Flow versus pressure triggering in mechanically ventilated acute respiratory failure patients

Background: The effects of flow triggering (FT) compared with pressure triggering (PT) on breathing effort have been the focus of several studies, and discrepant results have been reported; yet, it remains an area of conflict that warrants further studies. Objective: The aim of this work is to compare flow versus PT in ventilating patients with acute respiratory failure. Patients and methods: One hundred patients with acute respiratory failure of pulmonary origin were assigned randomly to two groups: 50 patients ventilated with PT and 50 patients ventilated with FT. The primary end points were weaning duration, evaluation of patient/machine synchronization, total duration of ventilation and ICU stay as well time under sedation and occurrence of complications. Mortality was considered the secondary end point. Patients were categorized into those with obstructive, restrictive, and combined pulmonary disease according to their medical history, and clinical and radiological assessment, and also more and less severe disease according to the APACHE II score level (cut-off point). Results: In all the patients studied, including those with restrictive pulmonary disorder and more severe disease (APACHE II score ≥32.5), there was a statistically significantly shorter duration of weaning, duration of ventilation, and duration of ICU stay in the FT group than the PT group. The pre-extubation oxygenation index was highly statistically significantly better in the FT group than the PT group (P < 0.001). In patients with obstructive pulmonary disorders, combined pulmonary disorders, and less severe disease (APACHE II <32.5), there was no significant difference between both PT and FT groups in these parameters. Conclusion: FT may be considered to be better than PT in ventilating acute respiratory failure patients with a restrictive pattern and those with higher severity scoring. In obstructive and mixed ventilatory impairment, use of either of them does not make a difference.


Introduction
Respiratory failure is a syndrome in which the respiratory system fails in one or both of its gasexchanging functions -that is, oxygenation of, and carbon dioxide elimination from, mixed venous (pulmonary arterial) blood [1].
Acute respiratory failure is characterized by an inability to maintain adequate oxygenation (a PaO 2 of <50-55 mmHg or an arterial oxygen saturation by pulse oxymetry of <85% on room air), ventilation (a PaCO 2 >50 mmHg or increase of >10 mmHg over the base line PaCO 2 ), or both, which develops over a short period of time. For practical purposes, the onset usually occurs over several hours or days [2].
Acute respiratory failure can be classifi ed into hypercapnic or hypoxemic respiratory failure, and can also be divided into those with normal versus abnormal chest radiography [2].
Th ere are three locked steps to the diagnosis of ARF: (a) Th e clinical suspicion that ARF might be present, (b) Confi rmation by arterial blood gas analysis that ARF is present, and (c) Further diagnostic steps that identify the specifi c etiology of the ARF [3].
One of the most serious complications of ARF is nosocomial pneumonia, usually caused by virulent organisms, frequently Gram-negative bacilli that cause necrotizing pneumonia, and may be resistant to common antibiotics. Acute stress ulceration with resultant upper gastrointestinal bleeding is a well-recognized complication in patients with respiratory failure [4].
Pulmonary embolism is a frequent fi nding in critically ill patients with ARF. When ARF and acute renal failure occur concurrently in critically ill patients, each condition may worsen or precipitate the other [5].
When correcting hypoxemia, the clinician must also address any coexisting hypercapnia and respiratory acidosis, and the immediacy of correction depends on the magnitude of the acidosis and its attendant eff ects [1].
When adequate oxygenation cannot be maintained by noninvasive means or if progressive hypoventilation and hypercapnia with respiratory acidosis occurs, endotracheal intubation and mechanical ventilatory support should be initiated. Mechanical ventilation can produce positive pressure at the airway opening or create negative pressure around the chest wall. Use of negative pressure ventilation is generally restricted to patients with chronic neuromuscular weakness or chest wall deformity [6].
For air to enter the lungs, a pressure gradient must exist between the airway and alveoli. This can be accomplished either by increasing pressure at the airway (positive-pressure ventilation) or by lowering pressure at the level of the alveolus (negative-pressure ventilation). Positive-pressure ventilation can be achieved by an endotracheal or a tracheostomy tube or noninvasively through a nasal mask or a facemask. In the past, this was invariably performed using an endotracheal or a tracheostomy tube, but in recent years, there has been an increasing trend toward the use of noninvasive ventilation, which can be accomplished using either a full face mask (covering both the nose and the mouth) or a nasal mask [7].
In acute care settings, critical care ventilators or portable positive-pressure devices are used in volumelimited or pressure-limited modes. Although either mode can be used with the expectation of similar rates of success, pressure-limited modes appear to be more readily accepted by patients. In the acute setting, nasal or oronasal masks are most commonly used [8].
In 1994, Tobin [9] listed the objectives of mechanical ventilation as follows: (1) Improve pulmonary gas exchange: (a) Reverse hypoxemia.  Selection of the initial ventilator setting is performed on the basis of the patient's size and clinical condition. Settings are entered and the proper function of the ventilator is verifi ed before connection to the patient. Ventilatory settings must be repeatedly reviewed to optimize ventilatory support while minimizing risks [10].
After initiating ventilatory support, the clinician should 'fi ne-tune' the trigger sensitivity setting. Fine-tuning is necessary because too sensitive a setting can cause some ventilators to autotrigger whereas an unresponsive trigger level can add to the breathing work load and cause patient-ventilator asynchrony [11].
Triggering refers to the mechanism through which the ventilator senses inspiratory eff ort and delivers gas fl ow or a machine breath in concert with the patient's inspiratory eff ort. In modern ventilators, the demand valve is triggered by either a fall in pressure (pressure trigger) or a change in fl ow (fl ow trigger). With pressure-triggered ventilation, a preset pressure sensitivity has to be achieved before the ventilator delivers fresh gas into the inspiratory circuit; with fl ow-triggered ventilation, a preset fl ow sensitivity is used as the trigger mechanism [12].
Th e most common trigger variables are time (the ventilator initiates a breath according to asset frequency, independent of the patient's spontaneous eff orts), pressure (the ventilator senses the patient's inspiratory eff ort in the form of a decrease in baseline pressure and starts inspiration independent of the set frequency), and fl ow (the ventilator senses the patient's inspiratory eff ort as a decrease in the baseline fl ow through the patient circuit or senses inspiratory fl ow directly with a sensor at the patient's airway opening) [13].
Pressure triggering (PT) is the oldest and simplest technique for the detection of patient eff ort. Th e sensitivity or trigger threshold is set in centimeters of H 2 O relative to the baseline pressure.
As an example, if baseline pressure is 5 cmH 2 O and the trigger threshold is 2 cmH 2 O, when patient eff ort causes pressure in the circuit to decrease to 3 cmH 2 O, the breath is triggered and if the baseline pressure is changed, for instance to 10 cmH 2 O and the trigger threshold remains the same (2 cmH 2 O), the ventilator is triggered when circuit pressure decreases to less than 8 cmH 2 O; the ability to maintain the trigger threshold constant irrespective of alterations of baseline pressure is frequently referred to as 'positive end expiratory pressure' (PEEP) compensation [14].
Flow triggering (FT) was introduced by Engestromin in the early 1980s, but did not become popular until it was reintroduced by Puritan Bennett in 1988; since then, FT has become standard on current ventilators. Like so much of ventilator technology, the methods of FT vary from manufacturer to manufacturer. FT systems vary in placement of the fl ow transducer, presence and absence of a continuous fl ow, and the ability to adjust bias fl ow and fl ow sensitivity [15].
FT is implemented using one of three methods; the fi rst simply measures a change of fl ow caused by the patient's inspiratory eff ort, there is no continuous fl ow in the circuit, and at end-expiration fl ow is 0. Th e second provides a preset non adjustable level of continuous fl ow in the circuit, from which a change of fl ow (the fl ow sensitivity) is detected. Th e third enables the clinicians to set the continuous fl ow and the fl ow sensitivity. In this case, a change in fl ow through the circuit caused by the patient's inspiratory eff ort reduces the fl ow below the fl ow trigger threshold setting and a breath is triggered. In the presence of a leak, this system can be tailored to overcome the leak while maintaining appropriate triggering [16].
FT reduces inspiratory eff ort during weaning from mechanical ventilation. Th ere was no change in breathing pattern, minute ventilation, and lung mechanics, and the magnitude of the inspiratory eff ort decreased signifi cantly with FT compared with PT in both instances [17].
Th e aim of this work is to compare fl ow versus PT in ventilating patients with acute respiratory failure.

Patients and methods
Th is study was carried out in the Respiratory Intensive Care Unit of Th oracic Medicine Department, Mansoura University Hospital, during the period from July 2011 to August 2013, after fulfi llment of departmental ethical committee requirements and obtaining oral consent from the patients or their surrogate.

Study design
Th is is a prospective clinical trial in which one hundred patients with acute respiratory failure of pulmonary origin were assigned randomly to two groups: group I included 50 patients who were ventilated through PT and group II included 50 patients who were ventilated through FT. Th e primary end points were weaning duration, patient/machine synchronization, total duration of ventilation, ICU length of stay, time under sedation, and occurrence of complications. Mortality was considered the secondary end point.

Inclusion criteria
Patients with acute respiratory failure (either de novo or on top of chronic) because of pulmonary causes and indicated for invasive mechanical ventilation were included in this study. Respiratory failure was defi ned as a PaO 2 measured at sea level of less than 8 kPa (60 mmHg) or PaCO 2 above 6.5 kPa (49 mmHg) [18]. For a more detailed analysis, patients' respiratory illnesses were categorized into obstructive, restrictive, and combined breathing disorders according to their medical history, and clinical and radiological assessment.

Methods
(1) All selected patients were intubated and connected to the ventilator using an Inspiration Events ventilator. Settings were tailored according to the clinical condition indicating mechanical ventilation and monitoring of clinical, laboratory, and lung mechanics data during the course of ventilation. Th e medical therapy and nursing care were individualized according to the original problem indicating mechanical ventilation. Analgo/sedation was achieved with Midazolam (bolus and/or infusion) and/or fentanyl (bolus and/or infusion) with dose adjustment according to the clinical indication and response considering morning sedation vacation for reassessment.
(2) Th e following was performed for every patient: (a) Clinical evaluation including assessment of history and examination. (b) Admission and follow-up chest radiograph. (c) Laboratory data on admission and during ICU stay including complete blood picture, electrolytes, arterial blood gases, and complete metabolic profi le. Th e thyroid profi le was added if indicated.
(d) Admission severity scoring through the APACHE II score. Patients were subdivided according to severity into two groups: group I included patients with APACHE II score of at least 32.5 and group II included patients with APACHE II score of less than 32.5 (statistically selected). (e) Patient-machine synchronization was assessed twice daily using the Riker Sedation/ Agitation Scale [19]. For statistical analysis, patients were categorized into comfortable (scale number 4), which was considered an indirect indicator of good synchronization, and noncomfortable (scale 5, 6, 7, 1, 2, 3), which was considered an indirect indicator of suboptimal synchronization. (f ) Recording duration under sedation, total duration of ventilation, weaning duration, and total ICU stay. (g) Recording complications (including reventilation within 24 h after extubation) and mortality. (h) Monitoring of patients according to ICU guidelines. (i) Mechanical ventilation for both pressure and fl ow groups was started by using the pressurecontrol or volume-control conventional mode or the pressure regulating volume control dual mode or alternating between them, that is combined mode. When the patients were placed on the spontaneous-breath mode, the pressure trigger was set at −2 cmH 2 O and the fl ow trigger was set at 2 l/min. Nursing care and pharmacological management were tailored dynamically according to every patient scenario. (j) Weaning of patients was achieved using either pressure support and or T piece trials after completing a daily weaning checklist according to Corrado et al. [20].

Statistical methods
Th e data were recorded on a report form, tabulated, and analyzed using the computer program statistical package for social science, version 16.

Descriptive data
Descriptive statistics were calculated for the data in the form of: (1) Mean and SD for quantitative data.
(2) Frequency and distribution for qualitative data.

Analytical statistics
In the statistical comparison between the diff erent groups, the signifi cance of diff erence was tested using one of the following tests: (1) Student's t-test and Mann-Whitney test (Z): used to compare the mean of two groups of quantitative parametric and nonparametric data, respectively. (2) Intergroup comparison of categorical data was performed using the χ 2 -test (χ 2 -value) and the Fisher exact test.
A P value of 0.05 or less was considered statistically signifi cant (S), a P value of more than 0.05 was considered statistically insignifi cant, and a P value of less than 0.01 was considered highly signifi cant (HS) in all analyses.

Results
Th e FT group had more severe illness compared with the PT group, with APACHE11 scores of 42.70 and 38.00, respectively, and with a statistically signifi cant diff erence (P = 0.04) (Tables 1-4).
Th ere was no signifi cant diff erence between both PT and FT groups in weaning duration (2.78 vs. 2.64 days with P = 0.867), duration of ventilation (4.67 and 4.07 days, P = 0.547), and total duration of ICU stay (6.56 vs. 4.93 days, P = 0.181) ( Table 5).
Weaning duration, duration of ventilation, total duration of ICU stay, and duration of use of sedation were signifi cantly shorter in the FT group than in the PT group (2.52 vs. 6.5, 4.6 vs. 9.7, 5.68 vs. 11.4, and 2.24 vs. 6.52 days, respectively, with P < 0.001 for all) ( Table 6).
Weaning duration, duration of ventilation, total duration in ICU stay, and duration of using sedation were insignifi cantly shorter in the FT group than in the PT group (

Discussion
Th e PT group included 50 patients; 46% were men, mean age 55.30 years, and the FT group included 50 patients; 54% were men, mean age 53.78 years, and there was no statistical diff erence in age and sex.
Th ere were signifi cantly higher numbers of smokers were in the FT group (46.0%) than in the PT group (32.0%) (P = 0.016).
Patients were classifi ed into three categories according to the main initial problem: obstructive pulmonary disorder group (23%) (which included patients with COPD, asthma, bronchiactasis, overlap syndrome).
Pneumonia was the most common cause of ICU admission and mechanical ventilation in both groups.
Goulet et al. reported that no study of triggering has assessed the outcome. Most studies, similar to theirs, have been short-term evaluations of physiologic response to various forms of triggering. It is unknown whether one approach to triggering is superior to others with respect to the duration of mechanical ventilation or other indices of morbidity. Because FT increases the cost and complexity of triggering, it would be of  interest to know whether the type of triggering aff ects outcome.
Th erefore, we focused on the evaluation of the two triggerings in terms of the following outcome parameters: weaning duration, total ventilation days, total duration of ICU stay, and duration of using sedations.
There was a statistically significantly shorter duration of weaning in the FT group (2.76 days) than in the PT group (5.48 days) (with P < 0.001). The total duration of ventilation was also shorter in the FT group (4.72 days) compared with (8.18 days) the PT group, which was statistically significant (P < 0.001).
Th e oxygenation index was better in the FT group than in the PT group (266.72 vs. 196.10, respectively), which was a statistically signifi cant diff erence (P < 0.001).   Ventilation was also better in the FT group compared with the PT group (pre-extubation PaCO 2 was 36.33 and 48.14 mmHg, respectively). However, this was a statistically insignifi cant diff erence (P = 0.149).
Most of self-extubation occurred in the PT group (20%) in compare to only (14%) of the FT group, without signifi cant diff erence (P = 0.425).
Th e total duration of ICU stay was signifi cantly shorter in the FT group than the PT group (5.80 vs. 9.86 days, respectively, with P < 0.001).
Although there were greater complications in the PT group (38%) versus (34%) in the FT group, there were no signifi cant diff erences (P = 0.677),   APACHE II score in pressure versus fl ow triggering g roups. but it is worth mentioning that pneumothorax was more common in the FT group as it occurred in eight patients either alone or in combination with other complications whereas it occurred in only two patients in the PT group, and this represents an advantage of PT.
Mortality was also higher in the PT group (44.0%) compared with the FT group (36.0%), but with no signifi cant diff erence (P = 0.414).
Reventilation was performed more in the FT group (22%) compared with the PT group (18%), but no signifi cant diff erence was detected (P = 0.62).
Th e patients in the FT group had more severe illness compared with those in the PT group, with APACHE11 scores of 42.70 and 38.00, respectively (P = 0.04).
Functional diagnosis of the cases st udied.

Fig. 3
Descriptive data of patients' primary diag nosis.

Fig. 4
Weaning duration and duration of ventilation in pressure versus fl ow trigg ering.

Fig. 5
Total duration of ICU stay in pressure versus fl ow trigg ering.

Fig. 8
Pre-extubation PaO 2 /FIO 2 and PaCO 2 in the pressure versus fl ow triggering group.

Fig. 6
Incidence of mortality in the pressure versus fl ow triggering g roups. Overall, in this study, FT was superior to PT with respect to the duration of mechanical ventilation, duration of weaning, ICU length of stay, and oxygenation index before extubation. Th ere was also a trend toward better ventilation as determined by the level of PaCO 2 before extubation and lower mortality in the FT group. Taking into consideration the fi nding that patients in the FT group had more severe illness compared with the patients in the PT group augments the satisfaction with the conclusion of FT superiority in the above outcomes.
Although the duration of weaning and the total duration of ventilation were shorter in the FT group, reventilation was performed more in the FT group. Th is may be related to the severity of illness, which was higher in the FT group than the PT group (APACHE II in the FT group was 42.70 and that in the PT group was 38).
Th is is with agreement with Sassoon et al. [22] and Giuliani et al. [23], who reported that FT is usually associated with a reduction in breathing eff ort compared with PT, although a signifi cant benefi t was not found consistently in all studies.
However, Correa et al. [24] concluded that during the pressure trigger ventilation, the minute ventilation was greater than that in the FT ventilation without aff ecting the other ventilatory parameters when they evaluated 20 mechanically ventilated adult ICU patients recovering from acute respiratory failure who were ventilated with a pressure support of 15 cmH 2 O, PEEP of 5 cmH 2 O, and FIO 2 of 40%. Th e patients were ventilated by two diff erent trigger systems during pressure support ventilation (PSV): a fl ow trigger of 2 l/min or a pressure trigger (−2 cmH 2 O) during PSV. Th ey measured the respiratory rate, expiratory tidal volume, minute ventilation, VCO 2 , VTCO 2 , ETCO 2 , SpO 2 , mean arterial pressure, and heart rate after 15 min in each study situation.
In another study, Goulet et al. [21] compared pressure versus FT during PSV in adult mechanically Weaning duration, duration of ventilation, need for sedation, and total duration of ICU stay in pressure versus fl ow triggering among restrictive pulmonary disorder pat ients.

Fig. 10
Weaning duration, duration of ventilation, need for sedation, and total duration of ICU stay in pressure versus fl ow triggering among patients with APACHE II score more than 32.5.

Fig. 12
Weaning duration, duration of ventilation, and need for sedation in pressure versus fl ow triggering among patients with combined pulmonary diso rders.

Fig. 11
Weaning duration, duration of ventilation, need for sedation, and total duration of ICU stay in pressure versus fl ow triggering among obstructive pulmonary disorder pat ients. ventilated patients. Th eir study included 10 patients recovering from acute respiratory failure: median PSV 10 cmH 2 O, median PEEP 5 cmH 2 O; all were in a hemodynamically stable condition and four trigger sittings were randomly applied: pressure −0.5 cmH 2 0, pressure −10 cmH 2 0, and base fl ow 5 l/min with a fl ow sensitivity of 2 l/min and base fl ow 10 l/min with a fl ow sensitivity of 3 l/min. Th ey found that a pressure trigger of −0.5 cmH 2 0 was consistently more sensitive than the other three triggering methods, but we cannot rely totally on these results because of the small population of this study. Also, they evaluated four triggering settings that they used in their clinical practice and the superiority was only evident with −0.5 cmH 2 O compared with −2 cmH 2 O in this current study. In addition, this study was carried out on stable weanable patients recovering from acute respiratory failure whereas the current study was carried out on patients with acute respiratory failure throughout the period of mechanical ventilation once they had resumed spontaneous breathing.
In another point of view, Tantucci et al. [25] concluded that the application of either a pressure-triggered or a fl ow-triggered system during pressure-support ventilation did not signifi cantly aff ect short-term changes in gas exchange, respiratory mechanics, and inspiratory workload in 16 orotracheally intubated adult patients recovering from acute respiratory failure of various etiologies, without chronic obstructive pulmonary disease. Again, this study was carried out on stable patients recovering from acute respiratory failure whereas the current study was carried out on severely ill patients throughout the period of mechanical ventilation once they had resumed spontaneous breathing.
The investigators consider work of breathing (WOB) an important issue that should have been studied and compared between the FT and PT groups. WOB assessment usually entails evaluation of trans-diaphragmatic pressures, which could not been evaluated in the current study for technical reasons and this is one of the limitations of this study. Nevertheless, WOB was substituted in this study with other parameters that indirectly reflect patient-ventilator synchrony, namely, patient sense of comfort on the ventilator as assessed by the Riker Sedation-Agitation Score and the need for sedation. Both parameters reflected better synchrony with FT. The finding of less breath effort with FT compared with PT was reported several years ago by Sassoon et al. [22], and confirmed by Giuliani et al. [23], who reported that inspiratory muscle effort (as reflected by the pressure-time product) was less with flow-triggered than with pressure-triggered SIMV during both mandatory and spontaneous breaths, with a better patient-ventilator interaction.
In addition, Branson et al. [26], concluded that FT reduces the WOB compared with PT, irrespective of the ventilator used. Th e reduction in WOB during FT is related to improved responsiveness and changes in the post-trigger phase, suggesting that FT is a superior technique.
When the patients were divided into subgroups, comparison between the two triggering methods in the obstructive pulmonary disorders group showed no signifi cant diff erence between both PT and FT groups in weaning duration ( Weaning duration, duration of ventilation, need for sedation, and total duration of ICU stay in pressure versus fl ow triggering among patients with APACHE II score less than 32.5.

Fig. 13
Self-extubation was performed more in the PT group (22.2%) than in the FT group (14.3%), with no signifi cant diff erences (P = 1.0).
Reventilation was performed more in the PT group (33.3%) compared with (14.3%) the FT group, with no signifi cant diff erence (P = 0.343).
In terms of complications, 93% of patients in the FT group were free from complications compared with 89% of patients in the PT group, with no signifi cant diff erence (P = 1.0).
In a study carried out by Ranieri et al. [27], it was concluded that application of FT requires less eff ort to initiate inspiration and provides a positive end-expiratory pressure level that can unload the respiratory muscles by reducing the eff ect of PEEPi.
Th is study was carried on six COPD patients with acute respiratory failure ready to be weaned; esophageal and gastric pressures were measured by intraesophageal and intragastric balloon pressure sensors, minute ventilation, and breathing patterns, and pressure-time product of the respiratory muscles and diaphragm was obtained during spontaneous ventilation through a mechanical ventilator (Puritan-Bennett 7200ae). Th ey found that the inspiratory muscles' eff ort necessary to overcome the triggering system overestimated PEEPi dyn measurement by an amount equal to 49 ± 2 and 58 ± 3% during pressure and FT, respectively. FT increased tidal volume and minute ventilation and decreased pressure-time product of the respiratory muscles and diaphragm. Th is benefi t in COPD patients was not investigated thoroughly in the current study, but FT showed a superior trend compared with PT in ventilation, self-extubation, reventilation, and occurrence of complications, and PT showed a superior trend in oxygenation, duration of sedation, comfortability, and synchronization, but with no statistical signifi cance in either case.
Also, Nava et al. [28]concluded that in patients with COPD recovering from an acute exacerbation, FT reduces the inspiratory eff ort during both PSV and assisted controlled mode (A/C) compared with PT. Th ey attributed the fi ndings to a reduction in PEEPi dyn and the time of valve opening with a fl ow trigger. Th e study compared the eff ect of FT (1 and 5 l/min) and PT (−1 cmH 2 O) on inspiratory eff ort during PSV and A/C delivered noninvasively using a full face mask. From the above results, we can conclude that FT is superior to PT in restrictive breathing disorders in the following: weaning duration, duration of ventilation, total duration of ICU stay, duration of need to sedation, and ventilation. In the FT group, there was better synchrony between the patients and the ventilator and the patients also felt more comfortable. However, the reventilation rate was lower in the PT group.
Th erefore, in this study, the superiority of FT was most evident in patients with restrictive compared with obstructive pulmonary disorders. Th is may be related to the nature and pathology of illness.
A combined pulmonary disorder study showed that weaning duration, duration of ventilation, total duration in ICU stay, and duration of using sedation were insignifi cantly shorter in the FT group than in the PT group ( In the PT group, 80.0% patients were free from complications compared with 72.7% patients in the FT group, but this was insignifi cant (P = 1.0).
In terms of APACHE II scoring, Forte et al. [29] reported that this score accurately refl ects the degree of physiological derangement and correlates with subsequent clinical course and length of ICU stay. Th e APACHE II scoring system is used widely in general ICU for comparative audit, evaluative research, and clinical management of individual patients. Th e number of acute organ failures has been shown to be an important determinant of prognosis in critically ill patients admitted to an ICU.
In this study, APACHE II scoring was applied and the patients were subdivided into those with the most severe illness (using APACHE II score >32.5 as a cut-off value), 74 cases, and less severe illness (APACHE II score <32.5), 26 cases.
Th e patients in the FT group had more severe illness than those in the PT group, with APACHE II scores of 42.70 and 38.00, respectively, and P equal to 0.04, which was considered statistically signifi cant.
It was obvious in patients with the most severe illness that FT was superior to PT in weaning duration, duration of ventilation, total duration of ICU stay, and duration of need for sedation. All these durations were signifi cantly shorter in the FT group than in the PT group (2.64 vs. 5.60, 4.64 vs. 7.94, 5.69 vs. 9.49, and 2.33 vs. 5.26 days, respectively, with P < 0.001 for all). Also, the FT patients showed better oxygenation as indicated by PaO 2 /FIO 2 (275.95 vs. 189.06 in PT). Although better ventilation was also evident (PaCO 2 level 35.67 vs. 48.67 in PT), it was statistically insignifi cant. In the less severe cases, there were no statistically signifi cant diff erences between FT patients and PT in all parameters. However, FT patients showed a trend toward better oxygenation and ventilation and shorter durations of weaning, ventilation, total duration of ICU stay, and duration of need or sedation. Th ese fi ndings suggest that the effi ciency and superiority of FT become obvious in challenging situations when patients have more critical illness and need the most optimized interventions. In patients with good recovery, the diff erence between the two triggering systems is too small to manifest.
In the current study, all patients were investigated using a single ventilator brand: Inspiration Events. Although the use of other brands may infl uence the results, other studies using other brands have shown similar results. Sassoon et al. [30], reported a reduction in WOB during FT with the Puritan Bennett 7200ae. Th e type of pulmonary illness, its severity, and the level of triggering (compared with other studies) were the most infl uencing factors in the current study.
From the above results, we can conclude that FT may be considered better than PT in ventilating acute respiratory failure patients with a restrictive pattern and those with a higher severity scoring. However, in obstructive and mixed ventilatory impairment, use of either of them does not make a diff erence.

Conclusion
FT may be considered to be better than PT in ventilating acute respiratory failure patients with a restrictive pattern and those with a higher severity scoring. In obstructive and mixed ventilatory impairment, use of either of them does not make a diff erence.

Recommendations
(1) FT should be considered in patients with acute respiratory failure because of restrictive ventilatory impairment and those with more severe illness. (2) Further studies are recommended for more justifi cation of this conclusion.