Abstract
Abstract
Introduction: In the prehospital setting, Emergency Medical Services (EMS) professionals rely on providing positive pressure ventilation with a bag-valve-mask (BVM). Multiple emergency medicine and critical care studies have shown that lung-protective ventilation protocols reduce morbidity and mortality. Our primary objective was to determine if a group of EMS professionals could provide ventilations with a smaller BVM that would be sufficient to ventilate patients. Secondary objectives included 1) if the pediatric bag provided volumes similar to lung-protective ventilation in the hospital setting and 2) compare volumes provided to the patient depending on the type of airway (mask, King tube, and intubation). Methods: Using a patient simulator of a head and thorax that was able to record respiratory rate, tidal volume, peak pressure, and minute volume via a laptop computer, participants were asked to ventilate the simulator during six 1-minute ventilation tests. The first scenario was BVM ventilation with an oropharyngeal airway in place ventilating with both an adult- and pediatric-sized BVM, the second scenario had a supraglottic airway and both bags, and the third scenario had an endotracheal tube and both bags. Participants were enrolled in convenience manner while they were on-duty and the research staff was able to travel to their stations. Prior to enrolling, participants were not given any additional training on ventilation skills. Results: We enrolled 50 providers from a large, busy, urban fire-based EMS agency with 14.96 (SD = 9.92) mean years of experience. Only 1.5% of all breaths delivered with the pediatric BVM during the ventilation scenarios were below the recommended tidal volume. A greater percentage of breaths delivered in the recommended range occurred when the pediatric BVM was used (17.5% vs 5.1%, p < 0.001). Median volumes for each scenario were 570.5mL, 664.0mL, 663.0mL for the pediatric BMV and 796.0mL, 994.5mL, 981.5mL for the adult BVM. In all three categories of airway devices, the pediatric BVM provided lower median tidal volumes (p < 0.001). Conclusion: The study suggests that ventilating an adult patient is possible with a smaller, pediatric-sized BVM. The tidal volumes recorded with the pediatric BVM were more consistent with lung-protective ventilation volumes.
Introduction
Protective lung ventilation or small volume ventilation has been a growing concept in Intensive Care Units (ICU's) around the world. Lowering the tidal volumes has been shown to decrease mortality and Acute Respiratory Distress Syndrome (ARDS) in the intensive care unit (ICU) settings.1,2 The findings have been so remarkable, that currently there is a movement to start protective lung ventilation earlier in the patient's hospitalization, beginning in the Emergency Department.3 However, many resuscitations occur prior to arrival to the Emergency Department, where the benefits of small volume ventilation may be even more pronounced. High tidal volume ventilation may be associated with decreased venous return and coronary perfusion pressure, thereby reducing the return of spontaneous circulation (ROSC).4 High tidal volumes have also been associated with increased evidence of gastric insufflation, thereby increasing the risk for aspiration and vomiting.5
It has been noted that a minimal volume is needed to produce chest rise in individuals who are ventilated with BVM.6 In response to this finding, the Airway and Ventilation Management Working Group of the European Resuscitation Council now recommends a tidal volume of 500 mL for positive pressure ventilation of non-intubated cardiac arrest patients.7 This is significantly less than the American Heart Association, who still recommends 800–1200 mL of air in non-intubated patients.8
Most prehospital providers use self-inflating bag valve masks during resuscitations. The adult bag has an anticipated volume of delivery between 600–1000 mL. By comparison, the pediatric bags have an anticipated delivery volume of 450 mL, which is much more in line with the recommendations made by the European Resuscitation Council. The primary objective of this study is to evaluate if EMS professionals could provide ventilations with a pediatric BVM that would provide volumes that were appropriate to ventilate adult patients. Secondary objectives included: 1) if the pediatric bag provided volumes that were similar to lung-protective ventilation in the hospital setting and 2) a comparison of the volumes provided to the patient depending on the type of airway (mask, King tube, and intubation).
Study Design
To test our hypothesis, we created an observational study using a simulator that was designed to train providers in mask ventilation, basic and advanced airway placement, and positive pressure ventilation skills with a bag-valve-mask or mechanical ventilator. It digitally records the respiratory rate, tidal volume, peak pressure, and calculated minute volume over a predetermined period of time. IRB approval was obtained from Washington University School of Medicine. All participants were conveniently recruited and completed written consent forms.
Setting
The researchers, JS and MK, travelled to the various EMS and Fire stations from a large, busy, urban fire-based EMS agency and recruited on-duty and available personnel. All participants were recruited were one of three levels of providers: Certified First Responders or Emergency Medical Responders, Emergency Medical Technicians, and Paramedics. The department allows for EMTs and Paramedics to either work as a single function provider on the ambulance or to be dual trained and function as both a firefighter and EMS provider on either the ambulance or fire apparatus. Certified First Responders can only be firefighters on fire apparatus. Fire apparatus are requested on most high acuity calls based on Emergency Medical Dispatch or if there is an expected delay in ambulance arrival.
The participants were asked to ventilate the simulator with three different airways in place for 60 seconds each. Each airway was ventilated with both the Ambu® SPUR® II adult-sized BVM and Ambu® SPUR® II pediatric-sized BVM. According to the Ambu® booklet placed with the device in each of the disposable bags, the adult-sized BVM should provide 600 to 1000 mL of tidal volume per squeeze depending if one or two hands were used and the pediatric-sized BVM should deliver about 450 mL for a single hand squeeze. The airways used in order of sequence where an oropharyngeal airway (OPA), a King® LTS-D supraglottic airway (SGA) that is currently used in the participants’ EMS system, and finally a 7.5 mM endotracheal tube (ETT) placed at 22 cm in depth, measured at the maxillary teeth and the cuff inflated to 30 cm H2O each time. Each airway was used in conjunction with an adult BVM first followed by the pediatric BVM then transition to the next airway device set up. The order of airways used was OPA, SGA, and then ETT for each of the participants. The participants were instructed that each test was the same scenario, “an average-sized adult male patient that was apneic but had a pulse.” The participants were asked to ventilate the patient as they had been instructed to do so according to their treatment protocols from their medical director, 10–12 breaths per minute for adults with chest rise and fall. The participants were able to visualize chest rise and fall on the simulator but were blinded to the data being recorded on the laptop that was recording each breath. The scenarios were advanced in order without debriefing the participants until the completion of all six tests. Following completion, the participants were able to see a breakdown of their data as well as getting to ventilate the simulator and watch the instantaneous feedback of the tidal volumes based on the amount the BVM was squeezed.
Analytical Methods
All data were entered into a structured study-specific database (Microsoft Excel® 2016, Microsoft, Redmond, WA). Data analysis was completed utilizing IBM® SPSS® v. 22 (IBM Corp., Armonk, NY) for summary statistics of participant demographic data. We calculated the median tidal volumes as well as the 95% confidence intervals for each of the 6 scenarios. Considering a simulated patient of 70 kg, we evaluated the number of breaths delivered that fell within the tidal volume range of 6–8 mL/kg of ideal body weight (420 mL–560 mL) comparison between levels of volume delivered was analyzed using Pearson chi-square with Bonferroni method for adjusted p-values for multiple comparisons. Comparisons between BVM size with each intubation device were analyzed using Wilcoxon signed ranks test for non-parametric related samples. An a priori p value < 0.05 was set for statistical significance.
Results
Fifty providers completed each of the six ventilation scenarios. Participants’ demographics are shown in Table 1. Over all scenarios, a total of 4,489 breaths were delivered. With an OPA in place and using mask ventilation, the providers had median tidal volumes (mL) of 796 (705–871) and 570.5 (542–637) for the adult and pediatric BVMs, respectively (Table 2). Additionally, there was one participant that did not recorded any successful ventilations for the entire minute of their Peds BVM and OPA scenario only. Based on researcher feedback, it was determined this was the result of a poor mask seal and large air leak that went unnoticed and uncorrected by the participant and not a simulation failure. When a supraglottic airway was placed, the median tidal volumes were 994.5 (880–1148) and 664 (647–714). Finally, with an ETT, 981.5 (901–1085) and 663 (615–696). The median tidal volumes were found to be statistically significantly lower when the pediatric BVM was used with the OPA (Z = 6.03, p < 0.001, r = 0.85), SGA (Z = 5.87, p < 0.001, r = 0.83), and ETT (Z = 6.15, p < 0.001, r = 0.87) airway devices. The majority of the ventilations recorded were considered to be an excessive volume and out of range for the weight (70 kg of predicted body weight) of the simulated patient (Table 3). A statistically greater percentage of breaths fell in the appropriate volume range when the Peds BVM was use (p < 0.001).
Table 1. Participant demographics
Table 2. Median Tidal Volumes (mL) by BVM size and airway device
Discussion
European Resuscitation Council recommend a decrease in tidal volumes in part to decrease flow rate and minimize stomach insufflation as these factors have been associated with aspiration and increased peak pressures during CPR.9 Our results show that most EMS personnel ventilate with volumes that are considerably larger than the 500 mL that is currently recommended by the European Resuscitation Council. In some instances, the volumes provided with the adult BVM were between 2–3 times the recommended volumes. The pediatric BVM ventilated with volumes that would be considered much more appropriate. It should be noted that even with the pediatric BVM, the average volumes were above the recommended 500 cc.
Table 3. Distribution of breaths by tidal volume delivered based on what is ideal for a 70 kg patient (PBW)
These large volumes with ventilation can have deleterious effects on the resuscitation, such as increased risk for gastric insufflation causing regurgitation and aspiration. In past studies, the rate of aspiration in unsuccessful resuscitations was almost 30%.10 It should be noted that the risk for aspiration in this population may be even higher, as 46% of patients were noted to have full stomachs at the time of their resuscitation. It has also been noted in small animal and human studies that the volume of air that enters the stomach is much greater when larger volumes are used for ventilation.11 There have been very few studies evaluating the effect of using a pediatric bag on the amount of air in the stomach or aspiration risks. The few small studies that have been performed did show evidence of decreased air entry into the stomach when the pediatric BVM when compared to the adult BVM.12
Elevated peak pressures have also been theorized to contribute to the risk of aspiration.13 While there are no studies looking at the lower esophageal pressures in adult resuscitations, studies in the pig model note that alive pigs have esophageal pressures close to 20 mmHg, which decreases to less than 5 mmHg during resuscitation.14 Our study showed elevated peak pressures as high as 30 mmHg, which would easily surpass the lower esophageal pressures of these models and begin to fill the stomach. In our model, air was re-routed from the lungs to the stomach when pressure reached above 30 mmHg. This may have contributed to the witnessed inflation of the stomach (although never measured). In one case, the volume and peak pressures provided via the adult BVM during respirations was such that the stomach was blown off of the mannequin.
Our study evaluated ventilation volumes based on ideal body weight. However, there is an increasing number of patients who have BMI's much greater than ideal weight. While ideal ventilation volumes are typically based on ideal body weight, there has been some concern that higher pressures are required for ventilation. While it has been noted that obese patients who are intubated do have high peak and plateau pressures, there are no studies evaluating these pressures and the effect of ventilating with supraglottic airways.15 Interestingly, one small study did note that while increasing the volume of ventilation in obese patients did increase peak pressures, it did not increase arterial oxygen saturation (but did produce significant hypocapnia).16 Another study compared volume controlled with pressure controlled ventilation and noted that pressure controlled ventilation strategies decreased peak inspiratory pressures without affecting oxygenation, ventilation, or hemodynamics, implying that the lower peak pressures may be of particular benefit in this group.17 Based on these studies, one could extrapolate that low tidal volumes may be sufficient to ventilate high BMI patients.
It has also been shown in this study and in prior studies, that EMS providers have a tendency to ventilate at a higher rate during times of stress.18 The large volumes along with faster rates can contribute to breath stacking, reduced pre-load, and potentially reduced rates of ROSC.19 While there have been limited studies evaluating this phenomena in humans, over-ventilation in pigs was found to have decreased coronary perfusion pressures and survival.20 Despite studies showing harm from large volume ventilation, many of the EMS personnel who volunteered in the study had been trained to give large breaths for resuscitations. Even if EMS personnel were trained to attempt to deliver smaller volumes, it is difficult to assess how much volume is too much volume with the adult BVM. It has also been noted in past studies that ventilation management is even more difficult in real life than when under experimental conditions.21
In extremely ill patients, EMS resuscitations represents the first step in care. While there is no definitive link between prehospital ventilation strategies and ARDS, there are very few studies evaluating for such a link. It has been noted that several characteristics associated with ARDS, including large volumes, faster rates, over oxygenation and lack of positive end expiratory pressure (PEEP) are unfortunately characteristics of BVM ventilation.22 Only one study has commented on the risk of ARDS from pre-hospital ventilation, and they did not find a link between prehospital ventilation volumes and ARDS.23 However, this study was small, and mixed both ventilator-dependent and BVM patients. Even in patients where small volume ventilation settings were not used, it was extremely rare to have volumes above 1,000 mL. In our study and prior studies, the volumes associated with BVM use may be much larger than is typically used with a ventilator (up to 1,500 cc per breath), increasing the risk for ARDS. This is especially concerning as the time to onset of ARDS can be as soon as 5 hours after arrival, thus implying early actions can have large impacts on the course of ARDS.24,25 It is especially concerning that patients who are septic or intubated in the field are among the highest risk for developing ARDS.26
Despite the large number of resuscitations performed by EMS in the prehospital environment, there are surprisingly few studies reviewing volumes given for ventilation. We found only four studies that looked directly at comparing the pediatric BVM to the adult BVM. While prior studies have shown that the adult BVM ventilation has been noted to have extremely large volumes, the evidence for pediatric BVM use has been controversial. Two of the studies noted that the volumes given with a pediatric BVM was inadequate.27,28 One study noted that the pediatric studies were ideal, and much closer to the range suggested by the European Council.29 One study raised concern noting the pediatric BVM had difficulty maintaining oxygen saturations without the use of supplemental oxygen, however they did maintain adequate ventilation with the pediatric BVM.30 To address the difficulty in oxygenation, they designed a medium sized bag (max volume 1000 mL), which did have better oxygenation but gave higher volumes (average volume 624 mL).31
The volumes obtained with the pediatric BVM in our study were larger on average than the prior studies. In addition, all of these prior studies were performed with nurse anesthetists, anesthesiologists, or ICU nurses. To date, this is the only study comparing the adult BVM to the pediatric BVM using EMS personnel.
Limitations
Our study was designed to simulate realistic conditions whenever possible. Because the simulator used a bellows device for the lungs, exhaled carbon dioxide was not possible and we were not able to provide end tidal capnography to monitor their ventilation rates and signs of hyper-/hypo-ventilation. The participants could only rely on chest excursion to judge the quality of ventilation. Also, while the investigators could visualize a gastric inflation balloon, the model we used was limited in that it does not record gastric volumes. However, the manufacturers of the simulator did mark a volume of 250 mL on the balloon. Another limitation was the possibility of the Hawthorne Effect because the participants knew that they were being studied and two of the researchers were watching them ventilate the simulator during each test. Looking at the tidal volumes delivered, it does not appear that this was the case since the majority of breaths delivered had excessive tidal volumes. We also did not randomize the order of airway scenarios because time was a limitation since the participants were all on-duty and available for emergency responses.
Conclusion
This study indicates that use of a pediatric BVM with 3 different airway devices provides a significantly lower median tidal volume with only 1.5% of breaths under the recommended tidal volume for a 70 kg patient. Further research will need to be performed on human patients in the pre-hospital setting to determine if there is a measurable benefit comparing adult BVM to pediatric BVM. Given this data in addition to the prior anesthesia studies, there is clinical equipoise with regard to the size of the BVM.
| Number | Percentage | Mean (SD) | |
|---|---|---|---|
| Males | 45 | 90% | |
| Provider Levels | |||
| CFR | 12 | 24% | |
| EMT | 25 | 50% | |
| Paramedic | 13 | 26% | |
| Age (years) | 40.76 (10.45) | ||
| Experience (years) | 14.96 (9.92) |
| Scenario | Median (mL) | 95% CI (mL) |
|---|---|---|
| Adult BVM & OPA | 796.0 | 705–871 |
| Peds BVM & OPA | 570.5 | 542–637 |
| Adult BVM & SGA | 994.5 | 880–1148 |
| Peds BVM & SGA | 664.0 | 647–714 |
| Adult BVM & ETT | 981.5 | 901–1085 |
| Peds BVM & ETT | 663.0 | 615–696 |
| Tidal Volume Range of Breath Delivered | Adult BVM | Peds BVM | Total | |
|---|---|---|---|---|
| Volume Low (< 420 ml) | Count | 10 | 34 | 44 |
| Percentage | 0.5% | 1.5% | 1.0% | |
| Volume in Range (420 ml –560 ml) | Count | 112 | 405 | 517 |
| Percentage | 5.1% | 17.7% | 11.5% | |
| Volume High (> 560 ml) | Count | 2073 | 1855 | 3928 |
| Percentage | 94.4% | 80.9% | 87.5% | |
| Total | 2195 | 2294 | 4489 | |
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