Three dental chairs (#1: EOMαⅡ®; GC, Tokyo, Japan, #2: EOM-PLUS SS®; GC, Tokyo, Japan, #3: EOM ∑®; GC, Tokyo, Japan) were used in this study. #1 exterior has a severely curved backrest exterior, #2 and #3 have a flatter shape.
The study procedure was performed according to a previously established method in which the CPR manikin (Resusci Anne® Torso Basic version 2011; Laerdal Medical AS) was positioned on the reclined dental chair with the upper end of the manikin torso aligned with the top edge of the backrest (Fig. 1)1,2. The superior surface of the backrest under the lower half of the manikin’s sternum was positioned horizontally using a levelling instrument. The hand position for MCC was the center of the chest. A metal indicator (point P) was secured to the inferior surface of the dental-chair directly under the area for MCC and made parallel to the floor with a level gauge. The distance of point P relative to the inferior surface of the backrest remained fixed for the duration of the study. Displacement of point P was captured using video recordings and measured while each health care provider performed MCC on the resuscitation manikin. MCC depth was kept between 5.1 to 6.0 cm during the study. The actual MCC depth was evaluated with the manikin‘s Skill-Reporter system which has a green light that indicates MCC depths of 5.1 to 6.0 cm. Any compressions outside of that range (i.e., without the green light) were excluded. When compression depths were within 5.1 to 6.0 cm, the vertical displacement of the backrest from its initial position was recorded and included for analysis. Video data were transferred to a computer, and the backrest vertical displacement measurements were determined using the simultaneously captured ruler for reference. The stabilizing stool placed under the backrest of the dental chair for this study was round with a hard seating surface (diameter 30 cm; height 45 cm; FB-01ALLBK, Fuji Boeki Co., Ltd.). The superficial edge of the stool’s seat was set to vertically contact the backrest either just under the area for MCC or under the shoulders.
Three basic life support provider in American Heart Association (A: 47-year-old male, 175 cm, 93 kg; B: 44-year-old male, 177 cm, 60 kg; C: 44-year-old female, 157 cm, 50 kg.) were tested at their convenience. Each study participant individually performed 10 contiguous rounds of MCC (20 compressions per round; 200 compressions total) at a pace of 100 compressions per minute in synchrony with a metronome for each of the 3 stool configurations (i.e., under MCC area, under shoulders, or no stool). A total of 600 chest compressions 5.1 to 6.0 cm in depth were recorded per participant.
Statistical analysis. The programming language R (version 4.0.2; The Comprehensive R Archive Network, USA) was used for statistical analysis. Displacement measurements (maximum distance of Point P from baseline during MCC) by three participants were analyzed separately at each of the three respective positional configurations. The change in vertical displacement for each of the two stool positions (under MCC or under the shoulders) compared with no stool position (baseline) was calculated using the following equation *
Data sets were analyzed using the Shapiro-Wilk test to determine normality of distribution. The non-parametric date sets were then analyzed using the Wilcoxon rank sum test to determine statistical significance (P < 0.001).