The laparoscopic technique has become a routine approach for a large number of surgical procedures, since it causes less postoperative pain, shortens hospital stay and allows patients to return to normal life more quickly than conventional laparotomic techniques [1-3]. At our Institution, laparoscopic procedures are currently performed in morbidly obese patients for laparoscopic gastric banding, which has been demonstrated to be associated with a lower perioperative morbidity and mortality than the laparotomic approach [3,4].
Both carbon dioxide pneumoperitoneum and patient position are known to markedly disturb cardiopulmonary function during laparoscopic procedures in healthy non-obese patients [5,6]. These negative effects might be further emphasized in the obese patient [7-9]; however, no investigations have been reported that evaluate the variations in gas exchange and cardiopulmonary function during laparoscopic gastric banding. Therefore, we prospectively assessed the respiratory and haemodynamic effects of inducing a pneumoperitoneum and positioning the patient in a reverse Trendelenburg position in anaesthetized, morbidly obese patients.
With Ethics Committee approval and written informed consent, 20 ASA physical status II or III patients, undergoing silicone-adjustable gastric banding for treatment of morbid obesity, were prospectively evaluated. Morbid obesity was defined as a body mass index (BMI) greater than 35 kg m−2, calculated as: body mass (kg) divided by height2 (m2) . After a complete medical history and physical examination had been obtained, patients with hypoventilation syndrome, chronic obstructive lung disease, myocardial infarction, and congestive heart failure were excluded.
After a standard oral premedication (diazepam 0.1 mg kg−1 ideal body weight), general anaesthesia was induced with fentanyl i.v. (1 μg kg−1 ideal body weight) and sodium thiopental i.v. (6 mg kg−1 ideal body weight). Orotracheal intubation with a cuffed tube was facilitated with succinylcholine i.v. (1 mg kg−1 ideal body weight), and further muscle relaxation was provided with atracurium (0.4 mg kg−1 h−1). Anaesthesia was maintained with 0.7-1.5% end-tidal isoflurane and 50% nitrous oxide in oxygen, to maintain arterial blood pressure and heart rate within ±20% of baseline values. Intermittent positive pressure ventilation (IPPV) was applied using a Dräger-Cato volume cycled ventilator (Dräger, Lübeck, Germany). During all measurements the ventilation was set with a tidal volume (VT) of 12 mL kg−1, ideal body weight , a ventilatory frequency of 12 bpm, and an inspiratory to expiratory time ratio of 1:2. Intraoperative monitoring included continuous electrocardiograph (ECG) (lead II) and heart rate, invasive arterial blood pressure, arterial blood gas analysis (IL BGM 1312, Instrumentation Laboratory, Lexington, AM, USA), pulse oximetry, inspired oxygen fraction (FiO2), end-tidal carbon dioxide partial pressure (ETCO2), end-tidal isoflurane concentration, peak airway pressure, and minute ventilation.
Blood-gas analysis variables were considered to evaluate the efficacy of pulmonary gas exchange during the procedure; the ratio between arterial oxygen partial pressure and inspired oxygen concentration (PaO2/FiO2) was also calculated as an indirect index of pulmonary shunt , while the arterial-to-end-tidal carbon dioxide tension difference [ΔP(a-ET) CO2] was calculated as an indirect index of physiological dead-space . The lung/chest wall compliance was calculated by dividing the expired tidal volume by the plateau pressure measured after a 1 s inspiratory pause .
According to the technique described by Belachew and colleagues , the laparoscopic procedure was performed using a carbon dioxide pneumoperitoneum, which was created and maintained through a paraumbilical trocar. The patients were then positioned in a modified lithotomy position, with a 25° reverse Trendelenburg position, according to the surgical technique. Intraoperatively, the ventilatory frequency was modified to maintain an ETCO2 ranging from 4.2 to 5.3 kPa (32-40 mmHg).
All the measured variables were recorded at the following times: 10 min after induction of anaesthesia (T0, baseline); 10 min after CO2 abdominal insufflation (T1), with the patient supine; 10 min after positioning the patient in a 25° reverse Trendelenburg position (T2); 30 min after T2 during the surgical time (T3); immediately before deflating the abdomen (T4); 10 min after deflating the abdomen (T5); and, finally, with the patient supine before the end of anaesthesia (T6). During all measurements the surgical procedure was stopped, and all the instruments were removed from the trocars.
Statistical analysis was performed using the program Systat 7.0 (SPSS Inc, Chicago, IL, USA). Analysis of variance for repeated measures was used to evaluate changes over time in the considered variables. Paired Student's t-tests with Bonferroni's correction were used to compare changes from baseline. A value of P ≤ 0.05 was considered as statistically significant. Continuous variables are presented as mean ±SD, while categorical data are presented as numbers (%).
Twenty patients, of whom three were male, were included with the following general characteristics: age 41 ± 10 years (range: 26-28 years), weight 120 ± 23 kg (range: 94-168 kg), height 164 ± 6 cm (range: 152-174 cm), BMI 44 ± 8 kg m−2 (range: 35-62 kg m−2). No complications consequent to either anaesthesia or surgery were reported during the study.
Systolic arterial blood pressure increased after pneumoperitoneum had been induced up to the T2 measurement time (Fig. 1); while no changes were observed in either diastolic arterial blood pressure or heart rate.
Oxygen arterial partial pressure and the PaO2/FiO2 ratio markedly decreased during the study period, and were lower than baseline values at the end of the study (Table 1). The arterial carbon dioxide partial pressure and arterial-to-end-tidal CO2 tension difference progressively increased throughout the study. After the carbon dioxide pneumoperitoneum had been resolved and patients turned to the supine position (T6), the PaCO2 returned to baseline values, but the arterial-to-end tidal CO2 tension difference was still above baseline (Table 1).
The lung/chest compliance markedly decreased from baseline after pneumoperitoneum had been established (Fig. 2); however, after the pneumoperitoneum had been resolved mean values of lung/chest compliance were not different from baseline values.
Recovery from anaesthesia was uneventful in all considered patients. No complications were reported during the first 24 h after surgery, and all studied patients were discharged from the hospital within one week after surgery.
This prospective, observational study demonstrated that, while producing only minor variations in haemodynamic variables, carbon dioxide pneumoperitoneum markedly affected pulmonary gas exchange and lung/chest compliance, producing major changes in patient oxygenation that were not corrected by placing the patient in a reverse Trendelenburg position and were still present at the end of surgery.
A major shortcoming of this investigation is the lack of an adequate control group, receiving gastric banding with the laparotomic approach, in order to differentiate those changes induced by anaesthesia from those related to the laparoscopic technique. However, previous investigations have demonstrated that laparoscopic placement of silicone-adjustable gastric banding was as effective as the laparotomic procedure, but was less invasive and improved the perioperative outcome [3,4,12-14]. For this reason, the inclusion of a control group with the laparotomic procedure was judged not ethical both by the surgical staff and the Ethics Committee.
The slight but significant increase in systolic arterial blood pressure after pneumoperitoneum induction was of short duration and not clinically significant. This effect was probably related to the increase in cardiac afterload because of the modified intra-abdominal pressure, as well as to the direct and indirect haemodynamic effects of carbon dioxide .
On the contrary, pulmonary gas exchange was markedly affected by abdominal insufflation. Changes in oxygenation were probably related to the observed reduction in lung/chest compliance. In the awake obese patient the chest wall compliance is known to be reduced up to 35% of normal-weight patients, due to the fat accumulation around the ribs, under the diaphragm, and intra-abdominally [8,16,17]. However, Pelosi and colleagues  recently demonstrated that in non-laparoscopic obesity surgery, the most important change is in lung compliance rather than chest wall compliance. In the present investigation we measured only the compliance of the whole respiratory system, then no differentiation can be made between the chest wall and lung components; however, it has been demonstrated that increasing the intra-abdominal pressure augments the central venous pressure by forcing blood from the abdominal organs into the central venous reservoir [19-21], while the increase in pulmonary blood volume markedly affects the lung elastance .
Moreover, we cannot exclude the development of small areas of atelectasis, since lung/chest compliance, but not oxygenation indicators, return to baseline values after pneumoperitoneum had been resolved. In fact, similar changes in lung/chest compliance have been described in non-obese young women undergoing carbon dioxide pneumoperitoneum for gynaecological procedures , but oxygenation was unaffected; while severe reduction in functional residual capacity (FRC) have been demonstrated in the anaesthetized, obese patient [22-24], leading to V/Q mismatch and hypoxaemia . A mismatched ventilation/perfusion ratio could also explain the increased arterial-to-end-tidal CO2 tension difference, which is strictly related to the physiological deadspace . The physiological deadspace is known to increase in obese patients , and both pneumoperitoneum and patient position on the operating table affect it further [5,27].
Another interesting observation is that the respiratory changes induced by abdominal insufflation were not modified by moving the obese patients from the supine to the reverse Trendelenburg position. In fact, the respiratory mechanics of the obese patient usually improve when the patient is placed in a semi-recumbent position [19,28], but this was not the case in the present investigation, suggesting that the elevated intra-abdominal pressure due to CO2 inflation prevented the beneficial effects of the reverse Trendelenburg position.
In spite of the relative intraoperative hypoxaemia, no severe clinical respiratory complications were reported during the postoperative course. However, it should be pointed out that the size of the studied population was very small, while patients with severe cardiovascular or respiratory diseases were not included. The observed changes in pulmonary function might have greater clinical relevance in obese patients with a poorer ASA physical status, or comorbid conditions such as obstructive airway disease, advanced age or underlying atherosclerotic disease.
In conclusion, this prospective observational study demonstrated that in obese patients, scheduled for laparoscopic silicone-adjustable gastric banding, abdominal insufflation with carbon dioxide markedly affected gas exchange and lung/chest compliance, while placing the patient in a 25° reverse Trendelenburg position had no beneficial effects on the considered respiratory variables. On the contrary only minor changes in systolic arterial blood pressure were observed after pneumoperitoneum was induced.
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