Postoperative invasive mechanical ventilation is an essential component of recovery following coronary artery bypass grafting (CABG), although its duration varies according to patient characteristics, surgical technique, and perioperative management. Contemporary enhanced recovery guidelines emphasize individualized extubation readiness, minimizing unnecessary sedation, and multidisciplinary postoperative care.¹˒² Structured recovery pathways have demonstrated favorable postoperative outcomes,³ while standardized early extubation protocols have reduced ventilation duration in cardiac surgical populations.⁴˒⁵ Evidence from Pakistan further supports the feasibility of fast-track extubation in appropriately selected patients.⁶ Nevertheless, prolonged mechanical ventilation (PMV) remains clinically relevant, with a pooled prevalence of 6.5% when defined as ventilation exceeding 24 hours following CABG.⁷

Identifying factors associated with prolonged ventilatory support is important for optimizing perioperative care. Advanced age, impaired functional status, renal dysfunction, smoking, obesity, and cardiopulmonary bypass exposure have been identified as potential determinants.⁸ However, existing prediction models demonstrate methodological limitations and insufficient external validation.⁹ Recent studies have identified associations involving operative risk, bypass duration, metabolic parameters, and perioperative characteristics,¹⁰⁻¹³ highlighting the multifactorial nature of postoperative ventilation. Comparisons between on-pump and off-pump CABG suggest differences in selected postoperative outcomes, including respiratory recovery, although findings remain heterogeneous and potentially confounded by patient selection.¹⁴⁻¹⁷ Regional investigations have further emphasized postoperative pulmonary complications,¹⁸˒¹⁹ while systematic reviews demonstrate considerable variation in extubation strategies, patient selection, and outcome definitions.²⁰⁻²

Despite these findings, the relationship between routinely documented preoperative clinical characteristics, surgical technique, and postoperative ventilation duration remains incompletely characterized in regional CABG populations. This study aimed to evaluate the associations of preoperative clinical characteristics and operative technique (on-pump versus off-pump CABG) with postoperative invasive mechanical ventilation duration in a comparative observational cohort. These findings may facilitate informed preoperative counseling, improve risk stratification, and guide postoperative resource allocation, while providing a foundation for future adequately powered studies incorporating appropriate adjustments for potential confounders.

Study design and setting

This prospective comparative observational study was conducted in the Department of Cardiac Surgery at the Punjab Institute of Cardiology, Lahore, Pakistan, between May and November 2023. The study population included patients with ischemic heart disease scheduled to undergo elective coronary artery bypass grafting (CABG).

Sample size and participant selection

A total of 230 patients were enrolled using a non-probability consecutive sampling technique. The sample size was determined using the single-population proportion formula, based on an anticipated frequency of on-pump coronary artery bypass grafting (CABG) of 97.8%,²³ a 95% confidence level, and an absolute precision of 2%. The initial estimated sample size was 207 patients, which was increased to 230 to account for an anticipated 10% rate of nonresponse or incomplete data. Patients of either sex, aged 20–70 years, scheduled to undergo elective CABG for ischemic heart disease were eligible for inclusion. Exclusion criteria comprised critical preoperative illness requiring inotropic support, intra-aortic balloon pump assistance, or respiratory support; acute renal failure; preceding cardiopulmonary resuscitation; acute myocardial infarction necessitating emergency surgery; concomitant valvular heart disease; previous cardiac surgery; left ventricular ejection fraction <30%; and intraoperative conversion from off-pump to on-pump CABG.

Data collection and outcome definition

Following written informed consent, patient data were collected using a structured proforma. Recorded variables included surgical technique (on-pump or off-pump CABG), age, sex, smoking status, diabetes mellitus, hypertension, dyslipidemia, height, weight, body mass index (BMI), New York Heart Association (NYHA) functional class, and postoperative invasive mechanical ventilation duration (hours). According to the study protocol, diabetes mellitus, hypertension, and dyslipidemia were defined by random blood glucose >200 mg/dL, blood pressure >160/90 mmHg, and total cholesterol >200 mg/dL, respectively as protocol-defined criteria. Postoperative Invasive Mechanical Ventilation Duration (hours) was defined as the total elapsed time, measured in hours, from admission to the cardiac intensive care unit (ICU) following CABG while receiving invasive mechanical ventilation through an endotracheal tube until the first successful extubation. Duration was calculated as the difference between the documented time of ICU admission and the time of extubation and was recorded as a continuous quantitative variable. Extubation readiness was assessed using predefined clinical criteria, including normothermia, hemodynamic stability, absence of significant mediastinal bleeding, adequate muscle strength and spontaneous respiratory effort (tidal volume >5 mL/kg), and sufficient consciousness to follow verbal commands. Prolonged mechanical ventilation (PMV) was defined as invasive ventilatory support exceeding 24 hours following CABG and was determined from the recorded ventilation duration. All surgical procedures were performed by cardiac surgeons experienced in both on-pump and off-pump techniques.

Statistical analysis

Categorical variables were summarized as n (%), and age and BMI as mean ± standard deviation (SD). Because recorded ventilation duration was right-skewed, it was reported using median (IQR), with the Mann–Whitney U test for two-group comparisons and Kruskal–Wallis test for the four NYHA classes. Associations of age and BMI with duration were screened using Spearman rank correlation. PMV proportions were compared by Fisher’s exact test when appropriate, and 95% confidence intervals for proportions were obtained using the Wilson method.

The primary adjusted analysis used ordinary least squares regression with natural-log-transformed ventilation duration as the dependent variable and heteroskedasticity-consistent HC3 standard errors. The prespecified covariates were operative technique (on-pump versus off-pump), age (per 10 years), male sex, BMI (per 5 kg/m²), NYHA III–IV versus I–II, hypertension, diabetes, hyperlipidemia, and smoking. Exponentiated coefficients are presented as adjusted geometric mean ratios (GMRs) with 95% confidence intervals. Because only 29 observations met the duration-derived PMV definition, no fully adjusted logistic PMV model with all available covariates was estimated. Two-sided p<0.05 was used for interpretation. Analyses were performed using SPSS v. 26.

Ethical considerations

The ethical approval was obtained from the institutional ethical review committee of the Punjab Institute of Cardiology, Lahore. Written informed consent was obtained from each participant before enrollment.

Population and baseline characteristics

Among 230 participants, mean age was 53.74 ± 11.60 years, and 160 participants (69.6%) were male. On-pump CABG accounted for 180 procedures (78.3%); off-pump CABG accounted for 50 (21.7%). Preoperative characteristics by operative technique are shown in Table 1.

Table 1. Baseline demographic and clinical characteristics by CABG technique

^{Values are mean ± standard deviation (SD) or number. BMI: body mass index; CABG: coronary artery bypass grafting; NYHA: New York Heart Association.}

Ventilation duration and PMV

The overall ventilation duration was median 5 (IQR 4–6) hours, with a mean of 7.74 ± 7.74 hours (range 2–30). In the on-pump group, the median was 5 (4–7) hours compared with 4 (4–5) hours in the off-pump group (Mann–Whitney U=5702.5, p=0.0032). Ventilation lasted no more than six hours in 130/180 (72.2%) on-pump patients and 45/50 (90.0%) off-pump patients. Based on duration >24 hours, PMV occurred in 29/230 (12.6%; 95% CI 8.9%–17.5%) observations overall. PMV was recorded in 26/180 (14.4%) on-pump and 3/50 (6.0%) off-pump observations (Fisher exact p=0.112); the unadjusted risk ratio was 2.41 (95% CI 0.76–7.63). The distributions of ventilation time by operative technique are illustrated in Figure 1; corresponding prolonged-ventilation proportions are shown in Figure 2.

Figure 1. Distribution of postoperative invasive mechanical ventilation duration by CABG technique.

Each dot represents one recorded observation. Boxes show the interquartile range, with the horizontal line indicating the median; whiskers extend to the most extreme observations within 1.5 interquartile ranges. The dashed line marks 24 hours; prolonged mechanical ventilation was defined as >24 hours. The between-group comparison used a two-sided Mann–Whitney U test (p=0.0032).

Figure 2. Proportion of patients with prolonged postoperative mechanical ventilation

(>24 hours) by CABG technique.

Error bars denote Wilson 95% confidence intervals for each proportion. The between-group comparison used Fisher’s exact test (two-sided p=0.112).

Unadjusted clinical associations

Smokers had a longer recorded ventilation duration than nonsmokers: median 5 (4–7) versus 4 (3–6) hours (p=0.0066). Ventilation duration differed among NYHA classes overall (Kruskal–Wallis p=0.0025). Associations with age (Spearman r=0.086, p=0.192) and BMI (r=-0.059, p=0.376) were weak and not statistically significant. Categorical unadjusted comparisons are presented in Table 2. Ventilation duration across the four NYHA functional classes is displayed in Figure 3.

Table 2. Unadjusted associations between clinical factors and ventilation duration

^{Two-group p-values were calculated using Mann–Whitney U tests; the four-level NYHA comparison used the Kruskal–Wallis test.}

Figure 3. Recorded postoperative mechanical ventilation duration across preoperative

New York Heart Association (NYHA) functional classes. Each dot represents one observation. Boxes indicate the interquartile range and median; whiskers extend to the most extreme observations within 1.5 IQR. The dashed line indicates 24 hours. The unadjusted overall comparison used the Kruskal–Wallis test (p=0.0025). Class sizes were I: 132, II: 70, III: 15 and IV: 13.

Multivariable analysis and sensitivity analyses

In the adjusted log-duration model (Table 3), on-pump CABG was associated with a 31.9% longer geometric mean ventilation duration (GMR 1.319, 95% CI 1.092–1.594; p=0.0041). Smoking history (GMR 1.364, 95% CI 1.124–1.655; p=0.0017) and recorded hyperlipidemia (GMR 1.331, 95% CI 1.153–1.536; p<0.001) also showed positive associations. No statistically significant adjusted association was observed for age, sex, BMI, NYHA III–IV, hypertension or diabetes. The model had limited explanatory power (adjusted R²=0.080). Adjusted estimates for all nine predictors are presented in Figure 4.

Table 3. Multivariable associations with log-transformed ventilation duration (n=230)

^{Exponentiated coefficients from OLS with HC3 robust standard errors; adjusted R²=0.080. GMR >1 indicates longer geometric mean ventilation.}

Figure 4. Adjusted associations of preoperative clinical factors and

CABG technique with recorded postoperative ventilation duration

Points represent adjusted geometric mean ratios (GMRs), and horizontal lines represent 95% confidence intervals. The vertical reference line denotes GMR=1. Estimates came from a multivariable ordinary least squares regression of natural-log-transformed ventilation duration, using heteroskedasticity-consistent HC3 standard errors (n=230; adjusted R²=0.080). A GMR >1 indicates longer geometric mean ventilation duration.

After excluding both observations with implausible recorded height/BMI (n=228), the associations for on-pump technique (GMR 1.307, p=0.0054), smoking (GMR 1.375, p=0.0013) and hyperlipidemia (GMR 1.306, p<0.001) remained directionally similar. Removing either observation associated with the duplicated identifier produced on-pump GMRs of 1.350 and 1.312, respectively, without materially changing the qualitative findings. A restricted secondary PMV logistic model yielded an adjusted odds ratio for on-pump technique of 2.59 (95% CI 0.75–8.99; p=0.134), with substantial uncertainty.

Table 4. Ventilation outcomes by CABG technique

The binary PMV outcome was derived uniformly using recorded duration >24 hours. Fisher exact p-values apply to PMV; Mann–Whitney U was used for duration.

This comparative observational study identified differences in the recorded duration of postoperative mechanical ventilation between on-pump and off-pump CABG patients. Despite a modest one-hour difference in the group medians, the distribution was skewed, and the adjusted geometric mean was approximately 32% greater after on-pump surgery. Smoking history and documented hyperlipidemia were also associated with longer ventilation in the multivariable duration model. These associations should be considered preliminary because allocation was nonrandom and available adjustment variables were limited.

The group finding is biologically rational. On-pump surgery exposes patients to cardiopulmonary bypass and associated physiologic changes that may influence postoperative lung function and clinical readiness for extubation. Nevertheless, the 2024 meta-analysis emphasized trade-offs between on-pump and off-pump approaches across clinical outcomes ¹⁴. In a systematic review focusing on patients with COPD, investigators observed shorter ventilation durations with off-pump surgery, although their evidence could not establish reductions across all respiratory complications ¹⁵.

Similarly, another study reported lower ventilatory support requirements among off-pump patients with COPD, while underscoring the need for larger trials ¹⁶. Because COPD status, pulmonary function, and intraoperative bypass time were unavailable in the present dataset, these mechanisms cannot be established directly. The association with smoking is important because smoking is common among patients undergoing CABG and represents a potentially modifiable preoperative exposure. The scoping review by Kumalasari and colleagues included smoking among one of the candidates of PMV risk factors ⁸.

The hyperlipidemia finding requires particularly careful interpretation. Its adjusted association with the continuous ventilation endpoint was statistically evident even though the unadjusted rank test was not significant. More concerningly, all 29 observations with duration-defined PMV were recorded as having hyperlipidemia, while no PMV events occurred in the 59 observations coded without it. This complete separation might arise from genuine clinical differences, treatment patterns, or documentation. Hyperlipidemia should not be promoted as a validated independent prognostic factor from these data, and confirmation of diagnosis coding against source charts is essential.

NYHA functional class appeared associated with ventilation time in unadjusted analysis but not when collapsed into classes III–IV versus I–II and entered into the adjusted duration model. The severe-NYHA strata were small, and a non-monotonic pattern was observed across classes. This is compatible with a cohort too small for stable assessment of effect modification or more flexible class-specific adjustment. Recent CABG prediction work incorporating functional and operative status demonstrates the value of richer risk data ^11,12. The present results should not be interpreted as evidence that NYHA status is clinically irrelevant.

Variation in ventilation duration is not attributable solely to preoperative risk. Early extubation depends on anesthesia, residual neuromuscular blockade, temperature, bleeding, hemodynamic support, and local protocols ²⁴. One study demonstrated a shorter time to extubation after sugammadex in a randomized cardiac surgery study ²⁵. Another study examined operating-room extubation after elective on-pump CABG without identifying clear outcome differences in their smaller cohort ²⁶. Additional randomized work has assessed post-extubation oxygen support in obese cardiac surgery patients ²⁷, and postoperative dexmedetomidine strategies in selected patients undergoing ultra-fast-track extubation ²⁸. These findings illustrate why differences in ventilation hours cannot be attributed to surgical technique alone.

The present study has several limitations. It is a single-cohort observational comparison with no randomization. The model explained only about 8% of variation after adjustment, supporting the view that perioperative determinants are important. Moreover, multiple exploratory comparisons increase the likelihood of chance findings, and results may not generalize beyond the study setting. Finally, the present dataset captured only binary smoking history. It cannot distinguish current smoking from former smoking or quantify smoking exposure, and it cannot establish whether smoking cessation before surgery would have reduced ventilation duration. The strong imbalance in smoking prevalence across operative groups also illustrates the potential for selection bias.

The practical implication is not to favor one bypass technique solely to shorten ventilation. Rather, the results support better documentation of smoking exposure and cardiometabolic status, transparent extubation protocols, and prospective collection of pulmonary, cardiac, and intraoperative variables. Large comparative cohorts with prespecified model development and external validation are needed to define clinically actionable risk estimates.

In this study, recorded postoperative ventilation duration was longer after on-pump than off-pump surgery. Documented smoking history and hyperlipidemia were also positively associated with ventilation duration after adjustment for available preoperative variables. These findings emphasize the importance of comprehensive preoperative clinical assessment and careful consideration of surgical technique when planning postoperative care in patients undergoing CABG. Early identification of patients with potentially unfavorable clinical characteristics may support individualized perioperative management, closer respiratory monitoring, and timely extubation strategies aimed at reducing prolonged mechanical ventilation. These observational associations do not establish causality. Larger, well-designed prospective studies incorporating pulmonary function, cardiac status, operative complexity, and standardized extubation protocols are warranted to validate these findings and determine their clinical implications.

DECLARATIONS

Acknowledgments

None declared.

Ethical Approval and Informed Consent

The ethical approval was obtained from the institutional ethical review committee of the Punjab Institute of Cardiology, Lahore. Written informed consent was obtained from each participant before enrollment, as confirmed by the investigators.

Funding

No external funding was received.

Conflicts of Interest Statement

The authors declare no conflicts of interest.

Data Availability

The de-identified analytical dataset may be considered for sharing by the corresponding author subject to institutional approval, consent restrictions and applicable privacy safeguards. Individual patient names and identifiers are not provided in this manuscript.

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