In recent years, several modes of mechanical ventilation traditionally considered only for use in ICUs have been incorporated in anaesthesia machines. Pressure-controlled ventilation (PCV), pressure-support ventilation (PSV) and pressure-regulated volume-controlled ventilation (PRVC, autoflow) are currently present in last generation anaesthesia workstations. That might be an expression of a growing interest in improving the mechanical ventilation performance during anaesthesia, especially in those cases of difficult conditions due to features of the surgery or the patient, or both. Pneumoperitoneum and Trendelenburg positioning used for laparoscopic gynaecological surgery have well known negative effects on respiratory mechanics and gas exchange: there is a decrease in functional residual capacity and respiratory compliance , an increase in respiratory resistances, impairment of arterial oxygenation and an increase in dead space [2,3]. Adjustment of ventilatory settings in order to prevent these deleterious effects may be difficult, especially when end-tidal CO2 and intrathoracic pressures (plateau airway pressure) are the only variables traditionally used as reference.
Adaptive support ventilation (ASV) is a mode of mechanical ventilation designed to automatically provide a continuous adaptation to changes in the patient's active and passive respiratory mechanics while maintaining a preset minute ventilation [4,5]. The main principle of ASV comes from the equation of Otis et al. , which aims to predict the respiratory rate associated with the lowest work of breathing (WOB) for a given level of alveolar ventilation  from the expiratory time constant (TCexp) of an individual patient. A recent study  showed that ASV was able to automatically adapt its ventilatory setting to different patterns of passive respiratory mechanics both in a physical lung model and in ventilated patients with acute, chronic respiratory failure and normal lungs. In addition, this mode has proved to be able to reduce the clinician's intervention related to the ventilatory management for weaning after cardiac surgery . Accordingly, with these features, it could be expected that ASV will prove beneficial for ventilatory support of patients undergoing surgical procedures in which significant changes in respiratory mechanics are produced. Using this mode of mechanical ventilation could help the anaesthesiologist to improve the adaptation of the ventilatory pattern to these changes by continuous monitoring of respiratory mechanics and automatic adjustment; however, most previous studies on ASV were aimed at evaluating its efficiency and its adaptability to changes in the patient's breathing activity during weaning trials [5,9,10] and for early extubation after cardiac surgery [11,12]. Only two previous studies [13,14] have tested ASV adaptability to changes in the patient's respiratory mechanics during anaesthetic procedures. They were carried out using adaptive lung ventilation (ALV), a mode preceding the current ASV that uses the same algorithm for the calculation of the optimal ventilatory pattern, and showed that this mode accurately adapted to changes in pulmonary mechanics during the transition from supine to lateral position and when shifting to and from one lung ventilation [13,14].
The present study was aimed at evaluating whether ASV adapts to changes in respiratory mechanics during gynaecologic laparoscopy guaranteeing adequate ventilation. We describe the changes in respiratory mechanics due to pneumoperitoneum and the Trendelenburg position and the automatic adaptation of the ASV ventilatory settings.
We designed a prospective, observational study involving patients scheduled for elective gynaecological laparoscopic surgery. After obtaining approval from the Ethics Committee of the Clinic Hospital of Valencia, the study was carried out between September 2005 and February 2006. Preoperative written informed consent was obtained from eligible patients. The preoperative exclusion criteria were age under 18 years, overweight (BMI > 30 kg m−2) and history of respiratory disease.
Standard monitoring and a 3 min period of preoxygenation with 100% oxygen preceded induction of general anaesthesia with fentanyl (2 μg kg−1) and propofol (2–2.5 mg kg−1). Cisatracurium (0.1 mg kg−1) was used to facilitate laryngoscopy and intubation using a cuffed endotracheal tube of proper size. The model and size of heat and moisture exchanger (HME) was the same for every patient. Anaesthesia was maintained with a continuous intravenous (i.v.) infusion of propofol adjusted for bispectral index (BIS) values between 40 and 50 and i.v. bolus of cisatracurium and fentanyl.
Mechanical ventilation protocol
Immediately after orotracheal intubation, ASV was initiated by means of a Galileo ventilator (Hamilton Medical AG, Rhäzuns, Switzerland) and was maintained throughout the intraoperative period. The main principle of ASV comes from the equation of Otis et al. , which is aimed at predicting the respiratory rate associated with the lowest WOB for a given level of alveolar ventilation :
In this equation, k stands for a flow wave-dependent constant; TCexp is the expiratory time constant of the respiratory system, calculated breath by breath, as the relationship between expiratory volume and expiratory flow at 75% of the maximum expiratory volume. VD (dead space) is the physiological dead space according to the lean body mass (VD = 2.2 ml kg−1)  and V′A is the alveolar ventilation, measured as minute ventilation (MV) – (RR*VD).
In the absence of spontaneous breathing ASV delivers PCV. When spontaneous respiratory activity is resumed it switches to PSV. The clinician sets only three parameters relating to minute ventilation: maximum inspiratory pressure, ideal body weight (which is used by the ventilator to calculate the ‘normal minute ventilation’ as 100 ml kg−1 for adults  and 200 ml kg−1 for paediatric patients) and the desired (targeted) minute ventilation (target MV) as a percentage of the normal MV. This percentage may range between 10 and 350%. Initially, the ventilator determines the expiratory time constant (TCexp) by analysis of the expiratory flow–volume curve  along five test breaths. Thereafter, the ventilator's software adjusts, breath to breath, with closed-loop algorithms based on online determination of the TCexp, and the respiratory system compliance (Crs) and resistance (Rrs) , the optimal combination of inspiratory pressure (Pinsp), inspiratory to total time ratio (Ti/Ttot) and respiratory rate to obtain the prescribed MV [18,19].
In our protocol, the initial settings of the ventilator consisted of the following four parameters: the ideal body weight of every patient, the target MV, which was preset and kept constant at 110% of the theoretical MV in order to compensate for the added dead space due to the HME filter , a positive end-expiratory pressure (PEEP) of 5 cmH2O and a fraction of inspired oxygen (FiO2) of 0.5. To maintain PaCO2 between 4 and 6.6 kPa (30 and 50 mmHg) and prevent severe hypoventilation or hyperventilation a simple protocol consisting of manually increasing or reducing the target MV by steps of ±10% in case of PaCO2 more than 6.6 kPa or PaCO2 less than 4 kPa was introduced.
Data and measurements
Data were collected at three time points: 5 min after induction, in the supine position (baseline); 15 min after pneumoperitoneum and Trendelenburg positioning were established (Pneumo-Trend); and 15 min after pneumoperitoneum withdrawal, in the supine position (final).
Respiratory mechanics variables (Csr, Rexp, TCexp and intrinsic PEEP) and ventilatory setting parameters (Pinsp, RR, Ti/Ttot, tidal volume and MV) were obtained directly from the ventilator using data acquisition software (Datalogger Version 2.4, Hamilton Medical AG). For every variable, values recorded every 5 s for 1 min were averaged. Arterial blood gas (ABG) samples were taken and analysed at every time point (Radiometer ABL 700 series; Radiometer Medical ApS, Brønshøj, Denmark).
Other records included intraabdominal pressure and any inefficient performance of the ASV such as episodes of ‘unable to meet target minute ventilation’ or when manual readjustments of ventilatory setting were needed according the above-mentioned protocol.
Statistical analysis was performed using SPSS (SPSS Inc., Chicago, Illinois, USA) for Windows (Release 12.0). One-sample Kolmogorov–Smirnov test was applied to test the normal distribution of data. The Student's t test was used for comparisons between variables measured at different time points. Variables are expressed as means and SD. A P value of less than 0.05 was considered statistically significant.
Twenty-two women were included in the study. The mean and range for age, body weight and height were 33.4 years (24–59), 63.0 kg (50–85) and 165.9 cm (160–176), respectively. All patients had a normal BMI (20–25 kg m−2). No patients were withdrawn for protocol failure. Surgical procedures were ovarium cystectomy (n = 11), unilateral adnexectomy (n = 6), bilateral adnexectomy (n = 2) and hysterectomy (n = 3). Intraabdominal pressure during pneumoperitoneum was kept between 1.6 and 2 kPa (12 and 15 mmHg) throughout the whole procedure in every patient and the Trendelenburg position was set between 30° and 50°. Pneumoperitoneum and Trendelenburg positioning were associated with a reduction of 44.4% (71.7 ± 12.9–39.8 ± 5.6 ml cmH2O−1) in the compliance of the respiratory system and an increase in expiratory airway resistances of 29.1% (10.3 ± 1.8–13.3 ± 2.4 cmH2O l s−1). As a result, the expiratory time constant was reduced by 33.3% (0.9 ± 0.1–0.6 ± 0.1 s) (Table 1). After pneumoperitoneum was removed (0°), all these values returned to the basal levels. Following these changes in respiratory mechanics, ASV readjusted the ventilatory parameters during the Pneumo-Trend period by increasing Pinsp by 16.1% (16.7 ± 2.0–19.9 ± 2.2 cmH2O) and respiratory rate by 9.3% (12.7 ± 0.6–14.0 ± 0.9 breaths per minute), whereas the Ti/Ttot ratio was increased by 43.3% (0.30 ± 0.04–0.44 ± 0.06) (Table 2). As a consequence, tidal volume was reduced by 10.2% (546 ± 66–490 ± 71 ml) but minute volume was kept nearly constant. During the final period, ASV ventilatory parameters returned towards the baseline values. Mild hyperventilation was observed during the baseline period. Pneumoperitoneum and Trendelenburg positioning caused an average increase of 8.7% (from 4.4 ± 0.6 to 5.0 ± 0.9 kPa; P < 0.05) in PaCO2 but normocapnia was maintained in all patients but one (Table 2). This patient (no. 2) was the only case in which clinician's intervention was required. That was due to a PaCO2 value of 6.9 kPa measured during the Pneumo-Trend period. According to the study protocol, this observation was followed by a manual increase by 10% of the target MV. This was associated with a reduction in PaCO2 near to normal levels (5.8 kPa) 15 min later. No variations were observed in PaO2 throughout the three periods (Table 2). No episodes of hypoxaemia, intrinsic PEEP, or ‘unable to meet target’ were observed in any patient.
This observational study showed that ASV automatically adapted the ventilatory settings keeping the MV unaltered in response to the changes in respiratory mechanics.
Introducing modes of mechanical ventilation other than volume and PCV in anaesthesia machines can contribute to improved ventilatory management in some groups of anaesthetized patients. This has been shown with PSV, which has proved to be beneficial for some anaesthetic procedures in both adults and paediatric patients [20,21]. We tested ASV because its characteristic features, especially its ability to continuously adapt the ventilatory settings to the mechanics of the respiratory system, could make this mode potentially beneficial for those surgical procedures in which changes in respiratory mechanics can be relevant. This may be the case for gynaecological laparoscopic surgery due to the effects in respiratory mechanics induced by pneumoperitoneum and the Trendelenburg position. In our experience, ASV resulted in an easy-to-use mode of mechanical ventilation in this surgical setting, with no need for any intervention by the anaesthesiologist. Our results agreed with previous studies [22–25]. Basically, a severe reduction in compliance of the respiratory system with a moderate increase in respiratory resistance and, as a consequence, a reduction in time constant. ASV adapted the ventilatory settings to these changes by slightly increasing respiratory rate and, especially, by prolonging the inspiratory to total time ratio. This way, the increase in inspiratory pressure needed to achieve the tidal volume required to keep constant the minute ventilation could be as low as 3.2 cmH2O in average. The reduction in the expiratory time caused by the increase in respiratory rate and in the inspiratory to total time ratio was not enough to cause intrinsic PEEP. This was consistent with the fact that the expiratory time constant was reduced by 30%. Therefore, the adaptation of the ventilatory setting made by ASV was adequate to achieve its three primary objectives: to keep the minute ventilation constant (increasing the respiratory rate to compensate for the reduction in tidal volume), to apply the lowest inspiratory pressure (prolonging the inspiratory to total time ratio to achieve the maximum tidal volume for a given inspiratory pressure) and to avoid intrinsic PEEP (the reduction in the expiratory time seemed to be calculated taking into account the reduction in the expiratory time constant).
Targets of minute ventilation were reached in all patients. We applied a target minute ventilation of 110% of the estimated minute ventilation, following the ASV user's guide recommendation, in order to compensate for the additional dead space (VD) due to the endotracheal tube, filter and Y piece. This led to a mild hyperventilation during the basal period, which turned into nearly normoventilation during the Pneumo-Trend period, as minute ventilation was kept constant. This behaviour was very similar for all the patients with the exception of only one, who showed a moderate hypercapnia during the pneumoperitoneum period. A mistake in the estimation of this patient's body weight and, as a consequence, in the calculation of minute ventilation was considered to be the most probable cause of the hypoventilation, as it was corrected by simply increasing minute ventilation by 10% and no other apparent causes were found after a subsequent detailed review. No other clinician interventions were needed in any other patient. This agrees with previous studies [9,12] that showed that the internal logic of ASV resulted in less manipulation compared with other ventilatory strategies for weaning after cardiac surgery and may simplify ventilatory management in these patients.
We applied a mild level of PEEP considering that this manoeuvre could not interfere with ASV behaviour and taking into account some studies which have shown it can preserve oxygenation during prolonged pneumoperitoneum . Whether this effect, the characteristic decreasing flow shape of the PCV or both had some influence on the gas exchange in our patients is merely speculative. Additionally, no extrapolations can be made with respect to patients with respiratory disease. This study was carried out by observational design; therefore, it does not allow any comparison with any other patient groups or different ventilatory strategies. Its main aim was to evaluate whether ASV would be able to guarantee an adequate level of ventilation in spite of the worsening of respiratory mechanics associated with laparoscopic surgery without the clinician's intervention. In consequence, only healthy patients were included. Further studies are needed to evaluate the potential benefit of ASV in patients with respiratory disease. Another limitation of this study comes from the small number of patients; however, it should be borne in mind that the group was very homogeneous with respect to anthropometric parameters and that all patients showed similar changes in respiratory mechanics to and from pneumoperitoneum and Trendelenburg positioning.
In conclusion, this study showed that ASV provided automatic, continuous adaptation of ventilatory parameters to changes in respiratory mechanics of patients who underwent gynaecological laparoscopy, maintaining a nearly constant minute ventilation and adequate gas exchange. Our results suggest that automatic decision-making, based on continuous respiratory mechanics monitoring, can offer a reliable alternative for the ventilatory management in this specific group of anaesthetized patients. Further studies are warranted to test whether ASV could benefit other groups of anaesthetized patients.
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