Effect of Switching between Pressure-controlled and Volume-controlled Ventilation on Respiratory Mechanics and Hemodynamics in Obese Patients during Abdominoplasty.

BACKGROUND: The ideal intraoperative ventilation strategy in obese patients remains obscure. This prospective, randomized study was designed to evaluate the effect of pressure-controlled ventilation (PCV) before or after volume-controlled ventilation (VCV) on lung mechanics and hemodynamics variables in obese patients subjected to abdominoplasty operation. PATIENTS AND METHODS: The study included forty patients with body mass index 30-45 kg/m2 subjected to abdominoplasty. All patients were randomly allocated in two groups after the induction of general anesthesia (twenty patients each), according to intraoperative ventilatory strategy. Group I (P-V): started with PCV until the plication of rectus muscle changes into VCV till the end of surgery. Group II (V-P): started with VCV until the plication of rectus muscle changes into PCV till the end of surgery. Lung mechanics, hemodynamics variables (heart rate and mean blood pressure), and arterial blood gases (ABGs) were recorded.
RESULTS: No significant difference in the hemodynamics and ABGs were recorded between the studied groups. The use of PCV after VCV induced the improvement of lung mechanics.
CONCLUSION: Switching from VCV to PCV is preferred to improve intraoperative oxygenation and lung compliance without adverse hemodynamic effects in obese patients.

RCT Entities:   Population Interventions Outcomes

INTRODUCTION

Abdominoplasty is a surgical procedure that effectively removes a considerable amount of abdominal skin and fat with tightening the abdominal wall.[1] Difficulties in ventilation are frequently encountered problems during anesthesia in obese patients undergoing abdominoplasty. Because of the restrictive ventilatory effects of obesity, these patients often show arterial hypercapnia, hypoxemia, and ventilation–perfusion mismatch.[2] An increase in the intra-abdominal pressure (IAP) to a variable degree is an additional factor worsens the ventilation during the procedure of abdominoplasty.[3]

The operation of abdominoplasty is usually carried out under general anesthesia. The use of volume-controlled ventilation (VCV) is common despite frequently seen high-pressure levels in obese patients. This high airway pressure may lead to volutrauma, barotrauma, and biotrauma. The mechanical consequences of reduced lung compliance and chest wall compliance, added to the reduction of functional residual capacity due to increased IAP explain impaired alveolar ventilation and the high airway pressures in those patients.[4]

Pressure-controlled ventilation (PCV) is a time-cycled mode in which square waves of pressure are applied and released using a decelerating flow. The decelerating flow often results in a higher mean inflation pressure when compared with the constant flow of VCV. PCV has been proposed as an alternative to VCV in the Intensive Care Unit (ICU) patients with adult respiratory distress syndrome and in obese patients to achieve adequate oxygenation and normocapnia.[5] Evidence has been accumulating that PCV compared with VCV during anesthesia for bariatric surgery improves gas exchanges without increasing ventilation pressures or causing any hemodynamic side effects.[6]

The aim of this study was to compare the use of PCV before and after VCV on respiratory mechanics and hemodynamics during abdominoplasty operations for obese patients.

Patients and Methods

This prospective randomized study was carried out in Mansoura university hospital included forty patients American Society of Anesthesiologists (ASA) physical Status I and II subjected to abdominoplasty. Approval of the study protocol was obtained from the institutional ethics committee, and all patients gave written informed consent before the operation. Inclusion criteria were age between 25 and 50 years old and body mass index (BMI) 30–45 kg/m2. All patients were randomly allocated in two equal groups (twenty patients each) by closed envelope method according to the intraoperative ventilatory strategy.

Exclusion criteria for this study were patient refusal, ASA physical Status III and IV, history of obstructive sleep apnea, and BMI below 30 or above 45 kg/m2. Patients with any cardiac dysfunction, respiratory problems, renal, and hepatic dysfunctions were excluded from the study. Intraoperative exclusion criteria were inability to perform tracheal intubation in the conditions of usual practice, inability to maintain stable mechanical ventilation settings for 30 min, inability to maintain an appropriate end-tidal CO2 (EtCO2), and inability to remove the tracheal tube in the operating room. The day before the operation, complete medical history, complete clinical examination, routine investigations (complete blood count, electrocardiogram [ECG], liver function, random blood glucose, serum creatinine, and coagulation profile), chest X-ray, and echocardiography were done.

After an overnight fasting, the patients were transferred to the anesthesia room and were premedicated with midazolam 0.03 mg/kg and metoclopramide 10 mg intravenous (IV) after insertion of 20-gauge IV cannula preoperatively, 2 h before the induction of anesthesia. In the operating room, following application of routine monitors (noninvasive blood pressure [NIBP], heart rate [HR], SpO2, and ECG), epidural catheter was inserted for intra- and post-operative pain control by injection of 8–12 ml of 0.25% bupivacaine (marcaine vial 0.5%, AstraZeneca, Australia) via median approach at level L3-L4.

An arterial sample was obtained for arterial blood gases (ABGs) analysis as basal values. NIBP (mean), HR, and SpO2 were recorded just before the induction of general anesthesia as a basal guideline.

Induction of general anesthesia was done by preoxygenation with 100% oxygen for 5 min, 1 µg/kg fentanyl followed by 2 mg/kg propofol mixed with 2 ml lidocaine 2% was injected to induce unconsciousness. The endotracheal tube was inserted using the direct laryngoscope after 1 mg/kg succinylcholine and its correct position was confirmed by EtCO2 curves with the capnography monitoring. Maintenance of anesthesia was done using one minimum alveolar concentration isoflurane. Loading dose of 0.5 mg/kg atracurium was used to maintain adequate muscle relaxation then incremental doses of one-fifth of loading dose when was needed. Controlled ventilation through the operation was done as follows: Group I (P-V): started with PCV until the plication of rectus muscle changes into VCV through postplication period till the end of surgery. Group II (V-P): started with VCV until the plication of rectus muscle changes into PCV through postplication period till the end of surgery.

PCV parameters were as follows: tidal volume (Vt): Pplateau has been set so that Vt = 10 ml/kg ideal weight (50 + [0.91 × (height in cm−152.4)] for men and 45.5 + [0.91 × (height in cm−152.4)] for women), respiratory rate (RR) = 12/min, the ratio of inspiratory-to-expiratory time (I: E) = 1:2, FiO2 = 0.5, and positive end-expiratory pressure (PEEP) = 5 cm H2O. VCV parameters were as follows: Vt = 10 ml/kg ideal weight, RR = 12/min, I: E = 1:2, FiO2 = 0.5, and PEEP = 5 cm H2O. Plateau time was 20% of inspiratory time allowing the ventilator to measure plateau pressure.

Continuation of mechanical ventilation has been done according to EtCO2, if <40 mmHg decrease RR by 2/min every 5 min till 10/min, decrease Vt by 1 ml/kg in VCV or decrease Pplateau by 2 cm H2O every 5 min till Vt6 ml/kg in PCV. If EtCO2 was more than 40 mmHg increase RR by 2/min every 5 min till 18/min, increase Vt by 1 ml/kg every 5 min till Vt12 ml/kg in VCV or increase Pplateau by 2 cm H2O in PCV (PPlateau limited to 40 cm H2O). If EtCO2 still high adjust parameter as RR at 25/min, Vt between 6 and 12 ml/kg in VCV, maintain Pplateau limited to 40 cm H2O in PCV, and inspiratory and expiratory flow curves have to return to zero every breath cycle.[6]

Fluid maintenance was done using 4-2-1 rule and according to intraoperative patient need. All operations were done by the same surgeon.

Intraoperative monitoring and recorded data

NIBP (mean), HR, and SpO2 were measured automatically and recorded every 15 min. RR, Vt, minute volume, peak pressure, plateau pressure, mean airway pressure, static compliance [Vt/(PIP-PEEP)], and dynamic compliance [Vt/(Pplateau-PEEP)] were recorded every 20 min. ABG was done after 20 min from the induction of anesthesia after the establishment of the ventilation mode. Another ABG was done after 20 min from plication of the rectus and switch to the other mode to obtain: pH, PaO2, PaCO2, PaCO2-EtCO2 gradient (CO2-g), and PAO2-PaO2 gradient. PAO2= (FiO2 × [Patmos-PH 2O]) − (PaCO2/RQ), i.e., PAO2= (FiO2 × (760-47)) − (PaCO2/0.8). The calculations above assume 100% of humidity at sea level and a respiratory quotient of 0.8, using the alveolar gas equation to determine PAO2. A-a gradient = PAO2-PaO2.[7]

Rectus plication time was marked as a target for switching between the ventilation modes. After termination of the surgery, discontinuation of the anesthesia and reversal of muscle relaxant was prepared and given to the patient in the form of 0.06 mg/kg neostigmine and 0.03 mg/kg atropine. Extubation was done after the fulfillment of the extubation criteria. The patient was transferred to the recovery room and the ward after fulfillment of discharge criteria.

Postoperative monitoring

NIBP (mean), HR, and ABG were recorded after 6 h for every patient in the postanesthetic care unit.

Any perioperative adverse effects in the form of hypoxia, hypercapnia, and atelectasis were recorded.

Statistical analysis

Statistical analysis was performed using SPSS Statistics version 22 software (IBM Corp. Released 2013. IBM SPSS Statistics for Windows, Armonk, NY: IBM Corp.). All data were tested for normality using Kolmogorov–Smirnov test. Parameters obtained during PCV and VCV were collected and calculated as mean values. Independent sample t-test was performed to compare values between the two studied groups (intergroup comparison). Paired-sample t-test was used to compare values within the two studied groups (intragroup comparison). All data were expressed as mean ± standard deviation, a value of P < 0.05 was considered to be statistically significant.

RESULTS

No significant difference was observed between two groups regarding the demographic data [Table 1].

Table 1

Patient characteristics of the studied groups (n=20)

Hemodynamic results

Insignificant inter- and intra-group differences were observed in the means of HR and mean arterial blood pressure values recorded as basal, intraoperative through PCV or VCV according to the studied group and 6 h postoperatively [Tables 2 and 3].

Table 2

Heart rate (beat/min) of the studied groups (n=20)

Table 3

Mean arterial blood pressure (mmHg) of the studied groups (n=20)

Arterial blood gases results

pH and HCO3 mean values recorded as basal, intra-operative during PCV and VCV (20 min after induction of anesthesia and 20 min after plication of the rectus muscle) according to the studied group and 6 h postoperative showed insignificant inter-group or intra-group differences [Tables 4 and 5]. PaCO2 and CO2-g values showed insignificant inter- and intra-group differences intraoperatively [Table 6].

Table 4

pH values obtained from arterial blood gases of the studied groups (n=20)

Table 5

HCO3 values (mEq) obtained from arterial blood gases of the studied groups (n=20)

Table 6

PaCO2 values (mmHg) and CO2-gradient (mmHg) obtained from arterial blood gases of the studied groups (n=20)

Mean values of SpO2 showed insignificant inter-group or intra-group differences changes intra- and post-operatively [Table 7].

Table 7

SpO2 values (%) obtained from arterial blood gases of the studied groups (n=20)

PaO2 was significantly increased 20 min after the induction of anesthesia and 20 min after the plication of rectus muscle in comparison to the basal values in either group. Use PCV before or after VCV did not significant affect the mean level of PaO2 intraoperatively in each group. Six hours postoperative PaO2 returned to the basal value in both groups [Table 8].

Table 8

PaO2 values (mmHg) obtained from arterial blood gases of the studied groups (n=20)

Group I (P-V) showed that the use of VCV after PCV caused a significant increase in A-a O2 g intraoperative pre- and post-plication of the rectus muscle. While Group II (V-P) showed that the use of PCV after VCV caused a significant decrease in A-a O2 g. Six hours postoperative A-a O2 g returned to the basal value in both groups [Table 9].

Table 9

Alveolar-arterial oxygen gradient values (mmHg) obtained from arterial blood gases of the studied groups (n=20)

Respiratory mechanics results

Group I (P-V) showed that the use of VCV after PCV caused a significant increase in the peak and plateau airway pressure and a significant decrease in static and dynamic lung compliance. While Group II (V-P) showed that the use of PCV after VCV caused a significant increase in the peak, plateau airway pressure, static, and dynamic lung compliance [Table 10]. Respiratory mechanics significantly improved after switching from VCV to PCV (Group II).

Table 10

Respiratory mechanics of the studied groups (n=20)

DISCUSSION

This study was designed to compare the use of PCV before and after VCV on respiratory mechanics and hemodynamics during abdominoplasty in obese patients. Nonsignificant difference in the hemodynamics and ABGs was recorded between the studied groups. Switching from VCV to PCV induced better improvement of lung mechanics than when used before VCV.

Pathophysiological changes due to obesity may complicate mechanical ventilation during general anesthesia. The ideal ventilation strategy has not yet been established, but it is expected to optimize gas exchange and pulmonary mechanics and to reduce the risk of respiratory complications.[8] Although patients undergoing abdominoplasty have healthy lungs, the pathophysiological changes induced by obesity make these patients prone to perioperative complications, such as hypoxemia, hypercapnia, and atelectasis.[9] Immediately after the induction of general anesthesia, atelectasis develops, leading to a reduction in both ventilation–perfusion ratio and pulmonary compliance. It has been demonstrated that in anesthetized patients, PaO2 inversely related to BMI.[10] Obesity is characterized by several alterations in the mechanics of the respiratory system[11] that tend to further exaggerate impairment of gas exchange.[4]

Intraoperative respiratory changes may extend to the postoperative period and may subsequently necessitate the use of supplementary oxygen. It may also delay discharge from the postanesthesia care unit, increase the need for respiratory physiotherapy or noninvasive ventilation, and also increase the probability of ICU admission. Furthermore, it has been demonstrated that obesity is a risk factor for postoperative tracheal reintubation, morbidity, and mortality.[12]

The selection of the optimal ventilation mode or the optimal control variable of ventilation for the obese patient is of interest to most of the anesthesiologists. VCV and PCV are not different ventilatory modes but are different control variables within a mode. During VCV, airway pressure increases in response to reduced compliance, increased resistance or active exhalation, and may increase the risk of ventilator-induced lung injury. During PCV, the inspiratory flow and flow waveform are determined by the ventilator as it attempts to maintain an inspiratory pressure profile. The clinician should titrate the inspiratory pressure to the measured Vt.[13]

Although previous studies showed that both modes were likely equivalent in pulmonary mechanics[14] and equally suited in morbidly obese patients,[13] this study showed that the respiratory mechanics were significantly better during use the of PCV after VCV. The possible explanation for our results is the different inspiratory flow wave form as PCV characterized by decelerating pattern while VCV is characterized by constant pattern. The ability of the decelerating flow waveform to distend the lung at the selected peak pressure throughout the inspiratory time was described by Davis et al.[15] This may facilitate alveolar recruitment, enhance diffusion, allow alveolar units with slow time constants to fill while preventing over distension of normal alveoli and augment collateral ventilation.

The change in dynamic lung compliance was associated with a change in gas distribution.[16] However, in isovolumetric conditions, variation of dynamic lung compliance depended not only on the elastic properties of the respiratory system but also on the resistive (flow-dependent) component of the airways and the endotracheal tube. The main disadvantage of PCV included variability in delivered Vt.[17] In contrast to VCV, PCV resulted in a smaller delivered Vt when respiratory system compliance was decreased. A smaller Vt might lead to atelectasis and might go undiagnosed because there is no change in peak pressure when PCV is used.[18]

These results are in agreement with Davis et al.,[15] who compared PCV and VCV with different flow wave form. De Beer and Gould[19] published review article about principles of artificial ventilation and mentioned the same results. In contrast, De Baerdemaeker et al.[13] concluded in his study that both VCV and PCV appear to be equally suited in morbidly obese patients. In addition, Karcz et al.[14] demonstrated that both modes are likely equivalent in pulmonary mechanics.

The current results showed nonsignificant difference in the hemodynamics variables (HR and mean blood pressure) between groups. This result is in agreement with De Baerdemaeker et al.,[13] who found that VCV and PCV may improve intraoperative oxygenation and respiratory system compliance without adverse hemodynamic effects.

In this study, the effects of switching between VCV and PCV on gasometry (pH, PaCO2, arterial-EtCO2 gradient, and HCO3) showed insignificant differences between two modes. The mechanical ventilation improves oxygenation parameters in obese patients[20] during both VCV and PCV explaining the significant increase in the PaO2 recorded in our results when compared intraoperative values with basal values.

Karcz et al. concluded in his systemic review “State-of-the-art mechanical ventilation” that both modes are likely equivalent in supporting gas exchange.[14] Similarly, another systemic review and meta-analysis published by Aldenkortt et al. confirmed our results and it was performed for randomized trials testing ventilation strategies in obese patients undergoing surgery. This review included all the studies that were published between 1978 and 2011 and came from 10 different countries. He concluded that intraoperative arterial oxygenation remained unchanged with PCV and VCV.[9] The same result was reported also by Tugrul et al.[21] who compared PCV and VCV during one-lung anesthesia.

Alteration of IAP during abdominoplasty is another problem affecting the respiratory mechanics.[22] The operative technique has at least two maneuvers that result in IAP elevation: plication of the rectus muscle and flap resection.[23] In this study, we found no change in respiratory dynamics regardless the type of ventilatory mode in response to change in IAP after plication of the rectus muscle.

CONCLUSION

Switching from VCV to PCV is preferred to improve intraoperative oxygenation and lung compliance without adverse hemodynamic effects in obese patients undergoing abdominoplasty operations.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.