Out of hospital cardiac arrest (OHCA) is a catastrophic event, which is equivalent to demise unless prompt cardiopulmonary resuscitation (CPR) and follow up care are available. Majority of the cardiac arrests occur at home (80%) and some of them at public places (20%). Only one fifth of them are in a shockable rhythm when the emergency medical services (EMS) arrive. There is a 10% decrease in survival on each passing minute after cardiac arrest. Bystander initiated CPR improves the chance of survival as it buys time till the arrival of EMS. The chance of receiving bystander CPR varies widely between countries and locations, with 43% in certain countries. Unwitnessed cardiac arrest naturally has a bad prognosis as there is no hope of bystander CPR. Prehospital achievement of ROSC (return of spontaneous circulation) is one of the best indicators of potential survival. Use of automatic external defibrillators (AED) and public access defibrillation go a long way in improving the survival of OHCA.
Key determinants of survival and the Chain of Survival
The key determinants of survival in OHCA are the time to the arrival of first emergency response team, witnessed cardiac arrest, effective bystander CPR, presence of an initial shockable rhythm (ventricular fibrillation or tachycardia) and prehospital achievement of ROSC. There is a concept of chain of survival in out of hospital cardiac arrest. The links of the chain of survival are recognition of cardiac arrest and activation of EMS, immediate high quality CPR, rapid defibrillation, basic and advanced EMS, advanced life support and post cardiac arrest care. The first three links can be taken care by a trained lay rescuer, while the next steps need medical services of increasing complexity including EMS, emergency department, cardiac catheterization laboratory and intensive care unit. Like in any chain, this chain is also as strong as its weakest link! Hence it is important to strengthen all the links of the chain of survival.
Triaging after resuscitation
ECG has an important role in triaging a survivor of OHCA. Detection of ST segment elevation on a post resuscitation ECG calls for rapid shifting to a cardiac catheterization laboratory for coronary angiography with intention for revascularization. A rapid echo screening is done prior to angio whenever possible. If a clear culprit lesion is identified on angio, immediate angioplasty is undertaken. Stable lesions are usually addressed only in case of hemodynamic instability, though future studies might throw light on this aspect. If coronary angiogram is normal, ergonovine / acetylcholine challenge for coronary vasospasm may be considered at a later date. During the index episode, normal coronary angiogram calls for a clinical reassessment with other investigations like imaging done if necessary. The same is applicable to those without ST elevation on initial ECG. They need clinical assessment, echo and CT scanning of the head and chest if needed, to look for other causes for cardiac arrest.
Role of bedside echo after OHCA
Role of bedside echo after an OHCA cannot be overemphasized. Ventricular function and regional wall motion abnormalities can be assessed as well as valvular heart disease. Echo is very useful to delineate cardiac tamponade and prompting relief by pericardial drainage. Aortic root dissection can be detected by echo while dissection of other parts of aorta are not likely to be picked up. Acute right heart dilatation may indicate massive pulmonary embolism as the etiology.
Identification of culprit vessel
Culprit vessel can be identified by features of acute occlusion, evidence of thrombus and by correlation with ECG and echo findings. Optical coherence tomography (OCT) can aid in identification of culprit vessel, though it is not likely to be widely available in the setting of OHCA.
Percutaneous coronary intervention (PCI) to culprit lesion reduces the infarct size, improves the hemodynamic stability and decreases the incidence of recurrent cardiac arrest.