The bag–valve–mask (BVM) ventilator is an essential device for basic life support (BLS), but effective BVM ventilation requires adequate training and frequent practice because it is a difficult skill to maintain.1 2 The optimal tidal volume during cardiopulmonary resuscitation (CPR) is not known,3 but adequate ventilation can be maintained with a lower tidal volume than normal.4 5 Excessive ventilation produces several complications,6 so an accurate volume of ventilation is mandated. The recommendation is to deliver each rescue ventilation over 1 s, give a sufficient tidal volume by BVM to produce a visible chest rise. An adequate volume can be delivered by squeezing a 1 litre adult bag approximately one-half to two-thirds of its volume or a 2 litre adult bag approximately one-third of its volume.7 However, both the operator and observer cannot be certain of the amount of tidal volume delivered, because it varies by the operator and squeeze method.8 9 We believe the problem arises from the inaccuracy of the bag of the manual resuscitator itself. We used an airtight sealed circuit free from mask fitting and gas leakage to observe how accurately the manual resuscitator can deliver.

METHODS

A total of 114 individuals educated in BLS and BVM participated in this study (table 1).

Education and experience level were described by a five-step Likert scale: (1) BLS education; (2) BVM education; (3) BVM experience with manikin; (4) BVM experience with human; (5) CPR experience. Physical characteristics were obtained by one observer. The size of the subject’s hand was obtained by having the subject place his hand flat on a board: two dimensions—from the middle edge of the palm to the distal thumb, from the middle fingertip to the distal skin crease at the wrist—were measured. The grip strength of both the subject’s hands was measured by using the baseline hydraulic hand dynanometer (Nexgen Ergonomics, USA). Tidal volume was generated by subjects using one and two-handed compression of the manual resuscitator (Solco resuscitator, Korea; adult, 1.6 litres) and was measured through the use of a microspirometer (Micromedical, England), which was connected by an endotracheal tube (fig 1).

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Figure 1 Measuring tidal volume. The manual resuscitator was connected to the microspirometer by an endotracheal tube with an airtight seal.

Subjects completed three procedures: one-handed, two-handed and two-handed half-compression. The objective of one-handed and two-handed half-compression was to deliver half the volume of the bag. Each ventilation was carried out for 1 s duration and the ventilation rate was 10 per minute according to the American Heart Association guidelines 2005.7 The participants were encouraged to rest every minute to avoid fatigue. Study results were entered into a database and analysed using SPSS 13.0. The Kruskal–Wallis H test was used for comparison between compression methods and Spearman’s correlation was used to determine the correlation between personal characteristics and tidal volume.

RESULTS

There were significant differences and wide variations between the squeezing methods. Tidal volume delivered by the two-handed method was significantly greater than that delivered by the one-handed method. Tidal volume delivered by two-handed half-compression was significantly less than the other methods (table 2).

There were no gender differences in any of the procedures. The physical aspects, including age, hand width, hand height, hand volume and grip power had no correlation with the volume delivered (table 3).

There was a slight increase in tidal volume by education and practice levels, but correlation was weak (table 4).

DISCUSSION

The manual resuscitator is frequently used in most emergency situations including CPR and during transport of the patient. Although many experts consider endotracheal intubation to be the optimal technique to secure airway protection, rates of success and complications vary widely.2 Recent evidence suggests that BVM ventilation may offer an acceptable alternative for airway stabilisation in out-of-hospital settings.1 When compressing the BVM, the two-handed compression technique was better than the one-handed. “Better” means first, less leakage by an effective seal of the mask to the face and second an increase in the tidal volume delivered.10 The effectiveness of two-handed compression was confirmed in terms of the greater mask pressure that it produced,11 but mask pressure is not the only factor influencing the tidal volume delivered. Two-handed compression provided higher tidal volumes per weight than one-handed compression, while generating peak pressures within an acceptable range in the infant and child manikin model.12 Little is known about the adequate tidal volume during CPR, but there is some evidence on oxygenation and ventilation. Excessive ventilation is unnecessary and harmful, because it increases intrathoracic pressure, decreases venous return to the heart and diminishes cardiac output.6 Excessive breaths induced by excessive ventilation may cause gastric inflation and from it complications.13 Smaller administered tidal volumes decrease the risk of stomach inflation or increases in intrathoracic pressure, whereas supplementary oxygen ensures the maintenance of oxygenation,14 15 but decreased tidal volumes are not always beneficial for oxygenation and ventilation. Without additional oxygen during BVM ventilation, over 11 ml/kg was able to maintain both sufficient oxygenation and carbon dioxide elimination.16 Aiming for an estimated ventilation of 10 ml/kg tidal volume might be expected to achieve normocarbia during CPR.3 In an observational study, trained BLS providers were able to detect “adequate” chest rise in anaesthetized, intubated and paralysed adult patients when a tidal volume of approximately 400 ml was delivered.17 Although it is not clear how much tidal volume is adequate, it is clear that both small and large tidal volumes are not good for resuscitation. BVM ventilation is a difficult skill that requires considerable practice and education.18 It has been challenged by alternative ventilating devices such as the pocket mask,19 laryngeal mask and combitube,1 20 laryngeal tube21 and mechanical ventilators.22 Compared with BVM, all other devices show relatively good ventilation performance, with more tidal volume, low gastric inflation and gas leakage, but these devices are not always readily available.1 Advanced airway device-related complications were more significantly related to training than the airway devices themselves.23 Although many emergency personnel are familiar with the use of invasive airway devices, in most apnoeic patients the fastest, easy to access, cost beneficial and most successful strategy is BVM ventilation.1 2 Unfortunately, BVM, especially the manual resuscitator, is not an accurate device to use. The tidal volume delivered differs widely by the squeezing method, individual characteristics and brands of BVM.8 9 The results of this study are consistent with previous similar studies;11 the tidal volume delivered by the two-handed method was significantly greater than that delivered by the one-handed method (Spearman correlation 0.398, p<0.01). Two-handed half-compression is not evidence based but based upon expert opinion and is believed to deliver a certain amount of tidal volume. There are too many resuscitators commercially available. In general, the capacity of an adult manual resuscitator is approximately 1–2 litres. If a doctor wants to give 500–600 ml of tidal volume to a patient, theoretically this volume can be delivered by squeezing a 1 litre bag approximately one-half to two-thirds of its volume or a 2 litre bag approximately one-third of its volume. In our study setting, two-handed half-compression of the 1.6 litre adult bag gave a significantly smaller tidal volume than the one-handed and two-handed compression method. We offer two possible explanations. First, it is difficult to achieve a half squeeze manually. Second, the meaning of half is misunderstood by the operators. The shape of the manual resuscitator is oval and so it would be difficult with a half squeeze method to produce half tidal volume. According to the previous literature, individual characteristics such as hand size8 and grip power9 influence the tidal volume delivered, but in our results, there were no significant differences in tidal volume between men and women. In addition, physical characteristics such as hand width, hand height, hand volume and grip power did not significantly influence the tidal volume delivered. All participants in this study were instructed to deliver each compression over 1 s, but previous studies do not mention any time limitations. We think that the time limit, 1 s, was not enough to show a difference among the individual characteristics with tidal volume. Many experts believe that education and practice are essential elements for the maintenance of adequate BVM skills.1 12 In our study, there was a slight increase in tidal volume by education and practice levels, but the correlation was weak. The importance of mastering the technique of BVM ventilation cannot be overemphasised. It is no longer viewed merely as a temporary measure, especially in out-of-hospital settings and urgent situations before advanced airways are in place, but the effect of education and practice are unclear. It remains uncertain how we can teach the BVM technique exactly, and how we can maintain this skill. Furthermore, increasing tidal volume is an important issue because of the leakage and stomach inflation, which would then require more tidal volume to be delivered.24 When an advanced airway is in place, an adequate tidal volume is more important than a large tidal volume. Our results show that two-handed compression can produce more tidal volume than one-handed compression and two-handed half-compression does not deliver sufficient tidal volume. According to the 2005 guidelines, during adult CPR, tidal volumes of approximately 500–600 ml (6–7 ml/kg) should suffice,7 17 but the rescuer cannot estimate the exact tidal volume. Therefore this guideline is only useful in the setting of automatic transport ventilators and as a reference for manikin manufacturers. Some study limitations should be noted. First, 43% of our participants were students, so they had relatively little clinical experience other than education. Second, this study was a simulated situation using a portable spirometer, thus normal physiological aspects such as airway resistance and lung compliance were not apparent.

CONCLUSIONS

The tidal volume delivered by the manual resuscitator shows large variations. There were significant differences in volume delivered by different compression methods, but physical characteristics are not a predictor of tidal volume delivery. Therefore, the manual resuscitator is not a suitable device for accurate ventilation. Practice and education may increase the tidal volume delivered.