The main objective of the trial is to find out if there is correlation between cerebral and skeletal muscle oxygenation values during cardiopulmonary resuscitation and if these values can predict return of spontaneous circulation. The investigators would like to find out which values, first measures, average, maximal, are better predictor of return of spontaneous circulation. All the patients with nontraumatic cardiac arrest in prehospital environment will be enrolled in the study. The measurements will be taken with NIRS device and special electrodes, which will be placed on patient's forehead and thenar eminence od right hand. No ALS procedure will be modified.

Out of hospital cardiac arrest (OHCA) is major cause of morbidity and mortality around the world. Annually there is around 350.000 deaths in the Europe because of OHCA. Despite a great progress in prehospital and emergency medicine in recent years, there is no major increase in surviving OHCA. There are many factors which influence to the end result of OHCA. Till now we still don't know how to predict the outcome of cardiac arrest in early phase of cardiopulmonary resuscitation (CPR).

Today emergency teams use end-tidal CO2 values (etCO2) as an indirect marker of cardiac output and as an auxiliary indicator of return of spontaneous circulation (ROSC). Higher values are also correlated with greater probability of ROSC.

Studies in recent years showed that there could be another method which could help to predict the result of OHCA. Near-infrared spectroscopy (NIRS) is a noninvasive spectroscopic method, which uses light near to infrared electromagnetic spectrum of light. The device can, based on Beer-Lambert law, calculates tissue oxygenation of underlaid tissue. It is also useful in the states, where there is little or even no blood flow (cardiac arrest). NIRS is currently used in operating rooms and ICU, but there is also option to use it in prehospital environment. During cardiac arrest the blood flow through organs is very low or absent. This causes hypoxic-ischemic brain injury and also injury of other vital organs. Based on previous studies with NIRS, we can measure cerebral oxygenation by placing electrodes on patient's forehead. Patients with higher values had greater probability of ROSC. It is presumed, that NIRS values are indirect indicator of coronary perfusion pressure. It is known that higher coronary perfusion pressure is connected with higher oxygen concentration in the brain during cardiac arrest. NIRS can also predict re-arrest even before losing a pulse.

Till today there are no studies which would look for correlation between cerebral and skeletal oxygenation during cardiac arrest.

The investigator's study is monocentric, prospective, observational, nonrandomized taking place at Emergency department of University Medical Centre Ljubljana (UMC Ljubljana). There is also a Basic emergency first aid infirmary (Unit SNMP) of Community Health Centre Ljubljana, which will with it's prehospital units make the measurements. After an eyewitness of cardiac arrest will call on emergency number 112 a medical dispatcher will activate medical team with emergency doctor and send them to the location. Upon arrival the team will start CPR based on ALS protocol. As soon as possible one of team members will put two NIRS electrodes on the patient - one on the patient's forehead and the other on patient's thenar eminence on the hand. ALS protocol will not be modified in any way. The NIRS device (Nonin SenSmart Model X-100) will be turned on till the end of resuscitation or till arrival to the hospital (UMC Ljubljana).

Inclusion Criteria:

• non-traumatic cardiac arrest in prehospital environment
• age 18 years or more

Exclusion Criteria:

• age 18 years or less
• pregnancy
• traumatic cause of cardiac arrest
• hypothermia
• drowning
• E-CPR protocol (candidates for ECMO CPR)
• spontaneous circulation upon arrival of emergency team at the scene

The adult population of city Ljubljana and surrounding municipality is which the emergency first aid is provided by Basic emergency first aid infirmary of Community Health Centre Ljubljana. The area is 1670 km2 big and there are up to 450.000 people.