Acute coronary syndrome (ACS) is the most important treatable cause of cardiac arrest. In contrast with cardiac arrest survivors with ST elevation myocardial infarction (STEMI), current guidelines do not recommend unselected/routine urgent invasive coronary angiography (ICA) in the patients with cardiac arrest without STEMI. This recommendation reflects existing evidence indicating that immediate invasive strategies may not confer significant benefit in this population and may even be harmful, while another ongoing randomized controlled trial is investigating this. Nevertheless, in COACT, a landmark trial investigating a strategy of immediate versus delayed coronary angiography in out-of-hospital cardiac arrest patients without STEMI, one or more culprit coronary lesions responsible for triggering cardiac arrest were identified in 40% of the total patient population. The question remains pertinent whether well selected cardiac arrest survivors without STEMI can benefit from early ICA. Yet, current available clinical tools fail to identify these patients. Markers such as clinical history, echocardiographic abnormalities, arrest rhythm (shockable/non-shockable), ECG changes other than ST-segment elevation and troponin levels lack sufficient sensitivity and specificity in a cardiac arrest setting for predicting ACS requiring intervention.
The diagnostic value of ECG-gated cardiac computed tomography angiography (CCTA) for detection of both acute and chronic coronary artery syndrome is well established, with recent evidence demonstrating the additional value of fractional flow reserve (FFR)-CT in ACS. Nonetheless, the need for ECG-gating remains a limitation.
Recently an ECG-less CCTA modality was developed, but its diagnostic accuracy is still under validation. ECG-less cardiac or coronary CT angiography (CCTA) allows cardiac imaging without requiring an ECG signal from the patient. Thus, it eliminates the steps associated with using a patient-attached ECG monitor: skin preparation, attaching the ECG leads, checking impedance, and confirming that the leads provide an adequate ECG signal to the scanning system. Therefore, workflow is optimized, which is critical in an emergency setting. In situations where it is difficult to attach the ECG leads, such as patients in a resuscitation setting who already have diagnostic ECG leads in place or other instrumentation, it is also advantageous that there is no need for an ECG signal.
Cardiac arrest patients without STEMI and with no evident non-cardiac cause generally undergo CT imaging of head and chest for evaluation of potential causes of cardiac arrest (e.g. pulmonary embolism, acute aortic dissection, intracranial hemorrhage). While ECG-gated CCTA is considered the optimal modality for non-invasive coronary imaging, ECG-less CCTA might offer a highly interesting alternative with the advantages mentioned earlier. Other benefits include no substantially longer scanning time, no need for additional contrast injection or administration of betablockers.
ECG-less Cardiac software is an FDA-approved cardiac scan mode that essentially utilizes existing CT system scan technology. The system uses a wide detector coverage of 160mm to provide full heart coverage and a fast gantry speed of 0.23 seconds per rotation to perform imaging in a single cardiac cycle. An estimate of the heart rhythm has to be provided, which is often readily available because emergency patients are already monitored. Based on the heart rhythm the scanner simulates an ECG signal. This simulated ECG signal provides virtual gating of the scan. The acquisition can be performed during a full heart cycle or three-quarters or half cycle, depending on how fast the heart rhythm is. The existing cardiac software options of SmartPhase (automated phase selection) and SnapShot Freeze 2 (optimized volume registration) amplify the quality of the images and correct for motion.
Patients are scanned using a Revolution Apex Elite system (GE Healthcare, Waukesha, WI -USA). The investigators use a hyperdrive pulmonary CT angiography (523mm/s with 0.28s/rotation gantry speed). After a short delay of a few seconds (5-12 sec), allowing the contrast to leave the pulmonary circulation, and enter the aorta and coronary arteries, a coronary CT angiography is performed within the same contrast bolus. No extra contrast is given to acquire the cardiac images. No intravenous beta-blocker nor sublingual nitroglycerin is administered.
The total added exam time (assessing heart rhythm, preparing the scan parameters, the delay time and the acquisition itself) is about one to two minutes.
The dose-length product (DLP) of the ECG-less cardiac scan depends on the duration of the scan time that is chosen. The average DLP is between 150 and 200 mGy.cm. The diagnostic reference level (DRL) as set by the Federal Agency for Nuclear Controle (FANC) for a coronary CT angiography is 300 mGy.cm.
The combined pulmonary CT angiography and ECG-less cardiac scan can be used to diagnose all the pathologies that can be assessed on a conventional pulmonary CT angiography scan (including, but not limited to, pneumonia, pleural fluid, pulmonary embolism, pulmonary infarct, pulmonary mass, pneumothorax, pericardial fluid, etc.) and provides extra diagnostic information about coronary artery disease.
In case the technique is well validated, future clinical questions could include whether ECG-less CCTA can help to identify a patient population of cardiac arrest survivors without STEMI that do benefit from early invasive coronary angiography and whether earlier treatment could improve outcome.
This study aims to investigate the feasibility and diagnostic accuracy of ECG-less gated CCTA in cardiac arrest survivors without STEMI, by means of agreement with ICA.
