Use of POCUS in the arrest setting typically begins with assessment of cardiac activity: present or absent. In human medicine, protocol-based ALS-compliant cardiac ultrasound use, such as the focused echocardiographic evaluation, has proven to be feasible and is associated with adequate image quality yield, no additional chest compression interruption time, and relevant diagnostic information capable of affecting management. In extreme situations with the greatest time sensitivity (such as cardiac arrest), even a single cardiac POCUS view may suffice in making a diagnosis of cardiac activity. Also, the accuracy in identifying major cardiac arrest causes is often maintained with a single view as the magnitude of pathologic findings to cause arrest are amenable to identification using POCUS. If myocardial activity is seen, cardiac POCUS can be used to help look for reversible causes of the shock state (pericardial effusion, massive pulmonary embolus, and pneumothorax) and can expedite treatment. On return of spontaneous circulation following cardiac arrest, cardiac POCUS has been used to qualitatively assess left ventricular systolic recovery over time. There are a few human studies that suggest cardiac POCUS, when limited to specific goals (cardiac activity present or absent) in a specific subset of patients (cardiac arrest) can predict patient outcomes.

Cardiac Point-of-Care Ultrasound During Active CPR

With adequate training, cardiac POCUS images can be acquired during the 10-second pauses allotted for pulse checks and a change of CPR providers. Given the ease with which a portable ultrasound device can be brought to the patient bedside, POCUS has been recommended as part of the code response. Cardiac POCUS can predict survivability to hospital admission in patients presenting without a pulse. The subxiphoid view in humans has been used to detect cardiac activity at the subxiphoid site during CPR; any non-fibrillating motion in humans is associated with greater survival to hospital admission (23.5% vs. 1.9%, respectively), and the presence of cardiac motion on ultrasound has been shown to be the most important predictor of survivability when trauma patients present in pulseless electrical activity (PEA) cardiac arrest (odds ratio 33.91). The objective is the visualization of cardiac activity, defined as any visible movement of the myocardium, excluding the movement of blood within the cardiac chambers or the movement of the isolated valve. The differential diagnosis between asystole and fine ventricular fibrillation is possible with experience. The demonstration of cardiac contraction on initial ultrasound is of vital prognostic importance as it indicates the possibility of return of spontaneous circulation (ROSC). It helps decision-making in terms of continuity of effort during CPR regardless of the time of resuscitation. Another objective of cardiac POCUS described in the human literature is the evaluation of effectiveness of compressions by providing direct, real-time observation of compression and relaxation of the cardiac chambers during the cardiac massage. Adequate or high-quality compressions are associated with ROSC and survival. Literature reports improper hand position during resuscitation may lead to compression of the ascending aorta, aortic root, or outflow tract of the left ventricle, but not the left ventricle, although this is typically done using transesophageal echocardiography. However, by using cardiac POCUS, it is possible to adjust the applied forces and the location of the hands to optimize compressions.

Although not reported in the veterinary literature, anecdotal evidence suggests the subxiphoid view can be used to assess the heart during active CPR in dogs and cats. Finally, cardiac POCUS has been used to evaluate for cardiac standstill when deciding to cease resuscitative efforts.

Point-of-Care Ultrasound and Identifying Reversible Causes of Arrest

Multiple protocols, ranging from only the subxiphoid window to multiple windows (SESAME, FEEL, FEER, CAUSE) have been published in human medicine to look for reversible causes of arrest. Reversible causes of arrest include pericardial tamponade, hypovolemia and other forms of shock, right ventricular failure (air emboli), and pneumothorax.

Pericardial tamponade: occurs when the pressure in the pericardium exceeds the pressure in the cardiac chambers, particularly the right atrium, resulting in impaired cardiac filling. To sonographically identify tamponade, pericardial effusion is typically identified first, although there are rare reports of tamponade being identified secondary to pleural effusion in the absence of pericardial effusion. It is therefore essential to know how to differentiate pericardial effusion from pleural effusion; pericardial effusion can be identified via transthoracic approaches, or via the subxiphoid approach. For patients in cardiac arrest, the subxiphoid view may be attempted first. If not sufficient to image the heart, the four-chamber long-axis view may be attempted next, as long as conforming to ALS protocols. If the origin of fluid is uncertain (pleural vs. pericardial) the pericardio-diaphragmatic window can be identified, which helps differentiate the two fluid types. Tamponade may be easiest to diagnose using the right parasternal four-chamber long-axis view and appears as a compression of the right atrial free wall into the atrium, intermittently reducing atrial chamber size (right atrial wall compressed inwards during systole). Asynchronous contractility of the right ventricular free wall can also be seen in short-axis views and can be identified via the subxiphoid view. However, it should be kept in mind that tamponade is a clinical diagnosis and the findings of shock in a patient with identifiable pericardial effusion should prompt a diagnosis of tamponade. Therefore, if pericardial effusion is present in arrested patients, particularly dogs, its emergent centesis is warranted.

Hypovolemia: can cause a combination of findings on cardiac and vascular POCUS. Cardiac POCUS may identify a pseudohypertrophy; sonographically visible cardiac changes that result from hypovolemia and decreased ventricular filling. Pseudohypertrophy is associated with decreased left atrial and ventricular chamber size and the appearance of a thickened intraventricular septum and ventricular free wall. This resolves with restoration of effective circulating volume. In addition, the caudal vena cava may appear small to collapse with large changes in respiratory variation, or if severe, complete collapse of the vena cava on inspiration. Therefore, the combination of a small La:Ao (≤1:1), decreased ventricular lumen size with thickened ventricular walls, and a thin ‘flat” CVC with no change in the CVC collapsibility index (CVCCI), or a change in the CVCCI of more than 50% suggests hypovolemia as a potentially reversible contributing cause of arrest. However, in the arrest setting, these parameters may be difficult to assess, and a suspicion of hypovolemia is often made when internal blood loss is noted in the abdomen or pleural spaces with POCUS.

Shock: hemorrhagic hypovolemic shock should be considered in the patient presenting with trauma, and cardiogenic, distributive, or obstructive shock should initially be considered in the medical patient. Each shock profile has different POCUS findings, but it is beyond the scope of these proceedings to describe in detail.

Right ventricular failure (air emboli) is reported as a reversible cause of arrest in human medicine. Massive pulmonary thromboembolism can be detected by ultrasound with indirect signs which include acute dilation of the right ventricle (RV), exceeding the normal relationship with the left ventricle (LV) (i.e., 1:1 RV to LV ratio) in the four-chamber view, and the finding of a D-shaped RV (flat intraventricular septum) in the short-axis window, both of which can be obtained from the subxiphoid window with practice, or from the right parasternal windows.

Pneumothorax: see proceeding on pneumothorax for rule-in and rule-out findings. Patients that arrest primarily due to pneumothorax will have a tension pneumothorax, which results in signs of cardiovascular shock. Hemothorax is another finding of the trauma patient, and if it is massive, it could generate hypovolemic shock.

Pitfalls

Causes of arrest identified on POCUS must be differentiated from non-arrest findings. For example, unilaterally absent lung sliding could indicate a simple pneumothorax or mainstem bronchial intubation, not a tension pneumothorax. Likewise, visualized peritoneal fluid could be ascites, not acute hemorrhage; a pericardial effusion could be present without tamponade physiology (e.g., common in cats); and right heart dilation may accumulate over the course of CPR without the presence of a massive pulmonary embolism. Treating an incorrect diagnosis suggested by POCUS may cause iatrogenic injury or delay identification of the real etiology.

Summary

POCUS in cardiac arrest when performed by trained clinicians can assess quality of compressions, detect reversible causes of arrest with non-defibrillable rhythms, and allow for monitoring interventions and their response to treatment. It also provides prognostic information regarding the possibility of a return to spontaneous circulation and survival. It should be part of a holistic approach that starts with using the subxiphoid window to assess the heart and caudal vena cava, and if not sufficient, advances to the visualization of the transthoracic right-parasternal windows, and then explores the next area, if necessary, based on cardiac imaging, with the clinical correlation.

References

References are available upon request.