The effects of abdominal opening on respiratory mechanics during general anaesthesia for open bariatric surgery in morbidly obese patients
*Tomasz M. Gaszyński
Department of Anaesthesiology and Intensive Therapy, Medical University of Łódź
Background. Morbid obesity (BMI >40 kg mL-2) is associated with several changes in anatomy and physiology of the respiratory system. Respiratory work is increased whereas compliance of the chest wall and lungs is decreased. Increased intra-abdominal pressure compresses the diaphragm and reduces its movability. Abdominal opening should influence respiratory mechanics.
Methods. Forty-seven morbidly obese patients (BMI 49.54±7.21 kg m-2) scheduled for elective bariatric surgery (open Roux-en-Y-Gastric Bypass − RYGB) were included in the study and divided into 3 groups: 40< BMI <50, 50< BMI <60 and BMI >60. Measurements were performed during general anaesthesia at two time points: before (T1) and after abdominal opening (T2). All patients were ventilated using the volume control mode.
Results. Preoperative spirometry revealed significantly decreased FEV1% in the BMI >60 group. The conditions of mechanical ventilation during general anaesthesia deteriorated with an increasing BMI. After abdominal opening, the mechanical ventilation conditions improved: increased lung compliance and decreased airway pressures. The difference was statistically significant in the entire population studied. After dividing the population into groups, however, the difference was no longer significant in patients with BMI >60. The most significant difference in peak and plateau pressures after abdominal opening was observed in patients with BMI <50.
Conclusion. Abdominal opening improves the conditions of mechanical ventilation in morbidly obese patients; when BMI is over 60 kg m-2, this improvement may not be relevant.
Morbid obesity, defined as BMI >40 kg m-2, is associated with several serious changes in anatomy and physiology of the respiratory system. Respiratory work increases whereas compliance of the chest wall and lungs decreases, which is related to higher amounts of fatty tissue around the ribs, under the diaphragm, and intra-abdominal fat as well as to pulmonary changes . Increased capacity of the pulmonary vessels is responsible for decreased compliance of the lungs. Mass loading results in a fall in static lung volumes. The functional residual capacity (FRC) is decreased resulting in more rapidly developing hypoxia . General anaesthesia in morbidly obese patients induces difficulties in artificial lung ventilation, which causes a further decrease in FRC and ventilation-perfusion mismatch. Adequate parameters of mechanical ventilation and FIO2 should be adjusted according to intraoperative tests of blood gases . Proper positioning of patients may improve ventilation. In the supine position, heart work and blood pressure in the pulmonary artery are increased , ventilation of the lower parts of lungs is worse and atelectasis develops . With the upper half of the body elevated, the tolerance to apnoea is prolonged . However, during general anaesthesia for the majority of bariatric surgeries, the supine position is required; this increases intra-abdominal pressure and results in compression of the diaphragm, which becomes less movably. Therefore, abdominal opening should improve respiratory mechanics.
The aim of the study was to assess the influence of abdominal opening on mechanical ventilation during general anaesthesia for bariatric surgery in morbidly obese patients.
After obtaining the approval of the Local Ethical Committee, obese patients were scheduled for elective bariatric surgery − open Roux-en-Y-Gastric Bypass (RYGB). The exclusion criterion was lung disease. Patients were divided into three groups: A – patients with BMI >40 kg m-2 and <50 kg m-2, B − with BMI >50 kg m-2 and <60 kg m-2, group C − with BMI >60 kg m-2. Spirometry was conducted on the day preceding surgery. Patients were anaesthetised with volatile sevoflurane and fentanyl as an analgesic. Muscle relaxation was provided with cisatracurium or atracurium. Muscle blockade was confirmed by neuromuscular stimulation, and BIS was installed for monitoring the depth of anaesthesia.
In order to standardize the measurements during anaesthesia, the lungs were ventilated using an anaesthetic ventilator and the volume control ventilation (VCV) mode, VT of 8 mL kg-1 of ideal body weight, a mixture of O2/N2O (FIO2=40%) and f adjusted to maintain ETCO2 at a level of 30-35 mm Hg (4.0-4.7 kPa); PEEP was set at 8 cm H2O (0.8 kPa) . When the airway pressures were high and proper blood oxygenation was difficult to achieve, the pressure control ventilation (PCV) mode was applied. The efficacy of artificial ventilation was confirmed by intraoperative blood gas testing.
Measurements during general anaesthesia included: peak airway pressure, plateau pressure and lung compliance. After induction, the measurements were taken at 5-minute intervals for about 15-20 min and mean values were calculated (time point T1). After abdominal opening the measurements were repeated (time point T2) and compared with those of T1.
Statistical significance was set at 5%. Data distribution was tested. Intergorup differences were analysed using the t-test for independent samples − two sample assuming unequal variations; intragroup differences were checked using the t-test for pairs with double samples for means.
Forty-seven patients were enrolled in the study: group A – 26, group B – 15 and group C – 6 patients. The preoperative spirometry revealed significant decreases in FEV1(%) in group C as expected (Table 1). Intraoperatively, the peak pressure values were significantly different at time point T1 and T2: 28.8±3.99 vs 26.8±4.2 cm H2O (2.8±0.4 vs 2.6± 0.4 kPa), respectively (p=0.013). Similar changes were observed in relation to lung compliance: 37.0±8.5 vs 43.4±8.8 mL cm H2O-1 (p=0.003) in T1 and T2, respectively. The differences in plateau pressure were not significant: 26.0±4.0 vs 24.5±4.5 cm H2O (2.5±0.4 vs 2.4±0.4 kPa).
After abdominal opening artificial ventilation substantially improved: an increase in lung compliance and a decrease in airway pressures were observed. The differences between T1 and T2 were significant for peak pressure, plateau pressure and compliance values in groups A and B but not in group C. The highest increase in peak and plateau pressures was observed in group A and a compliance decrease in group B (Table 2).
With increasing weight, intra-abdominal pressure increases whereas the functional residual capacity, expiratory reserve volume and total lung capacity decrease. The supine position and general anaesthesia together with muscle relaxation cause further reduction in lung volumes . Simple opening of the abdomen improves pulmonary compliance and lung volume . Our study findings disclose that the improvement is significant in patients with BMI <60 kg m -2.
If BMI is higher, the improvement is not so spectacular, which demonstrates that increased airway pressures and decreased compliance in obese patients result from several factors, including intra-abdominal pressure.
There is no consensus which mode of ventilation is preferable in morbidly obese patients; some authors suggest that PCV improves oxygenation  whereas the others demonstrate no significant difference between PCV and VCV [10, 11]. Our experiences, similarly to those reported by others , suggest that in obese patients with BMI slightly over 40 kg m-2, standard VCV is effective when VT equals 5-8 mL kg-1 of ideal body weight and f 10-12 min-1. An increase in volume or respiratory rate does not improve oxygenation and elimination of CO2 in this group of patients . Moreover, this does not protect them from atelectasis. In obese patients with BMI >50 kg m-2, PCV is found more efficacious. In both modes of ventilation, ETCO2 should be maintained at 45-50 mm Hg (6-6.6 kPa). Moderate hypercapnia is recommended as morbidly obese patients are usually hypercapnic. Hypocapnia with pCO2 <30 mm Hg (4 kPa) may lead to an increase in lung shunt.
Airway peak pressures up to 35 cm H2O (4.7 kPa) do not induce any problems in patients without lung diseases. In obese patients, PEEP may improve oxygenation, yet PEEP higher than 8 cm H2O (0.8 kPa) is not advised . Intraoperative arterial oxygenation improves once PEEP is supplemented with vital capacity manoeuvres .
In obese patients, laparotomy induces a significant increase in lung compliance and a decrease in resistance . In our study such improvement in artificial ventilation was present only in the patients with BMI <60 kg m-2; most likely, their chest and lungs were less affected by obesity. This was confirmed by preoperative spirometry as in the group of morbidly obese patients (BMI >60 kg m-2), FEV1(%) was significantly lower than in patients with BMI<60 kg m-2. Moreover, the mean age of patients with BMI >60 kg m-2 was higher than in the other two groups, which might be associated with prolonged changes in the cardiovascular system and worse circulatory performance during general anaesthesia.
Abdominal opening improves artificial lung ventilation in morbidly obese patients; if, however, BMI is over 60 kg m-2, the improvement may not be relevant.
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Klinika Anestezjologii i Intensywnej Terapii
Uniwersytet Medyczny w Łodzi
ul. Kopcińskiego 22, 90-153 Łódź