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Original Article

Respiratory mechanics in morbid obese patients with chronic obstructive pulmonary disease and hypertension during pneumoperitoneum

Salihoglu, Z.; Demiroluk, S.; Dikmen, Y.

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European Journal of Anaesthesiology: August 2003 - Volume 20 - Issue 8 - p 658-661

Abstract

Morbid obesity is defined as a body weight of more than 60% over the ideal weight or body mass index >40 kg m−2, or >35 kg m−2 with serious comorbidity [1]. Obesity has profound effects on respiratory function, which is characterized with a marked decrease in lung volumes, respiratory compliance, functional residual capacity and arterial oxygenation [2]. As the surgical treatment of morbid obesity has developed, interest in the anaesthetic management of this group of patients has increased [3]. It is known that obesity is an important respiratory risk factor during and after surgery and anaesthesia contributes to these adverse effects, leading to smaller functional residual capacity and promoting atelectasis [4,5].

Laparoscopic surgical techniques seem beneficial in obese patients in terms of respiratory morbidity with a faster reachievement of normal function [6]. Although many obese patients may also suffer from chronic obstructive pulmonary disease (COPD) and hypertension, information about the intraoperative course of these patients is very limited. Casati and colleagues [7] reported significant changes in blood-gas status and respiratory mechanics during laparoscopic gastric banding surgery in a group of otherwise healthy obese patients and speculated that these changes could be more important in patients with comorbidity such as COPD. This study investigated whether the reverse Trendelenburg position (head-up tilt) was of any use in ameliorating some of the adverse changes in respiratory mechanics and blood-gas status during laparoscopic surgery in morbid obese patients with coexisting COPD and hypertension.

Methods

Our Hospital Institutional Ethics Committee approved this study, and written informed consent was obtained from each patient. Sixteen morbid obese patients with coexisting COPD and hypertension, who were scheduled for laparoscopic gastric banding surgery, were enrolled in the study. All patients were diagnosed as having COPD according to the Global Initiative for Chronic Obstructive Lung Disease (GOLD) criteria [8]. Patients with arterial pressure = 160/90 mmHg were considered to be hypertensive.

All patients received low molecular weight heparin and ranitidine 12 h preoperation. Throughout the operation, the electrocardiogram, invasive arterial pressure, end-tidal PCO2 and arterial oxygen saturation were monitored continuously. All anaesthetics and other medications were administered according to the corrected body weight. Corrected weight was calculated using the following equation:

Anaesthesia was induced with fentanyl 2 μg kg−1 and propofol 1 mg kg−1; a propofol infusion was administered at a rate of 6 mg kg−1 h−1 until the end of the operation. Atracurium 0.6 mg kg−1 was given to facilitate orotracheal intubation with a cuffed tube. Throughout the operation, fentanyl 2 μg kg−1 and atracurium 0.1 mg kg−1 were given according to patients' needs. Lung ventilation was controlled in all patients and the ventilator set to deliver 10 mL kg−1 tidal volume, 10 min−1 frequency, I : E ratio 1 : 3, inspiratory pause 20% and a constant inspiratory flow rate. Inspired oxygen concentration was 50% in an oxygen-air mixture and fresh gas flow rate was 8 L min−1. Ventilator settings were kept constant throughout the operation. Flow-time waveforms were observed during surgery to check that neither expiratory flow limitation nor dynamic hyperinflation occurred by using a monitor of the ventilatory mechanics. Intra-abdominal pressure was held at the level of 12 mmHg during laparoscopy and a constant CO2 flow of 2 L min−1 was administered through a laparoscopic insufflator.

The VenTrak® respiratory mechanics monitor (Novametrix, Wellingford, CT, USA) was used to measure respiratory resistance (Raw), dynamic respiratory compliance (Cdyn) and peak inspiratory pressure (PIP). These measurements, together with mean arterial pressure (MAP) and heart rate (HR), were recorded at four time points: 5 min after induction of anaesthesia (T1), at 5 min after insufflation of the peritoneum (pneumoperitoneum) (T2), 5 min after adoption of a 20° reverse Trendelenburg position (T3), and 5 min after deflation of the peritoneum (T4). During these measurements, surgery was halted, but the cannulae were kept in place. Arterial blood-gas status was measured at the same time points. At the end of operation, and following emergence from anaesthesia, all patients were extubated and sent to the postanaesthetic care unit.

Statistical analysis

Values are expressed as mean ± standard deviation. Statistical analysis was performed using ANOVA for repeated measures with the Tukey-Kramer posttest to evaluate the differences between the study points. P < 0.05 was considered as being statistically significant.

Results

Sixteen morbid obese patients with ASA physical status III were included in this study. The main characteristics of the study group are listed in Table 1. During the pneumoperitoneum, the respiratory compliance decreased while respiratory resistance and peak inspiratory pressure decreased (Table 2). During the reverse Trendelenburg position, the respiratory compliance increased significantly again, although it remained well below the postinduction value. At the same time, peak inspiratory pressure and respiratory resistance decreased. Five minutes after deflation of the peritoneum, respiratory compliance and resistance, as well as peak inspiratory pressure, all approached the postinduction values. During the pneumoperitoneum, arterial pH decreased and PaCO2 increased, but during the reverse Trendelenburg position PCO2 again decreased (Table 2). Arterial plasma bicarbonate did not change during the procedure; PaO2 increased and was highest when the reverse Trendelenburg position was adopted (Table 2). Both mean arterial pressure and heart rate remained stable throughout the procedure (Table 3). No problems were encountered during the early postoperative period and all patients were discharged to the ward on the following day.

Table 1
Table 1:
Patients' characteristics.
Table 2
Table 2:
Respiratory mechanics and blood-gas status.
Table 3
Table 3:
Haemodynamic values.

Discussion

Long-term follow-up studies comparing weight reduction after the surgical and non-surgical treatment of morbid obesity have shown that the surgical approach has been more successful [10]. Recently, the laparoscopic gastric banding procedure has gained popularity since it is less invasive than laparotomy. For laparoscopic procedures, pneumoperitoneum to achieve an easy operative field is maintained by CO2 insufflation. However, pneumoperitoneum may have adverse effects mainly on the cardiovascular and respiratory systems [10,11]. During this period, absorption of CO2 from the peritoneum, increased intra-abdominal pressure, the patient's position and the provision of artificial ventilation of the lungs may influence haemodynamics, respiratory mechanics and the oxygenation of blood. The anaesthetic technique, the degree of obesity and coexisting diseases, such as COPD, are all important factors to consider when planning such an operation [12].

The mechanical properties of the respiratory system can be measured in mechanically ventilated patients by means of flow and pressure recordings. The VenTrak® respiratory mechanics monitor is one such device available to the anaesthetist to measure flow and pressure breath-by-breath at the bedside. It can also calculate respiratory resistance and dynamic compliance using a fixed orifice, differential pressure sensor attached at the opening of the endotracheal tube.

Pelosi and colleagues [12] demonstrated that the reduction in functional residual capacity in obese patients is closely related to body mass index. Casati and colleagues [7] and others [13] have shown that CO2 pneumoperitoneum might further restrict respiratory movements during laparoscopic gastric-banding operations. During pneumoperitoneum, increased intra-abdominal pressure causes a restriction in the movements of the diaphragm and decreases expansion of the lungs [14]. Recent studies investigating the effects of the reverse Trendelenburg position on respiratory mechanics have suggested that this position either had no effect [7] or caused a slight increase in compliance [15]. We did not study healthy obese patients but only those with coexisting diseases, i.e. COPD and hypertension - we found there was a significant increase in respiratory compliance and a decrease in respiratory resistance and airway pressure.

In patients with COPD, the main contributor to effective ventilation is movement of the diaphragm, but in morbid obesity the diaphragm is pushed upwards by the pressure created by the abdominal contents so resulting in diminished functional residual capacity. In the presence of COPD, this situation becomes more pronounced and contributes to further respiratory problems. In these patients, respiratory function can be improved by adopting the head-up position. Positioning patients in the reverse Trendelenburg position (i.e. a head-up tilt) works the same way and has a beneficial effect on gas exchange, as suggested by our results.

Vaughan and Wise have shown that respiratory compliance might decrease by 35% in morbid obese patients [16] - findings which we support. However, the initial mean compliance values after insufflation of the abdomen (pneumoperitoneum) were lower, probably because the patients in our study had COPD. In the reverse Trendelenburg position, the compliance rate increased only by 22%. Gas exchange in the lung may be impaired during laparoscopic surgery because of CO2 insufflation and the increased intra-abdominal pressure. It has been shown that CO2 removal is attenuated in otherwise healthy patients during the operative procedure, which causes a respiratory acidosis, with normal plasma bicarbonate concentrations, and a rise in PETCO2 and PaCO2. In the present study, blood-gas measurements showed similar changes observed in earlier studies.

Although the reverse Trendelenburg position may decrease venous return and compromise cardiovascular function thus offsetting the beneficial effects on gas exchange, it has been shown that morbid obese patients have a decreased venous compliance and relatively normal central blood volume that protects stroke volume from orthostatic stress [5].

There were no clinically relevant cardiovascular effects in our study. The mean decrease in MAP during the reverse Trendelenburg position was approximately 15%. It is suggested that adoption of the reverse Trendelenburg position in morbid obese patients with COPD and hypertension who are subjected to laparoscopic surgery has significant advantages compared with the horizontal supine position.

Acknowledgements

The authors gratefully acknowledge the valuable help of Dr Atilla Uysal, Chest Physician and Biostatistician, Yedikule Teaching Hospital for Chest Disease and Thoracic Surgery, Istanbul, for statistical and data analysis. This study was not financially supported by any external sources.

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Keywords:

LUNG DISEASES, OBSTRUCTIVE, chronic obstructive, pulmonary disease; OBESITY, morbid, obesity; POSTURE, head-up tilt; RESPIRATION, lung compliance, respiratory mechanics; VASCULAR DISEASES, hypertension

© 2003 European Academy of Anaesthesiology