European Journal of Cardiovascular Medicine

Lung atelectasis is a commonly encountered post-operative problem in patients who underwent general anaesthesia, particularly for upper abdominal and thoracic surgeries. Open abdominal surgeries are major procedures often requiring general anaesthesia for pain management and muscle relaxation. While these surgeries are essential for various conditions, they can lead to postoperative complications, including atelectasis. Atelectasis is the partial or complete collapse of lung units, which hinders gas exchange and can potentially lead to serious respiratory problems.

PEEP is a crucial strategy for preventing atelectasis, particularly in high-risk patients. By applying a small amount of pressure to the airway at the end of exhalation, PEEP:

• Maintains alveolar recruitment, improves ventilation distribution, reduces atelectasis formation, and promotes gas exchange.
• PEEP reduces the pressure difference between the alveoli and the surrounding tissues (transpulmonary pressure). This helps prevent the collapse, especially in dependent lung regions (areas of the lung that are lower down). [1]

While PEEP offers benefits, its use is not without potential drawbacks such as Barotrauma and hemodynamic effects. Determining the optimal PEEP level requires careful consideration of individual patient factors and lung mechanics. Diagnosing atelectasis traditionally relies on chest X-rays, but these have limitations. Chest X-rays can miss small or early atelectasis formations. [2] There are several benefits associated with the use of lung ultrasonography at the point of care, which has become an important instrument for diagnosing postoperative atelectasis. Ultrasound findings suggestive of atelectasis include: Absence of the "lung line", increased pleural sliding, and multiple vertical B lines.

Pulmonary complications following the surgery, particularly post-operative respiratory failure are significant contributors to morbidity and mortality perioperatively. (3,4) GA decreases the lung volume and helps in atelectasis formation. The atelectasis lung is associated with a reduction in both the gas exchange process (5). Evidence from high-level clinical trials indicates that mechanical ventilation may result in the worsening of the already injured lung among critically ill patients. Research done by Miskovic A. et.al., (6) and Haller G. et.al., (7) have put forward that low tidal volumes benefited the participants who require prolonged mechanical ventilation without having lung injury.

OBJECTIVES OF THE STUDY

Primary objective

The primary objective was to compare the effect of PEEP in patients undergoing general anaesthesia for open abdominal surgeries on postoperative atelectasis during the 30 minutes in the PACU.

Secondary objectives

1. To determine the incidence and degree of hypotension during intraoperative mechanical ventilation.

2.           To estimate the incidence of barotrauma and increased airway pressures during intraoperative mechanical ventilation.

Study design: A prospective randomized controlled trial Study setting: The study was conducted in the Department of Anaesthesiology, in Sri Devaraj urs Medical College, Tamaka, Kolar, in Karnataka. Study duration: The study was conducted between 01/01/2023 to 30/05/2024. Study participants: This study was conducted on patients above 18 years of age posted for open abdominal surgeries at R.L. Jalappa Hospital and Research Centre, Tamaka, Kolar. INCLUSION CRITERIA: • Patients aged >18 years • Patients who underwent open abdominal surgery • The estimated duration of surgery is > 2 hours. • Patients with American Society of Anaesthesiologists (ASA) physical status I – III and BMI less than 35kg/m2. Exclusion criteria: • Laparoscopic surgery • Previous lung or thoracic surgeries • Persistent hemodynamic instability in patients (systolic blood pressure< 90 mm of Hg) • Patients with chronic obstructive pulmonary disease history • Patients on systemic corticosteroid treatment. • Recent immunosuppressive medication and radiotherapy. • Patients with Severe cardiac disease is defined as New York Heart Association (NYHA) class III-IV. • Pregnancy • Acute lung injury or ARDS Sample Size: A total of 41 participants were recruited for the study. Sampling Method: Universal sampling was done. Randomization was done in a web-based random number generator available at www.random.org. No stratification factors were considered, and block sizes were either unequal or not specified. The sample size is determined using the G Power 3.1.9.6 software [8] by taking the length of anaesthesia in patients with the impact of positive end-expiratory pressure on post-operative atelectasis patients having open abdominal operations. This information was provided in research that was carried out by Xu, Q. et al. (9). The input values taken for the calculation are as follows: Number of groups: - 2 Calculated mean (as reported in the study) Group A: - 269 Calculated mean (as reported in the study) Group B: - 232 The standard deviation for Group A = 64 The standard deviation for Group B = 58 α error probability = 0.05 Power (1-β power probability) = 0.80 Allocation Ratio = 1: 1 Effect size f = 0.90 The minimum sample size needed for the study amounted to 41 So, for this study, we considered 50 patients in total and divided them into two groups for better statistical representation. Sampling procedure: The study was started after Institutional Ethical Clearance (IEC) and registration with the Clinical Trials Registry – India (CTRI). Patients were recruited after obtaining written informed consent. The study was conducted on patients who were fit according to the inclusion criteria and were planned to undergo open abdominal surgeries. Routine investigations such as complete blood count (CBC), serum electrolytes, urea, and serum creatinine, prothrombin time, international normalized ratio (INR), and activated partial thromboplastin time were done. Besides, an electrocardiogram (ECG) and a chest x-ray (CXR) were done before the surgery. Intravenous fluids (crystalloids) will be given according to the maintenance requirement calculated according to the body weight of the patient. Patients were assigned to two groups based on a computer-generated random table. Group A: Receiving PEEP of 4cm of H2O. Group B: Receiving PEEP of 8 cm of H2O. Throughout the intraoperative procedure, the following parameters were measured- • Heart rate (HR), • Mean arterial pressure (MAP), • End-tidal carbon dioxide(etco2), • Respiratory rate (RR), • Peripheral capillary oxygen saturation (SPO2), • Blood loss, urine output, plateau pressures, and peak pressures. Mephentermine and noradrenaline were among the vasopressors that were used intraoperatively to treat hypotension, which was defined as a systolic blood pressure that was below 90 mmHg or 20% below the initial preoperative level. During the immediate preoperative period and 30 minutes after surgery in the post-anaesthesia care unit (PACU), lung ultrasound was performed using the Philips InnoSight Diagnostic Ultrasound System (REF: 989605460371, FCC ID: VRSAPOLLO) with a 5-6 MHz curvilinear probe to check for any lung pathologies, including lung atelectasis. A lung ultrasound was performed by dividing the thorax into 12 segments. The lung ultrasound scoring (LUS) is done according to the modified lung ultrasound scoring system, for each segment of the thorax, as follows: Table 1: Lung ultrasound scoring (LUS) to classify aeration Method of collection of data: After the informed consent, the clinical, laboratory, and radiological information was abstracted from patients’ records in a predesigned proforma. The following parameters were abstracted from patients' records: • Heart rate • Spo2 (Oxygen saturation), • Mean arterial pressure, • Respiratory rate, • Peak airway pressures, • Plateau pressures, • Ultrasonography of the lung and • Modified lung ultrasound score The Statistical analysis: 1. Collected data was coded and entered in an Excel sheet database. 2. All the quantitative measures were presented by (mean + / - Standard Deviation (SD)), qualitative measures like gender, confidence interval, ASA physical status, etc., by proportions and confidence interval (CI). 3. To interpret the findings, it was determined that the Chi-square test or Fisher's exact test was suitable for categorical variables, and the independent sample t-test was appropriate for continuous variables to be distributed normally. 4. A p-value < 0.05 was considered statistically significant for all tests. Taking into account ethical issues, the Sri Devaraj Urs Medical College's institutional ethics council gave its blessing to the implementation of the research. The enrolment of patients did not take place until after the patients had provided their written informed consent.

We recruited a total of 50 participants, 25 in Group A: Receiving PEEP of 4 cm of H2O, and 25 in Group B: Receiving PEEP of 8 cm of H2O.

Age distribution:

There was a standard variation of 15.1 years in the participants' ages, and the mean was 41.7. The average age of the PEEP 8 group was 43.6 years, whereas that of the PEEP 4 group was 39.7 years, a little younger. In contrast, this disparity was not statistically significant (p > 0.05, t-test) (Table 2).

Table 2: Age distribution of the two groups

Gender distribution:

While there were 11 (36.4%) males and 14 (63.6%) females in the PEEP 4 group, the number of males and females were 17 (60.7%) and 8 (39.3%) in the PEEP 8 group, respectively. The variance was statistically insignificant (p=0.09, Chi-square test) (Table 3)

Table 3: Gender distribution of the two groups

Smoking history:

While four participants (16%) in the PEEP-4 group had a smoking history, three participants (12%) in the PEEP-8 group had a smoking history. The variance was not statistically noteworthy (p=0.68) (Table 4)

Table 4: Distribution of smoking status between the two groups

ARISCAT score:

For the purpose of assessing the likelihood of a patient experiencing pulmonary problems after surgery, the ARISCAT score is used as a tool for prediction. In the PEEP 4 group, the mean ARISCAT score was 26.7 (standard deviation: 3.1), whereas in the PEEP 8 group, it was 27.1 (standard deviation: 3.15). The change did not meet the criteria for statistical significance (p = 0.87).

Primary outcomes

Post-operative atelectasis: None of the patients developed atelectasis during the 30-minute post-operative period. While in the pre-operative period, none of the patients had an ultrasound score >0. In the postoperative period 30 minutes after shifting to Post operative anaesthesia care unit (PACU) only 2 (4%) patients developed an ultrasound lung score of 1 which signifies only a small loss of aeration. These two patients belonged to group A.

Secondary outcomes:

Duration of surgery: -

The participants in the PEEP 4 group had an average duration of surgery that was 209.4 minutes (standard deviation (20.3 minutes)), while the participants in the PEEP 8 group had a mean duration of surgery that was 209 minutes (standard deviation) in length. In terms of statistical significance, the difference was not significant (p = 0.94, t-test). Table 5

Table 5: Duration of surgery of the two groups

Duration of anaesthesia:

The average duration of anaesthesia in the PEEP 4 group participants was 164.2 minutes (SD 19.0 minutes) while the mean duration of anaesthesia in the PEEP 8 group was 162.2 minutes (SD 19.2 minutes). The difference was statistically insignificant (p 0.71, t-test) (Table 6)

Table 6: Duration of anaesthesia of the two groups

Urine output:

The mean urine output in the PEEP 4 group participants was 34.2 ml (SD 10 ml) while the mean urine output in the PEEP 8 group was 33.4 (SD 10 ml). The variance was not statistically noteworthy (p 0.78, t-test) (Table 7)

Table 7: Estimated urine output of the two groups

Hypotension:

 A total of 7 (14%) participants developed intraoperative hypotension. Out of these, 1 (4%) participant belonged to the PEEP 4 group, and 6 (24%) belonged to the PEEP 8 group. The difference was statistically insignificant (p=0.04) (Table 8)

Table 8: Distribution of intra-operative hypotension between the two groups

*Statistically significant

Pressure in the respiratory system:

Peak pressure:

The mean peak pressure of the PEEP 4 group was 20.68 cm of H₂O (SD 3.1 cm of H₂O), and the mean was 21.5 cm of H₂O (SD 3.5 cm of H₂O) in the PEEP 8 group. The difference was -0.84 cm of H₂O (95% CI: -2.7 to 1.1 cm of H₂O), which was statistically insignificant (p=0.38) (Table 9 & Figure 1).

Figure 1: Distribution of peak pressure between PEEP 4 & PEEP 8 group

Plateau pressure:

The average plateau pressure of the PEEP 4 group was 20.0 cm of H2O (SD 3.7 cm of H2O) and the mean was 23.0 cm of H2O (SD 3.7 cm of H2O) in the PEEP 8 group. The difference was 3.0 cm of H2O (95% CI: -0.34 to 4.86 cm of H2O) which was statistically insignificant (p=0.82) (Table 9 & Figure 2).

Figure 2: Distribution of plateau pressure between PEEP 4 & PEEP 8 group

Table 9: Distribution of the various pressures in the respiratory system in GROUP A and GROUP B

The study’s main aim was to determine the number of patients developing post-op atelectasis in a 30-minute PACU period in patients under PEEP 4 or PEEP 8 under GA for open abdominal surgeries. The study also had the following secondary aims to compare the degree and frequency of intraoperative hypotension, and the frequency of increased airway pressures and barotrauma during mechanical ventilation. Thus, addressing these objectives, the study endeavoured to give a desired evaluation of the advantages and drawbacks of employing PEEP 4 or PEEP 8 in this clinical scenario. The demographic and clinical data of the patients recruited in this study were also similar between the two groups, PEEP 4 and PEEP 8, which is crucial when comparing the primary and secondary results. In the PEEP 4 group, participants’ age was on average 39 years. seven years instead of 43. The patients in the PEEP 8 group had a mean duration of 6 years in the study group; however, it was statistically non-significant (p > 0. 05). This is important because the development of postoperative atelectasis depends on age since the lung compliance and response to mechanical ventilation in a patient varies with age. The proportion of participants with a smoking history was insignificant. There was non-significantly different between the groups (16% in PEEP 4 vs. 12% in PEEP 8, p = 0.68). Smoking can impair lung function and predispose to atelectasis, so this balance is relevant for the study's outcomes The average pre-operative heart rates and blood pressure measurements were comparable between the groups (heart rates: 89 beats/min in PEEP 4 vs. 85 beats/min in PEEP 8, p = 0.14; blood pressure: mean SBP, DBP, and MAP were 124/81/90 mmHg in PEEP 4 vs. 122/78/92 mmHg in PEEP 8, p > 0.05 for all). Stable cardiovascular parameters are essential for patient safety and can influence intraoperative and postoperative outcomes. Ensuring similar baseline characteristics between the PEEP 4 and PEEP 8 groups is critical for enhancing the validity and reliability of the study's findings. Comparable baseline characteristics help to ensure that any observed differences in outcomes, such as the incidence of postoperative atelectasis, are attributable to the intervention (PEEP levels) rather than to confounding variables. Thus, rigorous control of baseline characteristics strengthened the internal validity of the study and supported more robust and generalizable conclusions. some works of literature have reviewed the effects of PEEP on postoperative atelectasis and other related outcomes thus offering a rich background from which the results of the present study can be understood. Barbosa et al. (10) also did a study where they demonstrated that PEEP enhances intraoperative oxygenation and lessens atelectasis which is in line with the merits of PEEP highlighted above. This led them to conclude that the ideal PEEP levels should be in the range of 8 to 10 cm H2O because this level effectively helped to promote lung opening and reduce atelectasis while at the same time reducing the possibilities of barotrauma and hypotension. Another study by Brower et al. (11) on the subject added to the debate by assessing the impact of PEEP on patients with acute respiratory distress syndrome (ARDS). The authors proved that increased PEEP levels can enhance oxygenation but they also described the dangers of barotrauma and decreased blood pressure. While this study has been carried out on a different patient sample, the results also highlighted the need to consider the benefits and harms of PEEP. A meta-analysis done by Gildner et al. (12) assessing the effects of various PEEP levels on postoperative pulmonary complications in non-cardiac surgery revealed that the application of high PEEP levels (≥ 8 cm H2O) was likely to decrease the occurrence of atelectasis and enhance the oxygenation of the patient without increasing the adverse effects. This extensive literature review is in favour of the idea that moderate levels of PEEP should be used to help avoid postoperative pulmonary complications. Post-operative atelectasis: Atelectasis may develop in as many as ninety percent of individuals who are receiving GA. Following surgery, the disease may continue to have variable degrees of persistence, and it is often accompanied by pleural effusion. 34% of the amount of nonaerated lung tissue that is located next to the diaphragm might change based on the surgical treatment that is performed and the features of the patient. There is a possibility that it might be much greater when computed based on the tissue volume, although it is predicted to fall anywhere between 3 and 25 percent. Several processes have been suggested to contribute to atelectasis formation. These include- small airway collapse, lung compression, intra-alveolar gas absorption, and surfactant function compromise. Mechanical ventilation strategies during GA have been significantly shaped by oxygenation and lung compliance reduction. To combat the condition and enhance the end-expiratory lung capacity, it is advised that the tidal volumes be maintained at a maximum of 15 millilitres per kilogram of projected body weight during surgical procedures. Lung recruitment exercise & PEEP, unless contraindicated, may be beneficial to prevent or reverse the loss of EELV. (13) None of the two groups developed post-operative atelectasis in 30-minute PACU in our study. It indicates that PEEP might have a direct role in preventing postoperative atelectasis. Duration of Surgery The average duration of surgery was comparable between the PEEP 4 group (209.4 minutes, SD 20.3 minutes) and the PEEP 8 group (209 minutes, SD 20 minutes), with a statistically insignificant difference (p = 0.94). The similarity in surgery duration between the two groups aligns with findings from other studies that have assessed the impact of different PEEP levels on perioperative outcomes. For example: Almarakbi et al. (2009) (14) conducted a study on the effects of different PEEP levels during laparoscopic surgery and found no significant difference in the duration of surgery between groups receiving different PEEP levels (ASA Pubs). This suggests that the application of varying PEEP levels does not significantly impact the length of surgical procedures. Duggan et al. (2005) (15) examined the impact of PEEP on intraoperative and postoperative outcomes, noting that while PEEP influences lung mechanics and oxygenation, it does not affect the duration of surgery. This aligns with the results of the current study, where the duration was nearly identical across groups. Duration of Anaesthesia In the present study, there was no noteworthy variance in the mean duration of satisfactory anaesthesia between PEEP 4 (164.2 minutes, SD19.0) and PEEP 8 (162.2 minutes, SD19.2), p=0.71 While the observation of similar anaesthesia durations between groups is consistent with existing evidence and demonstrates that differing PEEP levels do not influence the overall duration of anaesthesia, the findings of Guldner et al. (16) showed that the baseline PEEP did not affect postoperative pulmonary problems. Furthermore, they observed that there was no significant variation in the length of anaesthesia with the degree of PEEP used. The study concluded that PEEP can improve respiratory function, but does not shorten anaesthesia time for surgical procedures. Talab et al. A study by Perilli et al (2009) (17) examined the impact of intraoperative PEEP on respiratory function in obese patients undergoing laparoscopic surgery. Anaesthesia duration was not significantly different between low and high PEEP groups, supporting the fact that alterations in the management of respiratory mechanics depend on the level of invasive mechanical ventilation applied for each patient respiratory settings changes were more influenced by type/titration than anaesthetic time. A similar study conducted by Hemmes et al. (18) also evaluated the effects of PEEP during anaesthesia and found no significant differences in time to extubation between different levels of PEEP. As the duration of anaesthesia was not affected, it is suggested that PEEP has major effects on respiratory mechanics. Hence, in all these experimental studies, the duration of anaesthesia has been reported to be unchanged in different PEEP levels. Data from the present study confirm and expand this assumption. Intra operative Hypotension: More notably, our findings regarding hypotension showed a considerable difference among the group groups. A total of 7 participants (14%) developed per-operative hypotension; however, the distribution was uneven, with only 1 participant (4%) in the PEEP 4 group and 6 participants (24%) in the PEEP 8 group experiencing this complication. This significant difference (p=0.04) suggests that higher PEEP levels could be related to an enhanced risk of hypotension, likely due to the increased intrathoracic pressure affecting cardiac output and vascular resistance. This association between higher PEEP and increased hypotension is particularly relevant in clinical practice. It underscores the need for careful monitoring and potentially more cautious management of fluid and hemodynamic status in patients receiving higher PEEP levels, especially in settings where patients may be at a higher risk for cardiovascular complications. For hemodynamics, the impact of high PEEP on MAP and hypotension development has been debated. Some evidence suggested that high PPEP ventilation might reduce MAP and promote hypotension in patients without ARDS. (19) However, a large meta-analysis found no significant changes in hospital mortality, ventilation duration, pulmonary complications, and overall hemodynamics between different levels of PEEP. (20) The findings of a retrospective cohort research revealed that moderate positive end-tidal pressure (PEEP) did not negatively alter arterial systolic blood pressure in trauma patients who need mechanical breathing, adding another layer of complication to the situation. Vulnerable patients who are ventilated with moderate positive end-expiratory pressure (PEEP) were shown to exhibit hemodynamic stability, which is an interesting finding. Consequently, this indicates that a modest PEEP may have a protective impact on the hemodynamics in such circumstances. (21) A prospective cohort research, on the other hand, found that high levels of positive end-tidal pressure (PEEP) may have a considerable effect on changes in blood pressure and cardiac function, especially in older individuals who diagnosed themselves with hypertension (22). From the findings of this research, it is clear that high PEEP has the potential to have a more significant impact on the dynamics of the cardiovascular system in particular patient groups. These findings collectively illustrate the nuanced and context-dependent effects of PEEP on hemodynamic, necessitating careful consideration of individual patient characteristics and conditions when applying PEEP in clinical settings LIMITATIONS: With a small sample, it’s challenging to generalize findings to a broader population. A study on a larger population could have generated more accurate results. We haven’t done any recruitment manoeuvres in our study before the application of PEEP.

PEEP might have a direct role in preventing postoperative atelectasis. In our study, patients who underwent general anaesthesia in either group A or group B did not show any development of atelectasis in the 30-minute postoperative period in PACU. PEEP 4 has a greater hemodynamic advantage than PEEP 8. Individual pre-operative assessment is important while deciding between PEEP 4 and PEEP 8.