Analysis of an Anesthesia and Intensive Care Ventilators during Suctioning through a Fiber-optic Bronchoscope

Background: The aim of this bench test was to compare the performance of an anesthesia and an intensive care ventilator during Fiber-optic Bronchoscopy (FOB) suctioning. Methods: In a lung model, a 5.9 mm OD FOB was inserted into an endotracheal tube through a tight-fitting connector. Pressure and flow sensors were placed at the airways opening (Pao) and distal to the bronchoscope at the trachea (Ptr). This model was tested under pressure and volume controlled mandatory ventilation and during Continuous Positive Airway Pressure (CPAP) delivered by both, anesthesia and intensive care ventilators. End-expiratory pressure was set at 10 cm H2O. Measurements were performed during suctioning with a mean negative flow of -12 ± 0.2 L/min at the tip of the FOB. Results: The anesthesia ventilator failed to keep end-expiratory pressure above zero at the trachea during FOB suctioning using both, continuous mandatory ventilation and CPAP. When using an intensive care ventilator, Ptr decreased below zero during FOB suctioning during volume controlled ventilation but remained slightly above zero under pressure controlled ventilation. When this ventilator delivered 10 cm H2O of CPAP, Pao remained constant while Ptr decreased by 2.5 cm H2O during suctioning. Conclusion: During FOB suctioning, targeted Ptr could neither be maintained anesthesia nor intensive care ventilators used in mandatory modes of ventilation. Only CPAP administered by the intensive care ventilator kept Ptr reasonably stable during suctioning.


Background
Fiber-optic Bronchoscopy (FOB) performed in patients under controlled mechanical ventilation is a common intervention in operating rooms and Intensive Care Units (ICU) [1][2][3] and it is known to be associated with severe cardio-respiratory complications [4][5][6].
FOB promotes lung collapse by the negative pressures created during suctioning. The subsequent loss of Functional Residual Capacity (FRC) induces atelectasis and potentiates Ventilator-Induced Lung Injury (VILI) any time partially collapsed lungs are ventilated with positive pressure [7][8][9][10][11]. This VILI is expected to be minimized if lungs were ventilated with a protective pattern and if FRC could be preserved during FOB suctioning [12][13][14][15][16][17]. Previous evidence created with closed system suctioning showed that ICU ventilators seem to be able to compensate at least partially the negative pressures of suctioning, especially under Continuous Positive Airways Pressure (CPAP) [18][19][20][21]. However, we observed that standard anesthesia machines failed to redo so during FOB performed in the operating room.
We hypothesized that intensive care ventilators are better than anesthesia machines to maintain tracheal pressure (Ptr) in FOB procedures. Thus, the aim of this bench test was to evaluate whether protective ventilatory settings delivered by standard anesthesia and intensive care ventilators in mandatory modes or CPAP could maintain tracheal pressure (Ptr) during FOB suctioning. We reasoned that the effects of FOB suctioning on Ptr would be compensated by CPAP as inspiratory flow is continuously delivered any time the actual circuit pressure decreases below the set pressure. This hypothesis was tested in two commercially available anesthesia and ICU ventilators.

Setup
A simulated trachea consisting of a tube of 2 cm ID and 15 cm length was connected to the test lung 190 (Maquet, Solna, Sweden) of 1 liter capacity ( Figure 1). This setup was incubated with an 8 mm ID cuffed Endotracheal Tube (ETT). A flexible bronchoscope Pentax VB-1530T2 (Pentax, Tokyo, Japan) with an OD of 5.9 mm, a length of 600 mm and a suction port of 2 mm ID was advanced into the trachea through a tight-fitting connector. A difference between the ID of the ETT and the OD of the bronchoscope was ≥ 2 mm to avoid unacceptable obstruction and auto PEEP [22].

Measurements
Pressure and flow sensors (Flux Med, MBMed, and Buenos Aires, Argentina) were placed at the airway opening between the Y-piece and the ETT (Figure 1). An additional pressure line was placed at the alveolar level (A') to detect the presence of auto PEEP (auto PEEP = PEEP at Y piece -PEEP at A´). Another pressure and flow sensor (NICO 2 , Respironics, and Wallingford, USA) was placed between the trachea and lungs. All sensors were calibrated according to the manufacturers' guidelines and data were recorded with two coordinated laptops.
The main parameters studied were Tidal Volume (VT), Peak Inspiratory (PIP) and Plateau (Pplat) Pressures, Peak Expiratory Flow (PEF), Dynamic Compliance (Cdyn) and expiratory airway resistances (Raw). Cdyn was calculated as measured VT divided by delta pressure (PIP-PEEP) and Raw as PIP minus Pplat divided by flow.

Suctioning system
The hospital's central suctioning system generated a negative pressure of -0.6 bar (-60 kpa or -450 mm Hg) which resulted in a suction flow of -55 L/min at the outlet. A 1 Liter bottle was hooked up to the outlet and connected to the suction port of the FOB via a 2 m hose of 8 mm ID. Then negative flow generated at the tip of FOB using this suction system was -12 ± 0.2 L/min mean (SD) as measured in triplicate.

Protocol
The setup was ventilated by the anesthesia workstation Advance (GE Healthcare, Madison, WI, US) and the intensive care ventilator Taema Extend XT (Taema, Antony, France). Mechanical breaths were delivered in Volume Controlled (VC-CMV) or Pressure Controlled (PC-CMV) Continuous Mandatory Ventilation modes. We selected a VT of 420 mL representing a VT of 6 ml/kg for an adult patient of 70 kg predicted body weight [12,14,15]. During PC-CMV the inspiratory pressure was adjusted until the target VT was reached. Respiratory rate was set to 15 bpm with a fixed inspiratory time of 30 % and 10% of inspiratory pause in VC-CMV and PEEP of 10 cm H 2 O. In PC-CMV inspiration lasted 33% of the respiratory cycle.
The system was pressurized with a constant pressure of 50 cm H 2 O for one minute to confirm the absence of air leaks. Afterward the following measurements were performed in random order:

Statistical analysis
Descriptive statistics were used to express the changes in variables during steps 1 and 2 in VC-CMV or PC-CMV. To assess the variability of the parameters we determined the coefficient of variation (CV = SD/mean). If the CV of the recorded breaths was < 10% we considered the measurements stable and acceptable for further analyses. Data is presented as mean (SD). A paired Student t-test with Bonferroni's correction was performed to compare variables at baseline with those when the FOB was in place. A p < 0.05 was considered statistically significant.

Results
During steps 1 and 2 the coefficient of variation for 30 values of the studied parameters was ≤ 3 %. Tables 1 and 2 show the effect of FOB on the ventilatory parameters when compared to baseline Setup consisting of an Endotracheal Tube (ETT) connected to an artificial trachea and lung model; a tight-fitting connection allows the leak-free insertion of the Fiber-optic Bronchoscope (FOB) through the ETT into the trachea. Pressure and flow sensors were placed proximal to the FOB at the airway opening (A), distally to the trachea (B) and at the alveolar level (A´). The suction system was connected to the 2 mm suction port of FOB. ventilation. In general, such effects were similar for the anesthesia and ICU ventilator.
FOB significantly increased PIP at the airways opening when compared to baseline ventilation using VC-CMV (+ 37% for the anesthesia ventilator and + 17% for the ICU ventilator) and PC-CMV (+ 27% for the anesthesia ventilator and + 26% for the ICU ventilator). This effect was not observed at the trachea where Ptr remained mostly unchanged. The obstruction generated by the FOB decreased PEF at the airway opening and trachea in VC-CMV and PC-CMV (all p < 0.05; tables 1-2). Figure 2 shows a representative example of the effect of the inserted FOB on pressure and flow tracings. Auto PEEP never exceeded 0.5 cm H 2 O in all conditions. The changes in pressure and flow induced by the FOB were associated with a significant decrease in Cdyn and increase in Raw measured at the airway opening in VC-CMV and PC-CMV. This effect was observed in the anesthesia and ICU ventilator alike. Cdyn showed no significant changes at the trachea whether the FOB was in place or not. Raw at the trachea, on the other hand, showed a small but significant increase with the FOB inserted only in the anesthesia ventilator (Tables 1 and 2     The effect of suctioning on airway pressure during protective mandatory ventilation was similar to the one observed in figure 3. In the anesthesia ventilator both, intermittent and continuous suctioning decreased end-expiratory pressures at the airway opening and at the trachea, from 10 to 0 cm H 2 O within a few breaths until the bellows of the anesthesia system emptied completely. Thereafter, with collapsed bellows, a negative pressure was immediately transmitted to the whole lung model reaching values of around -20 cm H 2 O. In the ICU ventilator, suctioning decreased pressures at the airway opening and the trachea, resulting in an end-expiratory pressure above 0 in PC-CMV and one close to zero or slightly negative in VC-CMV (Figure 3 and Table 3).
FOB suctioning performed during CPAP applied by the anesthesia ventilator decreased pressures at the airway opening and the trachea until the bellows of the anesthesia system deflated completely ( figure  4 and table 3

Discussion
This bench test demonstrated that using an anesthesia machine FOB suctioning decreased Ptr to sub-atmospheric levels during both, continuous mandatory ventilation and CPAP modes. An ICU ventilator used during suctioning maintained Ptr slightly supra-atmospheric in PC-CMV but not in VC-CMV. Only the latter was capable of maintaining desired CPAP levels at the airway opening and the trachea within ± 2.5 cm H 2 O during FOB suctioning.
Published data demonstrate that the effect of suctioning depends on the net balance between the suctioning outflow and ventilator's inspiratory inflow [18]. The pressure lost with VC-CMV and PC-CMV during FOB suctioning is an expiratory problem as the inspiratory valve is closed and the flow delivered by the ventilator is null [19].
CPAP, on the other hand, supplies gas flow any time airway pressure decreases thereby keeping airway pressures at the airway opening within the desired range. The principle of continuous gas insufflation during closed system suctioning in mechanically ventilated patients has previously been described [18,20,21]. Brochard et al., showed that a high O 2 flow insufflation via small capillaries molded in the wall of a modified ETT prevented a fall in FRC and arterial oxygenation during suctioning in mechanically ventilated patients [20]. Reissmann et al., described in animals another approach in which a modified double-lumen tube allowed the tracheal suctioning via one lumen while keeping alveolar volume and pressure by applying CPAP though the other [21]. Our results on Ptr during CPAP were quite similar to the above studies but having the advantage of not requiring modifications of both, ETT and ICU ventilator. Similar to Stenqvist et al., [18] we found that Ptr decreased less than 3 cm H 2 O of the set pressure during FOB suctioning under CPAP delivered by an ICU ventilator.   The anesthesia and critical care ventilator performed unequally during FOB suctioning. These differences are explained by the technical features of both machines and the objectives of these different devices. The anesthesia workstation uses a microprocessor-driven flow control valve like many ICU ventilators. This work station is built around a multi-breaths standing bellows placed in series with the other components of the circular system. Flow and pressure sensors placed at the inspiratory and expiratory limbs determine the total flow rate and compensate for any deviation from the desired VT. This Smart Vent system (GE Healthcare, Madison, WI, US) allows optimum ventilation for small babies all the way up to morbid obese patients. Although this machine can deliver inspiratory flows of approximately 120 L/min, such flows depend on the amount of gas stored within the internal volume of the circular system and the bellows. Once FOB suctioning empties the bellows this machine is no longer able to deliver the set VT or to maintain the CPAP level, which allows the negative pressure to be transmitted directly to the lungs figures 3 and 4. Thus, our data demonstrate that an anesthesia machine with a bag in the box ventilatory system can easily fail during bronchoscopy.
The Taema Extend is an ICU ventilator capable of generating inspiratory flows of up to 200 L/min. During suctioning in controlled modes of ventilation, inspiratory VT is still delivered despite a progressive decrease in the expiratory VT and PEEP. Even though the performance of this ventilator was superior to that of the anesthesia machine, the progressive loss of tracheal pressure emptied the artificial lungs "FRC". In the CPAP mode, however, this ICU ventilator maintained 10 cm H 2 O at the airway opening with a maximum drop in Ptr of 2.5 cm H 2 O. As the ventilator is controlled by the information received from the sensors placed within the device and not at the tracheal level the machine is not aware of the real Ptr. This technical limitation could be overcome simply by increasing the set CPAP by 2.5 cm H 2 O above the desired level of airway pressure (results not shown).

Limitations
We analyzed the performance of two standard anesthesia and ICU ventilators and, therefore, our results cannot be transferred to all ventilators in the market, which may have different technical and flow generation capabilities. We did not test different suctioning flows, ETTs or FOB sizes that would have change our results. Besides, this is a bench study where the effect of FOB suctioning on Ptr could be different than in real patients. For all these reasons the interpretation of our results should be done with caution.

Conclusions
During Fiber-optic Bronchoscopic suctioning both, standard anesthesia and ICU ventilators failed to maintain tracheal pressure with their continuous mandatory modes of ventilation. Only the ICU ventilator's CPAP mode maintains tracheal pressure.

Funding
Departmental funding.