups, but it may not eliminate all potential confounding factors.

Conclusion

In selective patients, using moderate PEEP compared with conventional lower PEEP in the early postoperative period after cardiac surgery correlated to shorter duration of mechanical ventilation and better oxygenation.

Availability of data and materials

No datasets were generated or analysed during the current study.

References

  1. Stephens RS, Whitman GJ. Postoperative critical care of the adult cardiac surgical patient. Part i: routine postoperative care. Crit Care Med. 2015;43(7):1477–97.

    Article  CAS  PubMed  Google Scholar 

  2. Apostolakis E, Filos KS, Koletsis E, Dougenis D. Lung dysfunction following cardiopulmonary bypass. J Card Surg. 2010;25(1):47–55.

    Article  PubMed  Google Scholar 

  3. Bignami E, Guarnieri M, Saglietti F, Belletti A, Trumello C, Giambuzzi I, et al. Mechanical ventilation during cardiopulmonary bypass. J Cardiothorac Vasc Anesth. 2016;30(6):1668–75.

    Article  PubMed  Google Scholar 

  4. Hansen JK, Anthony DG, Li L, Wheeler D, Sessler DI, Bashour CA. Comparison of positive end-expiratory pressure of 8 versus 5 cm H2O on outcome after cardiac operations. J Intensive Care Med. 2015;30(6):338–43.

    Article  PubMed  Google Scholar 

  5. Bignami E, Di Lullo A, Saglietti F, Guarnieri M, Pota V, Scolletta S, et al. Routine practice in mechanical ventilation in cardiac surgery in Italy. J Thorac Dis. 2019;11(4):1571–9.

    Article  PubMed  PubMed Central  Google Scholar 

  6. Borges DL, Nina VJ, Costa Mde A, Baldez TE, 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.

    Article  PubMed  Google Scholar 

  7. Li XF, Jiang RJ, Mao WJ, Yu H, Xin J, Yu H. The effect of driving pressure-guided versus conventional mechanical ventilation strategy on pulmonary complications following on-pump cardiac surgery: a randomized clinical trial. J Clin Anesth. 2023;89:111150.

    Article  PubMed  Google Scholar 

  8. 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.

    CAS  PubMed  Google Scholar 

  9. Lago Borges D, da José Silva Nina V, Pereira Baldez TE, de Albuquerque Gonçalves Costa M, dos Pereira Santos N, Mendes Lima I. Effects of positive end-expiratory pressure on mechanical ventilation duration after coronary artery bypass grafting: a randomized clinical trial. Ann Thorac Cardiovasc Surg. 2014;20(Suppl):773–7.

    Article  PubMed  Google Scholar 

  10. Michalopoulos A, Anthi A, Rellos K, Geroulanos S. Effects of positive end-expiratory pressure (PEEP) in cardiac surgery patients. Respir Med. 1998;92(6):858–62.

    Article  CAS  PubMed  Google Scholar 

  11. Acute Respiratory Distress Syndrome Network, Brower RG, Matthay MA, Schoenfeld AMD, Thompson BT, Wheeler A. Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute respiratory distress syndrome. N Engl J Med. 2000;342(18):1301–8.

    Article  Google Scholar 

  12. Boles JM, Bion J, Connors A, Herridge M, Marsh B, Melot C, et al. Weaning from mechanical ventilation. Eur Respir J. 2007;29(5):1033–56.

    Article  PubMed  Google Scholar 

  13. Schmidt GA, Girard TD, Kress JP, Morris PE, Ouellette DR, Alhazzani W, et al. Liberation from mechanical ventilation in critically Ill adults: executive summary of an official American college of chest physicians/American thoracic society clinical practice guideline. Chest. 2017;151(1):160–5.

    Article  PubMed  Google Scholar 

  14. Ho D, Imai K, King G, Stuart EA. MatchIt: nonparametric preprocessing for parametric causal inference. J Stat Softw. 2011;42(8):1–28.

    Article  Google Scholar 

  15. LAS VEGAS investigators. Epidemiology, practice of ventilation and outcome for patients at increased risk of postoperative pulmonary complications: LAS VEGAS - an observational study in 29 countries. Eur J Anaesthesiol. 2017;34(8):492–507.

    Article  Google Scholar 

  16. Peñuelas O, Muriel A, Abraira V, Frutos-Vivar F, Mancebo J, Raymondos K, et al. Inter-country variability over time in the mortality of mechanically ventilated patients. Intensive Care Med. 2020;46(3):444–53.

    Article  PubMed  PubMed Central  Google Scholar 

  17. Dyhr T, Laursen N, Larsson A. Effects of lung recruitment maneuver and positive end-expiratory pressure on lung volume, respiratory mechanics and alveolar gas mixing in patients ventilated after cardiac surgery. Acta Anaesthesiol Scand. 2002;46(6):717–25.

    Article  PubMed  Google Scholar 

  18. Setak-Berenjestanaki M, Bagheri-Nesami M, Gholipour Baradari A, Mousavinasab SN, Ghaffari R, Darbeheshti M. The prophylactic effect of different levels of positive endexpiratory pressure on the incidence rate of atelectasis after cardiac surgery: a randomized controlled trial. Med J Islam Repub Iran. 2018;32:20.

    Article  PubMed  PubMed Central  Google Scholar 

  19. Guérin C, Albert RK, Beitler J, Gattinoni L, Jaber S, Marini JJ, et al. Prone position in ARDS patients: why, when, how and for whom. Intensive Care Med. 2020;46(12):2385–96.

    Article  PubMed  PubMed Central  Google Scholar 

  20. Heunks LM, van der Hoeven JG. Clinical review: the ABC of weaning failure–a structured approach. Crit Care. 2010;14(6):245.

    Article  PubMed  PubMed Central  Google Scholar 

Download references

Acknowledgements

N/A.

Funding

This study was supported by the National High-Level Hospital Clinical Research Funding (2022-PUMCH-B-115, 2022-PUMCH-D-005) and the Fundamental Research Funds for the Central Universities (No. 3332022011).

Author information

Author notes

  1. Yi Chi and Qianling Wang are equal contributors.

Authors and Affiliations

  1. State Key Laboratory of Complex Severe and Rare Disease, Department of Critical Care Medicine, Peking Union Medical College Hospital, Peking Union Medical College, Chinese Academy of Medical Sciences, 1 Shuaifuyuan, Dongcheng District, Beijing, China

    Yi Chi, Qianling Wang, Siyi Yuan, Huaiwu He & Yun Long

  2. The First Clinical Medical College, Shanxi Medical University, 86 Xinjian South Road, Taiyuan, Shanxi, China

    Yutong Zhao

Authors
  1. Yi Chi
    View author publications

    You can also search for this author in PubMed Google Scholar

  2. Qianling Wang
    View author publications

    You can also search for this author in PubMed Google Scholar

  3. Siyi Yuan
    View author publications

    You can also search for this author in PubMed Google Scholar

  4. Yutong Zhao
    View author publications

    You can also search for this author in PubMed Google Scholar

  5. Huaiwu He
    View author publications

    You can also search for this author in PubMed Google Scholar

  6. Yun Long
    View author publications

    You can also search for this author in PubMed Google Scholar

Contributions

CY and WQL did literature search, designed the study, collected and analyzed the data. YSY and ZYT collected and check the data. CY and WQL wrote the initial manuscript draft. HHW and LY directed the critical revision of the final manuscript. All authors read and approved the final manuscript.

Corresponding authors

Correspondence to Huaiwu He or Yun Long.

Ethics declarations

Ethics approval and consent to participate

This study was approved by the Institutional Research and Ethics Committee of Peking Union Medical College Hospital. Informed consent was not applicable for the retrospective study and waived from the Institutional Research and Ethics Committee of Peking Union Medical College Hospital.

Consent for publication

N/A.

Competing interests

The authors declare no competing interests.

Additional information

Publisher’s Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Supplementary Information

Additional file 1: Table S1.

PEEP difference between groups according to different grouping criteria. Table S2. Outcomes comparison between PEEP < 6 and PEEP ≥ 6 after propensity-score matching. Table S3. Outcomes comparison between PEEP < 8 and PEEP ≥ 8 after propensity-score matching. Figure S1. Correlation between set PEEP and delta P/F ratio (P/F at T24h minus P/F on admission).

Rights and permissions

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data.

Reprints and permissions

About this article

Check for updates. Verify currency and authenticity via CrossMark

Cite this article

Chi, Y., Wang, Q., Yuan, S. et al. Maintaining moderate versus lower PEEP after cardiac surgery: a propensity-scored matched analysis. BMC Anesthesiol 24, 55 (2024). https://0-doi-org.brum.beds.ac.uk/10.1186/s12871-024-02438-4

Download citation

  • Received:

  • Accepted:

  • Published:

  • DOI: https://0-doi-org.brum.beds.ac.uk/10.1186/s12871-024-02438-4

Keywords

BMC Anesthesiology

ISSN: 1471-2253

Contact us