Introduction

Bag ventilation is sometimes provided to intubated patients as a short-duration alternative to mechanical ventilation [1]. As during intra-hospital transport, manual ventilation aims at mirroring as closely as possible the ventilator's parameters to maintain adequate respiratory exchanges and hemodynamic stability; however, the disconnection from the ventilator induces the loss of respiratory monitoring, manual ventilation then being provided without knowing either the pressures or the tidal volumes that are generated. Despite the risks of barotrauma described in the literature [13], poor attention is usually given to the bag ventilation patterns that differ among clinicians [1].

By squeezing the bag too hard, the operator can create higher airway pressures [2], higher tidal volumes [4] and higher respiratory rates than those generated by a correctly set ventilator. This increases the risk of barotrauma, pulmonary hyperinflation with hemodynamic instability, and alveolar hyperventilation with respiratory alkalosis [1, 2, 49]. Different measures, such as restriction of the bag volume [10] or pressure manometer mounted on the bag, have been proposed to decrease the risk of excessive airway pressures [1, 2]. Limitation of inspiratory flow rate is another approach. The Smart Bag (volume 1700 ml, 0-Two Medical Technologies Inc., Mississauga, ON, Canada) is a bag device equipped with a valve that, in response to the pressure exerted by the operator on the bag (Fig. 1), limits the inspiratory gas flow to approximately 40 l/min as opposed to 120 l/min obtained with a Standard bag [9, 11]. Several studies have shown, during simulated ventilation of non-intubated patients, that the Smart Bag reduces the airway pressures and decreases the incidence of stomach inflation [10, 12]. No study has been performed in intubated patients, in whom the cuff of the endotracheal tube increases the risk of excessive airway pressures.

Fig. 1
figure 1

The Smart Bag (0-Two Medical Technologies Inc., Mississauga, ON, Canada) is a bag device equipped with a valve that limits the inspiratory gas flow, in response to the pressure exerted by the operator on the bag. This response is proportional to the squeeze. If the operator squeezes the balloon too hard, the valve closes to limit the flow rate

The goal of our study was to analyze, in sedated and intubated patients without spontaneous respiratory activity, the respiratory and hemodynamic changes induced by manual ventilation with the Smart Bag and a Standard bag-valve device (Standard).

Materials and methods

This study was performed with approval of our local ethics committee and with the informed assent of eight patients (mean age 69 ± 6 years), obtained the day before admission in intensive care for a scheduled non-emergent coronary surgery. Twenty-four operators (intensive care workers) participated in this study. Three operators were allocated per patient. Each operator provided two sessions of manual ventilation in a precise chronological order, with the aim of closely maintaining adequate respiratory exchanges. During the first session the Standard bag was used (volume 1600 ml, Laerdal Silicone Resuscitator, Laerdal Medical AS, Stavanger, Norway) and the Smart Bag during the second. Each session lasted 5 min. Between sessions, the patient was connected to the ventilator until return of hemodynamic and respiratory parameters to the baseline values. Operators were not familiar with the Smart Bag and used it for the first time during the study. No recommendation was given; nevertheless, they were advised to squeeze the balloon less strongly if the resistance to flow was high.

Patients were sedated by an intravenous administration of propofol (1mg/kg h−1) and paralyzed with cisatracurium (0.15 mg/kg). Through an endotracheal tube, they were mechanically ventilated in pressure-control mode with an inspired fraction of oxygen of 1. The study started after stabilization of the patient's hemodynamic and respiratory conditions. Patients who required inotropic or vasopressor agents and/or those with severe lung dysfunction or chronic pulmonary diseases were excluded.

A compact monitor CS/3 with a gas analyzer M-COVX (Datex-Ohmeda GE Healthcare, Helsinki, Finland) was directly connected to the endotracheal tube after calibration with a 5% volume of CO2 calibrating gas. During the study, 13 respiratory parameters were measured and were recorded every 10 s on a computer by an acquisition program (S/5 Collect, version 3.0, Datex-Ohmeda).

A PiCCO plus monitor (Pulsion Medical Systems, Munich, Germany) measured simultaneously eight hemodynamic parameters. The calibration was performed according to the recommendations of the manufacturer, using a thermistor-tipped arterial catheter inserted into the right brachial artery (Pulsiocath-PV2014L22, Pulsion Medical Systems). A data-acquisition software (PiCCOwin; Pulsion Medical Systems) collected and stored the data every 12 son a computer. A paired Student's t-test was performed for statistical analysis. All values were expressed as mean ± standard deviation. Differences were considered statistically significant when p < 0.05.

Results

The results are presented in Table 1. The respiratory and hemodynamic parameters were similar during each session of mechanical ventilation, except for the end-tidal CO2 (ETCO2) which was lower just before the Smart Bag than the Standard. Compared with Standard, the inspiratory flow (Fig. 2a), the airway pressure (Fig. 2b), and the tidal volume decreased with the Smart Bag. The inspiratory time was higher, whereas the expiratory time was not different. Finally, the respiratory rate and the minute volume decreased, resulting in a significant rise in ETCO2.

Fig. 2
figure 2

Curve of respiratory flow (a) and airway pressure (b) in manual ventilation recorded during the study. a Compared with the Standard bag-valve device, the Smart Bag limits the inspiratory flow around 40 l/min (plateau aspect). b By limiting the inspiratory flow, the Smart Bag allows a reduction in the peak pressure as well as an increase in the inspiratory time and a decrease in the respiratory rate

Table 1 Respiratory and hemodynamic parameters according to chronological session. NS not significant

The change from mechanical to manual ventilation with the Standard and the Smart Bag allowed an improvement of stroke volume index and pulse contour cardiac index with a decrease of stroke volume variation. Hemodynamic state during the Smart Bag and the Standard was considered as clinically similar, although the mean arterial pressure was significantly higher with the Smart Bag.

Discussion

In some situations, such as during intra-hospital transport, manual ventilation is performed in intubated patients of intensive care, in order to temporarily replace the ventilator. The patient is then disconnected from any respiratory monitoring and the operator can unintentionally generate excessive airway pressures, tidal volumes, and respiratory rates that are sometimes deleterious to the patient.

Clarke et al. observed in patients manually ventilated, peak pressures of 51.5 ± 7.6 cm H2O and tidal volumes of 1120 ± 274 ml [4]. Turki et al. measured peak pressures as high as 100 cm H2O during manual ventilation of a lung model [2]. Although peak pressure is a poor indicator of the risks of barotrauma, such levels are certainly too high [1].

With the aim of limiting airway pressures, some strategies using smaller bag volume or pressure manometers have been previously implemented. Limitation of inspiratory gas flow is another method proposed with the Smart Bag, which incorporates a valve that limits the flow rate near 40 l/min [1, 2, 10, 11]. In 60 patients undergoing routine induction of anesthesia, Wagner-Berger et al. observed different flows from up to 120 l/min with the Standard bag against approximately 40 l/min with the Smart Bag [11]. We also observed a significant decrease of the average inspiratory flow with the Smart Bag (Fig. 2a).

In our study, compared to the Standard and whereas the airway resistances were similar, the limitation of inspiratory flow allowed the Smart Bag to significantly decrease the peak and mean airway pressures as well as the tidal volumes. Interestingly, the inspiratory flow and the tidal volumes measured with the Smart Bag were close to those observed during mechanical ventilation.

High respiratory rates and tidal volumes increase also the risk of respiratory alkalosis, by alveolar hyperventilation. In 28 patients requiring transport, Hurst et al. compared manual ventilation and ventilation by a transport ventilator. After manual ventilation, all patients presented a marked respiratory alkalosis [9]. Like von Goedecke [12], we observed in our study that the Smart Bag's valve induced higher inspiratory time than the Standard, while the expiratory time was identical, allowing a decrease of the respiratory rate. By decreasing the respiratory rate and the tidal volume, the Smart Bag reduced minute ventilation. Consequently, the ETCO2 was higher with the Smart Bag than with the Standard, although baseline ETCO2 was significantly lower before Smart Bag. This difference in baseline values can be explained by a significant decrease of ETCO2 during manual ventilation with the Standard, and duration of mechanical ventilation too short to allow a return to the previous level. Although arterial blood gases were not analyzed, the levels of ETCO2 let us assume that the Smart Bag can decrease the risk of severe respiratory alkalosis.

Pulmonary hyperinflation, resulting from an impairment to the expiration and promoted by the use of high tidal volumes and high respiratory rates, can also be observed during inappropriate manual ventilation, and can generate circulatory complications [1, 58]. In our study, the change from mechanical to manual ventilation allowed higher stroke volume and higher cardiac output, probably with a better venous return to heart. The stroke volume variation indeed decreased, whereas heart rate and dPmax were similar. The significantly lower end-expiratory pressures could explain this improvement.

Study limitation

Limitation of the inspiratory flow rate by the Smart Bag allows lower inspiratory pressures. Nevertheless, in some situations, such as the bronchospasm, higher insufflation pressures are required and flow limitation can be inappropriate. It would be interesting to evaluate the Smart Bag under these conditions.

Conclusion

When manual ventilation is provided to maintain adequate gas exchanges in intubated patients, the Smart Bag® generates lower airway pressures and lower tidal volumes than the Standard bag. It also decreases the respiratory rate and the fall of the ETCO2. No hemodynamic alteration is observed among patients hemodynamically stable and deprived of respiratory disorder.