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Compression characteristics of CPR manikins
RESUSCITATION
ELSEVIER
Resuscitation 30 ( 1995) 1I7- 126
Compression characteristics of CPR manikins Michael Angelicus Baubin* a, Hermann Gillyb, Alexander Poscha, Adolf Schinnerl”, Gunnar Anton Kroesena aDepartment of Anaesthesia and Intensive Care Medicine, Institute for Emergency and Disaster Medicine, The Leopold-FranzensUniversity of Innsbruck, Anichstra$e 35, A-6020, Innsbruck, Austria bL. Bollzmann Institute of Experimental Anaesthesiology and Research in Inlensive Care Medicine, Spitalgasse 23, A-1090. Vienna, Austria
Received 2 February 1995; revision received 12 May 1995;accepted 16 May 1995
AbStraCt We evaluated the force-depth compression characteristics of 8 different CPR manikins during mechanical cardiopulmonary resuscitation by a thumper. The force required to compressthe manikin’s thorax of 1, 2, 3, 4 and 5 cm was measured.It ranged between 6.3 and 14 kp at a depth of 1 cm, 11.6-30 kp at 2 cm, 17-38 kp at 3 cm, 22.5-54 kp at 4 cm and 28.5-69 kp at 5 cm. The manikins with a spring in the thorax (Ambu Man, Ambu MultiMan, Drager CPR-Max, Laerdal Resusci Anne) as well as one without (Ambu CPR Pal) showed a rather linear relationship between depth and force required to compressthe chest. Ambu Man, set at ‘High’, Laerdal ResusciAnne and Drlger CPR-Max revealed a slight increase in resistance, whereas 2 manikins without a spring (Laerdal Little Anne, Laerdal Family Trainer) and 1 manikin with a plastic spring-like construction (Actar 911) exhibited less resistance with increasing depth. According to our results, the manikins are not uniform in their compression characteristics; somebecomenonlinear when 3 cm of compression is exceeded.For correct CPR it is of utmost importance that the CPR trainee learns to compressin a sufficiently strong manner, but simultaneously to avoid an exceedingly high depth of compression irrespective of the thorax resistance.In order to prepare the CPR student for the varying chest resistancesof the human body, we recommend to train CPR on manikins with different chest resistances. Keywords:
Cardiopulmonary resuscitation; Training manikins; Compression characteristics
1. Introduction Since the development and publication of the combined cardiopulmonary resuscitation by Kouwenhoven et al. [l] in 1960, various training manikins for CPR have been developed. Dependl
Corresponding author.
ing on the particular design, their thorax resistance differs widely. It is to be assumed that manikins with a built-in spring show a linear relationship between depth and force of compression. Such a linear characteristic reflects the findings Ruben and Johansen made when carrying out experiments on 8 corpses [2]. Other manikins may not show a linear depth-force characteristic; their
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thorax resistance may decrease with increasing depth of compression and, thus, may differ considerably from the human chest resistance [3]. Furthermore, the manikins vary with respect to the in-built monitoring devices which either acoustically or visually indicate whether the correct compression point is hit or the correct depth of compression is reached. Moreover CPR manikins have been developed primarily with the aim of CPR training in mind. Both of the manikins Laerdal Resusci Anne and Ambu Man are already improved models which comply with the present international CPR guidelines. The more simple manikins Actar 911, Laerdal Little Anne, Ambu CPR Pal and Ambu MultiMan have been built in line with the concept that a separate manikin should be provided for each CPR student. The Laerdal Family Trainer is made of millboard and conceived to be taken home by the trainee to train his family. The chest of Ambu CPR Pal has been especially designed for training active compression-decompression CPR (ACD-CPR) with the Ambu CardioPump 141. Ambu MultiMan, equipped with a spring in the thorax is the latest product of the Ambu company and representsthe cheap version of Ambu Man. Ambu MultiMan is provided in pairs, i.e. 2 manikins at a time are connected at the abdomen. It is not surprising that the mechanical characteristics may vary, a fact which is also recognised by both CPR trainers and trainees. From this point of view, it is of utmost importance that the CPR trainee learns to compressin a sufticiently strong manner and, at the same time, avoids an exceedingly high depth of compression irrespective of the thorax resistance. As quantitative data on the thorax elasticity of manikins are virtually not available, the aim of this study was to measure the thorax elasticity of commonly used CPR manikins and to define their compression characteristics. 2. Materials and methods
CPR manikins of 4 different manufacturers were tested: Actar Trooper 911 (Actar-Airforce Inc., NJ, USA); Ambu Man, Ambu CPR Pal and Ambu MultiMan (Ambu International AIS,
30 (1995) 117-126
Copenhagen, Denmark); Draeger CPR-Max (Medical Plastics, TX, USA; distributed in Europe by Draeger Ltibeck, Germany); Laerdal Resusci Anne, Laerdal Little Anne and Laerdal Family Trainer (Laerdal, Stavanger, Norway). Laerdal Recording Anne and Laerdal Skillmeter Anne were not included in the study becausethey show the, same mechanical construction as Laerdal Resusci Anne. All manikins tested had been unused brand-new models; in addition, used models were investigated. Ambu Man and Resusci Anne (usedmodels) were tested for several hours. Out of all models tested (Table l), 2 were of female appearance. 2.1. Design characteristics of CPR manikins
Details about the construction of the models are given in Table 1. In 4 models (Ambu Man, Ambu MultiMan, Laerdal Resusci Anne, Draeger CPRMax) the thorax resistanceis provided by a spring, whereas in Ambu CPR Pal, Laerdal Little Anne and Laerdal Family Trainer the resistanceis caused by the thorax itself. In Actar 911, a plastic construction is used instead of a spring to generatethe thorax resistance. Ambu Man contains 2 springs, the stiffer one determining the compression characteristics and the other emptying the manikin’s lungs after ventilation. The spring providing the thorax resistance is not placed in line with the compression axis, but rather is inclined. By changing the respective angle (by shifting the dorsal part of the spring parallel to the back of the manikin), the thorax resistancecan be adjusted between ‘low’ and ‘high’. 2.2. Indication of correct compression depth and correct compression point
With Draeger CPR-Max and Laerdal Resusci Anne, electrical contacts indicate incorrect compression point (red light) and correct depth of compression (yellow light). With Ambu Man there is a mechanical linkage with which the respective alarm signals can be indicated at the abdomen of the manikin. With Laerdal Little Anne, a clicking sound (caused by the snapping of a small rectangular plastic plate) can be heard when the correct depth of compression is exceeded. Ambu MultiMan indicates the reaching of the correct
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119
Table I Physical characteristics of the training manikins investigated Manikin
Sex
Design
a.-p. diameter (rrn)
Thorax resistance caused by
Indicators for correct compression
Actar 911 Ambu Man Ambu CPR Pal Ambu MultiMan Dr&ger CPR-Max Laerdaf Resusci Anne Laerdal Little Anne Laerdal Family Trainer
n m m m m f f n
T T(B) T T B T(B) T T
17.5 17 20.5 19 20 16 16 14
Spring imitation Spring Chest cage Spring Spring Spring Chest cage Chest cage
Mechanical Mechanical Electronic Electronic Acoustic -
m, male; f, female; n, neutral, T, head and torso; B, body as a whole; (B), manikin also available with limbs; a.-p. diameter, anteriorposterior diameter at correct compression point.
depth of compression on an analogous scaleat the abdomen. Actar 911, Ambu CPR Pal, Laerdal Little Anne and Laerdal Family Trainer have no indicators at all. 2.3. Thumper assembly, measurements and registration An electrically controlled and pneumatically powered device (Thumper, Fig. 1, originally developed for animal CPR research [5]), was used
Fig. I. Experimental measurement set up with self-built thumper. I, piston with massager pad, 2, force transducer; 3, mechanical limitation for length of stroke (depth of compression); 5, steel girders (a, vertical; b, horizontal); 6, pneumatic valve assembly; 7, electronics for controlling the 2/3-way solenoid driven valves; 8, pressure regulator; 9, pressure reservoir; IO, hospital supply pressure. Vertical adjustment of the horizontal girder allows to accommodate the distance between massager pad and manikin.
to compress the thorax. The thumper assembly consists of a piston moving in a cylinder and allows to adjust the compression stroke range between 1 and 5 cm (* 0.5 mm). For measuring the force, a load cell (measurementrange 100 kp) was positioned at the lower end of the piston to which a reinforced polyester pad (100 x 100 mm) imitating the palm of the hand was fixed. By pressurizing either the front or back side of the piston the massagepad was moved either upward (relaxation phase) or downward (compression phase). The force of compression was recorded on a Gould chart recorder (Gould 3800 S; Gould, Cleveland, USA). Reading accuracy was 0.5 mm with a resolution of * 0.5 N. The force measuring systemwas calibrated with defined weights before each test. The manikin to be tested was secured against lateral movement. It was positioned in such a way that the thumper’s compressor pad would hit the sternum perpendicularly and exactly on the compression point. First, the compressor pad was adjusted without exerting pressure onto the sternum of the manikin, then the compression stroke was selected (range, l-5 cm). Ten compression/ relaxation cycles (compression time, 300 ms; relaxation phase, 300 ms) were applied at pre-set strokes of 1, 2, 3, 4 and 5 cm each. For each stroke length out of the 10 compression/relaxation cycles the 2nd and 9th cycles were evaluated. However, with the models Actar 911 and Ambu Man, only the second stroke was analy-
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sed. Ambu Man was tested for its resistance at both the ‘low’ and ‘high’ settings. Endurance tests of 8 h were carried out with Ambu Man and Laerda1 Resusci Anne, the depth of compression preset at 5 cm only. Every 300th compression cycle was evaluated in this test series. Compression frequency was set to 90 compressions per mitt, which complies with the recommendations of the American Heart Association [6] and the European Resuscitation Council [7]. In order to check whether the signals indicate that the correct depth of compression has been reached or even exceeded,the thoraxes were gradually compressed by using cardboard paddings of 1 mm each, which were put between the manikin’s back and the table while maintaining the contact between the compressor pad and the chest of the manikin. Finally, the chest geometry of the manikins was measured by determining the anterior-posterior diameter at the correct compression point (Table 1).
2.4. Data presentation and statistics
All results represent controlled single measurements. According to the physician’s preference,all force data are expressedin kp instead of SI units (N, Newton; 1 kp = 9.81 N). The relationship between force and depth of compression was approximated by polynomial regression analysis of 2nd degree (force K= a + bx + cx2; x = depth of compression).When the coefficient c did not differ significantly from zero, a linear relationship between depth and force of compression was assumed (K = a + bx). For fitting the software program Fig. P (version 6.0 c, Biosoft, Ferguson, MO, USA) was used. The level of significance was set at P < 0.05. 3. Results (Table 2, Fig. 2) 3.1. Evaluation of the individual manikins Actar 911. In order to reach a compression
depth of 3 cm, a compression force of 20.5 kp (ca.
Table 2 Results and analysis of compression force measured in order to compress the manikin’s thorax to a pre-set depth Manikin
Actar (n) Ambu Man - Low (n) Ambu Man - Low (u) Ambu Man - Low (u) Ambu Man - High (n) Ambu Man - High (u) Ambu Man - High (u) Ambu Man - High (u) Ambu Man - High (u) Ambu Man - High (u) Ambu MultiMan (n) Ambu MultiMan (n) Ambu MultiMan (n) Ambu CPR Pal (n) Ambu CPR Pal (n) Ambu CPR Pal (n) Draeger CPR Max (n) Laerdal Resusci Anne (n) Laerdal Resusci Anne (u-l) Laerdal Resusci Anne (u-2) Laerdal Resusci Anne (u-3) Laerdal Little Anne (n) Laerdal Family Trainer (n) Laerdal Family Trainer (n)
Depth of compression (cm)
Coefficient
of 2nd degree polynomial b
1
2
3
4
5
a
7.5 6.3 67 8 10 10 11 11 10 10 9.2 9.2 9.2 IO 8 8 7 7 8 9 9 10 14 II
14.5 11.6 13 15 17.8 19 21 21 24 22 17.6 18.3 17.8 16 16 16 21 15 16 17 17 21 30 26
20.5 17 19 20 27.8 30 32 33 35 34 26.1 26.3 26.1 24 23 24 37 23 25 24 26 30 38 38
25.5 22.5 23.5 28 38.5 42 43 43 47 46 34.6 34.6 34.8 33 32 32 54 32 29 32 35 36 45 41
28.5 30 34 35 50 55 57 57 60 61 40.3 40.3 40.5 42 40 40 69 46.5 37 41 43 38 52 55
-0.0893 0.4000 0.6607 0.4643 0.4750 0.3214 0.4643 0.3214 -0.3939 -0.0357 -0.182 -0.186 -0.121 0.7500 0.1429 0.0000 -1.4286 0.5179 -0.1429 0.4286 0.0357 -0.8571 -0.1786 -0.3929
n, new manikin; u, already used manikin.
8.3339 5.2214 5.4161 6.9464 8.1661 8.5036 9.5036 9.8893 11.4321 9.9107 9.738 10.002 9.675 7.5750 7.6143 8.0000 9.7OOu 5.3089 8.7714 8.0286 8.7179 13.2714 16.6536 13.7179
C
r2
-0.5268 0.1214 0.2054 -0.0179 0.3411 0.4821 0.3393 0.2679 0.1250 0.4464 -0.308 -0.3669 -0.296 0.1250 0.0714 0.0000 0.9286 0.7411 -0.2857 0.0000 -0.0179 -1.0714 - 1.2679 -0.5893
0.99953 0.99780 0.99020 0.99701 0.99913 0.99967 0.99917 0.99891 0.99894 0.99960 0.99940 0.99971 0.99976 0.99699 0.99944 1.ooooO 0.99641 0.99675 0.99555 0.99869 0.99969 0.99605 0.99605 0.98547
M. A. &z&n et al./Remcitation 30 (1995) 117-126
80 70 60 50 40 30 20 10
-.. -------
121
+DrSger CPR Max *Ambu Man “high” -+ Laerdal Resusci Anne -+Ambu Multiman *Ambu CPR Pal -+Ambu Man “low”
04
I
0
1
2
3
5
4
80 70 60
+
Laerdal Family Trainer +- Laerdal Little Anne * Actar
2oi
I0OJt 0
1
2
compression
3
I
i
4
5
depth (cm)
Fig. 2. (a) Compression depth-compression force plot of CPR manikins with ‘linear’ compression characteristics. (b) Compression depth-compressionforce plot of CPR manikins with ‘non-linear’compression characteristics. Convex shape indicates lessthorax stiffnesswhen exceeding 3 cm depth of compression.
7 kp/cm) was required. For each cm of further compression, an additional force of 4 kp was needed. Thus, for a depth of 4 and 5 cm, a spring ‘constam of 6 kplcm and 5.7 kplcm, respectively, can be calculated. Ambu Man. With the internal spring set at ‘low’ in the unused manikin, a force of 6.3 kp was re-
quired for compression of 1 cm, and further compression to a depth of 2-4 cm necessitated 5.8 kplcm. In order to achieve a thorax compression of 5 cm, a slightly higher force had to be applied (6 kp/cm). When testing the compression characteristics of the used manikin, a small increase in the thorax resistance was observed.
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With the thorax resistance set at ‘high’, the forces required for thorax compression were - as expected - higher. In the brand-new manikin 9-10 kp/cm were needed for a depth of l-4 cm and 10 kplcm for compression of 5 cm, thus yielding a spring constant of 9.5-10.5 kp/cm. In the used model, a force of 11-12 kplcm was required. Due to the linear thorax compression characteristics of Ambu Man, the difference between the settings ‘low’ and ‘high’ can be calculated using the coefficients b (unused manikin, 2.9 kp/cm; used manikin, 3.7 kp/cm; mean difference). The endurance test lasting for 8 h (altogether 27 000 compression cycles) did not reveal any changes in the compression characteristics. Ambu CPR Pal. For this manikin about 8 kplcm were necessaryto compressthe thorax, irrespective of the actual depth of compression. Acccordingly, the force-depth relationship was linear. Ambu MultiMan. In this manikin, a force of about 9 kp was neededto compressthe chest 1 cm, for 2-4 cm of compression a force of 17.8, 26 and 34.4 kp was needed.A depth of 5 cm was achieved when exerting a force of 40.5 kp. The force-depth relationship is almost linear, yielding a spring constant of 8.2 kp/cm. Driiger CPR-Max. To reach a depth of compression of 1 cm, a force of 7 kp was needed. For thorax compression, of 2-5 cm an additional force of 14 kp (l-2 cm), 16 kp (2-3 cm), 17 kp (3-4 cm) and 15 kp (4-5 cm) was required, which resulted in a spring constant between 10.5 and 13.8 kp/cm. The relationship between force and depth of compression was rather linear for the range of 2-5 cm. At a depth of 4 cm (force 54 kp) the signal indicating correct depth of compression flashed up. Laerdal Resusci Anne. In the brand-new model, a force between 7 and 8 kplcm was required to compressfrom l-4 cm. In order to compressfrom 4-5 cm, an additional force of 14.5kp was needed. In the usedmodels, a force of 8-9 kplcm was needed for compression of l-5 cm. Only after reaching a depth of 4.3 cm, the signal indicating the correct depth of compression was triggered in the manikin which had been precisely measuredusing the 1 mm cardboard paddings. The other 3 models failed to signal that the correct depth of compression had been reached at 4 cm. During an endurance test lasting for 8 h (27 000 compressions), no changes
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in the force required for a compression of 5 cm were observed. Laerdal Little Anne. Up to a depth of 3 cm, a force of about 10 kplcm was needed for compression. For 4 cm of compression, the additional force required decreasedto 6 kp. Finally, for compressing the thorax from 4 to 5 cm, an additional force of only 2 kp was required. Hence, up to a compression depth of 3 cm a linear relationship between force and depth of compression was observed, whereas between 3 and 5 cm this relationship was no longer linear. When compressing the manikin more than 5 cm, an acoustic signal (‘click’) was triggered. Laerdal Family Trainer. For 1 and 2 cm of compression, 14 and 30 kp, respectively, were needed in the first test series. Between 2 and 5 cm, a smaller force (7-8 kp/cm) was required for compression. When repeating the measurementson an other manikin of the same type, differences between 13-21% were observed at a depth of compression of up to 2 cm, at a depth of 3,4 and 5 cm the repeated measurements revealed only minor differences (< 8%). 4. Discussion Eight brand-new and 4 used CPR manikins of 4 different manufacturers were tested as to their resistanceduring dynamic chest compressionusing a mechanical test apparatus. The compression characteristics of each manikin were determined and assessedwith regard to possible differences. Then thesecharacteristics were compared with the data provided by the manufacturers. It was essential for the study that the reproducibility of each compression cycle was guaranteed. This was achievedby a pneumatically-driven and electricallycontrolled CPR thumper which had already been successfullytested in animal experiments [5]. Due to the rectangular, plateau phase-like force pattern chosen, the force applied could be exactly measured by the force transducer during the plateau (compression) phase. The depth of compression deviated by less than 5% (0.5 mm at a depth of 10 mm). Thus a simultaneous measurement of the stroke length was superfluous. Nevertheless, during the first pilot tests, the depth of compression was directly determined by a linear
M.A. Baubin et al./Resu.scitation
distance measurement system. As the transducer could only be fixed in a slanted position and not directly overhead the compression point (especially when testing the female manikins), this kind of distance measurement yielded rather unprecise results. The compression patterns remained identical within the individual compression cycles, regardlessof the depth of compression (seeFig. 3). The same applied for the endurance tests. Nevertheless,with the exception of some models, 2 data sets (2nd and 9th compressions) were evaluated. No differences could be found. Thus, the data evaluated represent single but controlled measurements and we may conclude that those measurementswhere only one data set was obtained may also be considered precise. The inaccuracies in the measurement and reading of force or depth (0.5 kp or 0.5 mm) result in an error of < 10%of the calculated spring constant (coefficient b of the polynomial) in the low range of either depth or force (< 15 kp). In the compression range as used for normal CPR (3-5 cm), these inaccuracies cause an error of only 4%. Thus, our measurements appear to be accurate enough to quantify even minor differences in the chest elasticity of the manikins. When comparing the results for small depth of compression, differences of up to 50% could be observed between the manikins: the force needed
rI I I:’LI Ill ii 11 ;:
l
time (10 mm/s)
Fig. 3. Original record of force during the compression and relaxation phase (10 cycles) with a depth of compression preset to 5 cm. For calculating the thorax resistancedata from the 2nd and 9th cycles were read off from the respective plateau phase (arrows). Time axis, 25 mm/s; scale, compression force as indicated.
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for a compression of 1 cm varied between 6.3 and 14 kp. The female models required a force of up to 9 kp/cm. The differences increased with greater depth of compression (about 124% at 3 cm; at 4 and 5 cm, the necessaryforce varied up to 140%). Out of the 5 cheaper manikins (Actar 911, Ambu MultiMan, Ambu CPR Pal, Laerdal Little Anne, Laerdal Family Trainer) only Ambu MultiMan has a built-in spring. Accordingly, it shows a linear characteristic which is similar to Ambu CPR Pal but differs from that of Ambu Man (set at ‘high’) with regard to the value of the spring constant. According to their design, all the manikins with a built-in spring in the chest, but also Ambu CPR Pal with its stiff plastic form, basically showed a nearly linear relationship between force and depth of compression. Ambu Man, set at ‘high’, revealed an even slightly increasing thorax stiffness (Table 2; positive coefticient in polynomial c), an unexpected result when considering the mechanical construction of Ambu Man. However, according to Tsitlik et al. [3] and Bankmann et al. [8], such a compression characteristic would resemble that of the human chest (resistance of the thorax increases with the depth of compression) and would therefore better meet the requirements for manikins. Ruben and Johansen [2], on the other hand, postulated a linear relationship between force and depth of compression, at least until a sternum or rib fracture occurs. Thus, in summary, it may be stated that the manikins DrHger CPR-Max, Ambu Man, Ambu CPR Pal, Ambu MultiMan, and Laerdal Resusci Anne, at least to some extent, meet the compression characteristics demanded by Tsitlik et al. [3] and Bankmann et al. [8], whereas the models Actar 911, Laerdal Little Anne and Laerdal Family Trainer fail to satisfy these demandsbecauseof their particular convex depthforce compression relationship. Especially the non-linear characteristic of Actar 911 in no way reflects the characteristics of a human thorax, becausethis manikin offers sufficient chest resistance only up to 2 cm (spring constant, approximately 8 kp/cm) and from 3 cm onwards its thorax can be further compressedwith a force of only 4 kp/cm. Such a compression characteristic does not convey a feeling of correct CPR. SimilQ, Ambu Man in
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‘low’ setting has a rather ‘soft’ compression characteristic (5.8 kp/cm); nevertheless,this model offers a linear force-depth relationship. Although the force-depth relationship of Laerdal Little Anne is to be considered non-linear, the measurement results of this manikin, as compared to the results of Ambu Man, are more satisfactory. This is due to the fact that at least a force of 36 kp is neededto attain a depth of compression of 4 cm, which would correspond to the female compression characteristic. In order to show the relationship between the force of compression and the resulting depth of compression mathematically, a polynomial of second degreewas adjusted to the experimental data. From the regression coefficient r2 > 0.995 it has becomeevident that the chosen function reflected the characteristic quite accurately. The nearer the polynomial coefficient c approacheszero the more linear the force-depth relationship becomes.With the ‘linear’ manikins the polynomial approximation showed data between 7.5 and 9.9 kp/cm for the sex-neutral and male manikins, for Ambu Man in ‘Low’ setting 5.2-6.9 kp/cm or in the caseof the female Laerdal Resusci Anne 5.3 kp/cm for the new model and 8.5 kp/cm for the used ones. It remains unclear why the spring constant was distinctly higher in the used Laerdal ResusciAnne. As this was confirmed in 3 manikins, differences between the individual manikins seem most probable; in any case,a smaller female chest resistance becameevident . No sign of fatigue could be detected either with Ambu Man or with Laerdal Resusci Anne during the tests of a duration of more than 8 h with a compression depth of 5 cm. With Laerdal Resusci Anne, however, the chest remained slightly compressed after the endurance test, also when exerting no force. There was a considerable discrepancy between our results and the specifications given by some of the manufacturers. For Ambu Man, when set at ‘low’, the manufacturer’s data claimed a resistance of 6 N/mm (0.6 kp/mm), which means that a force of 30 kp is neededto compressthe chest 5 cm, provided a linear force-depth relationship is assumed. Our 3 measurementsyielded forces of 30, 34 and 35 kp. At the y”gh’ setting, the manufacturer statesa resistanc&of 11 N/mm; our measurements
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(50, 55, 57, 60 and 61 kp) are in good agreement with the specified data. Among all the models, only Ambu Man reveals the technical feature of adjustable chest resistance. But as its resistance is only known for the maximum and minimum settings (‘high’ and ‘low’), additional data on the intermediate settings should be provided by the manufacturer. With Drager CPR-Max, we needed 69 kp in order to compressthe manikin by 5 cm instead of 300N (about 30 kp) as stated in the data sheet.According to the manufacturer the thorax resistance of this manikin should correspond to the thorax resistanceof a 30-year-old man, which, however, is irrelevant for CPR training becauseCPR victims rarely belong to this age group. As most of the CPR victims are between60 and 80 years old, CPR manikins should comply with the thorax resistance of this age group. Also with Laerdal Resusci Anne, our results did not fully agreewith the thorax resistanceas stated by the manufacturer (35 and 47 kp, for 38 and 51 mm compression, respectively). We found a somewhat smaller force for 4 cm (29-35 kp) and for 5 cm (37-46.5 kp) compression. It was interesting, that the data obtained with a brand-new manikin were in better agreement (46.5 kp for 50 mm compression). Information on how the manufacturers determine the thorax resistances of their manikins is lacking. In addition, some manikins show a nonlinear characteristic in their thorax resistance which was not pointed out, which underlines the importance of our measurements.To our knowledge, the present investigation is the first to provide quantitative data on training manikins. The American Heart Association recommendsa depth of compression of 1.5-2 inches (3.81-5.08 cm) for the adult [6], the European Resuscitation Council [7] recommends a depth of 4-5 cm. The manufacturers use these data when laying down the monitoring criteria for the indication of correct depth of compression. However, with the Laerdal Little Anne manikin we tested, a clicking sound warned the trainee not to increasethe compression any further when he had already reached 5 cm. This triggering characteristic invalidates the intention of such an alarm. According to the manufacturer, with future manikins the trainee will be
M.A. Baubin et al./Resuscitation
alerted when he compresses3.8 cm, which would more accurately reflect the guidelines. In the manikins with the option for monitoring both a sufficient (3.8 cm) or an exceedingly high (5 cm) depth of compression (Driiger CPR-Max and Laerdal Resusci Anne), the signal for sufficient depth of compression was triggered at 4 cm (Driiger CPR-Max), whereaswith Laerdal Resusci Anne this signal was set off at 4.3 cm rather than at 3.8 cm. If this holds true for all manikins of the Laerdal Resusci Anne type (and not only for the one we have precisely checked for its triggering characteristics), the trainee will be warned too late and might have already exerted too much force. The criterion relevant for efficient chest compression is the depth of compression rather than the force applied to the sternum. Cardiac output predominantly depends on the compression rate and on the mean arterial pressureachieved, which, in turn, depends on the depth of compression [9]. Therefore, it is of utmost importance that CPR trainees learn to assessthe correct depth of compression. In general, a depth of compression of 20-25% of the anterior-posterior diameter should be aimed at. Chest diameters of an average adult range between 15 and 25 cm. Thus, with the Laerda1Family Trainer (a.-p. diameter, 14 cm) the relevant depth would be 2.8-3.5 cm; apparently a depth of compression of 5 cm does not comply with the a.-p. diameter of this manikin. We found the male manikins to have a larger a.-p. diameter (measured in the lower part of the sternum) than the female ones. The manikins Actar 911 and Laerdal Family Trainer are sex-neutral. According to our results, neither the a.-p. diameter nor the appearance provide a reliable hint as to the manikin’s actual chest resistance. In this context, the chest resistanceof Laerdal Little Anne appears to be very high for a female manikin, at least in the range of up to 4 cm compression. Similarly, the used models of Laerdal Resusci Anne have a rather high spring constant. In contrast, Ambu Man, a male manikin, when set at ‘low’, shows a soft linear thorax resistance which resembles a typically female characteristic. This study was not primarily directed to investigate to what extent the human thorax characteristic is simulated by the manikins. There are not yet enough data available about the rela-
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tionship of force and depth of compression in humans [2,3,9]. CPR specialists are familiar with the great differences in thorax resistance of CPR victims and they also know that rib fractures, age and chest-compliancealter the thorax resistance.These real-life situations can, by no means, be satisfactorily simulated by the training manikins presently available. Therefore, the most important issue to be thoroughly discussed and paid attention to, during CPR training, is the thorax resistance of the individual manikin. Already Tsitlik et al. [3] pointed out the danger of injury as a result of incorrect CPR training. A trainee who practises only on a manikin with high thorax resistance will becomeaccustomedto applying too much force of compression, but also too little force results in insufficient CPR. From our tests it can be concluded that, in order to perform CPR correctly on any patient, CPR instructors and trainees should be encouraged to practise on different manikins, rather than to use the sametype of manikin all the time. The development of Ambu Man, the manikin with adjustable thorax resistance, may be considered a first step in this direction, even though it is highly questionable if a thorax resistance with a linear force-depth relationship is comparable to the human chest under CPR conditions. Acknowledgements The authors thank the companies Chemomedica Vienna, Driiger Austria and Roraco Vienna as well as the Vamed Schulungszentrum Vienna for the loan of the training manikins. Ing. R. Schwarz helped with the technical development of the measurementdevice and, together with Mr. S. Milovic, evaluated the recordings. The Department of Biostatistics of the University of Innsbruck assisted in the statistical analysis. We also thank Mag. I. Zach for her secretarial assistance.This research project was supported by the Fund of the Austrian Nationalbank (Jubillumsfonds, project No. 4929). References [I]
Kouwenhoven WB, Jude JR, Knickerbocker GG. Closed chest cardiac massage. J Am Med Assoc. 1960; 173: 1064-1067. [2] Ruben H, Johansen SH. Sternal displacement with different loads. Acta Anaesthesiol Stand 1966; 10: 31-36.
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[3] Tsitlik JE, Weisfeldt ML, Chandra N, Effon MB, Halperin HR, Levin HR. Elastic properties of the human chest during cardiopulmonary resuscitation. Crit Care Med 1983; 11: 685-692. [4] Cohen TJ, Tucker KJ, Lurie KG, Redberg RF, Dutton JP, Dwyer KA, Schwab TM, Chin MC, Gelb AM, Scheinmann MM, Schiller NB, Callaham ML. Active compression-decompression- A new method of cardiopulmonary resuscitation. J Am Med Assoc 1992; 267: 2916-2923. [5] Mauritz W. Hea-Lungen-Wiederbelebung Experimenteller Vergleich verschiedener Verfahren. Beitr Anlsth Intensiv Med, Bd. 28. Wien: Maudrich, 1989; 20-23. [6] American Heart Association. Guidelines for cardiopul-
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monary resuscitation and emergencycardiac care. J Am Med Assoc 1992;268: 2189-2191. [7] Basic Life Support Working Party of the European Resuscitation Council. Guidelines for basic life support. Resuscitation 1992;24: 103-I 10. [8] Bankman IN, Gruben KG, Halperin HR, Pope1 AS, Guerci AD, Tsitlik JE. Identification of dynamic mechanical parameters of the human chest during manual cardiopulmonary resuscitation. IEEE Trans Biomed Eng 1990;37: 211-217. [9] Omato JP, Levine RL, Young DS, Racht EM, Gamett AR, Gonzalez ER. The effect of applied chest compression force on systemicarterial pressureand end-tidal carbon dioxide concentration during CPR in human beings. Ann Emerg Med 1989; 18: 732-737.
ELSEVIER
Resuscitation 30 ( 1995) 1I7- 126
Compression characteristics of CPR manikins Michael Angelicus Baubin* a, Hermann Gillyb, Alexander Poscha, Adolf Schinnerl”, Gunnar Anton Kroesena aDepartment of Anaesthesia and Intensive Care Medicine, Institute for Emergency and Disaster Medicine, The Leopold-FranzensUniversity of Innsbruck, Anichstra$e 35, A-6020, Innsbruck, Austria bL. Bollzmann Institute of Experimental Anaesthesiology and Research in Inlensive Care Medicine, Spitalgasse 23, A-1090. Vienna, Austria
Received 2 February 1995; revision received 12 May 1995;accepted 16 May 1995
AbStraCt We evaluated the force-depth compression characteristics of 8 different CPR manikins during mechanical cardiopulmonary resuscitation by a thumper. The force required to compressthe manikin’s thorax of 1, 2, 3, 4 and 5 cm was measured.It ranged between 6.3 and 14 kp at a depth of 1 cm, 11.6-30 kp at 2 cm, 17-38 kp at 3 cm, 22.5-54 kp at 4 cm and 28.5-69 kp at 5 cm. The manikins with a spring in the thorax (Ambu Man, Ambu MultiMan, Drager CPR-Max, Laerdal Resusci Anne) as well as one without (Ambu CPR Pal) showed a rather linear relationship between depth and force required to compressthe chest. Ambu Man, set at ‘High’, Laerdal ResusciAnne and Drlger CPR-Max revealed a slight increase in resistance, whereas 2 manikins without a spring (Laerdal Little Anne, Laerdal Family Trainer) and 1 manikin with a plastic spring-like construction (Actar 911) exhibited less resistance with increasing depth. According to our results, the manikins are not uniform in their compression characteristics; somebecomenonlinear when 3 cm of compression is exceeded.For correct CPR it is of utmost importance that the CPR trainee learns to compressin a sufficiently strong manner, but simultaneously to avoid an exceedingly high depth of compression irrespective of the thorax resistance.In order to prepare the CPR student for the varying chest resistancesof the human body, we recommend to train CPR on manikins with different chest resistances. Keywords:
Cardiopulmonary resuscitation; Training manikins; Compression characteristics
1. Introduction Since the development and publication of the combined cardiopulmonary resuscitation by Kouwenhoven et al. [l] in 1960, various training manikins for CPR have been developed. Dependl
Corresponding author.
ing on the particular design, their thorax resistance differs widely. It is to be assumed that manikins with a built-in spring show a linear relationship between depth and force of compression. Such a linear characteristic reflects the findings Ruben and Johansen made when carrying out experiments on 8 corpses [2]. Other manikins may not show a linear depth-force characteristic; their
0300-9572/95/$09.05 0 1995 Elsevier Science Ireland Ltd. All rights reserved SSDI 0300-9572(95)00874-S
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thorax resistance may decrease with increasing depth of compression and, thus, may differ considerably from the human chest resistance [3]. Furthermore, the manikins vary with respect to the in-built monitoring devices which either acoustically or visually indicate whether the correct compression point is hit or the correct depth of compression is reached. Moreover CPR manikins have been developed primarily with the aim of CPR training in mind. Both of the manikins Laerdal Resusci Anne and Ambu Man are already improved models which comply with the present international CPR guidelines. The more simple manikins Actar 911, Laerdal Little Anne, Ambu CPR Pal and Ambu MultiMan have been built in line with the concept that a separate manikin should be provided for each CPR student. The Laerdal Family Trainer is made of millboard and conceived to be taken home by the trainee to train his family. The chest of Ambu CPR Pal has been especially designed for training active compression-decompression CPR (ACD-CPR) with the Ambu CardioPump 141. Ambu MultiMan, equipped with a spring in the thorax is the latest product of the Ambu company and representsthe cheap version of Ambu Man. Ambu MultiMan is provided in pairs, i.e. 2 manikins at a time are connected at the abdomen. It is not surprising that the mechanical characteristics may vary, a fact which is also recognised by both CPR trainers and trainees. From this point of view, it is of utmost importance that the CPR trainee learns to compressin a sufticiently strong manner and, at the same time, avoids an exceedingly high depth of compression irrespective of the thorax resistance. As quantitative data on the thorax elasticity of manikins are virtually not available, the aim of this study was to measure the thorax elasticity of commonly used CPR manikins and to define their compression characteristics. 2. Materials and methods
CPR manikins of 4 different manufacturers were tested: Actar Trooper 911 (Actar-Airforce Inc., NJ, USA); Ambu Man, Ambu CPR Pal and Ambu MultiMan (Ambu International AIS,
30 (1995) 117-126
Copenhagen, Denmark); Draeger CPR-Max (Medical Plastics, TX, USA; distributed in Europe by Draeger Ltibeck, Germany); Laerdal Resusci Anne, Laerdal Little Anne and Laerdal Family Trainer (Laerdal, Stavanger, Norway). Laerdal Recording Anne and Laerdal Skillmeter Anne were not included in the study becausethey show the, same mechanical construction as Laerdal Resusci Anne. All manikins tested had been unused brand-new models; in addition, used models were investigated. Ambu Man and Resusci Anne (usedmodels) were tested for several hours. Out of all models tested (Table l), 2 were of female appearance. 2.1. Design characteristics of CPR manikins
Details about the construction of the models are given in Table 1. In 4 models (Ambu Man, Ambu MultiMan, Laerdal Resusci Anne, Draeger CPRMax) the thorax resistanceis provided by a spring, whereas in Ambu CPR Pal, Laerdal Little Anne and Laerdal Family Trainer the resistanceis caused by the thorax itself. In Actar 911, a plastic construction is used instead of a spring to generatethe thorax resistance. Ambu Man contains 2 springs, the stiffer one determining the compression characteristics and the other emptying the manikin’s lungs after ventilation. The spring providing the thorax resistance is not placed in line with the compression axis, but rather is inclined. By changing the respective angle (by shifting the dorsal part of the spring parallel to the back of the manikin), the thorax resistancecan be adjusted between ‘low’ and ‘high’. 2.2. Indication of correct compression depth and correct compression point
With Draeger CPR-Max and Laerdal Resusci Anne, electrical contacts indicate incorrect compression point (red light) and correct depth of compression (yellow light). With Ambu Man there is a mechanical linkage with which the respective alarm signals can be indicated at the abdomen of the manikin. With Laerdal Little Anne, a clicking sound (caused by the snapping of a small rectangular plastic plate) can be heard when the correct depth of compression is exceeded. Ambu MultiMan indicates the reaching of the correct
h4.A. Baubin et al./Resuscitation
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119
Table I Physical characteristics of the training manikins investigated Manikin
Sex
Design
a.-p. diameter (rrn)
Thorax resistance caused by
Indicators for correct compression
Actar 911 Ambu Man Ambu CPR Pal Ambu MultiMan Dr&ger CPR-Max Laerdaf Resusci Anne Laerdal Little Anne Laerdal Family Trainer
n m m m m f f n
T T(B) T T B T(B) T T
17.5 17 20.5 19 20 16 16 14
Spring imitation Spring Chest cage Spring Spring Spring Chest cage Chest cage
Mechanical Mechanical Electronic Electronic Acoustic -
m, male; f, female; n, neutral, T, head and torso; B, body as a whole; (B), manikin also available with limbs; a.-p. diameter, anteriorposterior diameter at correct compression point.
depth of compression on an analogous scaleat the abdomen. Actar 911, Ambu CPR Pal, Laerdal Little Anne and Laerdal Family Trainer have no indicators at all. 2.3. Thumper assembly, measurements and registration An electrically controlled and pneumatically powered device (Thumper, Fig. 1, originally developed for animal CPR research [5]), was used
Fig. I. Experimental measurement set up with self-built thumper. I, piston with massager pad, 2, force transducer; 3, mechanical limitation for length of stroke (depth of compression); 5, steel girders (a, vertical; b, horizontal); 6, pneumatic valve assembly; 7, electronics for controlling the 2/3-way solenoid driven valves; 8, pressure regulator; 9, pressure reservoir; IO, hospital supply pressure. Vertical adjustment of the horizontal girder allows to accommodate the distance between massager pad and manikin.
to compress the thorax. The thumper assembly consists of a piston moving in a cylinder and allows to adjust the compression stroke range between 1 and 5 cm (* 0.5 mm). For measuring the force, a load cell (measurementrange 100 kp) was positioned at the lower end of the piston to which a reinforced polyester pad (100 x 100 mm) imitating the palm of the hand was fixed. By pressurizing either the front or back side of the piston the massagepad was moved either upward (relaxation phase) or downward (compression phase). The force of compression was recorded on a Gould chart recorder (Gould 3800 S; Gould, Cleveland, USA). Reading accuracy was 0.5 mm with a resolution of * 0.5 N. The force measuring systemwas calibrated with defined weights before each test. The manikin to be tested was secured against lateral movement. It was positioned in such a way that the thumper’s compressor pad would hit the sternum perpendicularly and exactly on the compression point. First, the compressor pad was adjusted without exerting pressure onto the sternum of the manikin, then the compression stroke was selected (range, l-5 cm). Ten compression/ relaxation cycles (compression time, 300 ms; relaxation phase, 300 ms) were applied at pre-set strokes of 1, 2, 3, 4 and 5 cm each. For each stroke length out of the 10 compression/relaxation cycles the 2nd and 9th cycles were evaluated. However, with the models Actar 911 and Ambu Man, only the second stroke was analy-
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sed. Ambu Man was tested for its resistance at both the ‘low’ and ‘high’ settings. Endurance tests of 8 h were carried out with Ambu Man and Laerda1 Resusci Anne, the depth of compression preset at 5 cm only. Every 300th compression cycle was evaluated in this test series. Compression frequency was set to 90 compressions per mitt, which complies with the recommendations of the American Heart Association [6] and the European Resuscitation Council [7]. In order to check whether the signals indicate that the correct depth of compression has been reached or even exceeded,the thoraxes were gradually compressed by using cardboard paddings of 1 mm each, which were put between the manikin’s back and the table while maintaining the contact between the compressor pad and the chest of the manikin. Finally, the chest geometry of the manikins was measured by determining the anterior-posterior diameter at the correct compression point (Table 1).
2.4. Data presentation and statistics
All results represent controlled single measurements. According to the physician’s preference,all force data are expressedin kp instead of SI units (N, Newton; 1 kp = 9.81 N). The relationship between force and depth of compression was approximated by polynomial regression analysis of 2nd degree (force K= a + bx + cx2; x = depth of compression).When the coefficient c did not differ significantly from zero, a linear relationship between depth and force of compression was assumed (K = a + bx). For fitting the software program Fig. P (version 6.0 c, Biosoft, Ferguson, MO, USA) was used. The level of significance was set at P < 0.05. 3. Results (Table 2, Fig. 2) 3.1. Evaluation of the individual manikins Actar 911. In order to reach a compression
depth of 3 cm, a compression force of 20.5 kp (ca.
Table 2 Results and analysis of compression force measured in order to compress the manikin’s thorax to a pre-set depth Manikin
Actar (n) Ambu Man - Low (n) Ambu Man - Low (u) Ambu Man - Low (u) Ambu Man - High (n) Ambu Man - High (u) Ambu Man - High (u) Ambu Man - High (u) Ambu Man - High (u) Ambu Man - High (u) Ambu MultiMan (n) Ambu MultiMan (n) Ambu MultiMan (n) Ambu CPR Pal (n) Ambu CPR Pal (n) Ambu CPR Pal (n) Draeger CPR Max (n) Laerdal Resusci Anne (n) Laerdal Resusci Anne (u-l) Laerdal Resusci Anne (u-2) Laerdal Resusci Anne (u-3) Laerdal Little Anne (n) Laerdal Family Trainer (n) Laerdal Family Trainer (n)
Depth of compression (cm)
Coefficient
of 2nd degree polynomial b
1
2
3
4
5
a
7.5 6.3 67 8 10 10 11 11 10 10 9.2 9.2 9.2 IO 8 8 7 7 8 9 9 10 14 II
14.5 11.6 13 15 17.8 19 21 21 24 22 17.6 18.3 17.8 16 16 16 21 15 16 17 17 21 30 26
20.5 17 19 20 27.8 30 32 33 35 34 26.1 26.3 26.1 24 23 24 37 23 25 24 26 30 38 38
25.5 22.5 23.5 28 38.5 42 43 43 47 46 34.6 34.6 34.8 33 32 32 54 32 29 32 35 36 45 41
28.5 30 34 35 50 55 57 57 60 61 40.3 40.3 40.5 42 40 40 69 46.5 37 41 43 38 52 55
-0.0893 0.4000 0.6607 0.4643 0.4750 0.3214 0.4643 0.3214 -0.3939 -0.0357 -0.182 -0.186 -0.121 0.7500 0.1429 0.0000 -1.4286 0.5179 -0.1429 0.4286 0.0357 -0.8571 -0.1786 -0.3929
n, new manikin; u, already used manikin.
8.3339 5.2214 5.4161 6.9464 8.1661 8.5036 9.5036 9.8893 11.4321 9.9107 9.738 10.002 9.675 7.5750 7.6143 8.0000 9.7OOu 5.3089 8.7714 8.0286 8.7179 13.2714 16.6536 13.7179
C
r2
-0.5268 0.1214 0.2054 -0.0179 0.3411 0.4821 0.3393 0.2679 0.1250 0.4464 -0.308 -0.3669 -0.296 0.1250 0.0714 0.0000 0.9286 0.7411 -0.2857 0.0000 -0.0179 -1.0714 - 1.2679 -0.5893
0.99953 0.99780 0.99020 0.99701 0.99913 0.99967 0.99917 0.99891 0.99894 0.99960 0.99940 0.99971 0.99976 0.99699 0.99944 1.ooooO 0.99641 0.99675 0.99555 0.99869 0.99969 0.99605 0.99605 0.98547
M. A. &z&n et al./Remcitation 30 (1995) 117-126
80 70 60 50 40 30 20 10
-.. -------
121
+DrSger CPR Max *Ambu Man “high” -+ Laerdal Resusci Anne -+Ambu Multiman *Ambu CPR Pal -+Ambu Man “low”
04
I
0
1
2
3
5
4
80 70 60
+
Laerdal Family Trainer +- Laerdal Little Anne * Actar
2oi
I0OJt 0
1
2
compression
3
I
i
4
5
depth (cm)
Fig. 2. (a) Compression depth-compression force plot of CPR manikins with ‘linear’ compression characteristics. (b) Compression depth-compressionforce plot of CPR manikins with ‘non-linear’compression characteristics. Convex shape indicates lessthorax stiffnesswhen exceeding 3 cm depth of compression.
7 kp/cm) was required. For each cm of further compression, an additional force of 4 kp was needed. Thus, for a depth of 4 and 5 cm, a spring ‘constam of 6 kplcm and 5.7 kplcm, respectively, can be calculated. Ambu Man. With the internal spring set at ‘low’ in the unused manikin, a force of 6.3 kp was re-
quired for compression of 1 cm, and further compression to a depth of 2-4 cm necessitated 5.8 kplcm. In order to achieve a thorax compression of 5 cm, a slightly higher force had to be applied (6 kp/cm). When testing the compression characteristics of the used manikin, a small increase in the thorax resistance was observed.
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With the thorax resistance set at ‘high’, the forces required for thorax compression were - as expected - higher. In the brand-new manikin 9-10 kp/cm were needed for a depth of l-4 cm and 10 kplcm for compression of 5 cm, thus yielding a spring constant of 9.5-10.5 kp/cm. In the used model, a force of 11-12 kplcm was required. Due to the linear thorax compression characteristics of Ambu Man, the difference between the settings ‘low’ and ‘high’ can be calculated using the coefficients b (unused manikin, 2.9 kp/cm; used manikin, 3.7 kp/cm; mean difference). The endurance test lasting for 8 h (altogether 27 000 compression cycles) did not reveal any changes in the compression characteristics. Ambu CPR Pal. For this manikin about 8 kplcm were necessaryto compressthe thorax, irrespective of the actual depth of compression. Acccordingly, the force-depth relationship was linear. Ambu MultiMan. In this manikin, a force of about 9 kp was neededto compressthe chest 1 cm, for 2-4 cm of compression a force of 17.8, 26 and 34.4 kp was needed.A depth of 5 cm was achieved when exerting a force of 40.5 kp. The force-depth relationship is almost linear, yielding a spring constant of 8.2 kp/cm. Driiger CPR-Max. To reach a depth of compression of 1 cm, a force of 7 kp was needed. For thorax compression, of 2-5 cm an additional force of 14 kp (l-2 cm), 16 kp (2-3 cm), 17 kp (3-4 cm) and 15 kp (4-5 cm) was required, which resulted in a spring constant between 10.5 and 13.8 kp/cm. The relationship between force and depth of compression was rather linear for the range of 2-5 cm. At a depth of 4 cm (force 54 kp) the signal indicating correct depth of compression flashed up. Laerdal Resusci Anne. In the brand-new model, a force between 7 and 8 kplcm was required to compressfrom l-4 cm. In order to compressfrom 4-5 cm, an additional force of 14.5kp was needed. In the usedmodels, a force of 8-9 kplcm was needed for compression of l-5 cm. Only after reaching a depth of 4.3 cm, the signal indicating the correct depth of compression was triggered in the manikin which had been precisely measuredusing the 1 mm cardboard paddings. The other 3 models failed to signal that the correct depth of compression had been reached at 4 cm. During an endurance test lasting for 8 h (27 000 compressions), no changes
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in the force required for a compression of 5 cm were observed. Laerdal Little Anne. Up to a depth of 3 cm, a force of about 10 kplcm was needed for compression. For 4 cm of compression, the additional force required decreasedto 6 kp. Finally, for compressing the thorax from 4 to 5 cm, an additional force of only 2 kp was required. Hence, up to a compression depth of 3 cm a linear relationship between force and depth of compression was observed, whereas between 3 and 5 cm this relationship was no longer linear. When compressing the manikin more than 5 cm, an acoustic signal (‘click’) was triggered. Laerdal Family Trainer. For 1 and 2 cm of compression, 14 and 30 kp, respectively, were needed in the first test series. Between 2 and 5 cm, a smaller force (7-8 kp/cm) was required for compression. When repeating the measurementson an other manikin of the same type, differences between 13-21% were observed at a depth of compression of up to 2 cm, at a depth of 3,4 and 5 cm the repeated measurements revealed only minor differences (< 8%). 4. Discussion Eight brand-new and 4 used CPR manikins of 4 different manufacturers were tested as to their resistanceduring dynamic chest compressionusing a mechanical test apparatus. The compression characteristics of each manikin were determined and assessedwith regard to possible differences. Then thesecharacteristics were compared with the data provided by the manufacturers. It was essential for the study that the reproducibility of each compression cycle was guaranteed. This was achievedby a pneumatically-driven and electricallycontrolled CPR thumper which had already been successfullytested in animal experiments [5]. Due to the rectangular, plateau phase-like force pattern chosen, the force applied could be exactly measured by the force transducer during the plateau (compression) phase. The depth of compression deviated by less than 5% (0.5 mm at a depth of 10 mm). Thus a simultaneous measurement of the stroke length was superfluous. Nevertheless, during the first pilot tests, the depth of compression was directly determined by a linear
M.A. Baubin et al./Resu.scitation
distance measurement system. As the transducer could only be fixed in a slanted position and not directly overhead the compression point (especially when testing the female manikins), this kind of distance measurement yielded rather unprecise results. The compression patterns remained identical within the individual compression cycles, regardlessof the depth of compression (seeFig. 3). The same applied for the endurance tests. Nevertheless,with the exception of some models, 2 data sets (2nd and 9th compressions) were evaluated. No differences could be found. Thus, the data evaluated represent single but controlled measurements and we may conclude that those measurementswhere only one data set was obtained may also be considered precise. The inaccuracies in the measurement and reading of force or depth (0.5 kp or 0.5 mm) result in an error of < 10%of the calculated spring constant (coefficient b of the polynomial) in the low range of either depth or force (< 15 kp). In the compression range as used for normal CPR (3-5 cm), these inaccuracies cause an error of only 4%. Thus, our measurements appear to be accurate enough to quantify even minor differences in the chest elasticity of the manikins. When comparing the results for small depth of compression, differences of up to 50% could be observed between the manikins: the force needed
rI I I:’LI Ill ii 11 ;:
l
time (10 mm/s)
Fig. 3. Original record of force during the compression and relaxation phase (10 cycles) with a depth of compression preset to 5 cm. For calculating the thorax resistancedata from the 2nd and 9th cycles were read off from the respective plateau phase (arrows). Time axis, 25 mm/s; scale, compression force as indicated.
30 (1995) 117-126
123
for a compression of 1 cm varied between 6.3 and 14 kp. The female models required a force of up to 9 kp/cm. The differences increased with greater depth of compression (about 124% at 3 cm; at 4 and 5 cm, the necessaryforce varied up to 140%). Out of the 5 cheaper manikins (Actar 911, Ambu MultiMan, Ambu CPR Pal, Laerdal Little Anne, Laerdal Family Trainer) only Ambu MultiMan has a built-in spring. Accordingly, it shows a linear characteristic which is similar to Ambu CPR Pal but differs from that of Ambu Man (set at ‘high’) with regard to the value of the spring constant. According to their design, all the manikins with a built-in spring in the chest, but also Ambu CPR Pal with its stiff plastic form, basically showed a nearly linear relationship between force and depth of compression. Ambu Man, set at ‘high’, revealed an even slightly increasing thorax stiffness (Table 2; positive coefticient in polynomial c), an unexpected result when considering the mechanical construction of Ambu Man. However, according to Tsitlik et al. [3] and Bankmann et al. [8], such a compression characteristic would resemble that of the human chest (resistance of the thorax increases with the depth of compression) and would therefore better meet the requirements for manikins. Ruben and Johansen [2], on the other hand, postulated a linear relationship between force and depth of compression, at least until a sternum or rib fracture occurs. Thus, in summary, it may be stated that the manikins DrHger CPR-Max, Ambu Man, Ambu CPR Pal, Ambu MultiMan, and Laerdal Resusci Anne, at least to some extent, meet the compression characteristics demanded by Tsitlik et al. [3] and Bankmann et al. [8], whereas the models Actar 911, Laerdal Little Anne and Laerdal Family Trainer fail to satisfy these demandsbecauseof their particular convex depthforce compression relationship. Especially the non-linear characteristic of Actar 911 in no way reflects the characteristics of a human thorax, becausethis manikin offers sufficient chest resistance only up to 2 cm (spring constant, approximately 8 kp/cm) and from 3 cm onwards its thorax can be further compressedwith a force of only 4 kp/cm. Such a compression characteristic does not convey a feeling of correct CPR. SimilQ, Ambu Man in
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M.A. Baubin et al./Resuscitation
‘low’ setting has a rather ‘soft’ compression characteristic (5.8 kp/cm); nevertheless,this model offers a linear force-depth relationship. Although the force-depth relationship of Laerdal Little Anne is to be considered non-linear, the measurement results of this manikin, as compared to the results of Ambu Man, are more satisfactory. This is due to the fact that at least a force of 36 kp is neededto attain a depth of compression of 4 cm, which would correspond to the female compression characteristic. In order to show the relationship between the force of compression and the resulting depth of compression mathematically, a polynomial of second degreewas adjusted to the experimental data. From the regression coefficient r2 > 0.995 it has becomeevident that the chosen function reflected the characteristic quite accurately. The nearer the polynomial coefficient c approacheszero the more linear the force-depth relationship becomes.With the ‘linear’ manikins the polynomial approximation showed data between 7.5 and 9.9 kp/cm for the sex-neutral and male manikins, for Ambu Man in ‘Low’ setting 5.2-6.9 kp/cm or in the caseof the female Laerdal Resusci Anne 5.3 kp/cm for the new model and 8.5 kp/cm for the used ones. It remains unclear why the spring constant was distinctly higher in the used Laerdal ResusciAnne. As this was confirmed in 3 manikins, differences between the individual manikins seem most probable; in any case,a smaller female chest resistance becameevident . No sign of fatigue could be detected either with Ambu Man or with Laerdal Resusci Anne during the tests of a duration of more than 8 h with a compression depth of 5 cm. With Laerdal Resusci Anne, however, the chest remained slightly compressed after the endurance test, also when exerting no force. There was a considerable discrepancy between our results and the specifications given by some of the manufacturers. For Ambu Man, when set at ‘low’, the manufacturer’s data claimed a resistance of 6 N/mm (0.6 kp/mm), which means that a force of 30 kp is neededto compressthe chest 5 cm, provided a linear force-depth relationship is assumed. Our 3 measurementsyielded forces of 30, 34 and 35 kp. At the y”gh’ setting, the manufacturer statesa resistanc&of 11 N/mm; our measurements
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(50, 55, 57, 60 and 61 kp) are in good agreement with the specified data. Among all the models, only Ambu Man reveals the technical feature of adjustable chest resistance. But as its resistance is only known for the maximum and minimum settings (‘high’ and ‘low’), additional data on the intermediate settings should be provided by the manufacturer. With Drager CPR-Max, we needed 69 kp in order to compressthe manikin by 5 cm instead of 300N (about 30 kp) as stated in the data sheet.According to the manufacturer the thorax resistance of this manikin should correspond to the thorax resistanceof a 30-year-old man, which, however, is irrelevant for CPR training becauseCPR victims rarely belong to this age group. As most of the CPR victims are between60 and 80 years old, CPR manikins should comply with the thorax resistance of this age group. Also with Laerdal Resusci Anne, our results did not fully agreewith the thorax resistanceas stated by the manufacturer (35 and 47 kp, for 38 and 51 mm compression, respectively). We found a somewhat smaller force for 4 cm (29-35 kp) and for 5 cm (37-46.5 kp) compression. It was interesting, that the data obtained with a brand-new manikin were in better agreement (46.5 kp for 50 mm compression). Information on how the manufacturers determine the thorax resistances of their manikins is lacking. In addition, some manikins show a nonlinear characteristic in their thorax resistance which was not pointed out, which underlines the importance of our measurements.To our knowledge, the present investigation is the first to provide quantitative data on training manikins. The American Heart Association recommendsa depth of compression of 1.5-2 inches (3.81-5.08 cm) for the adult [6], the European Resuscitation Council [7] recommends a depth of 4-5 cm. The manufacturers use these data when laying down the monitoring criteria for the indication of correct depth of compression. However, with the Laerdal Little Anne manikin we tested, a clicking sound warned the trainee not to increasethe compression any further when he had already reached 5 cm. This triggering characteristic invalidates the intention of such an alarm. According to the manufacturer, with future manikins the trainee will be
M.A. Baubin et al./Resuscitation
alerted when he compresses3.8 cm, which would more accurately reflect the guidelines. In the manikins with the option for monitoring both a sufficient (3.8 cm) or an exceedingly high (5 cm) depth of compression (Driiger CPR-Max and Laerdal Resusci Anne), the signal for sufficient depth of compression was triggered at 4 cm (Driiger CPR-Max), whereaswith Laerdal Resusci Anne this signal was set off at 4.3 cm rather than at 3.8 cm. If this holds true for all manikins of the Laerdal Resusci Anne type (and not only for the one we have precisely checked for its triggering characteristics), the trainee will be warned too late and might have already exerted too much force. The criterion relevant for efficient chest compression is the depth of compression rather than the force applied to the sternum. Cardiac output predominantly depends on the compression rate and on the mean arterial pressureachieved, which, in turn, depends on the depth of compression [9]. Therefore, it is of utmost importance that CPR trainees learn to assessthe correct depth of compression. In general, a depth of compression of 20-25% of the anterior-posterior diameter should be aimed at. Chest diameters of an average adult range between 15 and 25 cm. Thus, with the Laerda1Family Trainer (a.-p. diameter, 14 cm) the relevant depth would be 2.8-3.5 cm; apparently a depth of compression of 5 cm does not comply with the a.-p. diameter of this manikin. We found the male manikins to have a larger a.-p. diameter (measured in the lower part of the sternum) than the female ones. The manikins Actar 911 and Laerdal Family Trainer are sex-neutral. According to our results, neither the a.-p. diameter nor the appearance provide a reliable hint as to the manikin’s actual chest resistance. In this context, the chest resistanceof Laerdal Little Anne appears to be very high for a female manikin, at least in the range of up to 4 cm compression. Similarly, the used models of Laerdal Resusci Anne have a rather high spring constant. In contrast, Ambu Man, a male manikin, when set at ‘low’, shows a soft linear thorax resistance which resembles a typically female characteristic. This study was not primarily directed to investigate to what extent the human thorax characteristic is simulated by the manikins. There are not yet enough data available about the rela-
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tionship of force and depth of compression in humans [2,3,9]. CPR specialists are familiar with the great differences in thorax resistance of CPR victims and they also know that rib fractures, age and chest-compliancealter the thorax resistance.These real-life situations can, by no means, be satisfactorily simulated by the training manikins presently available. Therefore, the most important issue to be thoroughly discussed and paid attention to, during CPR training, is the thorax resistance of the individual manikin. Already Tsitlik et al. [3] pointed out the danger of injury as a result of incorrect CPR training. A trainee who practises only on a manikin with high thorax resistance will becomeaccustomedto applying too much force of compression, but also too little force results in insufficient CPR. From our tests it can be concluded that, in order to perform CPR correctly on any patient, CPR instructors and trainees should be encouraged to practise on different manikins, rather than to use the sametype of manikin all the time. The development of Ambu Man, the manikin with adjustable thorax resistance, may be considered a first step in this direction, even though it is highly questionable if a thorax resistance with a linear force-depth relationship is comparable to the human chest under CPR conditions. Acknowledgements The authors thank the companies Chemomedica Vienna, Driiger Austria and Roraco Vienna as well as the Vamed Schulungszentrum Vienna for the loan of the training manikins. Ing. R. Schwarz helped with the technical development of the measurementdevice and, together with Mr. S. Milovic, evaluated the recordings. The Department of Biostatistics of the University of Innsbruck assisted in the statistical analysis. We also thank Mag. I. Zach for her secretarial assistance.This research project was supported by the Fund of the Austrian Nationalbank (Jubillumsfonds, project No. 4929). References [I]
Kouwenhoven WB, Jude JR, Knickerbocker GG. Closed chest cardiac massage. J Am Med Assoc. 1960; 173: 1064-1067. [2] Ruben H, Johansen SH. Sternal displacement with different loads. Acta Anaesthesiol Stand 1966; 10: 31-36.
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[3] Tsitlik JE, Weisfeldt ML, Chandra N, Effon MB, Halperin HR, Levin HR. Elastic properties of the human chest during cardiopulmonary resuscitation. Crit Care Med 1983; 11: 685-692. [4] Cohen TJ, Tucker KJ, Lurie KG, Redberg RF, Dutton JP, Dwyer KA, Schwab TM, Chin MC, Gelb AM, Scheinmann MM, Schiller NB, Callaham ML. Active compression-decompression- A new method of cardiopulmonary resuscitation. J Am Med Assoc 1992; 267: 2916-2923. [5] Mauritz W. Hea-Lungen-Wiederbelebung Experimenteller Vergleich verschiedener Verfahren. Beitr Anlsth Intensiv Med, Bd. 28. Wien: Maudrich, 1989; 20-23. [6] American Heart Association. Guidelines for cardiopul-
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monary resuscitation and emergencycardiac care. J Am Med Assoc 1992;268: 2189-2191. [7] Basic Life Support Working Party of the European Resuscitation Council. Guidelines for basic life support. Resuscitation 1992;24: 103-I 10. [8] Bankman IN, Gruben KG, Halperin HR, Pope1 AS, Guerci AD, Tsitlik JE. Identification of dynamic mechanical parameters of the human chest during manual cardiopulmonary resuscitation. IEEE Trans Biomed Eng 1990;37: 211-217. [9] Omato JP, Levine RL, Young DS, Racht EM, Gamett AR, Gonzalez ER. The effect of applied chest compression force on systemicarterial pressureand end-tidal carbon dioxide concentration during CPR in human beings. Ann Emerg Med 1989; 18: 732-737.