Metronome Use Positively Affect Cardiopulmonary Resuscitation Parameters: Prospective Observational Multicenter Manikin Study

2.1 Study design

This was a multi-center, prospective, randomized, crossover, manikin-based, two-rescuer CPR simulation study. The study was conducted between October 21, 2015, and December 1, 2015, in the emergency medicine departments of Izmir Tepecik Education and Research Hospital (Center 1), Ege University Hospital (Center 2), and 9 Eylul University Hospital (Center 3). Each of these departments receive about 150-200 adult out-of-hospital cardiac arrest cases annually. Before starting the study, ethical approval was obtained from the local ethics committee (Tepecik Educational and Research Hospital Local Ethical Committee, Izmir; June 30${}^{\mathrm{th}}$, 2015 - No:5).

2.2 Equipment

The AMBU${}^{\mathrm{\circledR }}$ Adult Advanced Life Support (ALS) Training Manikin was used to assess the CPR performance of participants. Using the “StudioOne 3.3.3.41198 Win x64” program, a melodic rhythm consisting of two different sounds for compression and ventilation was created. The rhythm was determined based on the 2015 AHACPR guidelines, with a compression rate of 110/min and a ventilation rate of 10 breaths/min [3]. (Appendix: Metronome guidance rhythm audio file)

2.3 Study setting

In order to standardize ventilation during CPR, all manikins were intubated with a size 8 Portex${}^{\mathrm{\circledR }}$ endotracheal intubation tube. The endotracheal tube was taped at 22 cm. A standard bag-valve device (AMBU${}^{\mathrm{\circledR }}$1.6 L) was used for ventilation. The manikin was placed on a CPR board on a stretcher. The height of the stretcher was adjusted according to each participant. In order to create a realistic simulation, the chest stiffness of the manikin was adjusted to the highest level.

The periodic ventilation and chest compression skills of each participant were recorded using the built-in software feature of the manikin (AMBU${}^{\mathrm{\circledR }}$manikin software). This software enables saving each compression and ventilation data derived from the manikin so that they can be assessed individually. All manikins were calibrated prior to each group. The fatigue levels of participants were evaluated using the 10-step “Modified Borg Fatigue Scale” (MBFS) [19, 20]. The evaluation was repeated at the end of each 2-minute cycle.

2.4 Selection of participants

Emergency medicine residents with more than 6 months of professional experience who had previously received training on basic and advanced cardiac life support and who volunteered to participate in the study were recruited. The simulation experiments were carried out in three different centers. In order to render the conditions similar, the experiments were conducted in the resuscitation unit of each center. The participants were divided into groups of two. Each participant performed 2 sets of CPR: 8 minutes of conventional CPR and 8 minutes of metronome-guided CPR. The 8 minute-duration was chosen to create a moderate level of fatigue.

The participants switched places every 2 minutes during the 8-minute CPR. After conventional CPR was complete, a 30-minute break was given before metronome-guided CPR in order to avoid fatigue. In this break, the audible metronome melody was introduced to the participants.

2.5 Choosing the reference standard

In this simulation CPR study, the parameters recommended by the AHA CPR and ECC Guidelines were used as the reference standard. Thus, a compression rate of 110/min, compression depth of 50-60 mm, ventilation rate of 10/min, tidal volume of 0.5-0.6 L, and minute ventilation of 1.5-6 L/min, with the hands positioned on the lower half of the sternum were deemed acceptable.

A shallow compression was defined as a chest compression depth of less than 50 mm, and a deep compression was defined as a chest compression depth of greater than 60 mm. A ventilation with a tidal volume of less than 0.5 L was defined as a low-volume ventilation, and a ventilation with a tidal volume of greater than 0.6 L was defined as a high-volume ventilation. Allowing the chest to return to its initial (static) state after each chest compression was defined as “full chest recoil”.

The data on compression rate, compression depth, full chest recoil, ventilation rate, mean tidal volume, mean minute ventilation and incorrect hand position were recorded by the manikin software program as graphics and mean values.

2.6 Statistical analysis

All statistical analyses were performed using the Statistical Package for Social Sciences® (SSPS) software (IBM SPSS for Windows, Version 22, SPSS Inc., USA). The distribution of the data was analyzed using the Shapiro-Wilk Normality test. The data were not normally distributed, so the Wilcoxon Signed Ranks test was used for comparison. Quantitative variables were presented as median and interquartile range (IQR), and qualitative variables were expressed as numbers and percentages. Based on the mean compression rate values in secured airway scenario reported by Kern et al, the sample size was calculated to be 28 participants with a 95% confidence interval and 80% power [7]. Considering the margin of error, 30 participants were included. All analyses were performed within a 95% confidence interval and a p-value $<$ 0.05 was considered statistically significant.

4.1 Ventilation rate and volume

In their studies on the effect of metronome use on ventilation during CPR, Kern and Chung reported that metronome use prevented hyperventilation by keeping the ventilation rate within the range recommended in the guidelines [7, 18].

Na et al. [16], adopted to the 30:2 compression to ventilation ratio, performed ventilations according to different compression rates (e.g. 100, 110, 120 compressions/min). They measured the mean airway pressure and tidal volume in order to assess the quality of ventilation. They reported that as the speed of the metronome rhythm increased, the quality of ventilation decreased. In addition, they recommended a chest compression rate of 100/min in case of a respiratory etiology [16].

Both in the out-of-hospital study by Aufderheide [5] and in the in-hospital study by Milander et al. [11], trained rescuers were found to perform significant hyperventilation which compromised hemodynamic and pulmonary functions and decreased the survival rates.

As stated by Aufderheide et al. [5], the impact of hyperventilation on survival is not related to a decrease in the coronary perfusion pressure caused by hypocapnia. The physiological outcomes of hyperventilation are an increase in positive intrathoracic pressure during the decompression phase of CPR, a decrease in cardiac preload and output, and a reduction in right ventricular function. An increase in the tidal volume is also known to affect cardiac output negatively. This whole process leads to a decrease in survival rates [5].

The above-mentioned studies have emphasized that despite all the training they received, trained rescuers still hyperventilated patients and that the training programs were insufficient in preventing hyperventilation [5, 11].

The present study is the first objective simulation study performed using a metronome with different sound alerts for compression and ventilation. It differs from previous studies in that ventilation was simulated through an advanced airway. The metronome model used in our study succeeded in ensuring that the ventilation rate is within the range recommended in the guidelines. This way, ventilation rate was lowered without causing a change in the tidal volume, resulting in a decrease in minute ventilation and in hyperventilation. This seems to be an effective solution for hyperventilation which has a negative effect on survival as pointed out by Aufderheide et al.

Even though the metronome-guided CPR model used in our study managed to yield minute ventilation values close to the ones recommended in the guidelines, these values are still slightly above the target. This stems from the fact that the mean ventilation volume was high in both groups. The high ventilation volume was attributed to the fact that ventilation was performed by the same participant using the same bag-valve device.

4.2 Fatigue

Chung [18] and Jantti [6] reported that metronome use had no effect on fatigue levels. When the CPR duration was set at 8 minutes to induce a moderate level of fatigue, it was observed that metronome use could decrease the fatigue levels of rescuers, even if the chest wall stiffness of the manikin was adjusted to the highest level. This was attributed to the fact that compression rate, compression depth, and ventilation rate were higher in the conventional CPR group compared to the metronome-guided CPR group, which caused the rescuers in the conventional CPR group to exert more effort.