Critical Care, Cardiology

Deep Dive into the Evidence: Cardiac Ultrasound in Cardiac Arrest

Advanced Cardiac Life Support protocols for the management of cardiac arrest are presently based on clinical examination findings and limited cardiopulmonary monitoring equipment. 

Despite significant advances in cardiac imaging since the inception of protocolized management for cardiac arrest, imaging has not yet been incorporated into these protocols. The adjunctive use of cardiac ultrasound in the setting of cardiac arrest resuscitation has been demonstrated to have a diagnostic and intervention-driving impact. Accordingly, it has been proposed that Transthoracic Echocardiography (TTE) may be used adjunctively during cardiac arrest resuscitation to improve clinical outcomes. TTE has been shown to aid with prognostication for patients cardiac arrest. Further, it can be used to assess for reversible causes of cardiac arrest by quickly examining for pericardial tamponade, pneumothorax or by looking for DVTs. POCUS can even be utilized to perform more accurate pulse checks.   

However, the role of TTE during cardiac arrest management is limited from a technical standpoint as there is only minimal opportunity to attempt visualization of meaningful cardiac windows during brief CPR interruptions for rhythm assessment. Similarly, resuscitation equipment attached to the patient (eg, defibrillator pads, automated compression devices) impede further the technical feasibility of TTE in the setting of cardiac arrest. These effects can compound into ultrasound actually being detrimental during cardiac arrest. One prospective study showed that utilizing echo during cardiac arrest doubled the length of pulse checks. Another study that the inter-rater reliability of what the definition of cardiac standstill on echo is. Thus, TTE cardiac ultrasound should be utilized in a protocolized way during cardiac arrest to reduce the length of pulse checks, quickly identify reversible causes and improve inter-rater reliability. However, there are frequently times when TTE is limited due to the lack of windows, poor image quality or body habitus.    

Enter transesophageal echocardiography (TEE).

TEE offers superiority in image quality, while also circumventing the technical barriers to the meaningful implementation of TTE in the setting of cardiac arrest. In addition to technical feasibility, several studies have examined the role of TEE in cardiac arrest and its impact on clinical outcomes.

From September 2017 to 2018, in their prospective observational trial, investigators at the University of Pennsylvania evaluated patients with out of hospital cardiac arrest by employing TEE in the ED. Although this study was limited by a smaller cohort of 33 patients, possible bias as investigators both lead resuscitations and employed TEE, and by being a single-center study, the results have opened the gates that have long prevented us from prognosing and from improving our management of patients with cardiac arrest. In this study, 7 emergency physicians were certified to perform TEE.  Four primary views were obtained in this study’s TEE imaging with the table below showing the location, anatomy viewed, and diagnostic significance. Please see our chart for the trial’s pertinent findings and results.

 

When compared to TTE conducted via the 2018 outlined CASSA exam, TEE offers advantages regarding quality of images and lack of limitation by physical barriers, as discussed in the introduction. In this trial, images altered 97% of decisions. TEE provided continuous feedback in which it identified the area of maximal compression (AMC) with improved ROSC experienced with AMC appropriately repositioned over the left ventricle (LV). The significance of LV compressions on hemodynamics will be further discussed below. TEE also confirmed the cardiac pump theory of the aortic valve opening during compression and the mitral valve opening during recoil which affirms the theory that TEE may provide further insight regarding the physiology of CPR which has mostly been based off of theory. Even more impressive was that TEE identified VFib in 2% of cases thought to be asystole (which led to defibrillation), identified intracardiac thrombosis (which lead to thrombolysis), and identified pseudo-PEA in two cases.

Although it is important to remember that this study had insufficient power to correlate the rhythm and activity identified by TEE on patient outcome, the possibility that TEE could help us to further stratify PEA as well as improve prognostic decisions is exciting and has encouraged further research on this topic.

One of the points this study made was that RV dilation on ultrasound which has previously been correlated with pulmonary embolism (PE) as the cause of PEA, was actually a common finding in that 57% of intra-arrest and 8% of post-arrest cases demonstrated RV dilation defined as RV/LV ratio > 0.6. There was no follow up as to if patients with RV dilation had PEs but with such a high percentage of OHCA patients demonstrating RV dilation, it was presumed that not all were related to PEs. Similarly, a 2017 study using swine models, identified RV dilation in the majority of cardiac arrests irregardless of the etiology. This study randomly assigned swine models to arrest from dysrhythmia, hyperkalemia induced diastolic arrest, and hypovolemia, and demonstrated that the RV increased throughout the resuscitation of all swine, with it measuring the largest in hypovolemic cases. That finding is a direct contraindication to the prior school of tthought that hypovolemia caused ventricular collapse in hypovolemic cardiac arrest. The study further suggests that RV dilation is likely from cardiac arrest itself rather than from a specific etiology. Although PE can cause RV dilation, it is not the only etiology for RV dilation, and should not necessarily prompt us to reach for tPA unless clinically correlated first.

The other interesting focus of the University of Pennsylvania study was the effect that AMC over the LV had on resuscitation vs when positioned elsewhere. When repositioned over the LV, TEE was able to confirm appropriate function of the AV and MV as described above by the cardiac pump model. In 3 cases, repositioning over the LV resulted in ROSC and in one case it prompted the initiation of Extracorporeal Cardiac Life Support (ECLS) as the repositioning lead to signs of consciousness. However ,it is noted that the data was not statistically significant regarding the initial position of AMC and survival to ED, survival to discharge, or ROSC. Interestingly, a 2016 Swine Model used TEE to evaluate the optimal position for chest compressions. In this animal study, AMC over the LV vs the aortic root was compared regarding the ability to achieve ROSC and regarding the effect on Coronary Perfusion Pressure (CPP) and on aortic systolic and diastolic pressures. It was concluded with statistical significance that during all time intervals, aortic diastolic and systolic pressures, right atrial systolic pressures, and EtCO2 were all higher in AMC over the LV when compared to the aortic root. Also impressive was that 69% of swine models with AMC over the LV achieved ROSC whereas 0 models in the aortic root group achieved ROSC. However mean CPP was only higher in two time intervals, the 12 to 14 minute interval and 22-30 minute interval, when compared to the aortic root group. Only the 12 to 14 minute interval was statistically significant. This was a prospective randomized comparative trial in which swine models were randomly assigned to the LV or to the standard chest compressions groups. The swine were intubated and TTE was used to identify the left ventricle and the aortic root and these locations were externally marked. TEE was placed at the MELAX view. Cardiac arrest was induced and all swine remained in V fib without intervention for 10 minutes, and then BLS was initiated with recorded MELAX views. ROSC was defined as SBP > 60 mmhg for one minute and swine were declared expired with SBP < 60 mmhg for 10 minutes. Although the study was limited by the difference in physiology between swine and human models and in the difficulty blinding investigators, it is the first study to demonstrate that chest compression location may have an effect on achievability of ROSC.

Although these articles studied various aspects in which TEE may be useful in Cardiac Resuscitation, collectively they represent an advanced way to resuscitate our patients who present in cardiac arrest. Furthermore, these studies support TEE as an improved tool for diagnosing the etiology of cardiac arrest as well as for prognosing patients on an individual basis. We have discussed the improved views obtained, the ability to assess the quality of CPR, and the clarification on diagnosis as to etiology and reversibility of arrest.

Access the PowerPoint deck or view the individual slide gallery below.

Deep Dive: Echo in Cardiac Arrest

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