ABSTRACT
Objective:
Position (supine and prone) changes have their essential effects on respiratory mechanics and pulmonary perfusion in patients under general anesthesia. These effects on respiratory mechanics, arterial blood gas, and hemodynamic parameters in patients who underwent percutaneous nephrolithotomy operation were compared in pressure and volume control ventilation (VCV) modes.
Methods:
This study prospectively evaluated 50 patients who underwent percutaneous nephrolithotomy. Patients were divided into groups of VCV and pressure control ventilation (PCV). Each group was divided further into two subgroups with supine and prone positions. General anesthesia was applied to all patients. Respiratory mechanics were recorded every 5 min. Arterial blood gas samples were repeated at each position change. Hemodynamic and respiratory parameters were simultaneously recorded.
Results:
Peak inspiratory pressure (Ppeak), plateau pressure (Pplato), and driving pressure (DP) of the VCV group were higher in the prone position than in the supine position. Ppeak, Pplato, and DP in the prone position were higher in the VCV group than the PCV group, and Horowitz ratio and compliance were lower. The Horowitz ratio of both groups was significantly higher in the prone than in the supine position.
Conclusion:
Despite the advantages, the superiority of PCV to VCV cannot be mentioned at the present.
INTRODUCTION
Percutaneous nephrolithotomy is an invasive surgery that is performed in the supine and prone positions under general anesthesia (1). Depending on the surgical position, various changes occur in all body systems. As preferred by the surgical team, a position that will facilitate the surgical approach but will not endanger cardiovascular and pulmonary functions should be applied (2). Position changes have essential effects on respiratory mechanics and pulmonary perfusion in patients under general anesthesia (3). A 10% and 12.5% decreased vital capacity in the prone and supine positions were found, respectively (2).
Healthy people have decreased lung compliance during the prone position due to thoracal expansion restriction, decreased chest wall elasticity, obesity, neuromuscular blockers, and abdominal compression (4). However, prone positioning, which is also used to improve oxygenation in patients with acute respiratory distress syndrome (ARDS) in the intensive care unit, is a safe and most effective lung-protective ventilation strategy component that includes low positive end-expiratory pressure (PEEP), low tidal volume (TV), and low driving pressure (DP) (5).
Pulmonary blood flow and gas distribution differ according to the supine position in patients who are mechanically ventilated in the prone position (6). Thoracic wall movement is limited by compression. With decreased muscle tone due to neuromuscular blockers, the diaphragm is directed toward the cephalus by intra-abdominal pressure. The resulting changes in the lung volume and pulmonary blood flow differentiation affect the respiratory mechanics (7,8).
Respiratory system compliance decreases by 17-30% when paralyzed patients under general anesthesia are turned to the prone position (9). However, some studies revealed no significant changes in the compliance when the appropriate position (with chest wall and pelvic supports) was given (10,11). A significantly increased functional residual capacity is seen in the prone position, which can be explained by dependent alveoli reopening that tends to close in the supine position (12).
Better oxygenation in the prone position compared to the supine position is achieved by improving the ventilation-perfusion ratio and eliminating lung compression by the heart. Thus, an increased ventilable lung is obtained (13).
Close monitoring of respiratory mechanics is vital in the operating room and the intensive care unit (14). Mechanical ventilation is life-saving; however, it causes ventilator-induced lung injury (VILI) (15). Respiratory parameters, such as TV, DP, flow, respiratory rate, and PEEP, were associated with VILI (16). However, DP was considered the main mediator to cause VILI. Additionally, intraoperative high DP is associated with increased postoperative pulmonary complications (17).
This study compared the respiratory mechanics and blood gas parameters of patients who underwent percutaneous nephrolithotomy and were ventilated with volume control ventilation (VCV) or pressure control ventilation (PCV) in supine and prone positions.
METHODS
Study Population
This study was prospectively conducted in 50 patients who underwent percutaneous nephrolithotomy. Patients with the American Society of Anesthesiologists classification I-II, between the age of 18 and 65 years, without chronic obstructive pulmonary disease, diabetes, and cardiopulmonary diseases was included in the study. All patients underwent preoperative anesthetic evaluation.
Study Design
Patients were sequentially randomized into two groups, first from the VCV group and then from the PCV group. Vascular access was provided to patients after electrocardiogram, noninvasive blood pressure, and oxygen saturation (SpO2) monitoring. At a rate of 2-4 mL/kg/h, 0.9% NaCl infusion was started. Initial heart rate (HR), mean arterial pressure (MAP), and peripheral SpO2 were recorded as baseline values. Intravenously, propofol of 2 mg/kg, fentanyl of 2 μg/kg for induction, and vecuronium of 0.1 mg/kg for neuromuscular blockage were given. After endotracheal intubation with a 7.5 mm inner diameter spiral tube, radial artery cannulation was performed. Balanced general anesthesia with sevoflurane of 1 MAC and remifentanil was maintained. The VCV (n=25) and PCV group (n=25) were ventilated with Dräger Primus anesthetic machine (Lübeck, Germany). Ventilation parameters were constant with a respiratory rate of 12/min, PEEP of 5 cm H2O, and inspiration-expiration rate of 1:2. These settings remained since the end-tidal carbon dioxide (EtCO2) was in the range of 30-35 in all patients. At the beginning of the operation, all patients were ventilated with a TV of 6-8 mL/kg, and the DP is adjusted to provide this TV in the PCV group. Respiratory and hemodynamic parameters of all patients were recorded at 0, 5, 10,15, 20, 25, and 30 min with 5-min intervals in the supine period after induction. Similarly, 5 min after the patient was turned to the prone position, the data were recorded at 5, 10,15, 20, 25t, 30, and 35 min with 5-min intervals. Arterial blood gas samples were taken at the 30th min in the supine position and the 35th min in the prone position.
Thoracal gel supports were placed on both sides of the chest during prone positioning. After the patients were turned to the supine position at the end of the surgery, inhalation agents were stopped. Atropine of 0.01 mg/kg and neostigmine of 0.03 mg/kg were administered to eliminate residual neuromuscular blockade after spontaneous breathing started. Patients were extubated when spontaneous breathing was sufficient. All patients were hemodynamically stable, without complications.
Compliance, peak inspiratory pressure (Ppeak), plateau pressure (Pplato), DP, ETCO2, HR, MAP, pH, partial arterial carbon dioxide pressure (PaCO2), bicarbonate (HCO3), and partial arterial oxygen pressure (PaO2) parameters were recorded as excel file.
The mean values of all data in supine and prone positions were recorded for statistical analyses.
Calculation of DP and Compliance
The gas flow is constant in VCV and 5% pause time (Tpause) is set at the end of inspiration, thus Pplato and compliance are automatically calculated by the ventilator and screen display. In PCV, the airway pressure is considered constant from the beginning to the end of inspiration. Therefore, Ppeak and alveolar pressure (Pplato) are calculated as equal (18-20).
The presence of auto-PEEP was evaluated with the expiratory hold maneuver. However, auto-PEEP was not detected in our patients (Auto - PEEP = PEEPtotal - PEEPset). Since no auto-PEEP exists, DP is calculated as (DP) = Ppeak - PEEP, and compliance is calculated as (C) = Tve ÷ (Ppeak - PEEP) in PCV.
Sample Size Calculation
A pilot study was conducted with five patients to determine the number of patients to be included in the study. The comparison of VCV and PCV in the prone position determined the DP difference as the primary outcome. In the prone position, DP was calculated as 13±2 cmH2O in VCV and 11±2 cmH2O in PCV. The sample size was calculated as at least 23 patients per group based on a pilot study (power=95%; α=0.05) (G*Power version 3.1.9.4, Germany).
Statistical Analyses
The gender distribution of the groups was compared using the Chi-square test. Demographic data (age, height, and weight), arterial blood gas, and respiratory parameters of the groups were homogeneous in the Shapiro-Wilk test and were evaluated with the Student’s t-test. Subgroup comparisons were made with the paired t-test. The mean and standard deviation (SD) values for each parameter were used for statistical representation. Results were evaluated at the significance level of p<0.05. Statistical analyzes were made with Number Cruncher Statistical System 2007 Statistical Software (Utah, USA).
RESULTS
No statistical difference was found between gender distribution and mean ± SD values of age, height, predictive bodyweight of the VCV and PCV groups (Table 1).
A statistically significant difference was observed between the mean values of Ppeak, TV, PaO2/FiO2 in the supine position of the VCV and PCV groups, whereas no statistically significant difference between the mean values of Pplato, TV, DP, compliance, ETCO2, PaCO2, HCO3, pH, HR, and MAP.
A statistically significant difference was observed between the mean values of Ppeak, Pplato, DP, TV, compliance, PaO2/FiO2 in the prone position of the VCV and PCV groups, whereas no statistically significant difference between the mean values of ETCO2, PaCO2, HCO3, pH, HR, and MAP.
The mean ± SD of the above-mentioned respiratory parameters of the VCV and PCV groups in the supine and prone positions are shown in Table 2.
In the VCV group, Ppeak, Pplato, DP, PaO2, and PaO2/FiO2 values were statistically significantly higher, and compliance values were significantly lower in the prone position than the supine position. However, no statistically significant difference was observed between ETCO2, PaCO2, HCO3, and pH values.
The mean ± SD values of the respiratory parameters in the supine and prone positions of the VCV group are shown in Table 3.
In the PCV group, PaO2, PaO2/FiO2 values were statistically significantly higher, and TV and compliance values were significantly lower in the prone position than the supine position. However, no statistically significant difference was observed between Ppeak, Pplato, and DP values.
The mean ± SD values of respiratory parameters in the supine and prone positions of the PCV group are shown in Table 4.
DISCUSSION
PCV and VCV have been compared for a long time (21-24). Our study re-discussed this issue with current topics, such as DP, under the guidance of previous studies.
The mean PaO2 values of the PCV group were higher than the VCV group in supine and prone positions. Studies show that PCV provides better PaO2 values in supine and prone positions than VCV mode (24-29).
No significant difference was found between the PaCO2 values between the PCV and VCV groups in supine and prone positions. Additionally, a recent meta-analysis reported no difference in PaCO2 values between the VCV and PCV groups in patients who had elective surgery in the supine position (24).
In VCV and PCV groups, mean PaO2 values were statistically significantly lower in the supine than in the prone position. In the prone position, significantly increased PaO2 values have been attributed to the reduced dependent lung areas and the lesser gravitational effect of the heart and great vessels on the lung (25,26,30,31). Thus, a better perfusion/ventilation ratio is obtained (32). Additionally, increased functional residual capacity and secretion mobilization also contribute to this improvement (26). Many studies have indicated that the prone position positively affects the arterial blood gas parameters (33-35). Our study revealed no difference between the PCO2 values in position changes in both ventilation modes. Previous studies have shown that PaCO2 values in the prone position are equal or lower than in the supine position (33-35).
At the beginning of the operation, despite higher TV in the PCV group, no significant difference was found between the Pplato values in the supine position of the PCV and VCV groups. However, after the patient was turned to the prone position, the Pplato values of the PCV group were lower than the VCV group, although the other set of respiratory parameters (DP in PCV and TV in VCV) were constant. Similarly, Ppeak values of the PCV group were also lower in both positions. Studies report that Ppeak and Pplato values of the VCV group in the prone position are equal or higher than in the supine position (10,26,36). Another study that ventilated patients with VCV after anesthesia induction and pneumoperitoneum revealed decreased Pplato values when the ventilation mode was changed to PCV after 40 min (35). A meta-analysis reported lower intraoperative Ppeak and Pplato values in PCV (29).
The lower Ppeak and Pplato values in the PCV group were attributed to the different gas flow patterns between the two groups (35). Therefore, no difference was found between the DP values of the VCV and PCV groups in the supine position but a significant difference in the prone position. Similarly, DP values of the VCV group were significantly different between positions. The DP is constant in both positions in PCV, thus homogeneous ventilation of the alveoli with different time constants is ensured and prevented excessive bronchoalveolar unit stretching (26,35-37).
No difference was found between the compliance of the VCV and PCV groups in the supine position. However, when the patients were turned to the prone position without changing their respiratory settings, the compliance was significantly higher in the PCV group than the VCV group. The compliance of the VCV and PCV groups in the supine position was statistically significantly higher than in the prone position. The reduced compliance of the VCV group when patients are turned to the prone position is thought to be due to the increased Ppeak and Pplato. Contrarily, Ppeak and Pplato are constant in PCV, thus the reduced compliance is due to the decreased TV. Reduced lung compliance in the prone position is due to the increased abdominal pressure to the thorax due to neuromuscular blockers, thoracal movement restriction, and chest wall compression by the support materials (26,35,37). The study of Sen et al. (38) in percutaneous nephrolithotomy patients compared the PCV and VCV modes in the prone and supine positions, and compliance was found to be lower in both modes in the prone position. The compliance of the PCV group in the prone position was found to be higher than the VCV group (38). The compliance decreases from supine to the prone position by approximately 17% to 30% (9). However, studies report that compliance in the prone position will not change with correctly placed thoracal and pelvic supports (10,11). Compliance was observed to significantly increase when switching from VCV to PCV after anesthesia induction (35). In patients who underwent laparoscopic gynecological surgery, lung compliance was higher in PCV than VCV (39). Changes in compliance are due to the gas flow pattern. The decreasing flow pattern in the PCV is argued to reduce lung tension. However, changes in compliance while isovolumetric depend not only on the elastic properties of the respiratory system but also on the resistive component of the airway and endotracheal tube (26,35).
Respiratory dynamic changes in the prone position mentioned above are due to the need for a higher DP to reach the set TV in VCV and the lower TV to reach the set DP in PCV.
High DP was strongly associated with VILI and mortality (40). Additionally, high DP on the first day of mechanical ventilation is a risk factor for ARDS development later on (41) and is also associated with postoperative pulmonary complications (17). Therefore, obtaining a better gas exchange with lower DP gave PCV an advantage over VCV. Compared to the VCV group, the PCV group had lower DP and better gas exchange. However, this advantage of PCV has recently become controversial (42) since reaching inspiratory pressure (DP) in a short time interval (T-slope) with the highly variable gas flow in PCV can have a damaging effect. These theoretical concerns regarding the PCV should be seen as worthy of investigation.
Postoperative pulmonary complications develop in 5%-33% of patients. These complications were shown to reduce with lung-protective ventilation. For lung-protective ventilation in the surgical patient, an international expert panel recommends that Cdyn, DP, and Pplato should be monitored on all mechanically controlled ventilated patients, and currently, a preferred specific ventilation mode is not recommended, as studies are reporting conflicting results (43).
Our study has some limitations. First, airway resistance parameters were not compared between ventilation modes since inspiratory resistance values in PCV were not automatically calculated by the ventilator, and we did not have the opportunity to calculate with the least-square fit method (44). Second, the effects of both ventilation modes and position changes on advanced hemodynamic parameters (central venous pressure, cardiac output, systemic vascular resistance, and lung water) have not been studied. Third, neuromuscular blockade monitoring could not be performed.
CONCLUSION
Prone position was beneficial as it increased oxygenation in both the VCV and PCV groups without causing any adverse effects on hemodynamic and respiratory mechanics. The PCV group had better respiratory mechanics (lower DP and Pplato) and blood gas parameters (higher PaO2) than the VCV group in the prone position. However, today, the superiority of PCV and VCV over each other cannot be mentioned.