The present work demonstrated that in patients who were hospitalized in the Neurosurgical intensive care unit, the use of ASV mode compared with the SIMV mode made a significant difference in some respiratory parameters. Although it had no special effect on the patients arterial blood gas variable the use of ASV mode ventilation will lead to a significantly less peak airway pressure, dead space, expiratory tidal volume and at the same time more dynamic compliance (although not significant). This means the patients could be ventilated easier and safer with the lower probability of lung injuries.
In the ASV mode, the selection of tidal volume, an appropriate frequency for mandatory breathing and a suitable tidal volume for spontaneous breathing is made automatically based on mechanical properties of the respiratory system and target alveolar ventilation per minute. One of the advantages of this mode compared to other ventilation modes like pressure regulated volume controlled ventilation (PRVC) is earlier extubation after cardiac surgery (11). Tidal volume and the respiratory rate were calculated by using the Otis equation, which leads to the lowest effort of breathing. In this mode, tidal volumes are regulated through the microprocessors by careful and continuous monitoring of patients respiratory system.
According to results, expiratory tidal volume in ASV mode is less than the mandatory breaths of SIMV mode, which was set by the operator. It seems that two factors contribute to the amount of tidal volume in ASV mode; the rate of breathing that increases the lower the tidal volume, and vice versa. These factors are not significant clinically in that they are established by machine microprocessors. The second factor is body weight correction and the use of ideal weight rather than the actual weight for calculations of tidal volume needed for ventilation. This factor is defined by the operator and needs more consideration. It seems that the use of weight correction could lead to the prescription of less tidal volumes (12).
In present study, expiratory tidal volume in ASV mode was 6.8 ± 1.8 mL/kg. This amount was found to be less than conventional modes in other investigations. For example, 8.7 ± 1.4 ml/kg IBW in Casina et al. study (13) who found this ventilation mode applies lower tidal volume and plateau pressure to patients and allowed rapid extubation after cardiac surgery. The main difference between their study and ours is measuring the type of airway pressure. They evaluated plateau pressure that was significantly higher than our findings in peak pressure. Maybe the patients they selected were the main cause of this difference (cardiac surgery patients).
Arnal et al. also found 8.3 ± 1.3 mL/kg IBW, which is the mean tidal volume used for ventilation in ASV mode in polyvalent ICU patients, which reported no incident with the use of this mode (14).
In fact, due to respiratory monitoring and automatic adjustment of proper tidal volume in the ASV mode, we can reach the minimum respiratory work also the least resistance load and lung elasticity. In conclusion, it seems that using lower tidal volume with higher respiratory frequency (to maintain adequate minute ventilation) is the main strategy in ASV mode to decrease the effort of breathing. This finding could help and guide practitioners for better ventilation of patients. All of above findings was as favorable as our results.
In this study, peak inspiratory pressure in the ASV mode is less than the SIMV mode. Petter et al. who compared the ASV mode with SIMV followed by pressure support in terms of weaning the patients after cardiac surgery from the ventilator observed lesser peak pressure alerts in the ASV mode (5). Additionally, ASV patients required fewer ventilatory settings manipulations (P < 0.05) and endured less high-inspiratory pressure alarms compared to standard protocol in their study.
Tassaux D et al. found that during the ASV of patients with acute respiratory failure in the medical ICU the tidal volume increased and total respiratory rate decreased compared to SIMV-PSV ventilation mode (15). They showed that with AS, central respiratory drive was markedly reduced, suggesting decreased inspiratory load and improved patient-ventilator interactions in patients under partial ventilator support.
In the Choi et al. study, maximum inspiratory pressure in patients with ARDS using the ASV mode is less than the volume-controlled ventilation (VCV) (24.6 ± 6.0 vs. 32.2 ± 6.8 cmH2O) (16). Actually, they found that after changing the mode of ventilation from VCV to ASV in ALI/ARDS patients, inspiratory and expiratory tidal volumes increased and conversely, the total respiratory rate and maximum pressure decreased. All of those changes mean the patients were better ventilated and ASV had provided better respiratory mechanics in terms of peak airway pressure and tidal volume than VCV.
In fact, in the ASV mode the respiratory parameters are regulated automatically regarding the patients needs, which the lung has least resistance to. This feature of ASV mode could affect the maximum pressure and lead to the decrease of the P-Peak of the patient. In this regard present study supporting these finding and comparable with previous studies.
This study showed that, respiratory dead space in ASV mode was less than SIMV mode. The ventilator machine computed the required minute ventilation based on patient’s IBW and subsequently estimated the patient’s dead space. Various studies have suggested that the increased dead space is related with the increase of patients’ mortality (17) and lower dead space could decrease the intrapulmonary shunt, which would improve the resultant complications (18).
Bhalla et al. showed the initial and day 1 mean. The end-tidal alveolar dead space fraction was associated with the mortality in pediatric critical care unit (17). Although we didn’t assess mortality in our patients included this study, but decreasing dead space in ASV mode is obviously a useful finding in lung mechanics.
While this paper is not about the duration of ventilation and its difference in different respiratory modes, many of the studies were conducted on the effect of ventilation types and the time of deterring. Numerous studies suggested using the ASV mode for lowering ventilation time (19, 20).
According to lower peak airway pressure, better compliance and less respiratory dead space, which causes less work of breathing, lower ventilation time of patients by ASV mode is reasonable (21).
Generally, we found that applying the ASV mode can lead to improved ventilation conditions compared to SIMV, but reducing the medication and hospitalization time that consequently could lower the expenses of the health system in intensive care units is not evaluated in our study and it could be suggested to other researchers. Undeniably, the increase of number of patients and the number of studies on patients with more acute respiratory diseases, such as ARDS, further studies are recommended concerning the effect of this mode on pathological lung cases.
Subsequently, in the ASV mode, the lower tidal volume is specified by the processor than the regulated volume in SIMV mode for the patients (6.8 mL/k). We recommend the same amount used for ventilation of patients instead of more volumes in traditional modes (8 - 12 mL/kg in VCV or SIMV).
5.1. Limitations
The time of extubation, ICU staying time and patients’ mortality wasn’t evaluated in our study.
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