Identification of reversible causes
Hernandez et al. developed a systematic algorithm to identify the four most critical causes of cardiac arrest in the C.A.U.S.E. protocol (Cardiac Arrest Ultra-Sound Exam) . These potentially reversible causes include a dilated right ventricle in pulmonary embolism, fluid around the heart in pericardial tamponade, collapsed ventricles in hypovolemia and absent lung sliding in tension pneumothorax. The authors have also suggested additional views that can be obtained in hypovolemia such as the inferior vena cava to confirm an ‘empty tank’, as well as the abdominal aorta to evaluate for aneurysm as a cause of hypovolemia.
The American Society of Echocardiography and the American College of Emergency Physicians 2010 guidelines recommend the use of PoCUS only in pulseless electrical activity (PEA) or asystole and discourage its use in shockable rhythm . Their justification is rational as identification of ventricular fibrillation or pulseless ventricular tachycardia should be followed by immediate shock delivery and resumption of chest compressions. Detecting pathologies such as wall motion abnormality or hypertrophic cardiomyopathy is unlikely to affect the management during CPR, but should be identified after ROSC. However, we believe there are exceptions where ultrasound might be valuable in such scenarios, especially if the ventricular fibrillation is refractory. Cardiac arrest due to pulmonary embolism can present with ventricular fibrillation in 5% of cases . Ultrasound in this scenario may prompt the physician to administer thrombolytic therapy. Patients with wall motion abnormality in refractory ventricular fibrillation might benefit from coronary intervention even with ongoing CPR .
Right ventricular heart strain in cardiac arrest
A number of studies incorporating PoCUS in cardiac arrest mention investigation of the right ventricle for dilatation to evaluate for the presence of a pulmonary embolism [11, 24]. However, an increasing amount of evidence suggests that this may be an ineffective method to rule in the diagnosis. While it has been well established that the presence of a pulmonary embolus can undoubtedly result in right heart strain that could manifest as a dilated RV, a number of other factors, such as hypovolemia, hyperkalemia, primary arrhythmias and pre-existing chronic right ventricular strain, have also been shown to produce similar right-sided enlargement during cardiac arrest [30,31,32].
In fact, an interesting aspect to consider is the phenomenon that some degree of RV dilatation may even be a normal consequence of resuscitation during arrest. Gabriel Wardi et al. found that RV strain and dilatation was more demonstrable when a greater amount of time had elapsed into resuscitation .
The 2019 European Society of Cardiology guidelines on pulmonary embolism have addressed the accuracy of RV dilatation in pulmonary embolism . Although RV dilatation plays an important prognostic role in stable patients with pulmonary embolism, it has a rather weak positive predictive value for PE-related deaths. This frequently encountered relative insensitivity is partially attributable to the inherent difficulty in standardizing ultrasound parameters for any study .
In light of the presence of such false-positives when visualizing the right side of the heart during cardiac arrest, the diagnosis of pulmonary embolism and any subsequent intervention based on the same should be further augmented by factors other than isolated right heart strain on PoCUS. Historical details of pre-arrest signs and symptoms, as well as possible intra-arrest evaluations for deep vein thrombosis in high-risk patients could prove useful measures to dependably diagnose and treat pulmonary embolism during cardiac arrest . Furthermore, increased presence of false-positive findings late into resuscitation prompts consideration of integrating ultrasound evaluations as early as possible into cardiac arrest protocols.
Identification of cardiac standstill
Cardiac standstill, also known as true asystole, is defined as the complete absence of any cardiac motion including the ventricles, atria and valves [36, 37]. Patients recognized to have standstill with concomitant electrical activity on the monitor are often described to have true PEA. Pseudo-PEA is the presence of ventricular contractility visualized by ultrasound with electrical activity but no palpable pulse . The M-mode option on ultrasound detects any motion along a given line against time. If any movement is identified, that part of the heart will look hazy like “sand on a beach” (Fig. 3a). When there is a complete absence of cardiac contractility, the image will resemble a “barcode” appearance (Fig. 3b).
Identifying true PEA or cardiac standstill on ultrasound carries an important prognostic value. In 2001, Blaivas et al. conducted one of the earliest and largest prospective studies on cardiac standstill . Of the 169 patients, 136 were found to be in standstill and had 0% survival regardless of the electrical rhythm they presented with. On the other hand, 20 patients survived to hospital admission out of 33 patients with cardiac activity on initial ultrasound. Mean patient age in this study was 71 years which may admittedly represent a more senior population than is usually encountered in other centers. The study’s considerable limitation was that it included only out-of-hospital arrests where overall survival is less than with in-hospital arrests. No data was provided about survival to hospital discharge or neurological outcomes.
Salen et al. had similar outcomes in his two prospective studies in 1999 and 2005 [36, 37]. However, in the earlier study, out of 59 patients with no cardiac activity, 2 had survived. Several other studies have been published during the last two decades with similar findings of poor outcome associated with cardiac standstill, but most of them still reported ROSC incidence in a small number of patients with cardiac standstill [39,40,41,42]. One of the highest survivals of patients with no wall motion was reported by Breitkreutz et al. in 2010 wherein a total of five (10%) out of 50 patients with no cardiac movement survived . His results also confirmed that the presence of wall motion can predict a much higher survival rate (n = 30/75, 40%). This was further validated in more recent study, the US-CAB protocol where cardiac activity identified in 47 cases (26.6%) out of a total of 177 arrest patients being studied was associated with higher rates of ROSC (95.7% vs. 21.5%, p < 0.0001) and survival to hospital discharge (25.5% vs. 10.0%, p < 0.01). Furthermore, detection of cardiac activity after 10 min of CPR exhibited 100% sensitivity, specificity, positive and negative predictive value for ROSC .
The largest, multi-center, observational prospective study was published in December 2016 by Gaspari et al. . The REASON 1 trial (Real-time Assessment and Evaluation with Sonography—Outcomes Network), included over 20 hospitals and enrolled 793 patients. The study looked at rate of ROSC, rate of survival to hospital admission and to hospital discharge. Overall survival to discharge was 0.6% (n = 5) for patients in cardiac standstill and 3.8% for patients with cardiac activity (n = 30). A subgroup analysis of patients who had no bystander CPR, presented with asystole and had no cardiac activity on arrival, had no ROSC. Furthermore, REASON was the first trial to prove that sonographic identification and treatment of a reversible cause of a cardiac arrest increases survival. Fifteen percent of identified pericardial tamponades in the trial achieved ROSC and were discharged out of the hospital.
In March 2019, Members of the Sonography in Hypotension and Cardiac Arrest (SHoC) Investigators published a meta-analysis on the reliability of PoCUS to predict outcome in non-traumatic out-of-hospital and in-hospital cardiac arrests . Ten studies with 1486 participants were included. Presence of cardiac activity on PoCUS had a pooled sensitivity of 60.3% (95% CI 38.1–78.9%) and specificity of 91.5% (80.8–96.5%) for ROSC. In asystole, the sensitivity of cardiac activity on PoCUS for predicting ROSC was 26.1 (7.8–59.6%) compared with 76.7% (61.3–87.2%) in PEA. Cardiac activity on PoCUS had higher odd ratios of 16.9 for ROSC, 10.3 for hospital admission and 8.03 for hospital discharge. Unlike previous metanalyses by Blyth et al. Tsou et al. and Wu et al. the SHoC group excluded studies with traumatic arrests or shockable rhythms [44,45,46]. One of the major discrepancies among the included studies was the definition of cardiac activity and operator experience.
Physicians who use bedside ultrasound in their resuscitations feel more comfortable terminating codes if no cardiac activity is found . Although the chances of survival for cardiac activity in asystole are small, they are still three times higher than those in true standstill. Therefore, it would stand to reason that those particular patients would benefit more from longer resuscitation efforts.