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Original Article

Effect of vital capacity manoeuvres on arterial oxygenation in morbidly obese patients undergoing open bariatric surgery

Chalhoub, V.*; Yazigi, A.*; Sleilaty, G.*; Haddad, F.*; Noun, R.; Madi-Jebara, S.*; Yazbeck, P.*

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European Journal of Anaesthesiology (EJA): March 2007 - Volume 24 - Issue 3 - p 283-288
doi: 10.1017/S0265021506001529
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Abstract

Introduction

Maintenance of adequate arterial oxygenation is often a problem in morbidly obese patients during general anaesthesia [1–3]. Obesity negatively affects respiratory mechanics and lung volume by weighting the chest wall and exerting increased upward pressure on the diaphragm [4,5]. These alterations are further aggravated in the supine position and during general anaesthesia [6]. Sedated and paralysed morbidly obese patients have a significantly decreased total respiratory system, lung and chest wall compliance as well as an increased airway resistance and reduced functional residual capacity [4,7–10] associated with atelectasis and airway closure [11,12]. The resulting augmentation in ventilation–perfusion mismatching, intrapulmonary blood shunting and oxygen consumption will widen the alveolar-arterial oxygen tension difference and cause significant hypoxaemia [4,9,10,13,14].

Several ventilatory strategies were evaluated in order to improve intraoperative arterial oxygenation in morbidly obese patients. It was shown that mechanical ventilation with large tidal volumes improves oxygenation in obese patients [15]. However, the use of large tidal volumes may result in high inspiratory airway pressure and cause lung damage. Even more, the beneficial effect of large tidal volumes was not reproduced in recent studies [16,17]. Pressure-controlled ventilation was assessed in morbidly obese patients undergoing gastric bypass and it did not improve gas exchange [18]. Another strategy to improve oxygenation in obese patients was the use of positive end-expiratory pressure (PEEP). Some authors found that in anaesthetized and paralysed morbidly obese patients, PEEP improved arterial oxygenation [19,20], but others had not [21].

The vital capacity manoeuvre (VCM) is an alveolar recruitment technique performed by inflating the lungs with a positive pressure of 40 cm H2O and maintaining this pressure for 10–15 s [22]. By increasing lung volumes to vital capacity and providing a sustained high peak inspiratory pressure above alveolar opening pressure, a VCM expands collapsed alveoli and relieves lung atelectasis [23,24]. Several studies reported that, in normal weight patients, the VCM was an efficient technique in decreasing intrapulmonary shunting related to atelectasis and in improving arterial oxygenation [24,25]. Other studies also suggested that a VCM should be followed by ventilation with a PEEP in order to prevent the newly recruited alveoli from collapsing and to maintain the improvement of arterial oxygenation [26,27]. A single and very recent report indicated that VCM and PEEP effectively increased respiratory system compliance and arterial oxygen partial pressure (PaO2) in obese patients undergoing laparoscopic bariatric surgery [28]. The aim of our study was to assess the effects of VCM, followed by ventilation with a PEEP, on arterial oxygenation in morbidly obese patients undergoing open bariatric surgery.

Methods

From January to July 2005, morbidly obese adult patients, American Society of Anesthesiologists Grade III, scheduled for open bariatric surgery, were enrolled in this single-centre, prospective, randomized study. All patients had a body mass index (BMI) above 40 kg m−2. Cardiac, pulmonary and neurological diseases were considered as exclusion criteria. Institutional review board approval and written informed consent were obtained.

Patients were premedicated with hydroxyzine 1 mg kg−1. General anaesthesia was induced using propofol 2 mg kg−1, fentanyl 1 μg kg−1 and succinylcholine 1 mg kg−1. After tracheal intubation, anaesthesia was maintained with 1–2 minimum alveolar concentration sevoflurane and fentanyl 1–2 μg kg−1 h−1. Neuromuscular blockade was obtained by cisatracurium with a loading dose of 0.2 mg kg−1 followed by a continuous infusion of 2–4 μg kg−1 h−1. Anaesthesia drugs were calculated on an ideal body weight basis. Intraoperative monitoring parameters included five-lead electrocardiography, invasive radial arterial pressure, pulse oximetry, capnography, urine output and rectal temperature. All patients were mechanically ventilated (Datex-ohmeda Aestiva/5; Helsinki, Finland) with a tidal volume of 10 mL kg−1 of ideal body weight, an inspiratory/expiratory time ratio of 0.4 and a mixture of oxygen (O2) and nitrous oxide (N2O) adjusted to have an inspired oxygen concentration of 40%. The use of nitrous oxide rather than air in the inspired gas was a part of the anaesthetic protocol adopted in our institution for obese patients, despite the increased risk of perioperative atelectasis. End-tidal carbon dioxide (ETCO2) was continuously monitored and respiratory rate was subsequently adjusted to maintain ETCO2 at a level of 30–35 mmHg.

Patients were randomized to one of two groups by opening sealed envelopes. In Group 1 patients, a PEEP of 8 mmHg was added to the ventilation regimen. In Group 2 patients, a VCM was applied before adding a PEEP of 8 cm H2O to the ventilation regimen. The VCM was performed by inflating the lungs to a peak airway pressure of 40 cm H2O and maintaining this pressure for 15 s. VCM and/or PEEP were applied 10 min after abdominal opening and reverse Trendelenburg positioning of the patient. PEEP was maintained until tracheal extubation. The use of surgical retractors on the lower end of the rib cage was standardized for all patients and for all time points.

All patients underwent open gastric bypass performed by the same surgical team. Appropriate amounts of crystalloid infusion for replacement of fluid deficit and blood loss were calculated and administered. Urine output was maintained above 0.5 mL kg−1 h−1. Haematocrit was kept at or above 30%. A warming blanket was used to maintain rectal temperature between 36.5 and 37.5°C. Heart rate (HR) and arterial pressure variations were maintained within 20% of preoperative normal values by adjustment of general anaesthesia depth, fluid challenge and vasoactive drugs as appropriate.

At the end of surgery, tracheal extubation was performed in the postanaesthesia care unit. Signs of barotraumas as pneumothorax or subcutaneous emphysema were actively searched for by clinical examination and a systematic chest X-ray. Patients were discharged from the postanaesthesia care unit at the discretion of the attending anaesthesiologist. The study was not designed to follow up patients postoperatively for respiratory complications.

Arterial blood gases, ETCO2, peak airway pressure, expired tidal volume, respiratory rate, mean arterial pressure and HR were measured in the two study groups at the following times: T0 = before the application of VCM and/or PEEP, T1 = 10 min after the application of VCM and/or PEEP and T2 = at the end of surgery before abdominal closure. All measurements were performed while the abdomen was open and the patient was in the reverse Trendelenburg position. Patient characteristics data, surgical time, time interval between T1 and T2, intraoperative total fluid administration, urine output and vasoactive drug requirement were recorded. Extubation time after the end of surgery, length of stay in the postanaesthesia care unit and the incidence of prolonged haemodynamic instability or pulmonary barotraumas were also noted.

Sample size was based on the protocol of Frison and Pocock for repeated measurements (one preliminary measurement followed by two iterative measurements, pre–post correlation set to 0.7). For an expected difference of 40 points on alveolar-arterial oxygen pressure gradient (A-aDO2) with a standard deviation of 40, an alpha error of 0.05 and a power of 95%, 23 subjects are needed in each group of treatment.

Statistical analysis was performed using a general linear model providing a two-way analysis of variance with repeated measures. The between-subjects factor was the treatment group and the repeated within-subjects factor was the studied haemodynamic, ventilation and oxygenation parameters. Repeated contrasts, comparing the mean of each level (except the first) with the mean of the previous level, were used to test for differences among the levels of the factor. Sphericity was tested separately for each effect. If any P value was below 0.05, the assumption was violated and corrections to the F tests by Huyn–Feldt test for that effect were made. Initial categorical data were compared using X2-test. Initial patients’ characteristics were compared using t-test if normal conditions were satisfied; otherwise, U statistic was used. All tests were two-sided. Statistical significance was accepted at a P value less than 0.05. Data are presented as mean ± standard deviation or as median with interquartile range for non-normally distributed data.

Results

Fifty-two patients were included in the study. Patient characteristics and perioperative data are presented in Table 1. The two groups were comparable regarding age, sex, BMI, surgical time, time interval between T1 and T2, intraoperative total fluid administration, urine output, extubation time and length of stay in the postanaesthesia care unit. Vasoactive drugs were not required in either of the two study groups. Haemodynamic and ventilatory measurements at T0, T1 and T2 are presented in Table 2. There was no significant difference between the two groups regarding mean arterial pressure, HR, PaCO2, ETCO2, peak airway pressure, expired tidal volume and respiratory rate at any measurement time.

Table 1
Table 1:
Patient characteristics and intraoperative data.
Table 2
Table 2:
Haemodynamic and respiratory parameters.

Measured arterial oxygenation parameters included PaO2 and SaO2. A-aDO2 values were calculated as PAO2 − PaO2. PAO2 was calculated using the equation PAO2 = (atmospheric pressure − water vapour pressure) × FiO2 − PCO2/0.8. Arterial oxygenation parameters were presented in Table 3. Baseline values were comparable between the two groups of patients. In Group 1, PaO2 and SaO2 were significantly increased and A-aDO2 decreased at T2, when compared with T0 and T1. In Group 2, PaO2 and SaO2 were significantly increased and A-aDO2 decreased at T1 and T2, when compared with T0. Arterial oxygenation parameters at T1 and T2 were significantly improved in Group 2, when compared with Group 1.

Table 3
Table 3:
Arterial oxygenation parameters.

All patients were extubated within 20–40 min and discharged from the postanaesthesia care unit within 4 h. No prolonged haemodynamic instability or pulmonary barotraumas related to the application of VCM or PEEP was detected in the postanaesthesia care unit.

Discussion

This study evaluated the addition of VCM to PEEP and its effects on arterial oxygenation in morbidly obese patients undergoing open bariatric surgery. As reported in earlier clinical investigations [19,20], PEEP improved intraoperative PaO2 and decreased the alveolar–arterial oxygen gradient. However, the onset of this improvement was not rapid and its magnitude was moderate. In sedated and paralysed patients, the efficiency of PEEP in providing enough transmural pressure to inflate collapsed lung units is limited [29]. Even more, it may be associated with overstretching of already opened alveoli and redistribution of pulmonary blood flow towards non-ventilated units [6,30]. These consequences may partially offset the beneficial effects of PEEP on lung volume and small-airways closure [31]. The application of a VCM, in our study, significantly accelerated and magnified the beneficial effects of PEEP. The amelioration of arterial oxygenation was within 5 min and lasted as long as the lungs were under positive pressure ventilation. By providing a sustained high peak inspiratory pressure, the VCM overcame the opening pressure of the collapsed alveoli and accelerated the recruitment of atelectatic areas. Even though neither venous admixture nor intrapulmonary shunt (Qs/Qt) was determined in our study, the mechanism by which arterial oxygenation was improved was most likely due to increased ventilation–perfusion ratio and decreased shunt fraction.

Recently, Whalen and colleagues reported that VCM followed by ventilation with a PEEP of 12 cm H2O improved arterial oxygenation during laparoscopic bariatric surgery [28]. However, the patients in the control group were ventilated in a standard fashion with a PEEP of 4 cm H2O. Therefore, the specific beneficial effect of the VCM vs. a PEEP of 12 cm H2O is not clearly demonstrated in this study. Also, the high level of PEEP used by Whalen and colleagues might explain the intraoperative frequent use of vasopressors. Our study showed that a low level of PEEP (8 cm H2O) associated to a VCM was sufficient to significantly improve arterial oxygenation and to avoid haemodynamic instability.

The results of our study should be put into perspective. In the study population, baseline arterial oxygen saturation values were above 95% and the improvement in PaO2 after an alveolar recruitment strategy was not expected to significantly improve arterial oxygen content. Increasing PaO2 when arterial oxygen content is optimal will not change anaesthetic management. However, a vital capacity manoeuvre followed by a PEEP, as described in this report, may serve as a resort if arterial oxygenation is a problem intraoperatively in morbidly obese patients.

Inflating the lungs to vital capacity with a high and sustained peak inspiratory pressure carries potential negative effects such as barotrauma, haemohyphen-qj;dynamic depression and increased intracranial pressure. Bein and colleagues showed that, in patients with cerebral injury, lung recruitment manoeuvre leads to deterioration of cerebral haemodynamics and oxygenation [32]. We did not have pulmonary barotrauma, prolonged haemodynamic instability or clinical signs of increased intracranial pressure in our investigation. However, our study group was small and haemodynamic parameters as well as intracranial pressure were not measured during the performance of the vital capacity manoeuvre. Until larger studies will document the safety of VCM, this technique should not be performed in patients with cerebral injury, pulmonary emphysema or haemodynamic instability.

This study has several limitations. First, it was performed during open bariatric surgery with patients in reverse Trendelenburg position and not during laparoscopic surgery and in supine position. The impact of pneumoperitoneum and supine position on respiratory mechanics and oxygenation is more deleterious than the impact of abdominal opening and reverse Trendelenburg position [33–35]. Nevertheless, and according to Whalen and colleagues [28], our results were reproducible during laparoscopic bariatric surgery. Second, this study has evaluated the effect of a single VCM in obese patients ventilated with regular tidal volumes and respiratory rate. It might be interesting to evaluate the effect of repeating the VCM or its association with large tidal volumes and high respiratory rate. All these ventilatory techniques were used as recruitment strategies, but their effects on PaO2 were inconsistent [16,17,36]. Third, technical procedures for measurements of lung volumes and respiratory mechanics were not available in our institution. The assessment of such parameters as end-expiratory lung volume, compliance and resistance of the total respiratory system, lung and chest wall would have added valuable information on the effects of VCM in morbidly obese patients. Tusman and colleagues reported that in normal-weight patients, an alveolar recruitment manoeuvre increased the respiratory system compliance [26]. Similar results were recorded in obese patients during laparoscopic bariatric surgery [28]. Fourth, we have not assessed arterial oxygenation in the postoperative period. In a previous study, we showed that a VCM performed before tracheal extubation will not improve postoperative PaO2 in morbidly obese patients after bariatric surgery [37]. Similarly, Whalen and colleagues reported that all the beneficial effects of alveolar recruitment manoeuvres on oxygenation disappeared after tracheal extubation of morbidly obese patients [28].

In conclusion, a VCM followed by ventilation with PEEP improved intraoperative arterial oxygenation in open bariatric surgery. This recruitment strategy was more efficient on gas exchange than on PEEP alone. VCM showed no complications in our small study group with healthy lungs. Further studies are needed to define the optimal use of VCM as well as its effects on oxygenation and respiratory mechanics in anaesthetized morbidly obese patients.

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Keywords:

SURGERY, abdominal, bariatric; RESPIRATORY FUNCTION TESTS, gas exchange; MECHANICAL VENTILATION; MORBID OBESITY; POSITIVE PRESSURE RESPIRATION

© 2007 European Society of Anaesthesiology