Quality of chest compressions during pediatric resuscitation with 15:2 and 30:2 compressions-to-ventilation ratio in a simulated scenario

Abstract

The main objetive was to compare 30:2 and 15:2 compression-to-ventilation ratio in two simulated pediatric cardiopulmonary resuscitation (CPR) models with single rescuer. The secondary aim was to analyze the errors or omissions made during resuscitation. A prospective randomized parallel controlled study comparing 15:2 and 30:2 ratio in two manikins (child and infant) was developed. The CPR was performed by volunteers who completed an basic CPR course. Each subject did 4 CPR sessions of 3 minutes each one. Depth and rate of chest compressions (CC) during resuscitation were measured using a Zoll Z series defibrillator. Visual assessment of resuscitation was performed by an external researcher. A total of 26 volunteers performed 104 CPR sessions. Between 54–62% and 44–53% of CC were performed with an optimal rate and depth, respectively, with no significant differences. No differences were found in depth or rate of CC between 15:2 and 30:2 compression-to-ventilation ratio with both manikins. In the assessment of compliance with the ERC CPR algorithm, 69.2–80.8% of the subjects made some errors or omissions during resuscitation, the most frequent was not asking for help and not giving rescue breaths. The conclusions were that a high percentage of CC were not performed with optimal depth and rate. Errors or omissions were frequently made by rescuers during resuscitation.

Full text

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Quality of chest compressions
during pediatric resuscitation with
15:2 and 30:2 compressions-to-
ventilation ratio in a simulated
scenario
Gema Manrique1,2,3,4, Araceli González2, Maitane Iguiñiz2, Ana Grau2, Blanca Toledo5,
Miriam García1,3,4 & Jesús López-Herce1,2,3,4 ✉
The main objetive was to compare 30:2 and 15:2 compression-to-ventilation ratio in two simulated
pediatric cardiopulmonary resuscitation (CPR) models with single rescuer. The secondary aim was to
analyze the errors or omissions made during resuscitation. A prospective randomized parallel controlled
study comparing 15:2 and 30:2 ratio in two manikins (child and infant) was developed. The CPR was
performed by volunteers who completed an basic CPR course. Each subject did 4 CPR sessions of
3 minutes each one. Depth and rate of chest compressions (CC) during resuscitation were measured
using a Zoll Z series debrillator. Visual assessment of resuscitation was performed by an external
researcher. A total of 26 volunteers performed 104 CPR sessions. Between 54–62% and 44–53% of
CC were performed with an optimal rate and depth, respectively, with no signicant dierences. No
dierences were found in depth or rate of CC between 15:2 and 30:2 compression-to-ventilation ratio
with both manikins. In the assessment of compliance with the ERC CPR algorithm, 69.2–80.8% of the
subjects made some errors or omissions during resuscitation, the most frequent was not asking for help
and not giving rescue breaths. The conclusions were that a high percentage of CC were not performed
with optimal depth and rate. Errors or omissions were frequently made by rescuers during resuscitation.
Pediatric cardiac arrest (CA) is an important health problem since it has high mortality (52–80%) and a large
proportion of survivors suer from permanent and severe neurologic disability (poor outcome in 20–50%)1–4.
Previous studies have shown that optimizing maneuvers of cardiopulmonary resuscitation (CPR) in children
could improve survival and prognosis4,5. Nevertheless, many studies documented that CPR is oen not optimally
performed6,7.
The 2015 European Resuscitation Council (ERC) guidelines recommend a synchronized 15:2
compression-to-ventilation ratio during basic pediatric CPR1,8,9. However, there is not a strong evidence to indicate
the best compression-to-ventilation ratio in pediatric CPR10,11. Firstly, maintaining an adequate ventilation is crucial
during CPR in children because pediatric CA is primarily caused by respiratory failure2. e 15:2 ratio delivers more
ventilations whereas the 30:2 ratio delivers more chest compressions (CC). In the other hand, dierent recommen-
dations for the compression-to-ventilation ratio between children (15:2 ratio) and adults (30:2 ratio) could increase
the errors or omissions and impair learning. In adult simulation models12–14, there are dierent studies that compare
the quality of CC in both compression-to-ventilation ratio. However, only one study conducted in pediatric models,
but it was developed by volunteers with pediatric advanced life support (PALS) accreditation15. In our knowledge,
there are no previous studies comparing the quality of CC between 15:2 and 30:2 compression-to-ventilation ratio
during pediatric CPR conducted by volunteers training in basic life support.
1Pediatric Intensive Care Department, Gregorio Marañon University Hospital, Madrid, Spain. 2Public Health,
Maternal and Child Department, School of Medicine, Complutense University of Madrid, Madrid, Spain. 3Health
Research Institute of the Gregorio Marañón Hospital, Madrid, Spain. 4Research Network on Maternal and Child
Health and Development (RedSAMID), Madrid, Spain. 5Pediatric Department, Gregorio Marañon University Hospital,
Madrid, Spain. ✉e-mail: pielvi@hotmail.com
OPEN
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Hypothesis and objectives. e hypothesis of the study is that the quality of chest compressions is the
same with 15:2 as 30:2 ventilation-to-compression ratio in a pediatric simulation model. For this purpose, the
main objective was to compare the quality of chest compressions and performance of single rescuer CPR com-
paring two compressions-to-ventilation ratios 15:2 and 30:2 in a simulated scenario with two manikins (infant
and child model). e secondary aim was to describe the visual assessment of the quality of resuscitation in both
scenarios.
Methods
Design. We designed a prospective randomized parallel controlled study of simulated pediatric CPR to com-
pare the depth and rate of CC using a 15:2 vs 30:2 compression-to-ventilation ratio in manikins. e CPR was
performed by medical students and pediatric residents and two manikins were used: infant and child model. e
study was approved by the Gregorio Marañón Ethics Committee of Madrid, Spain.
Participants. Twenty-six volunteer senior medical students who recently completed a basic pediatric CPR
course. All participants provided their informed consent.
Equipment and materials. Two CPR training manikins were used: Resuscijunior and Resuscibaby
(Laerdal, Wappingers Falls, NY, USA). CPR electrodes were applied onto sternum and connected to a Zoll R Series
Monitor/Debrillator (ZOLL Medical Corporation, Chelmsford, MA, USA) to record CPR-quality parameters.
Intervention. Before beginning CPR, participants received a practical demonstration of pediatric and infant
basic life support algorithm in both manikins by one of the senior researchers. All participants could practice
chest compressions and ventilation in both manikins prior to CPR session.
Each rescuer performed four CPR sessions, of three minutes duration each of them, with a resting period of
at least 5 minutes between each period. Two of the CPR sessions were performed on the infant manikin and two
on the child manikin. Two CPR sessions with dierent compression-to-ventilation ratio (30:2 and 15:2) were
developed on each manikin. e order of beginning each session were randomized.
Child CPR was delivered while kneeling beside the manikin on the oor. On the infant manikin, an encircling
technique was used for chest compressions with the manikin lying on a table1.
For child manikin, compression depth between 3.8 and 5.1 centimeters (cm) and a rate between 100 and
120 min−1 were considered optimal. For the infant manikin optimal compression depth was between 3 cm and
4 cm.
e rescuers did not receive feedback of depth and rate of CC from the monitor during CPR sessions.
Measurements. ree researchers trained in pediatric cardiopulmonary resuscitation participated in CPR
sessions. One researcher coordinated CPR sessions, other researcher made sure that the debrillator measured
properly and the third researched visually assessed whether the maneuvers were properly performed.
Data measured and stored in Zoll debrillator were analyzed with RescueNet Code Review program. e
following variables over one-minute intervals were analyzed: depth and rate of CC, release velocity (mm/s), time
without CC and the percentage of optimal compressions (in depth and rate).
Video recordings of the CPR sessions were performed to check the researcher’s visual assessment of the over-
all quality of resuscitation. One of the researchers (always the same one) checked whether the basic life support
algorithm and maneuvers were satisfactorily followed by each rescuer. e opening of the airway was considered
inadequate if any of these events occurred: in the child manikin the rescuer did not perform a good neck exten-
sion or did not pinch the nose, and, in the infant manikin, the rescuer did not have a good placement of the hands
in the front-chin maneuver. Ventilation maneuver was inadequate when the observer researcher considered it to
be too fast, too slow, or excessive or shallow chest rise. Other items assessed were whether the rescuer requested
help, if they checked breathing or pulse and the accomplishment of the order of the basic life support algorithm.
At the end of all sessions, participants were asked which compression-ventilation ratio (15:2 or 30:2) they
preferred in each manikin.
Statistical analysis. e SPSS statistical package, version 20.0 (SPSS Inc, Chicago, USA) was used for statis-
tical analysis. Normal distribution of variables was tested with the Kolmogorov-Smirnov test. Continuous varia-
bles are expressed as mean and standard deviations or medians with interquartile ranges and categorical variables
as percentages. Paired Student t-test was used to compare continuous variables. McNemar’s test was used to
compare categorical variables with paired data. P values less than 0.05 were considered signicant.
Statement. All methods were carried out in accordance with relevant guidelines and regulations.
Results
Twenty-six volunteers, 53.8% men, performed 104 resuscitation sessions (four per rescuer). All of them had pre-
viously completed a pediatric CPR course, 7.8 ± 1.0 months before the study.
Parameters of CPR quality. Table1 showed CC rate in the infant and child manikin (including variables
of data collection sheet and those analyzed by the program RescueNet Code Review). Only 59.6% and 54.9% of
the CC, respectively, were performed with a guideline recommended rate. ere were no dierences in the rate of
CC between the ratio 15:2 and 30:2.
ere were no dierences in the percentage of optimal compressions in depth between the 15:2 and 30:2
ratios, neither in the child or the infant manikin (Table2).
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Deeper of CC was found in the rst minute (3.1 ± 0.6) than in the third minute (3 ± 0.5) of resuscitation in
infant model with 30:2 ratio (p = 0.018). In the rest of resuscitations, although depth and rate were slightly higher
in the rst than in the third minute, no signicant dierences were found with both ratios (15:2 and 30:2) and
manikins. No dierences were found in rate of CC, nevertheless, the number of CC performed in each minute in
the third minute was higher than in the rst minute with both compression-to-ventilation ratios and manikins
(p < 0.05).
In the infant manikin, 15:2 ratio had higher percentage of CC performed with optimal depth than 30:2. With
both manikins the ratio 30:2 achieved a higher number of compressions and less time without compressions than
the ratio 15:2. e release velocity was higher with the ratio 15:2 at the end of the resuscitation (Table2).
Visual assessment of the quality of resuscitation. Compliance with European CPR protocol was ana-
lyzed: in the child manikin some errors or omissions were found in 69.2% of resuscitations with 30:2 ratio and in
80.8% with 15:2 ratio. In the infant manikin there were CPR algorithm errors in 69.2% of the resuscitations with
30:2 ratio and in 73,1% with 15:2 ratio. Over the half of the volunteers (53,8%) performed at least one CPR session
without any errors or omissions, most of them (64,3%), executed without errors more than one session. e most
common error in both manikins and both ratios was do not give rescue breaths (23.1%) and not ask for help or do
it too late (21.2%). Errors or omissions performed during resuscitation are shown in Fig.1. No dierences were
found in errors or omissions between the 15:2 and 30:2 ratio in both manikins.
In the visual assessment of the resuscitation: airway opening in the child manikin was adequate in 65.4% of the
resuscitations with 30:2 ratio and in 50% of 15:2 ratio. In the infant manikin it was adequate in 84.6% of the resus-
citations with both ratios. Ventilation maneuver in child manikin was adequate in 92.3% of the resuscitations
Parameters
Children manikin Infant manikin
15:2 ratio 30:2 ratio
p
15:2 ratio 30:2 Ratio
p(n = 26) (n = 26) (n = 26) (n = 26)
Mean rate (min-1)
Mean (SD)
Minute 1 109.0 (9.2) 109.0 (11.8) 0.98 106.3 (10.7) 107.8 (11.4) 0.54
Minute 2 109.3 (11.1) 108.2 (12.3) 0.63 106.2 (10.4) 107.3 (11.9) 0.64
Minute 3 108.9 (10.4) 108.2 (11.8) 0.74 105.4 (12.1) 106.6 (13.0) 0.64
Overall 109.1 (11.5) 108.5 (12.7) 0.78 108.3 (13.7) 107.2 (12.4) 0.68
Percentage of chest compressions
Mean (SD)
Lower rate than optimal 14.3 (22.6) 20.3 (30.6) 0.35 18.5 (21.0) 23.8 (33.2) 0.39
Optimal rate 55.8 (35.1) 54.1 (39.1) 0.83 62.1 (31.5) 57.0 (31.1) 0.55
Higher rate than optimal 29.4 (34.5) 28.8 (40.1) 0.94 19.4 (31.5) 19.2 (25.8) 0.98
Number of chest compressions
Mean (SD)
Minute 1 54.4 (10.4) 67.5 (12.4) <0.01 56.1 (9.5) 67.3 (12.2) <0.01
Minute 2 64.0 (11.9) 81.0 (10.7) <0.01 66.1 (10.8) 80.9 (10.6) <0.01
Minute 3 63.1 (10.5) 80.6 (11.9) <0.01 65.3 (11.8) 80.9 (10.1) <0.01
Mean 60.5 (10.2) 76.4 (9.0) <0.01 62.5 (8.9) 76.4 (9.0) <0.01
Time without compressions (seconds in each CPR session)
Mean (SD) 74.0 (12.6) 45.3 (7.1) <0.01 68.4 (18.6) 42.9 (11.3) <0.01
Table 1. Rate of chest compressions. Paired T-student test.
Parameters
Children manikin Infant manikin
15:2 ratio 30:2 ratio
p
15:2 ratio 30:2 r atio
p(n = 26) (n = 26) (n = 26) (n = 26)
Mean depth (cm) Mean
(SD)
Minute 1 4.4 (0.8) 4.4 (0.9) 0.84 3.1 (0.5) 3.1 (0.6) 0.62
Minute 2 4.4 (0.8) 4.3 (0.8) 0.81 3.0 (0.5) 3.0 (0.6) 0.99
Minute 3 4.4 (0.8) 4.3 (0.8) 0.45 3.1 (0.5) 3.0 (0.5) 0.07
Global 4.4 (0.8) 4.3 (0.8) 0.66 3.1 (0.5) 3.0 (0.5) 0.41
Percentage of chest
compressions Mean (SD)
Lower depth than
optimal 27.7 (37.6) 31.5 (41.3) 0.35 44.9 (39.2) 51.8 (41.3) <0.01
Optimal depth 48.4 (37.3) 44.1 (38.1) 0.83 52.7 (36.8) 43.7 (38.0) <0.01
Higher depth than
optimal 23.9 (36.4) 24.4 (36.5) 0.94 2.4 (4.9) 4.5 (14.5) 0.015
Release velocity (mm/s)
Mean (SD)
Minute 1 300.7 (59.4) 296.5 (79.9) 0.71 239.0 (44.3) 243.2 (61.6) 0.011
Minute 2 284.2 (56.6) 273.2 (64.7) 0.22 226.7 (45.3) 213.8 (44.4) <0.01
Minute 3 288.9 (58.9) 266.0 (65.1) 0.017 223.5 (47.0) 208.8 (41.6) <0.01
Mean 291.3 (54.4) 278.6 (67.3) 0.13 229.8 (42.7) 221.9 (45.8) 0.23
Table 2. Depth and release velocity of CC. Paired T-student test.
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with 30:2 ratio and in 76.9% with 15:2 ratio. In the infant manikin it was adequate in 88.5% with 30:2 ratio and in
80.8% with 15:2 ratio. No dierences were found between both ratios for any of the two manikins.
Preferred compression-to-ventilation ratio. For the child manikin, 65.4% of the rescuers preferred
30:2 ratio. In contrast, for infant manikin, 76.9% of the participants reported that 15:2 ratio was more comfortable
because 30:2 produced more weariness of the ngers.
Discussion
e main objective of our study was to compare CC quality between 30:2 and 15:2 compression-to-ventilation
ratio in pediatric and infant manikins. In our study, there were no dierences in the rate or depth of CC between
the ratio 15:2 and 30:2. A high percentage of compressions were not performed with optimal rate and depth.
Furthermore, depth and rate of CC were not modied throughout the CPR session. e results could not allow
to conclude that a compression-ventilation ratio is better than the other. Our study highlights the relevance of
periodic recertications.
No differences were found in depth or rate of CC in both manikins between 15:2 and 30:2
compression-to-ventilation ratio. In our knowledge, there are no studies in pediatric CPR comparing CC quality
in 15:2 or 30:2 compressions-to-ventilations ratios developed by volunteers without PALS accreditation. ere is
only one study11 that compared, in manikin-simulated pediatric resuscitation, three compressions-to-ventilations
ratios (5:1, 10:2 and 15:2). In this study there were no dierences in depth and rate between the three ratios.
Although depth and rate of chest compressions is the same with both compression-to-ventilation ratios, it is
not clear whether survival increases with increasing number of ventilations or prioritizing chest compressions.
In a study of adults with out-of-hospital cardiac arrest, continuous CC without ventilations did not result in sig-
nicantly higher rates of survival or favorable neurologic function than 30 compressions to two ventilations16.
In other similar study the percentage of one-month survival patients with good neurological outcome was lower
with continuous CC, however, in the multivariate analysis the continuous CC group showed better neurological
outcome than the CC plus ventilation group17. In a pediatric asphyxial arrest animal model18 CC plus ventilation
produced better oxygenation, ventilation, and cerebral oxygenation than compression-only CPR. In observa-
tional studies of children resuscitation who had out-of-hospital CA19,20 CPR with CC plus ventilation produced
better outcomes than compression-only CPR. e fact that the main cause of pediatric CA is respiratory failure
could be the reason for better outcomes of CPR with CC plus ventilations than compression-only CPR.
We found slightly higher chest compression release velocity with 15:2 ratio but these dierences are not rel-
evant. One study showed the association of chest compression release velocity with higher survival and favora-
ble neurologic outcome aer out-of-hospital cardiac arrest in adults patients21. In this study the adjusted odds
of survival increased from slow (<300 mm/s) to fast release velocity (≥400 mm/s) and from moderate (300–
399.9 mm/s) to fast (≥400 mm/s) release velocity. It should be noted that in our study the release velocity in all
groups oscillated between 220 and 300 mm/s, corresponding in all cases with the slow velocity. However, the
inuence of CC release velocity and survival is still controversial because other studies have found no relationship
between them22. More studies are needed to analyze the role of this parameter in the quality of resuscitation.
ere is good evidence supporting the use of CPR feedback/prompt devices during CPR training to evaluate
and improve CPR quality23. In our study, we use one device to analyze CC quality. We also used video recordings
to asses pediatric resuscitation management and to detect errors or omissions during resuscitation because pre-
vious studies showed that it is a useful tool for this purpose24.
Some studies have investigated optimal depth of CC but there is a lack of evidence in infants and children
CC25–27. Although deeper CC have shown higher arterial blood pressure28, excessive CC depth may cause serious
mechanical complications25. In both manikins but, especially, in the infant model, high percentage of CC were
shallower than recommended. is fact may be due to the diculty of performing CC in an infant manikin encir-
cling the chest and compressing only with the thumbs. Smereka and Ladny designed a new technique for chest
compressions in infants using two thumbs directed at the angle of 90 degrees to the chest while closing the ngers
of both hands in a st. ey described this new technique and compared it in two studies performed on manikins
with those maneuvers used routinely29,30 showing that a higher simulated blood pressure was reached with this
new technique29,30. As in our study, other studies that have analyzed the quality of CC in pediatric manikins,
Figure 1. CPR algorithm errors or omissions during resuscitations.
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found a depth of CC lower than recommended7. erefore, shallow CC could also be related to low delity of the
manikin used.
Vaillancourt et al.13 developed an adult simulated CPR manikin cross-over study with elderly volunteers that
compare 15:2 to 30:2. is study showed that the 15:2 ratio resulted in proportionally more adequate compres-
sions (dened as depth of 4–5 cm followed by full decompression during each minute). ey used a metronome
to encourage the administration of chest compressions at a rate of 100 per minute, so rate of CC should not be
evaluated.
In our study, in both compression-to-ventilation ratios, an important percentage of CC were performed with
a higher or lower rate than optimal (>120 or <100 cpm). It should be noted that, although half of CC were per-
formed with optimal rate, the number of CC performed in each minute is not high, which can be explained by
the time used for ventilation. With the ratio 30:2 the number of CC performed was higher than the ratio 15:2. In
a study by Haque et al.15 more compression cycles were achieved with 30:2 ratio without eect in compression
depth and rate, similar to our results. However, they reported higher subjective fatigue in the 30:2 ratio.
Depth and rate of chest compressions did not change signicantly during the three minutes of CPR with
any of the compressions-to-ventilation ratios in our study. is might have been because each CPR session
lasted only 3 minutes. Nevertheless, other published studies with simulated CPR sessions and shorter periods
found a decrease in depth over time26. Vaillancourt et al.13 measured objective fatigue by changes in heart rate,
mean arterial pressure and venous lactate, and perceived level of exhaustion using the validated Borg Rating of
Perceived Exertion scale. ey have found that the 30:2 ratio resulted in similar objective measures of fatigue,
but higher perceived fatigue than the 15:2 ratio. In our study, 60% of the rescuers felt that 30:2 ratio was more
comfortable than 15:2 in the child model. is could be due to the smaller number of position changes for
ventilation that needs to be done in 30:2 ratio. In contrast, 82% of the volunteers reported that the 15:2 ratio
was more comfortable for infant CPR, attributing it to the fact that the 30:2 ratio caused greater weariness of
the ngers.
We have observed that, despite having done a pediatric CPR course in the last year, a high percentage of res-
cuers made some errors or omissions in CPR algorithm, the most frequent giving ve initial recue breaths and
forgetting to ask for help. Rescuers who performed without error or omissions one CPR session, usually did it
without errors in another session. is may be because some volunteers internalize knowledge better than oth-
ers. ere is no agreement in which the best time interval for doing CPR recertication courses is and the best
method to keep the skills acquired. Traditionally it has been recommended that recertication should be done
every 1 or 2 years at most31. Our results highlight the importance of frequent refreshment of CPR training, pos-
sibly between 6 months and 1 year.
e study is a pediatric simulation model with manikins that compare to compression-to-ventilation ratios.
Experimental or clinical studies should be carried out, in which other factors that may aect survival, such as
ventilation, oxygenation, blood pressure, etc., will be analyzed in addition to the quality of CC.
e scope of the study is limited to single rescuer CPR. It could be dierent in situations in which two rescuers
are providing CPR. e pauses of transitions between compressions and ventilations by a single rescuer should
be longer than when CPR is performed by more than one rescuer. e study also has the inherent limitations
of those performed with manikins. Experimental and clinical studies are needed to analyze the eect of the
two compression-to-ventilation ratios and the interaction between quality of chest compressions and ventilation
and oxygenation achieved during resuscitation. Other potential limitation was that the researchers could not be
blinded to resuscitation method and the small sample size. e assessment of the resuscitations maneuvers as
opening of the airway and ventilation was carried out subjectively, although evaluation criteria were dened pre-
viously. e qualitive assessment was performed by the same researcher to avoid interobserver variability. Finally,
in our study participant expressed their preference between 15:2 or 30:2 ratio from and subjective point of view,
but no other objective fatigue scale was used as in Vaillantcourt13 stu dy.
Conclusions
In this model of pediatric CPR simulation, a high percentage of chest compressions were not performed
with optimal frequency and depth. No dierences were found in depth or rate of CC between 15:2 and 30:2
compression-to-ventilation ratio with both manikins. Depth and rate of CC were not modified during de
CPR session. e participants preferred 30:2 ratio in the child manikin and 15:2 ratio in the infant manikin.
Approximately three quarters of the participants performed errors or omissions during resuscitation.
Received: 17 January 2020; Accepted: 6 April 2020;
Published: xx xx xxxx
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Acknowledgements
We acknowledge to all volunteers who participated in the study.Funding:This work was supported by the
PI15/00743 of Carlos III Health Institute, responsible for Spain’s Health Research and Development Strategy,
co-nanced by the European Union ERDF funds (European Regional Development Fund) and the Mother-Child
Health and Development Network (Red SAMID) - RETICS funded by the PN I+D+I 2013-2016 (Spain), ISCIII-
Sub-Directorate General for Research Assessment and Promotion and the European Regional Development Fund
(ERDF), ref. RD16/0022. Funders did not participate in the study design, dataset collection and analysis and
writing the manuscript.
Author contributions
G.M. was a major contributor in writing the manuscript and contributed in interpretation of data and statistical
analysis. A.G., M.I. and A.G. contributed with the acquisition of the data and developedCPR sessions. B.T.
contributed with the acquisition of the data. M.G. contributed to dra the work and with the acquisition of the
data. J.L.H. contributed to the conception, design of the work and revised the manuscript. All authors read and
approved the nal manuscript.
Competing interests
e authors declare no competing interests.
Additional information
Correspondence and requests for materials should be addressed to J.L.-H.
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