Diagnostic performance of the cardiac FAST in a high-volume Australian trauma centre

Background: Cardiac injury is uncommon, but it is important to diagnose, in order to prevent subsequent complications. Extended focused assessment with sonography in trauma (eFAST) allows rapid evaluation of the pericardium and thorax. The objective of this study was to describe cardiac injuries presenting to a major trauma centre and the diagnostic performance of eFAST in detecting haemopericardium as well as broader cardiac injuries. Methods: Data of patients with severe injuries and diagnosed cardiac injuries (Injury Severity Score .12 and AIS 2008 codes for cardiac injuries) were extracted from The Alfred Trauma Registry over a four-year period from July 2010 to June 2014. The initial eFAST results were compared to those of the final diagnosis, which were determined after analysing imaging results and intraoperative findings. Results: Thirty patients who were identified with cardiac injuries met the inclusion criteria. Among these, 22 patients sustained injuries under the scope of eFAST, of which a positive eFAST scan in the pericardium was reported in 13 (59%) patients, while nine (41%) patients had a negative scan. This resulted in a sensitivity of 59% (95% CI: 36.7%–78.5%). The sensitivity of detecting any cardiac injuries was lower at 43.3% (95% CI: 26.0–62.3). Conclusions: The low sensitivities of eFAST for detecting cardiac injuries and haemopericardium demonstrate that a negative result cannot be used in isolation to exclude cardiac injuries. A high index of suspicion for cardiac injury remains essential. Adjunct diagnostic modalities are indicated for the diagnosis of cardiac injury following major trauma.


BACKGROUND
Cardiac injury in the setting of major trauma can be an insidious cause of death.Myocardial contusions are common, but most do not progress to the significant sequelae of arrhythmia and death. 1 Myocardial lacerations and cardiac tamponade are less common, but can progress to death. 2 Rapid diagnosis is essential in patients with cardiac tamponade to reduce mortality.Historically, surgical intervention via a pericardial window was used to investigate a suspected pericardial effusion.The advent of portable ultrasound technology has enabled a rapid and non-invasive assessment of the pericardium.
The focused assessment with sonography in trauma (FAST) has been incorporated into the Advanced Trauma Life Support (ATLS) w recommendations to be performed directly after the primary survey. 3This is a focused examination, which is carried out using portable ultrasound equipment to detect free fluid in the perisplenic space, perihepatic space, pericardium and pelvis.This has been further expanded upon by creating an extended FAST (eFAST) examination, which includes an examination aimed to detect pleural pathology. 4,5eFAST was originally developed to identify the presence of pericardial fluid 6 and initial studies reported a sensitivity and specificity of approximately 100% in detecting a haemopericardium. 7,8However, more recent case series and reports 9,10 have described cases of false negative haemopericardium eFAST results.
Although the primary design of eFAST is to detect the presence or absence of haemopericardium, there may be a misconception among clinicians that eFAST can be used to exclude all cardiac injuries.When considering the overall population of major trauma patients with any cardiac injury, the sensitivity of eFAST is likely to be even lower than previously reported due to the broad spectrum of cardiac injuries, beyond haemopericardium, that exist.For instance, a recent retrospective study in South Africa demonstrated an eFAST sensitivity of only 20% in detecting any cardiac injury following penetrating cardiac trauma. 11Although, beyond cardiac injuries, eFAST has been shown to have a comparable sensitivity to chest X-rays in the detection of post-traumatic pneumothoraces. 12he aim of this study was to determine the diagnostic value of eFAST for haemopericardium, and other conditions intended to be identified by eFAST, such as pericardial lacerations and tears and cardiac injury.

Setting
This study was performed at The Alfred Hospital, one of three Major Trauma Services in Victoria, Australia, that receive approximately 85% of major trauma cases in the state of Victoria. 13The Alfred Hospital has an overall annual census of 17,000 injured patients with more than 7,000 adult trauma patients admitted each year.Patient data from all major trauma admissions are recorded in the Alfred Trauma Registry (ATR).Major trauma admissions are classified as those with an injury severity scale .12 (using AIS 2008 codes), all trauma admissions necessitating an urgent surgery or ICU admission, and all injuries causing death.

Design
This was a single centre retrospective cohort study.Patients with cardiac injury were retrospectively identified using the ATR.The database search was limited to include only patients with AIS codes pertaining to cardiac injury and an injury severity $3 (Table 1).Patients presenting between 1 July 2010 and 30 June 2014 were considered eligible.All patients included in the study sustained cardiac injury and were hence considered disease positive.Data extracted from the ATR included patient demographics, vital signs on presentation, eFAST results, investigation findings, operative findings, injury type, and mortality.The Shock Index (SI), defined as the heart rate divided by the systolic blood pressure (HR/SBP), was used as a surrogate for haemorrhagic shock. 14The eFAST results were validated from the clinical documentation and compared with radiological and intraoperative findings and final discharge diagnoses.The eFAST results for injuries specifically regarded under the scope of eFAST (i.e.cardiac tamponade, pericardial effusion or haemopericardium) were also looked at.Data on additional potential confounders of eFAST results -obesity, subcutaneous emphysema and haemothorax were also extracted.

Statistical analysis
Microsoft Excel was used to collate data.Analysis of data was performed using MedCalc and STATA v.13.Sensitivities, unadjusted odds ratios (ORs), 95% confidence intervals (CIs) and p-values (,0.05 considered statistically significant) are reported.

Ethics
The Alfred Hospital Research and Ethics Committee approved this study.

RESULTS
Over the 4-year study period, a total of 16,312 patients were included in the ATR.From this, a total of 31 patients were identified.One patient was excluded from the analysis, given that the eFAST result was not documented.The remaining 30 patients formed the study population (Figure 1).
There were no statistically significant differences between patients with and without a positive eFAST result for pericardial fluid (Table 2).Of the 22 patients with injuries regarded under the scope of eFAST, 13 (59%) were eFAST positive, while nine (41%) were eFAST negative.This results in a sensitivity of 59% (95% CI: 36.7% -78.5%).
Thirteen (43.3%) patients had a positive eFAST scan of which all were true, with 17 (56.7%)patients having a false-negative eFAST scan, of which all were true for cardiac injury.This resulted in a sensitivity of 43.3% (95% CI: 26.0 -62.3) for detecting cardiac injury.Detailed descriptions of the 17 false negative eFAST results are listed in Table 3.
Only three of 27 patients (11%) were hypotensive (SBP # 100).Two SBP values were not recorded and one patient, who was soon declared as deceased, had a recorded SBP value of zero.Nineteen patients (63.3%) had tachycardia, defined as heart rate $100 beats per minute.Twelve (63.7%) of these patients were eFAST positive and seven (37.3%) were eFAST negative.Seven patients had a SI $ 1, of which two (16.7%)patients had a positive eFAST and five (33.3%) had a negative eFAST (Table 4).Five patients died during their hospital admission as a result of their cardiac injury.Three patients had a positive eFAST scan for pericardial fluid (60%) and two had a negative scan result (40%).
Six patients had concurrent subcutaneous emphysema, as per their chest X-ray.Only two of the six patients with subcutaneous emphysema were identified as eFAST positive (33.3%).The presence of subcutaneous emphysema made the occurrence of a false negative result 1.7 times more likely (OR: 1.69, 95% CI: 0.26-11.07,p ¼ 0.58), although not statistically significant.
The weights of 27 out of 30 patients were documented.Three patients weighed 100 kg or more, of which one was eFAST positive (33.3%).Patients weighing $ 100 kg were 1.43 times more likely to have a false negative result (OR: 1.43, 95% CI: 0.11 -18.00, p ¼ 0.78), which again is not of statistical significance.
Among the 14 patients with penetrating wounds, six had a false negative eFAST result, out of which five (83.3%) had either a coexisting haemothorax or pericardial fluid in the thorax.Three patients with blunt trauma also had negative eFAST scans and concurrent haemothoraces.
The patients with cardiac contusions (n ¼ 4) and chordae tendinae rupture (n ¼ 1) had a negative eFAST scan, which was to be expected given there was no free fluid.Three patients sustained injuries to the great vessels, all of whom were also eFAST negative.

DISCUSSION
We aimed to review cases with cardiac injuries and correlate those cases with the initial eFAST results.Cardiac injury was uncommon in this single centre Australian setting.Injuries range from benign, such as myocardial contusion, to fatal, such as chamber rupture.Bedside sonography during trauma resuscitation was not adequate to exclude cardiac injury.The primary design of eFAST was to identify pericardial fluid rather than all cardiac injuries.When classified according to injury type, patients with solid organ damage and patients with injury to the great vessels had negative eFAST results.This is expected due to the absence of pericardial fluid.
When detecting cardiac injuries regarded under the scope of eFAST, the sensitivity for eFAST was only 59%.While eFAST was initially developed to exclude the presence of pericardial fluid, clinicians may incorrectly assume that a negative eFAST result can be used to exclude cardiac injuries not regarded within the scope of eFAST.In reality, given the large variety of cardiac injuries, more definitive imaging such as an echocardiogram or computer tomography (CT) should be sought, and eFAST should only be used to help detect the absence or presence of haemopericardium.
However, a significant proportion of those with free fluid in the pericardium or with cardiac lacerations or tears also had false negative results (41%).The reliability of eFAST in detecting the presence of pericardial fluid is limited by many factors.Image quality may be reduced by either subcutaneous emphysema or abundant adipose tissue.It is widely accepted that it is difficult to perform ultrasound on obese patients, due to excess adipose tissue attenuating the ultrasound waves. 15Our results, although not statistically significant, suggest that a false negative result may be more likely in patients weighing more than 100 kg or with concurrent subcutaneous emphysema.Another important consideration for eFAST false negatives, as reported by Ball et al., 8 is the potential of penetrating wounds to cause decompression of a haemopericardium into the thorax through a laceration in the pericardial sac.This drainage of blood into the thoracic cavity prevents the accumulation of blood in the pericardium, leading to a false negative pericardial eFAST result.This is especially relevant for patients with concurrent haemothoraces.It was observed that 83% of patients in our study with penetrating wounds and false negative eFAST results had concurrent haemothoraces or pericardial fluid in the hemithorax.Given the retrospective nature of this study, it is difficult to determine from the clinical documentation whether the blood in the haemothoraces originated from the pericardium or from elsewhere.Nonetheless, these results support the warning from Ball et al., 9 to be wary of a negative eFAST result with a concomitant haemothorax in the setting of penetrating thoracic trauma.
The three blunt injuries which caused a haemothorax and a false negative result were a right atrium tear, a right pulmonary artery laceration, and right pulmonary artery pseudoaneurysm.This supports findings by Baker et al., 10 that blunt trauma can also cause a pericardial laceration, thereby leading to a false negative eFAST result.
Nonetheless, the use of ultrasound in a trauma setting confers many benefits.eFAST can rapidly provide valuable diagnostic information without delaying resuscitation measures.
In addition, a limitation of this retrospective study design is that we were restricted to the information recorded in the clinical documentation and the available vital signs.We did not have data on the ultrasound operator's level of training.The reliability of an operator's eFAST assessment would depend on their level of training, but such differences in inter-operator reliability are reflective of clinical practice.From the retrospective data, we were unable to establish the exact region which elicited a positive eFAST result.This study is also limited by the small sample size, as cardiac injury in a trauma setting is uncommon in Australia.
To combat these limiting factors, we recommend widespread education on the limitations of eFAST and development of guidelines on the use of eFAST, particularly with a negative result in high risk patients.We accept that eFAST should not be used as a definitive diagnostic tool, and that its place is as an adjunct to clinical assessment.In the setting of a negative eFAST result, further investigations such as point of care troponin, ECG, CXR and CT should be considered.Emergency echocardiography plays an important role in diagnosing cardiac injury and hence the training of pre-hospital and emergency clinicians to perform echocardiography should be encouraged.Despite this study demonstrating eFAST's low sensitivity in detecting haemopericardium, eFAST remains a highly beneficial tool in the rapid evaluation of thoracic pathology, especially pneumo-and haemothoraces.Hence we support the recommendation to include eFAST in the ATLS w algorithm, 16 provided negative pericardial eFAST results are regarded with caution.

CONCLUSIONS
Our study demonstrates an eFAST sensitivity of less than 50% for detecting any cardiac injury and only 59% for detecting findings under the intended eFAST scope.This may largely be attributed to the fact that cardiac injuries do not always produce pericardial effusions.When pericardial fluid is present, there may be factors affecting the reliability of eFAST, including patient weight and subcutaneous emphysema.Cardiac injury or fluid cannot be excluded from a negative eFAST result.Updated guidance on the use and limitations of eFAST in cardiac injuries, especially in patients with a negative eFAST, may improve the utility of ultrasound during initial trauma reception and resuscitation.

Figure 1 .
Figure 1.Schematic of study population identification.

Table 1 .
List of abbreviated injury scale codes.

Table 2 .
Demographics and presenting clinical variables.
HR: heart rate; SBP: systolic blood pressure; SD: standard deviation; IQR: interquartile range.a One value missing.b Two values missing.

Table 3 .
Descriptive data of false negative eFAST results.

Table 4 .
eFAST results categorised by injury type.