Intraoperative Mechanical Ventilation and Postoperative Pulmonary Complications After Cardiac Surgery

Michael R. Mathis, M.D.; Neal M. Duggal, M.D.; Donald S. Likosky, Ph.D.; Jonathan W. Haft, M.D.; Nicholas J. Douville, M.D., Ph.D.; Michelle T. Vaughn, M.P.H.; Michael D. Maile, M.D., M.S.; Randal S. Blank, M.D., Ph.D.; Douglas A. Colquhoun, M.B., Ch.B., M.Sc., M.P.H.; Raymond J. Strobel, M.D., M.S.; Allison M. Janda, M.D.; Min Zhang, Ph.D.; Sachin Kheterpal, M.D., M.B.A.; Milo C. Engoren, M.D.

Disclosures

Anesthesiology. 2019;131(5):1046-1062. 

In This Article

Results

Of the 5,365 cardiac surgical cases reviewed, 4,694 met study inclusion criteria (Figure 1). Among these cases, 513 (10.9%) experienced a postoperative pulmonary complication. Individual nonmutually exclusive components of postoperative pulmonary complications included pneumonia (121 cases, 23.6% of postoperative pulmonary complications), prolonged ventilation longer than 24 h, (302, 58.9% of postoperative pulmonary complications), reintubation (115, 22.4% of postoperative pulmonary complications), and PaO2/FIO 2 below 100 mmHg (164, 32.0% of postoperative pulmonary complications).

Figure 1.

Study inclusion and exclusion criteria.

Patient Population – Baseline Characteristics and Univariate Analyses

As described in Table 1, our study population had a median age of 62 yr, and 64% were men. Cardiac surgeries performed included coronary artery bypass grafting (20.6%), valve (44.3%), aorta (2.1%), and combination (33.0%). Cases were primarily elective (79.7%); remaining cases were urgent (20.3%). Our study population included cases across four time partitions by Society of Thoracic Surgeons Adult Cardiac Surgery Database version, including 349 (7.4%) from 1/1/2006 to 12/31/2007; 1,286 (27.4%) 1/1/2008 to 6/30/2011; 1,679 (35.8%) 7/1/2011 to 6/30/2014; 1,380 (29.4%) 7/1/2014 to 5/31/2017. An overall lung-protective ventilation strategy was used in 1,913 cases (40.8%); among components of a lung-protective ventilation strategy, a VT below 8 ml/kg predicted body weight was achieved in 64% of cases, modified driving pressure below 16 cm H2O in 71% of cases, and PEEP at or above 5 cm H2O in 63% of cases. Adherence to varying thresholds and independent associations with postoperative pulmonary complications are provided in Supplemental Digital Content 1A through 1C (http://links.lww.com/ALN/C26). Crude incidence of postoperative pulmonary complications among cases using an overall lung-protective ventilation strategy was 6.6%, compared with 13.9% among cases without an overall lung-protective ventilation strategy (Table 2). Postoperative pulmonary complications were associated with increased postoperative mortality as well as longer postoperative mechanical ventilation, ICU stay, and hospital stay (Table 3). Patients receiving a lung-protective ventilation strategy were more commonly tall, nonobese, male, and nonsmokers (Supplemental Digital Content 2, http://links.lww.com/ALN/C27).

Intraoperative Ventilator Management

Patients were ventilated with a cohort mean ± SD VT of 7.8 ± 1.5 ml/kg predicted body weight, median (interquartile range) driving pressure of 13 (11 to 16) cm H2O, and PEEP of 5 (4 to 5) cm H2O. Compared with pre-CPB ventilator parameters, we observed no significant differences in post-CPB parameters (Table 1). We observed distributions of overall per-case median ventilator parameters to be unimodal and rightward-skewed for VT and driving pressure, versus a bimodal distribution (0 cm H2O and 5 cm H2O) for PEEP (Figure 2). Over the study period, we observed significant linear trends in ventilation practices: providers used decreasing VT and driving pressure, and increasingly used PEEP (P < 0.001 for all trends; Figure 3).

Figure 2.

Frequency distributions of per-case median intraoperative ventilator parameters, including tidal volume per predicted body weight, modified driving pressure, and positive end-expiratory pressure (in A, B, and C, respectively).

Figure 3.

Temporal trends in intraoperative ventilator strategies, including tidal volume per predicted body weight, modified driving pressure, and positive end-expiratory pressure (in A, B, and C, respectively).

Impact of Ventilator Parameters–Multivariable Analyses

Of the 4,694 cases studied, we observed data completeness rates greater than 99% for all but two risk adjustment variables, preoperative respiratory rate (97.0%) and total intraoperative crystalloid (98.8%). Peak inspiratory pressure and weight were removed from the model due to multicollinearity (variance inflation factor greater than 10). Platelet count, international normalized ratio, total intraoperative opioid, preoperative respiratory rate, and history of cancer were removed, given a lack of use in previous validated cardiac surgery or postoperative pulmonary complication risk score models.[9,10,43] Multiple additional variables were removed via least absolute shrinkage and selection operator (denoted by "–" in Supplemental Digital Content 3, http://links.lww.com/ALN/C28). Through multivariable analyses adjusting for postoperative pulmonary complication risk factors, an intraoperative lung-protective ventilation bundle was independently associated with reduced postoperative pulmonary complications (adjusted odds ratio, 0.56; 95% CI, 0.42–0.75, figs. 4 and 5). Modelling lung-protective ventilation exposure as a treatment, we observed a number needed to expose of 18 (95% CI, 14–33) to prevent one postoperative pulmonary complication.

Figure 4.

Independent associations between intraoperative lung protective ventilation strategies and postoperative pulmonary complications.

Figure 5.

Significant independent associations between multivariable model components and postoperative pulmonary complications.

We observed no associations between a lung-protective ventilation bundle and minimum postoperative PaO2/FIO 2 while intubated, initial postoperative ventilator duration in hours, length of ICU stay in hours, or length of hospital stay in days (Supplemental Digital Content 4, http://links.lww.com/ALN/C29). We observed similar findings for logarithmically transformed secondary outcomes. Postoperative mortality occurred in 49 cases (1.0%); our study was not adequately powered to analyze independent associations between lung-protective ventilation and mortality.

Among individual pulmonary complications (pneumonia, prolonged ventilation longer than 24 h, reintubation, and PaO2/FIO 2 less than 100 mmHg postoperatively while intubated), a lung-protective ventilation bundle demonstrated univariate associations across all postoperative pulmonary complication components; after multivariable adjustment, a lung-protective ventilation bundle remained protective against all postoperative pulmonary complication components except for prolonged ventilation longer than 24 h (Supplemental Digital Content 5, http://links.lww.com/ALN/C30, and Supplemental Digital Content 6, http://links.lww.com/ALN/C31).

Sensitivity Analyses

When analyzing each component of the lung-protective ventilation bundle separately, we found that modified driving pressure driving pressure less than 16 cm H2O was independently associated with reduced postoperative pulmonary complications (adjusted odds ratio, 0.51; 95% CI, 0.39–0.66) whereas VT below 8 ml/kg predicted body weight and PEEP at or above 5 cm H2O did not demonstrate significant independent associations (adjusted odds ratios [95% CIs] 0.99 [0.75–1.30] and 1.18 [0.91–1.53], respectively; Figure 4). Furthermore, driving pressure less than 16 cm H2O was independently associated with improvements in all secondary outcomes.

When analyzing the lung-protective ventilation bundle as partitioned into pre-CPB and post-CPB periods, we observed no collinearity between corresponding pre-CPB and post-CPB variables (variance inflation factors below 10) and thus included all variables into a single model. We found that adherence to the post-CPB lung-protective ventilation bundle was associated with less postoperative pulmonary complications (adjusted odds ratio, 0.53; 95% CI, 0.38–0.74) whereas the pre-CPB lung-protective ventilation bundle was not associated with postoperative pulmonary complications (adjusted odds ratio, 1.19; 95% CI, 0.84–1.68, Supplemental Digital Content 7, http://links.lww.com/ALN/C32). Similarly, when analyzing the lung-protective ventilation components individually partitioned into pre-CPB and post-CPB periods, we observed no collinearity between corresponding pre-CPB and post-CPB components and thus included all variables into a single model. We observed post-CPB driving pressure less than 16 cm H2O was associated with lesser likelihood of postoperative pulmonary complication (adjusted odds ratio, 0.57; 95% CI, 0.42–0.78), but neither the pre-CPB driving pressure below 16 cm H2O (adjusted odds ratio, 0.77; 95% CI, 0.56–1.07) nor VT below 8 ml/kg predicted body weight nor PEEP at or above 5 cm H2O pre-CPB and post-CPB components was associated with postoperative pulmonary complications.

Logistic regression models using either least absolute shrinkage and selection of operator restricted to 24 covariates, or forward selection of univariate association thresholds (P < 0.10) found independent associations between lung-protective ventilation, driving pressure, and postoperative pulmonary complications, but not VT or PEEP (Supplemental Digital Content 8, http://links.lww.com/ALN/C33, and Supplemental Digital Content 9, http://links.lww.com/ALN/C34). Finally, sensitivity analyses of clinically important subgroups yielded similar independent associations between the lung-protective ventilation bundle and outcomes. The protective association of the lung-protective ventilation bundle was observed in both males and females, in elective but not urgent cases, across all body mass index ranges, only in patients without chronic lung disease, and in patients undergoing valve procedures (Supplemental Digital Content 10, http://links.lww.com/ALN/C35).

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