SCD is the most common cause of death worldwide. It affects about 350,000 to 700,000 people in Europe annually and its incidence is expected to rise in the coming years [18].

Although the SCA incidence at schools accounts for only 2.6% of all public location SCAs [19], a trained student could witness a medical emergency that requires CPR in any location. According to the Teenmark Survey 2003 [20], American teenagers (12-17 years of age) spend a good amount of their free time in shopping malls. Becker et al. reported that large shopping malls are public locations with a high incidence of cardiac arrests (ten in five years) [21]. 35 and 11 cardiac arrests per site were registered in international airports and public sports venues, respectively, over five years [21].

Earlier studies have shown that even nine-year-old school children have the cognitive skills to perform CPR after specific training [22, 23]. The limiting factor for these children to perform adult CPR correctly is their body mass. Students who can attain a compression depth of at least 4 cm can resuscitate other children [24]. In accordance with the science advisory of the American Heart Association [25], we chose giving courses to 9th graders. Younger students might have been discouraged or disinterested in CPR, resulting from a physical incapability to deliver high-quality CPR.

We found that the teenagers' baseline BLS skills were poor when they entered the training. We saw significant improvement after our theoretical and practical training course. BLS skills after four months were at nearly the same level as directly after training (Figure 1).

Figure 1 lllustration of the overall basic life support performance of teenagers. Full size image

The psychological phenomenon of the "bystander effect" occurs when someone witnesses an emergency but does not help the victim because other people are present. It is often observed in emergencies [26]. Students who feel competent and understand the concept of "diffusion of responsibility" will also be more likely to help [27–29]. Furthermore, Roppolo et al. stated that early CPR training contributes to better retention rates for subsequent courses [30]. We also hope to decrease the most common fears associated with CPR. Explaining the theoretical background on SCA makes students understand the necessity of providing immediate CPR and reduces the reluctance to help in a critical situation.

BLS and AED training in schools should emphasize the recognition of an emergency and the provision of high-quality chest compressions [31, 32]. Our BLS training also focused on these two aspects. We stressed the concept of agonal breathing (gasping) because it is important that students do not mistake this as normal breathing, which would prevent them from starting CPR. If adolescents know how to identify a dangerous situation, they can interpret a situation as an emergency and provide at least indirect help by calling the ambulance, for example.

Concerning AED usage, our main goal was to explain its purpose to the teenagers. Ventricular arrhythmias are typically the cause of sudden cardiac arrest and are more common than severe bradycardia or pulseless electrical activity [18]. For each minute that passes before defibrillation, the chance of survival is reduced by about 10-12% [33]. We did not assess the correct deployment of an AED in the control sessions because earlier studies showed that even elementary school students already have the cognitive skills to correctly apply an AED [34].

The depth, rate and interruption duration of the compressions directly influence the outcome of cardiac arrest [35, 36]. Those quality-determining factors are best learned through practice [37]. Although other studies advocate the use of video-based self-training kits to reach a larger number of people and reduce costs [38, 39] we favored hands-on training within small groups led by skilled instructors, who can give individual feedback and advice.

We did not use the Skillmaster Manikin for the first assessment of compressions because we saw that the vast majority of students in pre-study testing did not perform correct CPR. Therefore, we thought that the expensive manikin would not be advantageous in the pre-training examination because it would yield limited, if any, useful data.

Putting the patient in a stable recovery position was only assessed in the pre-training session. After the training, all students correctly realized that the fake scenario required CPR; consequently, the item had become irrelevant.

There were two unexpected results in our study. First, we found that fewer students than before the training called for help in the post-training assessment. This may be due to the fact that students were focused on performing CPR. We strongly emphasized the importance of chest compressions in the training. Before the training, most students were unsure of how to act in an emergency and reacted "naturally" by calling for help. We assume that even a trained student would become insecure in a real emergency and call for help before starting CPR.

Second, fewer students reported confidence in putting the patient in a stable recovery position after the training than before, which we found to be astonishing. Despite, or perhaps because of the hands-on practice of the stable recovery position recommended by the AHA, it seems that students perceived the position as a very complex task. This needs to be considered and prevented in future training sessions.

Students that had already received first aid training in primary school did not perform better in the pre-training than their peers. Interviewing the responsible persons showed that the training had focused on the social responsibility of helping others. There was hands-on training for the stable recovery position and bandaging. CPR was only briefly demonstrated.

Educating school children about BLS is an excellent strategy to reach a broad public and increases the percentage of trained adults in a community [31]. If students share the acquired knowledge with their families and non-trained friends, we indirectly introduce a larger part of the community to the topic of BLS and AED deployment [30]. This raises the likelihood that a trained person will be present at an SCA event scene to perform immediate bystander CPR.

Limitations

The high schools that participated offer only honors classes. Students at those schools are considered to have a readiness of mind and good comprehension. In addition, the schools are situated in a middle-class neighborhood. These two factors may lead to a selection bias for the learning effect and retention capacities.

Absence of students due to scheduling issues, as well as class turnover and organizational difficulties on the schools' side, led to a reduction of the data at the four-months' evaluation (Table 1, group 5).

Due to a technical problem, post-training chest compression data of 26 students were incomplete.

The BOscore is not yet validated, because it was specifically designed for this study and was used here for the first time.