Factors associated with long-term mechanical ventilation in patients undergoing cardiovascular surgery

In the present study, 21.4% of patients who underwent cardiovascular surgery were LTMV. One out of every five people who underwent heart surgery had MV longer than 24 h. In other studies, including the study of Trouillet et al. (2009), this rate was 6.2% [18]. In the study by Rajakaruna et al. (2005), the rate was 2.6% [19]. Also, Sharma and colleagues reported the rate of LTMV between 6 and 7% [20]. The possible reason for the discrepancy in the results can be due to the difference in the number of factors examined, the number of samples, the existence of interference between variables, the method of anesthesia, the method of operation, and the quality of care after surgery, which needs further investigation. In Iran and many medical centers, despite achieving the clinical weaning criteria, patients are subjected to MV for long hours without needing a ventilator from a respiratory therapist's viewpoint. Therefore, a standard national protocol for weaning from MV after cardiovascular surgery seems necessary.

The present study showed a significant relationship between age and the time of MV so this time was longer in older people, which is in line with the results of most studies conducted in this field. Also, in the current study, the average age was 61.9 years. In the study by Gomes et al. (2015), the average age of the people was 60.7 [6]. Serrano et al. (2005) reported the average age of the people to be 64 years [21]. Hasanzadeh et al.'s study (2017) also reported that the average age of people was 62.9 years [17]. Elderly people probably experience more medical conditions. Therefore, more attention is recommended in the care of this group. Since the nurses of the heart surgery units have an effective role in examining and recognizing patients' problems in the initial examination before accepting this group, it is advised to pay attention to the risk factors that increase the patient's problems with increasing age.

In the present study, people with a history of baking bread had a higher risk for LTMV (P = 0.010). The results of the searches showed that a similar study had not investigated the effect of baking bread on MV. Wood, animal dung, crop residues, and coal, typically used in fires or poorly functioning gas stoves, may lead to severe indoor air pollution. In Iran, most households in rural communities use biomass fuel for cooking, and the type of fuel used plays an important role in health and disease. This role becomes more important when cooking is done in the living room, which is common in winter [22].

In the current study, the variables of kidney disease, intra-aortic balloon pump, chronic obstructive pulmonary disease, creatinine level, chest secretions, ejection fraction, and cardiac enzyme level were predictors of long-term ventilation.

In the study of Alejandro et al. (2019), there was a significant relationship between the history of kidney disease and LTMV [23]. In the study of Totonchi et al., a significant association was observed between the history of kidney disease and the duration of MV after heart surgery [24]. The results of these studies are consistent with our study. In comparison, Imanipour et al. (2006) did not observe a significant relationship between the history of kidney disease and the time of weaning in patients undergoing bypass surgery [25]. In Judy et al.'s study (2001), there was no significant relationship between the history of kidney disease and weaning time [26]. The reason for these differences can be related to the number of cardiopulmonary pumps (CPB). Due to the harmful effects of the artificial CPB on liver function and renal filtration and the occurrence of changes in the pharmacokinetics of anesthetic drugs, such as pre-operative disease suffered from liver and kidney problems, the average duration of MV after the operation would increase. Hessel et al. (2019) mentioned a statistically significant relationship between LTMV and the use of an intra-aortic balloon pump. And people who had an intra-aortic pump had a higher chance of LTMV [27]. Also, Prapas et al. (2007) reported a significant relationship between LTMV and the use of an intra-aortic balloon pump [28].

Furthermore, Sharma et al. (2017) showed a significant relationship between the history of chronic obstructive pulmonary disease and prolonged MV [20]. The study by Faghani et al. (2017) also indicated that having a respiratory disease has a statistically significant relationship with the length of time the tracheal tube remains in cardiac patients [29]. Also, the study by Fitch et al. (2013) showed that there is a statistically significant relationship between the history of chronic obstructive pulmonary disease and the duration of separation from mechanical ventilation in patients after cardiovascular surgery, and a lack of these disorders has been reported as a protective factor [30]. There is a possibility that many people with respiratory disorders have reduced lung volumes. However, due to the lack of specific clinical symptoms, they are not subjected to more detailed lung examinations and are not reported as people with respiratory diseases. Therefore, it seems that patients with respiratory disorders, due to the decrease in lung capacities and inability to coordinate with MV after surgery, need more attention from the medical staff and the cardiac surgery intensive care unit nurses.

In line with the results of our study, in Totonchi et al.'s study, a significant relationship was observed between the amount of serum creatinine and kidney disease with the duration of weaning [24]. Also, Reddy et al. (2007) showed that pre-operative serum creatinine level is an independent risk factor for LTMV after heart surgery in adults [31]. Therefore, taking the necessary measures to reduce the amount of mechanical ventilation in patients can be effective against kidney damage and changes in creatinine levels. Siddiqui et al. (2012) showed that individuals with higher secretion rates were more likely to be on LTMV [32]. In the study of Jafarudi et al. (2014), the relationship between the number of chest secretions after the operation, as a quantitative variable, with the duration of MV indicated a meaningful positive correlation between these two variables [33]. Since the establishment of proper chest drainage with the appropriate discharge of secretions prevents pressure on the heart and lungs and improves the functioning of these organs, this improves the patient's breathing, reduces the need for ventilation support, and reduces the duration of MV. However, this issue, LTMV with increasing secretions, might be related to other factors, such as bleeding, which need further studies.

The results of different studies were in line with our study [18, 23, 34]. Regarding the ejection fraction (EF), in people with EF < 30%, compared to those greater than 30%, it was associated with a higher risk for longer MV. Disturbance in the function of the left ventricle leads to the aggravation of heart problems during surgery. In addition, the use of MV reduces the efficiency of the left ventricle and causes a decrease in end-diastolic and cardiac output volume, ultimately causing a reduction in cardiac preload and afterload. Therefore, patients with left ventricular dysfunction and low ejection fraction need more time to reach hemodynamic stability, so they achieve weaning criteria in a longer time, and they should be kept on MV for a longer time. No similar studies were found about the effect of cardiac enzymes on LTMV.

From the present study's findings, the number of breaths in 24 h after the operation has a statistically significant relationship with the duration of MV and acts as a protective factor. By increasing each unit of RR, LTMV decreases by 0.921 h. Jafroudi et al. (2014) also observed a significant relationship between the number of breaths and the duration of MV [33]. Also, the results of our study conform with the results of Annapoorna's research, which concluded that patients are at risk of increasing the duration of MV with a decrease in breathing rate [35]. Furthermore, in the study of Subic and colleagues, it was concluded that the ratio of respiratory rate to tidal volume is a sensitive indicator for predicting the outcome of weaning from MV [36]. It seems very high or low respiratory rate leads to a disturbance in the patient's hemodynamic and physiological condition, which can play an important role in the outcome of patients' weaning.

Another finding of the present study was that postoperative systolic blood pressure has a statistically significant relationship with the duration of MV and acts as a protective factor. The results of the study by Siddiqui et al. (2012) also showed that people with hypertension had a higher chance of LTMV [33]. Yeganeh et al. (2018) have also reported a significant positive relationship between blood pressure changes in patients until the time of endotracheal tube removal [37]. Considering that blood pressure is directly related to the amount of ventilation, it can be understood that if decreases or an increase in blood pressure, the volume of blood in the exchange with air in the lungs changes, and as a result, it will have a direct effect on the ventilation rate.

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