Ventilation of Nonparalyzed Patients Under Anesthesia with Laryngeal Mask Airway, Comparison of Three Modes of Ventilation: Volume Controlled Ventilation, Pressure Controlled Ventilation, and Pressure Controlled Ventilation-volume Guarantee.

BACKGROUND: Pressure controlled ventilation (PCV) is the preferable mode of ventilation of nonparalyzed patients undergoing anesthesia with laryngeal mask airway (LMA) as compared to volume controlled ventilation (VCV) and spontaneously breathing patient. In this study, we compared the PC-volume guarantee (PC-VG) mode of ventilation with VCV and PCV modes.
MATERIALS AND METHODS: A total of 30 patients, American Society of Anesthesiologists (ASA) physical status Classes I and II, scheduled for elective surgery under general anesthesia with a classic LMA were ventilated, subsequently, with the three modes of ventilation: VCV, PCV, and PC-VG for 10 min each mode. Tidal volume set for all patients was 8 ml/kg of ideal body weight. Parameters measured with modes of ventilation include peak inspiratory pressure (PIP), compliance, measured tidal volume, O2 saturation, end-tidal CO2, and presence of an oropharyngeal leak.
RESULTS: The PIP was significantly higher with the application of VCV mode of ventilation than PCV and PC-VG modes. The compliance was significantly lower when using the mode of ventilation VCV than PCV and PC-VG. The PIP and the compliance were not statistically different between the PCV and PC-VG modes of ventilation.
CONCLUSIONS: Ventilation of nonparalyzed patients with LMA under anesthesia with PC-VG is advantageous over VCV in reducing PIP and increasing lung compliance. No difference was noted between PCV and PC-VG in ASA Classes I or II under the adequate depth of anesthesia in patients with normal pulmonary function.

INTRODUCTION

The laryngeal mask airway (LMA), in use since 1998,[1] was initially designed as an alternative to bag valve mask ventilation.[2] In short time, the LMA received wide recognition and has had a major impact on airway management in anesthesia practice.[34] It has been successfully used as an alternative to the endotracheal tube in general anesthesia of short procedures with spontaneous or assisted ventilation.[56]

Spontaneous breathing is a popular mode of ventilation with LMA, but it provides less effective gas exchange than does positive pressure ventilation (PPV),[789] especially when the depth of anesthesia is increased or when muscle relaxation is needed for the procedure. Ventilation and operation with or without muscle relaxants using LMAs were achieved in patients who underwent different types of surgery. At the proper depth of anesthesia, satisfactory conditions for ventilation without muscle relaxants are attractive because the side effects of muscle relaxants such as prolonged neuromuscular block and the need to reverse with a neuromuscular antagonist are avoided. Pressure controlled ventilation (PCV) can be usefully and successfully delivered through the LMA between 15 and 20 cm H2O to prevent gastric insufflation or oropharyngeal leak.[10] PCV advantage over volume controlled ventilation (VCV) is that the peak inspiratory pressure (PIP) in the former is reduced at the same tidal volume and inspiratory time.[1112]

PC-volume guarantee (PC-VG) is a new ventilator modality that combines the advantages of PCV with those of VCV. In another way, it leads to a stable tidal volume in the face of changing lung dynamic compliance and any leak with a variability of inspiratory pressure under a limited preset PIP.[13]

The aim of this study performed as a case–control study, was to compare the respiratory parameters between three modes of ventilation PCV, VCV, and PC-VG in adults undergoing elective surgery under general anesthesia with LMA.

MATERIALS AND METHODS

The study was approved by the hospital ethics committee and an informed written consent was obtained from all patients. Thirty American Society of Anesthesiologists (ASA) physical status Classes I and II patients scheduled for elective surgery under general anesthesia with a classic LMA and mechanical ventilation were included in the study. Exclusion criteria were gastroesophageal reflux, morbid obesity (body mass index [BMI] above 30), ASA physical status above Class II, and emergency cases.

No sedative premedication was given before surgery. On arrival to the operating room, all patients were monitored with electrocardiogram, pulse oximetry, noninvasive blood pressure, and entropy monitor (Carescape Monitor B650, GE Healthcare, Hilsinki, Finland) (values between 40 and 60 indicate the adequate depth of anesthesia).

Induction of anesthesia was performed with propofol 2.5 mg/kg, sufentanil 0.15 µg/kg and remifentanil was started at a rate of 0.15 µg/kg/min. Following the loss of consciousness, the patient's lungs were manually ventilated with 100% oxygen through a face mask connected to a semi-closed anesthesia circuit of a Datex Ohmeda Aisys machine (GE Company, Madison, USA). Muscle relaxants were not used. After reaching Entropy between 40 and 60 and appropriate jaw relaxation was achieved, a classic LMA (The Laryngeal Mask Company Limited Le Rocher, Victoria, Mahé, Seychelles) was inserted (size 3 for patients with weight inferior to 50 kg, size 4 for those with weight between 50 kg and 70 kg, size 5 for patients superior to 70 kg). Adequate LMA positioning was checked clinically by chest rise, equal bilateral alveolar sounds, and the presence of square CO2 wave on capnography with manual ventilation. Oropharyngeal leak and gastric inflation were evaluated by auscultation of the neck and epigastric area with PIP at 20 cm H2O. Anesthesia was maintained with remifentanil started at induction and sevoflurane 1.4% in air/oxygen mix of 40% to maintain entropy level between 40 and 60 and blood pressure and heart rate within 20% of baseline values during surgery.

Mechanical ventilation was applied with the subsequent modes: VCV, PCV, and PC-VG with 10 min of ventilation for each mode. The set tidal volume in CVV and PCV-VG modes was 8 ml/kg of the ideal body weight of the patient. In the PCV mode, the pressure was adjusted to obtain a volume of 8 ml/kg of the ideal body weight. The set respiratory rate was 14 breaths/min in all patients with all modes of ventilation. Parameters recorded at the end of each mode of ventilation from the monitoring and ventilator screens (GE Carespace Monitor B650 and GE Aisys) were: PIP, dynamic compliance, measured tidal volume, O2 saturation, end-tidal CO2, and oropharyngeal leak as well as systolic blood pressure, diastolic blood pressure, mean arterial blood pressure, and heart rate. To note that oropharyngeal leak was evaluated qualitatively by neck auscultation before every reading (0 = no leak was detected, 1 = Leak was detected).

After completing the surgery, sufentanil 1–2 µg/kg was given, and the LMA was removed when the patient opened his eyes spontaneously or responded to verbal commands.

Data were computerized and analyzed using the SPSS 15.0 software (Statistical Packages for Social Science; SPSS Inc., Chicago, Illinois, USA). Normality of the distribution of data was assessed using the Kolmogorov–Smirnov test. We expressed continuous variables as mean ± standard deviation (SD). Means were compared using ANOVA test, P < 0.05 was considered statistically significant. Post hoc analysis using Tukey HSD test was used when applicable, P < 0.01 was statistically significant.

RESULTS

Thirty patients were enrolled into the study. The mean (SD) age, weight, ideal body weight, BMI, and total anesthesia time are shown in Table 1 as well as the male to female ratio and the ASA physical status.

Table 1

Demographic and anesthetic characteristics

The PIP, the dynamic compliance and the expiratory tidal volume with different modes of ventilation (VCV, PCV, and PC-VG) are shown in Table 2 as mean ± SD with the subsequent statistical analysis in Table 3.

Table 2

Lung mechanics

Table 3

Post hoc analysis using Tukey honest significant difference test

The end-tidal CO2(mmHg) and O2 Saturation (%) with different modes of ventilation (VCV, PCV, and PC-VG) are shown in Table 4.

Table 4

Data on gas exchange

The PIP was significantly higher with the application of VCV mode of ventilation than PCV and PC-VG modes (14.70 ± 2.83 cm H2O vs. 12.53 ± 2.16 cm H2O and 12.60 ± 2.13 cm H2O). The dynamic compliance was significantly lower when using the mode of ventilation VCV than PCV and to PC-VG (40.74 ± 12.47 mL/cm H2O vs. 47.18 ± 12.54 mL/cm H2O and 47.11 ± 12.23 mL/cm H2O). The expiratory tidal volume, the end-tidal CO2, and the O2 saturation were not statistically different when using the VCV mode of ventilation as compared to PCV and PC-VG.

The PIP, the dynamic compliance, the expiratory tidal volume, the end-tidal CO2 (mmHg) and O2 Saturation (%) were not statistically different between the PCV and PC-VG modes of ventilation.

No leak was detected at different time points with different modes of ventilation studied.

DISCUSSION

LMA is now in routine use during anesthesia for management of the airway. Stated advantages with respect to tracheal intubation include efficiency, the absence of need to use muscle relaxants and improved recovery profile.[56] Different modes of ventilation used with LMA in general anesthesia has been studied and compared,[71112] but no one had investigated the use of PC-VG.

In our study, we compared VCV, PCV, and PCV-VG with LMA. All patients underwent satisfactorily lungs ventilation with the three modes as assessed by expiratory volumes, O2 saturation, ETCO2 and absence of a leak. Our data show that PCV and PCV-VG are more effective methods of ventilation with LMA classic as compared to VCV. They insure higher dynamic compliance and lower PIP.

The principal characteristic of VCV mode of ventilation is the set tidal volume at a constant flow during inspiration phase. This resulted in a variation of the PIP according to the lung compliance and the airway resistance.[14] The set tidal volume with VCV mode in our study was adjusted to 8 mL/kg of ideal body weight to maintain ETCO2 within normal range. In the PCV mode of ventilation, the set desired PIP is adjusted to achieve adequate tidal volume. In this mode, the flow is high in the initial phase of inspiration and then decreases in the later phase of inspiration to maintain the set pressure through the inspiratory time. It controls inspiratory pressure and allows the inspired volume to vary with changes in compliance and airway resistance.[14] Clinical studies proved that in laparoscopic procedures if the PIP is high for VCV mode of ventilation, PCV may offer increased tidal volume at a lower PIP because of the increased flow of gas earlier in the inspiratory phase.[15] If there is a concern for high PIP danger, PCV is recommended to limit the pressure in the airway and lungs like in patients with emphysema, and in neonates.[1617] If there is low dynamic compliance, the use of PCV lead to higher tidal volume in pregnancy, laparoscopic surgery, morbid obesity, ARDS, and the presence of leak in the system (uncuffed ET tube, LMA).[17] In our study, when using the PCV mode, the target PIP was decided to insure the tidal volume calculated to be 8 mL/kg of the body weight. The PIP was lower statistically than that of VCV mode with equal tidal volume. Furthermore, the compliance was higher significantly in the PCV mode than that of VCV. Similar results were reported by Natalini et al.,[12] he concluded a decrease in the work of breathing with PCV demonstrated by the increase in PIP and the decrease in compliance in VCV mode as compared to PCV.

The PCV-VG mode of ventilation is theoretically a pressure based mode where the ventilator operate as PCV not only with a set maximum PIP but also a set tidal volume. PC-VG ensures a set tidal volume for all mandatory breaths with the necessary minimum pressure. If resistance or compliance changes, the pressure adapts gradually to administer the set tidal volume in this mode of ventilation.[17] In our study, the set tidal volume was calculated in PC-VG mode as for the other modes, 8 ml/kg of the ideal body weight. The compliance and PIP were not significantly different from those of the PCV, but they were advantageous over the VCV mode as for the PCV mode. Also to note that the PCV-VG was not different than PCV because of the adequate depth of anesthesia in all patients and subsequently there was no documented secondary changes in airway resistance or the lung compliance. Also to note that all patients studied were ASA Classes I and II with no previous pulmonary problem, and there was no evidence of leak with the LMA or ventilation.

CONCLUSION

Ventilation of nonparalyzed patients with LMA under anesthesia with PC-VG is advantageous over VCV in reducing PIP and increasing lung compliance. No difference was noted between PCV and PC-VG in ASA I or II under the adequate depth of anesthesia in patients with normal pulmonary function.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.