Eur J Cardiothorac Surg 2003;23:143-148
© 2003 Elsevier Science NL
Rupture of the aorta following road traffic accidents in the United Kingdom 1992–1999. The results of the co-operative crash injury study
D. Richensa*,
K. Kotidisa,
M. Nealeb,
C. Oakleyb,
A. Failsb
a Department Cardiothoracic Surgery, Nottingham City Hospital, Hucknall Road, Nottingham NG 5 1PB, UK
b Transport Research Laboratory (TRL Limited), Crowthorne, UK
Received 19 June 2002;
received in revised form 15 October 2002;
accepted 21 October 2002.
* Corresponding author. Tel.: +44-115-969-1169; fax: +44-115-840-2605
e-mail: drichens{at}ncht.trent.nhs.uk
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Abstract
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Objective: The true incidence and survivability of blunt traumatic aortic rupture following road traffic accidents in the UK is unclear. The objective of this study was to determine the extent of blunt traumatic aortic rupture in the UK after road traffic accidents and the conditions under which it occurs. Methods: Data for the study was obtained from the Co-operative Crash Injury Study database. Road traffic accidents that happened between 1992 and 1999 and included in the Co-operative Crash Injury Study database were retrospectively investigated. Results: A total of 8285 vehicles carrying 14 435 occupants were involved in 7067 accidents. There were 132 cases of blunt traumatic aortic rupture, of which the scene survival was 9% and the overall mortality was 98%. Twenty-one percent of all fatalities had blunt traumatic aortic rupture (130/613). Twenty-nine percent were due to frontal impacts and 44% were due to side impacts. Twelve percent of the blunt traumatic aortic rupture cases in frontal vehicle impacts were wearing seat belts and had airbag protection and 19% had no restraint mechanism. The Equivalent Test Speed of the accident vehicles, (where equivalent test speed provides an estimate of the vehicle impact severity and not an estimate of the vehicle speed at the time of the accident), ranged from 30 to 110 km/h in frontal impacts and from 15 to 82 km/h in side impacts. Conclusion: Blunt traumatic aortic rupture carries a high mortality and occurred in 21% of car occupant deaths in this sample of road traffic accidents. Impact scenarios varied but were most common from the side. The use of an airbag or seat belt does not eliminate risk. The injury can occur at low severity impacts particularly in side impact.
Key Words: Blunt trauma • Aortic rupture
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1. Introduction
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The incidence of blunt traumatic aortic rupture (BTAR) following road traffic accident (RTA) in the UK is unknown, though it is generally thought to be a significant cause of death following trauma. Recent studies reveal that in the USA and Canada around 7500–8000 victims die of this condition a year [1], which in the majority of cases is the result of an automotive accident. These figures represent 5–16% of motoring fatalities in North America.
The fundamental mechanisms responsible for the initiation of BTAR have yet to be fully determined, other than characterisation of the lesion as a deceleration injury. The majority of the published literature detailing this injury tends to concentrate on the diagnosis, pathology and treatment of the disorder in survivors [2–5], while few investigators have attempted to establish the mechanisms that initiate the injury. The reproducible character of the pathological lesion in BTAR, namely a transverse tear at the level of the isthmus of the aorta immediately distal to the left subclavian artery in over 90% of cases [1], intuitively suggests that there is a consistent mechanism leading to the injury which if characterised could be modified by re-designing vehicle interiors and restraint systems. Knowledge of the historical incidence of this injury would enable assessment of the influence of improvements in vehicle and highway engineering to date.
This study was performed in order to assess the incidence and mortality of BTAR following RTAs in the UK over the period 1992–1999 and to investigate the types of impact conditions under which BTAR arises.
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2. Methods
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The co-operative crash injury study (CCIS) database at the Transport Research Laboratory (TRL Limited) contains vehicle accident data gathered from seven specific areas of the UK (Hampshire, Bristol, Manchester, Warwickshire, Staffordshire, Birmingham and Leicestershire) and includes RTA data gathered from June 1992 up to and including the present day. The major sponsor of the work is the UK Government Department for Transport, Local Government and the Regions (DTLR) in addition to a number of co-sponsors from the automotive industry. Current and previous sponsors of the project include Autoliv, Ford Motor Company Ltd, Honda R&D (Europe) Ltd, LAB, Nissan Motor Company Ltd, Toyota Motor Europe, Volvo Car Corporation, Rover Group Ltd and Daewoo.
The study is managed by TRL and numerous institutes, which include the Vehicle Inspectorate in Guildford, Bristol, Manchester, Warwickshire and Staffordshire, the Birmingham Automotive Safety Centre at the University of Birmingham and the Vehicle Safety Research Centre at the University of Loughborough, gather the data. To be included onto the database, accidents must fulfil all of the following criteria:
- at least one of the vehicles in the accident must be a car or car derivative;
- the vehicle is less than 7 years old at the time of the accident;
- the vehicle was sufficiently damaged for it to have been removed (towed away) from the accident scene by a removal agent;
- the vehicle contained at least one injured occupant according to the Police.
The variables recorded include: injuries sustained by the accident victims (categorised using the Abbreviated Injury Scale), where the casualty was seated, whether restraint/safety systems in the vehicle were used, direction of the impact, severity of the impact and behaviour of the vehicle after impact, such as overturning for example.
The study did not include pedestrian injuries.
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3. Results
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3.1. Incidence and survivability
Fig. 1
plots the cumulative frequency of all accidents in the database and shows a reduction of these in the final year of the study. This reduction is mainly due to a time lag between the incidence of RTAs and the time taken to collect all the accident data, after which the RTA data is then recorded in the database. Fig. 2
plots the cumulative frequency of the BTAR cases illustrating that the occurrence of BTAR has had a constant progression over the lifetime of the database study. The percentage of all reported BTAR injury is constant: 22/994 (2.2%) in the 1st year of the study and 8/372 cases (2.1%) in the final year.
At the time of reviewing the database it contained information on a total of 7067 accidents, involving 8285 vehicles and 14 435 occupants. 613 of these occupants were fatalities, 3719 were seriously injured and the remaining 10 103 were slightly injured/uninjured or their injuries were unknown. In the database a total of 132 cases of BTAR had been recorded of which 130 were fatal (21% of all deaths).
The 132 cases of BTAR occurred in 123 vehicles that contained 104 additional passengers who did not suffer BTAR. Of these additional 104 passengers there were 18 fatalities, 41 were seriously injured and the remaining 45 were either slightly injured/uninjured or their injuries were unknown.
Of the 132 cases of BTAR, 120 died before reaching a hospital, six subsequently died before being admitted to hospital and of the other six, only two managed to survive their injuries. This effectively equates to a scene survival rate of around 9% and an overall survival rate of 1.5%. BTAR was observed in over 21% of the fatalities contained on the CCIS database.
3.2. BTAR in vehicles with multiple occupants
Fig. 3
provides a histogram detailing the number of occupants in each of the 123 vehicles in which at least one case of BTAR was recorded. It shows that over 50% of these vehicles contained only one passenger and that 8 were the highest number of passengers contained in a single vehicle in which at least one case of BTAR occurred. Seven multiple cases of BTAR within a single vehicle were found in the database. These findings are presented in Table 1. These results show that multiple cases of BTAR in a single vehicle are not an indication that all the occupants of the vehicle will experience BTAR. There is a wide variation in the injuries sustained by the other non-BTAR vehicle occupants, from only minor injuries in the fourth occupant of vehicle 7 to fatalities due to injuries other than BTAR in vehicles 2 and 5 of Table 1.
3.3. Occupant location in the accident vehicle
BTAR was found in both front and back seat vehicle occupants. Eleven of the 132 BTAR victims were located in the back seat of the vehicle, the location of five of the BTAR cases were unknown and the remaining 116 victims were located in the front seats. The higher number of front seat BTAR cases will be due to the higher frequency of vehicle occupants normally travelling in this part of the vehicle.
3.4. Use of restraint mechanism
For the BTAR cases involved in pure frontal impacts the occupant restraint methods used by the individuals were investigated. These findings are presented in Fig. 4
and show that the use of a belt or airbag does not eliminate the risk of BTAR. In total, the number of BTAR cases known to be wearing a seat belt was 74 out of 132.
3.5. Impact scenarios
Table 2 shows that BTAR cases arose in all conceivable types of vehicle accidents, with the majority of cases arising through frontal and side impacts. It is anticipated that this result is most likely due to the larger number of frontal and side impact vehicle accidents rather than as a result of any other relevant factors associated with these types of vehicle impact.
3.6. Vehicle impact severity
Vehicle impact severity is quantified with a calculated Equivalent Test Speed (ETS). Although quoted in units of km/h, ETS is not an estimate of a vehicle's impact speed prior to an accident, but an estimate of vehicle impact severity based on the stiffness and deformation experienced by the accident vehicle. The ETS is the anticipated speed the accident vehicle would need to impact a rigid wall in order to experience the same amount of deformation as that observed in the accident.
Figs. 5 and 6
illustrate the ETS frequency that BTAR occurred for both the pure frontal and pure side impact cases respectively. These figures show that BTAR occurs under a wide range of vehicle impact severities, with cases of BTAR shown to occur in accidents with an ETS as low as 30 km/h-1in both front and side impacts.
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4. Discussion
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The main aims of this study were to establish the incidence of BTAR resulting from motor vehicle accidents in the UK and to investigate the accident conditions under which it occurs.
The review of the CCIS database at TRL established that over a 7-year period there were 132 cases of BTAR with 130 fatalities, which represents 21% of the total number of recorded fatalities in the CCIS database (130/613). UK government accident statistics reveal that in 1998 there were a total of 238 923 personal injury accidents on the roads with 3421 fatalities. Of these fatalities 1696 were car occupants [6]. The percentage of fatalities reported in this study is higher than in the UK Government statistics. This is due to the data in the CCIS database being biased towards serious accidents involving serious to fatally injured occupants. Extrapolating the 21% incidence of BTAR fatalities discovered in this study to all car occupant deaths in the UK would produce an estimate of up to 360 deaths in 1 year due to BTAR following RTAs. Scaling for the proportions of drivers versus passengers, and for the different impact types would increase the accuracy of this estimate, however this has not been addressed as part of this work.
The incidence of BTAR should be viewed in context with the increasing volume of traffic on the UK roads. The average distance travelled by car per year has increased by 41% from the mid 1980’s to 1999. Over the same time period the number of trips per person per year by car has increased by 25% and the time spent travelling by car per person per year has increased by 27% [7].
In the last three decades there has been a significant reduction in the number of fatal road accidents. Between 1967 and 1998 the number of fatalities had fallen by 57% [6]. The total number of accidents involving personal injury had only fallen by 12% over this time frame showing a shift from fatal to less severe injury after road accidents at a time when road usage has increased. This shift may be related to safer vehicle design, safer driving, increased seat belt usage and better highway engineering.
The survival rate in this study following BTAR is low (1.5%) but is consistent with the literature. One of the earliest reports to review the incidence and pathology of BTAR was presented by Parmley [8]. In this combined autopsy and clinical review of 275 cases of BTAR it is reported that only 38 managed to survive the initial insult, a survival rate of just 13%. It is further documented that of those surviving the initial insult, only two survived their injuries, the majority of the rest died within 15 days of the accident. Similar statistics on the mortality of BTAR are provided in more recent publications. Fabian [1] reviewed 274 patients received in 50 trauma centres spread throughout North America and Canada. In this study of the 274 patients 54 died directly as a result of BTAR. The total number of fatalities in this group was 86. Dunn and Williams [9] quote a scene survival rate of 20% in patients with BTAR. Greendyke [10] determined from reviewing the published literature that the initial survival rate of BTAR was between 10 and 20%. Even though these statistics derive from literature produced over the past half century, the scene survival of BTAR would not seem to have been influenced by the changes in vehicle design and safety.
Previous authors discussing BTAR have surmised that the primary initiator of the injury is either high deceleration loads or a result of crushing of the aorta [11,12]. However, considering the accident conditions under which BTAR occurred in this study it is impossible to differentiate if BTAR is the result of a deceleration/acceleration load or a crushing load. Many authors have stated that BTAR arising from falls is generally the result of high decelerations. However, the deceleration response will inevitably bring about a deformation in the spine and thoracic cage due to their flexibility. This response will in itself apply a crushing mechanical load on the organs within the thorax. Only if the thoracic cage were a purely rigid system would the loading on the internal organs be a pure deceleration load. Alternatively, slow compression of the chest (similar to the effect of a mechanical press) would apply a pure crushing load, but there are no examples in this series in which BTAR was initiated by these precise loading conditions. The thorax is crushed during vehicle impacts, but the crushing load will also apply a deceleration pulse on the body. Based on this rationalisation, it seems to suggest that BTAR arises under conditions of both crushing and deceleration/acceleration, though it is not possible, based on available data, to determine which of these is the primary mechanism for BTAR.
Furthermore, there are a number of conflicting theories in the literature attempting to explain the fundamental mechanisms that cause BTAR after the initiating mechanism has been applied to the chest. The presence of passengers in this series who did not suffer BTAR in the same vehicle as other occupants who did sustain the injury implies that these secondary mechanisms may be passenger specific. In future studies looking into these mechanisms the aorta should not be examined as a structure in isolation, rather the full dynamics of the aorta within the thorax at the time the initiating force is applied should be considered. Such work is outside the scope of this study but is necessary considering the predicted annual mortality in the UK from this injury following RTAs.
It would be simpler to design restraint mechanisms within vehicles that modify the primary initiating mechanism particularly as in this study it is frequently an impact from the side at comparatively low speeds. Design of safety features which may modify occupant specific variables at the time of impact, such as gating airbag deployment to certain phases of the cardiac cycle, would clearly be more complex. The introduction of airbags into vehicles in the late 1980s early 1990s is not seen to influence the regularity that BTAR occurs in this study.
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5. Conclusion
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BTAR carries a high mortality and occurred in 21% of car occupant deaths in this sample of RTAs. Impact scenarios leading to the injury vary but are most common from the side. The use of airbags or a seat belt does not eliminate risk. The injury can occur at low severity vehicle impacts particularly in side impact. More research into the prevention of this injury is needed to complement that into the diagnosis and treatment of survivors.
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Acknowledgments
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The work for this paper was funded by Nottingham City Hospital Trust Funds, the UK Government DTLR and TRL Limited. The paper uses accident data taken from the United Kingdom Co-operative Crash Injury Study. CCIS is managed by TRL Limited, on behalf of the Department for Transport, Local Government and the Regions (Vehicle Standards and Engineering Division) who fund the project with Autoliv, Ford Motor Company, Honda R&D Europe, LAB, Toyota Motor Europe, and Volvo Car Corporation. The data were collected by teams from the Birmingham Automotive Safety Centre of the University of Birmingham; the Vehicle Safety Research Centre of the University of Loughborough; and the Vehicle Inspectorate Executive Agency of the DTLR. Further information on CCIS can be found at http://www.ukccis.com/.
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Footnotes
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Presented at the joint 15th Annual Meeting of the European Association for Cardio-thoracic Surgery and the 9th Annual Meeting of the European Society of Thoracic Surgeons, Lisbon, Portugal, September 16–19, 2001.
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Top
Abstract
1. Introduction
