Background:
The general approach to management of the patient in cardiac arrest has not changed significantly since the inception of ACLS protocols. Recent research has indicated that perhaps the use of ultrasound can offer important diagnostic and prognostic information and allow for a more thoughtful approach to the cardiac arrest patient
Everything You Think You Know is a Lie...Maybe
Of the many interventions performed during cardiac arrest, the only ones known to improve outcomes are:
High Quality CPR
Early Defibrillation in Shockable Rhythms
The remainder of common interventions, including those recommended in ACLS are of questionable benefit. Though it is extremely difficult to elucidate survival benefit and especially survival with good neurologic outcomes in these patients, studies have not been able to demonstrate benefit to:
Advanced Airway Management
Use of Pressors/Antiarrythmics
Use of IV fluids
IV vs IO access
Part of the reason why this is so difficult is that the proportion of patients that survive OHCA is so small, and the proportion that survive when initially in a non-shockable rhythm is even lower
Overall Survival Rate (to hospital discharge) in OHCA ~10% (AHA data)
Survival Drops to 1-2% in those with initial non-shockable rhythm
Researchers have recently tried to further risk stratify these patients using ultrasound and determine if there is a group of patients in the non-shockable category that may have a better prognosis than others
One of the inherent difficulties in managing cardiac arrest is the lack of information available, not only about the patients history and current illness, but also regarding the patient's physiology during the arrest
Under standard protocols, the primary modes of information for physiologic data are:
Presence/Absence of a pulse
Cardiac Rhythm on monitor
Research has shown, however, that healthcare providers perform poorly when using digital palpation as the mode of determining pulses. In one study:
10% of providers believed a patient had pulses when in fact they were pulseless.
45% of providers felt no pulse when a pulse was present (and SBP >80).
The median time to perform a pulse check was 24 seconds
Additionally, the use of pulses as a surrogate marker for cardiac activity is problematic. While presence of pulses may indicate blood pressure is adequate to perfuse vital organs, absence of pulses alone does not give reliable information about presence or lack of cardiac activity
Further, as we'll discuss below, the electrical rhythm can also be misleading and lead to inappropriate classification of cardiac arrest patients, compared to a classification system based on underlying mechanical rhythm. This is information that we believe can be obtained by point-of-care cardiac ultrasound.
Cardiac Arrest Myth-Busting
Myth #1: Never Shock Asystole
The ACLS management of Cardiac Arrest is significantly different for shockable vs nonshockable rhythm is significantly different. Similarly, as noted above, the prognosis is also very different for these groups. The determination of cardiac rhythm is made initially based on the electric activity of the heart, either on ECG or cardiac monitor.
Case reports have shown that while the electrical rhythm may generally be reflective of the actual mechanical rhythm of the heart, there are instances where this may not be true, or that the electrical rhythm may be misleading or difficult to interpret such as when the fibrillation amplitude appears extremely low or "fine v-fib"
While the above rhythm may be easily mistaken for asystole, ultrasound can provide valuable information and may help to identify cases where this misattribution could occur. Ventricular fibrillation can be easily seen on cardiac ultrasound:
In these cases, we’ve seen that the heart responds to therapy for its mechanical rhythm, fibrillation. As such, knowing this information can vastly change your management strategy, and outcomes, given the greatly improved survivability in a shockable rhythm.
Myth #2: All PEA is Created Equal
Although it lacks a single unifying definition, pulseless electrical activity is most clearly defined as presence of organized electrical activity (QRS complexes on ECG) with absence of mechanical activity OR activity insufficient for organ perfusion (AHA).
Given the lack of ability to determine mechanical rhythm in typical ACLS protocols, it is often difficult to differentiate the two subgroups that make up the overall population of 'PEA'
Those WITH organized mechanical activity or "pseudo-PEA"
Those WITHOUT organized mechanical activity
It is possible, based on recent data, that these two groups have a significantly different prognosis. Fortunately, cardiac ultrasound can provide a window into what cardiac activity is occurring in a cardiac arrest situation
Subxiphoid view: Intra-arrest cardiac ultrasound Revealing lack of organized mechanical activity
Parasternal Long axis view: Intra-arrest cardiac ultrasound Revealing presence of organized mechanical activity
A recent large multicenter trial (REASON trial) conducted by Gaspari et al. found that patients with organized mechanical activity determined by ultrasound had significantly better outcomes with an overall survival rate to hospital admission) of 29% vs 7% as well as survival to hospital discharge (3.8% vs 0.6%) compared to those who did not have organized mechanical activity by ultrasound.
This distinction may also have significant downstream implications. A follow up subgroup analysis of the above study found that those patients who were found to have organized cardiac activity on intra-arrest ultrasound and were treated with titrated vasopressor infusions rather than bolus dose epinephrine (per ACLS protocol) may fare better with increased rates of survival to hospital admission (46% vs 38%). This gives some additional evidence to the concept that these patients may have physiology more consistent with profound cardiogenic shock rather than what we think of as cardiac arrest.
Myth #3: Mind your H's and T's
A commonly taught ACLS paradigm for management of non-shockable rhythms is to consider H's and T's as possible reversible causes of arrest. Classically, these reversible causes are listed as:
H's |
Hypovolemia |
Hypoxia |
Hydrogen Ion (acidosis) |
Hypokalemia/Hyperkalemia |
Hypothermia |
T's |
Tamponade |
Toxins |
Tension Pneumothorax |
Thrombosis (Pulmonary Embolism) |
Thrombosis (Myocardial Infarction) |
While the above may certainly cause or contribute to arrest, it can be cumbersome and increase the cognitive load on providers during an arrest. Additionally, those that are easily ruled out (hypoxia, hypovolemia, hypothermia) are part of routine assessments of all patients, while the remainder can be difficult to diagnose without a classic history (PE, MI, Tension Pneumothorax, Tamponade, Toxins, Hypo/HyperKalemia) which is often absent in the acute stages of managing cardiac arrest. While controversial, it is generally not advised to treat any of these empirically in cardiac arrest unless there is truly an increased suspicion for the pathology.
Fortunately, ultrasound can provide significant information which can identify several of these reversible causes, and allow providers to promptly address them if present. Pericardial effusion and Tension pneumothorax can be seen clearly on ultrasound.
Subxiphoid view: Example of hemorrhagic pericardial effusion/tamponade with clot formation
Subxiphoid view: Example of large pericardial effusion with tamponade- note compression of RV and
In the REASON trial, Gaspari et al. also showed that those individuals who had a pericardial effusion identified and underwent pericardiocentesis emergently had much higher survival to discharge rates than others with effusion at over 15% survival to hospital discharge compared to 1.3%
Ultrasound may also help to identify secondary findings which could be consistent with massive pulmonary embolism as a cause of arrest. Right ventricular dilation is also rapidly identifiable on beside US.
Parasternal Long Axis view in periarrest patient: Enlarged RV and underfilled hyperdynamic LV
While Gaspari et al found improved survival to hospital discharge with thrombolysis in patients with suspected PE based on a combination of clinical and ultrasound findings (6.7%), it is important to note that several other studies have raised concern that there is likely rapid dilation of the RV during cardiac arrest and that this may not be a reliable marker when evaluating for right heart pathology, and particularly pulmonary embolism. Given the findings in the REASON trial, however, there may still be a role for ultrasound when combined with other clinical and historical findings/factors.
Additionally, cardiac ultrasound has been considered a tool to assist with prognosticating those who are unlikely to survive in cardiac arrest. Many studies have looked at its role in identifying patients who are unlikely to survive as a tool to assist with determination of when to cease resuscitation efforts. Overall, it appears that lack of organized cardiac activity on ultrasound is a very poor prognostic indicator for ultimate survival, although a very small proportion (0-1%) may survive with resuscitation efforts. As such, in those patients with other poor prognostic indicators, ultrasound may be helpful in identifying those who are unlikely to benefit from prolonged resuscitation.
Trouble in Paradise
While cardiac ultrasound can offer significant benefits when performed during cardiac arrest, investigators have also identified some potential pitfalls in its use which should raise some concern to clinicians. Huis in't Veld et al. described an association between ultrasound use and increased pauses in CPR in their observational study. While this was not necessarily a direct cause/effect, they did find that in resuscitations which utilized US for cardiac evaluation, there was an increase in pauses or "off the chest' time of 8.4 seconds when ultrasound was used. However, even in their patients who did not undergo ultrasound imaging, mean pause time was greater than the recommended amount at 21 seconds. As such, this is a good reminder that providers should be mindful of pause times in CPR during resuscitation.
Clattenburg et al evaluated this concept in their paper implementing the "CASA" protocol, a focused stepwise method aimed at ultrasound assessment during cardiac arrest. Providers in their study were given education on how to limit pause time in CPR while still obtaining important ultrasound information. In this pre/post intervention observational trial (clinicians were not forced to follow this protocol), they found that exposure to this information alone was able to decrease the pause time by 4.0 seconds. In their protocol, they focused on several concepts to minimize time off the chest in arrest
Place ultrasound probe on the chest prior to stopping compressions
Have a separate provider perform ultrasound from that which is running the resuscitation (most experienced ultrasonographer)
Have another individual count to 10 while ultrasound is being performed in order to limit the total time CPR is paused
Additionally, they recommended serializing the assessment, such that providers are only assessing for 1 focused question during each pulse check period. For example, assess only for pericardial effusion during the first pulse check, right heart dilation during the second check, and cardiac contractility on the third pulse check thereby hopefully limiting the amount of time needed to assess the image each time. Others have suggested separating image acquisition and interpretation by saving a clip of the ultrasound findings limited to ~3s. By doing this providers can use the time between pulse check to interpret the image and limit compression pauses even further.
Overall these findings tell us that while ultrasound use CAN increase pauses during CPR, a strict protocol and awareness to time constraints may be helpful in limiting this effect and allow for maximal benefits of ultrasound without the significant downsides that comes with CPR pauses.
Additional concern was raised in a study performed by Hu et al. which evaluated variability in clinician's ability to determine cardiac standstill. In their study, investigators showed a series of cardiac images to providers and asked them to determine whether or not they interpreted cardiac activity/contraction in each. They found that there was only moderate inter-rater agreement with α=0.47. The authors determined that lack of a consensus definition of what constitutes cardiac activity was a large contributor to this result. Indeed, in their later study, Gaspari et al. showed substantial inter-rater agreement on cardiac standstill (k=0.63) when using the definition of cardiac activity as "any visible movement of myocardium" excluding valvular movement. As such, it may be prudent for clinicians to use this definition going forward, especially if using data from this study to influence decision making.
The Future
Since the REASON trial data was published, several others have raised interesting study questions which give us insight into the future of cardiac arrest resuscitation. Teran et. al performed an analysis on the original REASON trial participants in which they determined that there may be a positive association with ejection fraction in PEA with ROSC. Dr. Teran and co-investigators also have proposed feasibility of and possible benefit to ED physician performed TEE during cardiac arrest to guide not only the clinical questions listed above, but also as a measure of adequate external compression location.
We still have many questions with regards to optimal cardiac arrest management. These investigators have shown that ultrasound can provide essential information that may impact management, however we have much to learn on how we can best wield this additional power to guide our resuscitations without causing any unintentional harm in the process.
Author: Aalap Shah, MD
References
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