The investigators propose a multicenter randomized controlled trial in South Korea and Taiwan to observe the clinical effects of REBOA on nontraumatic out-of-hospital-cardiac-arrest (OHCA) patients. While REBOA has been traditionally used in trauma for hemorrhage control, it has also shown promising results in nontraumatic cardiac arrests by rerouting circulation to increase perfusion in the coronary and brain.

Nontraumatic OHCA patients arriving at the 2 participating hospitals between the ages of 20 to 80 will be eligible. If the patient meets the enrollment criteria, he/she will be randomized into the control group (treatment with conventional ACLS according to the 2020 AHA guideline) or the REBOA group (ACLS according to the 2020 AHA guideline with REBOA application). A sheath catheter is inserted with ultrasound guidance to gain access to the common femoral artery in both groups. This is followed by insertion of the REBOA catheter, if the patient is allocated into the intervention group. The aortic balloon is inflated. If ROSC is achieved, the balloon is deflated slowly. Patients who achieve ROSC will receive post cardiac arrest management according to the 2020 AHA guideline.

Inclusion Criteria:

• Adult nontraumatic OHCA
• Witnessed arrest
• Arrival at ER from 9AM to 5PM (in South Korea) and 8AM to 8PM (in Taiwan)

Exclusion Criteria:

1. age below 20 years old or over 80 years old,
2. traumatic cardiac arrest,
3. those with unwitnessed cardiac arrest,
4. pregnant patients,
5. patients who have already achieved ROSC upon arrival at the emergency department
6. pre-cardiac arrest cerebral performance category of 3-4
7. those showing evidence of cardiac arrest due to bleeding (such as gastrointestinal bleeding)
8. those suspected of having aortic disease, such as dissection, intramural hematoma, or aneurysm, by bedside ultrasound performed immediately after ED arrival or have a previous history of aortic disease
9. whose legal representative has requested termination of resuscitation efforts before study enrollment
10. declared dead at scene before randomization
11. Patients who meet the criteria for extracorporeal cardiopulmonary resuscitation (ECPR), and therefore the decision is made to perform ECPR, will not be enrolled. The ECPR criteria applies when all of the following criteria are met: pre-cardiac arrest CPC of 1-2, witnessed cardiac arrest with bystander CPR, ages between 20-70, initial shockable rhythm, ECMO pump-on available within 60 minutes of onset of cardiac arrest, and patients without end-stage diseases such as cancer, liver cirrhosis, or end-stage renal failure.