|Year : 2014 | Volume
| Issue : 2 | Page : 96-100
Does pressure-controlled ventilation-volume guaranteed differ from pressure-controlled ventilation in anesthetized patients
Nermin S. Boules, Mohamed A El Ramely
Department of Anesthesiology and Pain Management, National Cancer Institute, Cairo University, Cairo, Egypt
|Date of Submission||16-Sep-2013|
|Date of Acceptance||18-Nov-2013|
|Date of Web Publication||31-May-2014|
Nermin S. Boules
Department of Anesthesiology and Pain Management, National Cancer Institute, Cairo University, 1 Kasr El Aini Street, Fom El Khalig, Cairo
Source of Support: None, Conflict of Interest: None
General anesthesia causes depression of both respiratory centers and respiratory muscles. Hence, patients under general anesthesia require ventilatory support to maintain arterial oxygenation and eliminate carbon dioxide. Mechanical ventilation can improve patients outcome. The purpose of this study was to determine whether pressure-controlled ventilation-volume guaranteed (PCV-VG) provide better oxygenation than pressure-controlled ventilation (PCV) in anesthetized patients.
Patients and methods
A total of 30 patients scheduled for abdominal cancer surgery under general anesthesia were enrolled in the study. Mechanical ventilation was started with PCV for 60 min then PCV-VG was applied to all patients with the same parameters, targeting the obtained tidal volume (Vt). Arterial blood pressures, heart rate, ETCO 2 , SpO 2 , pH, PaCO 2 , and PaO 2 were measured after 60 min of intubation during PCV and after 60 min of initiation of PCV-VG. Vt, mean airway pressure, and peak airway pressure (during PCV-VG) were recorded. Oxygenation index calculation was performed at the preset times.
All parameters were comparable with no significant difference between both modes of ventilation in anesthetized patients ( P ≥ 0.05).
While maintaining constant Vt and I/E ratio, there were no significant differences in respiratory and ventilatory parameters, and also the oxygenation index was comparable between both PCV and PCV-VG modes of ventilation.
Keywords: General anesthesia; oxygenation index assessment; pressure-controlled ventilation; pressure-controlled ventilation-volume guaranteed
|How to cite this article:|
Boules NS, El Ramely MA. Does pressure-controlled ventilation-volume guaranteed differ from pressure-controlled ventilation in anesthetized patients. Ain-Shams J Anaesthesiol 2014;7:96-100
|How to cite this URL:|
Boules NS, El Ramely MA. Does pressure-controlled ventilation-volume guaranteed differ from pressure-controlled ventilation in anesthetized patients. Ain-Shams J Anaesthesiol [serial online] 2014 [cited 2018 Jan 17];7:96-100. Available from: http://www.asja.eg.net/text.asp?2014/7/2/96/133303
| Introduction|| |
General anesthesia causes depression of both respiratory centers and respiratory muscles. Hence, patients under general anesthesia require ventilatory support to maintain arterial oxygenation and eliminate carbon dioxide. In addition to its depressing effect on respiratory drive and mechanics, general anesthesia also affects gas exchange , which can contribute to postoperative respiratory complications; hence, perioperative optimization of mechanical ventilation is considered essential to improve patients outcome .
Pressure-controlled ventilation (PCV) is one of the modes that has been proposed to improve gas exchange in anesthetized patients, as PCV has a constant set pressure during inspiration but no minimum tidal volume (Vt). The ventilator calculates the inspiratory time (TI) from the rate and I/E ratio settings. A high initial flow pressurizes the circuit to the set inspiratory pressure, which potentially facilitates recruitment of unstable alveoli . The flow then decreases to maintain the set pressure, resulting in a decelerating inspiratory flow. The pressure sensors in the ventilator measure the patient airway pressures. When ventilator automatically adjusts the flow to maintain the set inspiratory pressure, the resulting Vt depends on the pressure limitation and on the chest compliance .
However, in pressure-controlled ventilation-volume guaranteed (PCV-VG), the Vt and the rate are predetermined and the ventilator delivers the Vt using also a decelerating flow but a constant pressure. The ventilator adjusts the inspiratory pressure needed to deliver the Vt breath-by-breath so that the lowest pressure is used. PCV-VG begins by first delivering a volume breath at the set Vt. The patient's compliance is determined from this volume breath and the inspiratory pressure level is then established for the next breath. Hence, PCV-VG combines the benefits of decelerating flow of PCV with the safety of a volume guarantee at a lowest possible titrated inspiratory pressure .
Oxygenation index assessment [PaO 2 /(FiO 2×P mean )] may indicate the presence of atelectasis and reflect intrapulmonary shunt . The PaO 2 /FiO 2 ratio and the PaO 2 /PAO 2 ratio are most commonly used; however, they are not equally sensitive across the entire range of FiO 2 and they do not account for changes in the functional condition of the lung that result from alterations of PEEP, or other techniques used to adjust the lung volume during mechanical ventilation ,.
The aim of the study was to examine the following hypotheses.
- Whether PCV-VG provides the same Vt provided by PCV with lower peak inspiratory pressure.
- Whether there are changes in the flow pattern in both modes; as the beneficial effect of decelerating flow on oxygenation is only demonstrated when the flow reaches zero level at the end of inspiration, we use the oxygenation index as a parameter for more adequate flow pattern.
- To test the result that has been confirmed in spontaneously breathing patients on paralyzed patients, as dual-control mode proved to be more effective in spontaneously breathing patients.
| Patients and methods|| |
After approval by the National Cancer Institute ethics committee and after obtaining informed written consent from each patient, 30 patients (ASA I, II, and III) undergoing abdominal cancer surgery were enrolled in the study. The inclusion criteria were age more than18 years and less than 60 years and no major obstructive or restrictive pulmonary disease (defined as less than 70% of the predicted values). Exclusion criteria were patient refusal, suspected difficult intubation, and inability to maintain stable mechanical ventilation settings for 30 min (inability to maintain an appropriate end-tidal CO 2 and SpO 2 less than 94%).
Preoperatively, all patients underwent pulmonary function tests, arterial blood gases, cardiac assessment, and physical examination. All patients were fasted for at least 8 h. A 14-G catheter was inserted in the upper extremity vein and 2 mg midazolam was given 10 min before arrival in the operating room. Upon arrival in the operating room, monitoring consisted of five-lead electrocardiogram, noninvasive blood pressure, and pulse oximetry. A 20-G catheter was inserted in a radial artery under local anesthesia for continuous monitoring of arterial blood pressure and arterial blood gas measurement. After 5 min of preoxygenation in the supine position by facemask, anesthesia was induced with a bolus of propofol (2 mg/kg) and fentanyl (2 μg/kg). One minute after a 0.9 mg/kg bolus of rocuronium, direct laryngoscopy was performed and the trachea was intubated with a cuffed endotracheal tube (7.5 mm for men and 7 mm for women). Endotracheal intubation was confirmed by capnography and chest auscultation. Anesthesia was maintained with sevoflurane (1-2% expiratory concentration), and neuromuscular block was maintained with administration of supplementary rocuronium.
All patients were ventilated with a Datex-Omeda Ventilator (S/5 Avance, Aisys). Mechanical ventilation was started with PCV using upper pressure limit of 20 cmH 2 O and respiratory rate of 12/min (respiratory rate is adjusted to maintain ETCO 2 between 30 and 35 mmHg). The inspiration/expiration ratio (I/E) was set 1 : 2.
After 1 h of the previously mentioned ventilation strategy, PCV-VG was applied to all patients with the same parameters, targeting the obtained Vt during the first phase. A PEEP of 3 cmH 2 O was applied to all patients. FiO 2 was maintained at 50%. Crystalloid solutions (8-10 ml/kg) were used as maintenance fluid intraoperatively.
At the end of the surgery, prostigmine 0.05 mg/kg and atropine 0.015 mg/kg were given after obtaining 0.7 train-of-four ratio. Before extubation, FiO 2 was increased to 1.0 in patients breathing spontaneously. Extubation occurred when train-of-four ratio reached 0.9. After extubation, nasal cannula was given, if necessary, providing a SpO 2 above 95%.
Systolic, diastolic, and mean arterial blood pressures, heart rate, ETCO 2 , and SpO 2 were continuously monitored on a Datex-Ohmeda monitor. In addition, arterial blood samples were withdrawn to measure pH, PaCO 2 , and PaO 2 after 60 min of intubation during PCV and after 60 min of initiation of PCV-VG. Vt, mean airway pressure, and the peak airway pressure (during PCV-VG) were recorded. Oxygenation index calculation [PaO 2 /(FiO 2×P mean )] was performed at the preset times.
Sample size calculation
From previous study with similar modes of ventilation , we anticipated that the oxygenation index would be ~40 ± 5 in both modes of ventilation. Using an α of 0.05 and desired power of 90%, we estimated that 20 patients will be needed to demonstrate a statistically significant difference.
In addition, from previous studies with similar modes of ventilation , , we anticipated that the mean airway pressure would be ~15 ± 4 in both modes of ventilation. Using an α of 0.05 and desired power of 90%, we estimated that 25 patients will be needed to demonstrate a statistically significant difference.
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 by the Kolmogorov-Smirnov test. We expressed continuous variables as mean ± SD. Numerical data were compared using paired Student's t-test. Changes in hemodynamic parameters were analyzed using repeated measures analysis of variance followed by the Scheffe test, as appropriate. A P-value of 0.05 or less was considered statistically significant.
| Results|| |
Thirty patients who underwent upper abdominal surgery were studied in the National Cancer Institute during the period between March 2013 and July 2013. All patients completed the study. Demographic characteristics of the patients, ASA status, and blood loss are summarized in [Table 1].
Venilatory parameters such as the Vt, mean airway pressure, ETCO 2, and the peak airway pressure (during PCV-VG) were measured at the end of each ventilation period, and all parameters were comparable with no significant difference (P ≥ 0.05) [Table 2].
Arterial blood gases (PaCO 2 , PaO 2 , and pH) showed no statistically significant difference (P ≥ 0.05) when performed at the end of each ventilation period. In addition, the oxygenation index assessment was comparable in both periods, with no significant difference (P ≥ 0.05) [Table 3].
Hemodynamic parameters measured preoperative and at the end of each ventilation period are shown in [Table 4]. All data were comparable with no significant difference.
| Discussion|| |
Our finding showed that, while maintaining constant Vt and I/E ratio, there were no significant differences in the mean and peak airway pressure, ETCO 2 , PaO 2, and PaCo 2, and the oxygenation index assessment was comparable between both PCV and PCV-VG modes of ventilation in patients under general anesthesia. This can be explained by the fact that both PCV and PCV-VG use a decelerating flow pattern in which the bulk of Vt is delivered early during inspiration and its residency time in the lung is longer, providing better arterial oxygenation.
In PCV, the ventilator produces an inspiratory flow aimed at achieving and maintaining the preset pressure in the proximal airway . This pressure progressively equilibrates with alveolar pressure, resulting in an exponentially decelerating inspiratory flow. The Vt becomes dependent on the preset pressure, the TI, and the respiratory system compliance and resistance. However, PCV does not guarantee minute ventilation because any change in the respiratory system compliance or resistance will affect the Vt delivery . In addition, when using PCV, the flow-time curve is continuously monitored to adjust the optimal inspiratory and expiratory times. The minimum TI is the time required for inspiratory flow to reach zero, which ensures that inspiratory pressure reaches the alveolar level. Further lengthening of TI increases the mean airway pressure and can improve the arterial oxygenation . Irrespective of the I/E ratio and the duration of the end inspiratory pause, attention should be paid to keep a sufficient expiratory time to allow end-expiratory flow to reach zero and avoid the development of intrinsic PEEP .
However, in pressure control ventilation-volume guaranteed, the microprocessor compares the Vt of the previous breath, using exhaled Vt to minimize the possible artifacts due to any leak of the endotracheal tube, and adjusts the working pressure up or down within preset limits to try to deliver the set Vt. However, the term 'guarantee' is somewhat misleading, because the Vt does fluctuate around the target value .
In agreement with our study, Muñoz et al. failed to demonstrate any important difference between PCV and controlled mechanical ventilation, with decelerating inspiratory flow waveform. They concluded that the differences in the airway pressures detected by the ventilator were spurious and were due to the place (inspiratory line) where these pressures were measured. The difference between the peak pressure measured in the endotracheal tube has statistical, but not clinical, value and was lower in controlled mechanical ventilation with the decelerating flow waveform .
In addition, Davis and colleagues suggested that VCV with decelerating flow waveform and PCV can provide improved oxygenation compared with VCV with square flow waveform when Vt, I/E, and PEEP were held constant in patients with acute respiratory distress syndrome. They believed that this was because of an increase in mean airway pressure and the salutary effects of the decelerating flow waveform (square pressure waveform) on intrapulmonary distribution of gases .
PCV-VG is considered one of the dual-control, breath-to-breath, pressure-limited, time cycled modes of ventilation. However, inspecting the waveforms leads clinicians to realize that dual control does not guarantee a set Vt . Jaber et al.  showed that, during VSV (VSV is a pressure-limited mode that uses a target Vt and minute ventilation for feedback control; thus, the level of pressure support is continuously adjusted to deliver the preset Vt), a dual-control mode responsive to Vt, a ventilator demand increase induced by the addition of dead space leads to a decrease in pressure support, whereas no change occurs during standard PSV. The response to an added respiratory load required greater effort during VSV than during PSV. These findings are in agreement with ours, as pressure control ventilation-volume guaranteed added nothing more than pressure control ventilation either in respiratory mechanic or oxygenation.
However, Cheema et al.  examined the feasibility and efficacy of volume guarantee in 40 premature newborn infants. In a cross-over trial, they compared synchronized intermittent positive pressure ventilation with and without VG in infants with acute respiratory distress syndrome, and synchronized intermittent mandatory ventilation with and without VG during the weaning phase. In both VG groups, infants were able to achieve equivalent gas exchange using statistically significant lower peak airway pressure, and fewer excessively large Vt s during the volume guarantee periods were recorded. Because of their short duration of the study, no major conclusions could be drawn, other than that the ventilator performs as intended and no short-term adverse effects were evident.
In addition, Samantaray and Hemanth  concluded that, during mechanical ventilation in postcardiac surgical patients without pre-existing lung disease, pressure-regulated volume control mode was found to be advantageous in the later stages of ventilation as it results in significantly lower mean airway pressure and improved oxygenation index compared with the PCV mode. They explained their results by the fact that, although both PCV and PRVC use a decelerating flow pattern, which has been shown beneficial in acute lung injury, PRVC combines the benefits of decelerating flow of PCV with the safety of a volume guarantee at the lowest possible titrated inspiratory pressure. The results of both studies are different from ours as our patients were healthy and paralyzed and we used fixed Vt, whereas they made excessive Vt one of the beneficial effects of the guarantee mode. Consequently, their finding cannot be generalized.
In conclusion, there is no significant difference between PCV-VG and PCV with respect to oxygenation and airway pressure in healthy and anesthetized patients undergoing abdominal surgery. Further studies are required to detect which mode is better in different types of patients, for example in morbid obese patients.
| Acknowledgements|| |
Conflicts of interest
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[Table 1], [Table 2], [Table 3], [Table 4]