Effect of Mechanical Ventilation Mode Type on Prioperative Blood Loss in Patients Undergoing Posterior Lumbar Inter body Fusion Surgery: A Randomized Controlled Trial

this study has examined modes of mechanical ventilation, pressure or volume-controlled ventilation (PCV or VCV) on intra- and post-operative surgical bleeding in posterior lumbar inter body fusion (PLIF) surgery Methods This research was a randomized, single-blinded, and parallel study, that 78 patients were selected. They mechanically ventilated using either PCV or VCV in PLIF surgery. In this regard, a permuted block randomization was used with a computer-generated list. After induction of anesthesia in supine position, the hemodynamic and respiratory parameters were measured


Results
The mean bleeding was 431.281 ± 361.04cc in the PCV group and 465.26 ± 338.16 cc in the VCV group (p = 0.669). Moreover, blood transfusion rates in the PCV and VCV groups were 0.40 ± 0.74 and 0.43±78 0.78 pack cell (p = 0.836), respectively. Notably, surgeon satisfaction was more observed in the PCV group (82.1% vs. 74.4%, p = 0.548). In addition, the other variables were similar in these two groups.

Conclusions
The mean bleeding volume was higher in the VCV group compared to the PCV group; however, no signi cant difference was observed between these two groups. Hemoglobin levels in the patients included before and after surgery showed that the two groups were in a similar condition.

Background
It was indicated that posterior spinal xation surgery is often associated with heavy bleeding that sometimes requires a blood transfusion or blood product (1,2) The need for blood transfusion in spinal fusion surgeries without any implantation of the device was reported in 70% of the cases and in case of using the device, it was up to 50% of the cases. Additionally, the estimated bleeding rates have been reported in studies to be between 80 and 3100 cc. (3, 4, 5, 6, 7, and 8) In spinal deformity correction surgeries performed using posterior reversal methods, the amount of bleeding has been reported to be from 1 to 3 liters (9, 10, 11, 12, 13 and 14). Accordingly, there are similar reports in interior correction (15,16). Reduction of intraoperative bleeding is important for maintaining hemodynamic stability and the desirability of the surgical eld. On the other hand, it was found that the convenience of the surgeon reduces the duration of the operation, which is accompanied with a reduction in the amount of bleeding (2,17). Moreover, reducing bleeding reduces the need for blood transfusions and blood products, which consequently decreases the risks of hemolytic and non-hemolytic complications, lung damage, viral and bacterial infections, and hypothermia and coagulation disorders caused by blood transfusions (2, 18 and 19).
The techniques used to reduce intraoperative bleeding and the suitability of the surgical eld are divided into two groups as follows: 1-Reduction of bleeding by reducing intravenous blood ow such as the controlled hypotension and local vascular contraction, and 2-Accelerating and stabilizing coagulation using chemical and biological agents such as desmopressin, aprotinin, and tranexamic acid; estrogen; and bone wax. Correspondingly, these techniques have been widely studied as contradictory results, and nally many contradictory ndings have been obtained (19, 20, 21, 22, 23 and 24).  In this prospective, randomized, parallel and one-way blinded clinical trial, 78 patients with ASA class 1 and 2, aged between 18 and 75 years old, who were candidates for posterior spinal xation surgery, were included and then randomly divided into two groups of 39 subjects after obtaining the written informed consent.

Ethics and patients
This study was prospective, randomized, and parallel (allocation ratio = 1:1).
For the participants' allocation, the permuted block randomization was used by applying a computergenerated list of random numbers and using sealed envelopes. Although surgeons and nurses who were involved in patient care, were aware of the study they were blinded to the details of the study's protocol. The surgical procedure was performed by one surgeon and a single surgical team using the same method.
The permuted block randomization was used for the participants' allocation (allocation ratio=1:1). in this regard used random numbers that extracted from computer-generated list and sealed envelopes.
Anesthetic protocol: Anesthesia was induced and then maintained by the attending anesthesiologist, who managed anesthesia using a standard regimen in the study protocol; however, he was blinded to the details of the study protocol. After establishing routine patient monitoring (Heart rate, noninvasive blood pressure, pulse oximetry, and end tidal CO2), anesthesia was induced with an intravenous premedication of 2µg/kg fentanyl, 0.03 mg/kg midazolame, 5 mg/kg sodium thiopental, and 0.5 mg/kg atracurium. Endotracheal tube with a suitable size, a high-volume, and a low-pressure cuff was inserted for the patients. Measurements hemodynamic changes BP, HR, MAP, amount of blood loss, and transfusion were recorded intera and post-operative duration of surgery was recorded as well.
In this study, the amount of bleeding was calculated based on the volume of blood lost in the suction, the number of completely impregnated gases (4 x 4 cm, 15 cc each), large amount of completely impregnated blood (30 x 30 cm per 50 cc), and the amount of blood in the hemovac drain of the patients and then registered until the end of the operation.
Additionally, the amount of blood and blood products injected during the operation along with blood pressure and heart rate were recorded before the induction, immediately after the intubation, after being in the prone position, during the operation every 15 minutes, at the end of the operation, after being in the supine position, and after endotracheal tube exit and recovery time.
Afterward, postoperative hemoglobin levels were checked after the arrival to general ward.
Surgeon's satisfaction from the eld of surgery was also questioned and recorded as good, moderate, and weak, which were stated based on the volume of bleeding, the bleeding conditions of the operation eld, and the surgeon's personal opinion.
All the obtained data were measured and recorded by one trained observer who was not aware of each group. All the patients operated by one surgeon who was not informed of the method of ventilation.

Statistical analysis
The ndings were described with central and environmental indicators and then displayed with the related tables and graphs. The ndings analyzed using Kolmogorov-Smirnov test, parametric t-test and Fisher's exact test and also with Chi-square tests. SPSS-16 software was used to analyze data.
In this study, details of the protocol were blinded for surgeons and nurses. A surgeon was applying surgical procedure and a single surgical team used the same method

Patient's inclusion criteria
The patient's inclusion criteria were as follows: 1. Patients with ASA class 1 or 2, 2. Ages between 18 and 75 years old,

Patient consent to enter the study
The patient's exclusion criteria were as follows: 1. Respiratory diseases like Asthma, and chronic obstructive pulmonary diseases.
2. Severe heart diseases such as heart failure, myocardial infarction, and valvular diseases.

Result
In this study, 78 patients who were candidates for posterior spinal xation surgery were studied in the two groups of volume control ventilation and pressure ventilation in terms of the amount of bleeding caused by the operation. The variables of sex, age, height, weight, and body mass index (BMI) of the studied patients showed that these were not signi cantly different between these two groups ( Table 1). The mean bleeding volume in the studied groups was shown in table 2. the volume of bleeding due to surgery in the VCV group was 465.26 cc and in the PCV group, it was 431.28 cc; however, no signi cant difference was observed between these two groups (p = 0.67) ( Table 2).
Most of the patients in both treatment groups required no blood transfusions. In the VCV Group 17.9% and in the PCV Group 15%, although the patients needed two pack cells of blood. The difference between these two groups was not statistically signi cant (p = 1) ( Table 3).
Surgeon evaluation of the surgical eld in 74.4% of the patients in the VCV group and 82.1% of the patients in PCV Group has shown good results (p = 0.58) ( Table 4).
The frequency of the need for antihypertensive drugs during surgery in the VCV group was 10.3% and in the PCV Group was 5.1%, which were not statistically signi cant (p = 0.22) ( Table 5).
Comparison of hemoglobin levels of patients before and after surgery showed that these two groups were in a similar condition. After removing the effects of age, sex, body mass index, duration of operation, and basal hemoglobin no changes were observed in hemoglobin levels between the two groups. (Table   6).

Discussion
This study reported that the mean volume of bleeding due to surgery in the ventilation group with volume control was 465.26 cc, and in the ventilation group with pressure control, it was 431.28 cc, but no signi cant difference was observed between these two groups (p = 0.67). The results of the Kang's study showed that the rate of intraoperative bleeding in the PCV group was lower than the VCV group in patients undergoing similar surgery (253 and 382 ml), respectively (28). However, in the study of Le Guen and et al (2019) that examined the effect of mechanical ventilation modality, showed no bleeding in trans sphenoidal pituitary surgery (29). Accordingly, the difference can also be related to the impact of some other factors such as the length of surgery and the number of surfaces that should be operated. The average duration of surgery in Kang's study was 156 minutes and in the present study, it was 146 minutes. In addition, another variable was the age of the include patients that was less than 46 years old in both groups, but in the Kang's study, it was more than 64 years old. It was indicated that age alone with cardiovascular changes can have a great impact on the amount of bleeding directly or indirectly by exacerbating the other factors.
The Guen's study challenged the possible effect of ventilation type as well as the increased intrathoracic pressure on venous return in positive pressure ventilation and as a result rejected its clinical effect, which is consistent with the results of the present study. However, the sitting position could still be considered as a contributing factor in these differences.
Comparison of hemoglobin levels of the patients before and after surgery showed that the two groups were in a similar condition. This nding is similar with the ndings of other studies (11, 18, 19 and 27).
Although the amount of intraoperative bleeding was different between these two groups, the measured hemoglobin and hematocrit were not signi cantly different between them at any time. Therefore, this indicates that hemoglobin and hematocrit are not good indicators of intraoperative bleeding, which can be due to various causes like uid shifts. Furthermore, there was no difference in the amounts of postoperative bleeding at 24 and 72 hours between the two groups.
In the present study, most of the patients in both treatment groups required no blood transfusions.
Notably, in the ventilation group with volume control of 74.4% and in the ventilation group with pressure control of 75%, the difference between the two groups was not statistically signi cant (p=1).
The surgeon evaluated the surgical eld in 74.4% of the patients in the ventilation group with volume control and 82.1% of the patients in the ventilation group with a good pressure control (p = 0.58). This nding is in agreement with a similar study (27).
Due to the limited studies performed similar to our study, the comparison of most of the results of this study with other studies was limited.

Conclusion
The mean volume of bleeding due to surgery was higher in the volume-controlled ventilation group compared to the pressure-controlled ventilation group, but no signi cant difference was observed between these two groups. Due to the inconsistencies in few studies available, the role of ventilation type in the rate of bleeding cannot be con rmed as an independent variable, so it needs further investigations.