Cerebral Oximetry: A New Tool for Out-of-Hospital Cardiac Arrest Resuscitations?

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Cardiac arrest resuscitation science and technology have improved over time, yet we still struggle to achieve the ultimate goal of neurologically intact survival. Advances have been made in the chain of survival: phone applications such as PulsePoint exist to alert lay rescuers to nearby cardiac arrests for more rapid responses, the American Heart Association has simplified its algorithms to enhance rates of bystander CPR, dispatchers guide CPR and AEDs are now more widely available in public locations. On the EMS response side of the equation, high performance “pit-crew” CPR is commonly used, technologies such as mechanical compression, heads-up CPR, and impedance threshold devices have an increased presence, and ECMO centers are becoming more numerous with some locations even implementing pre-hospital ECMO.

Related

Despite all these advances, neurologically intact survival to discharge after out-of-hospital cardiac arrest (OHCA) is quite poor. In the case of a witnessed arrest with an initial rhythm of ventricular fibrillation, the chance of a good outcome may approach 50% whereas an unwitnessed arrest with an initial rhythm of asystole is essentially unsurvivable. Many cases of OHCA fall somewhere in between. Given an inability to prognosticate survivability, resuscitation is undertaken with no standardized criteria for termination.   

The missing link in most cases is the ability to rapidly and non-invasively assess brain function, or at the very least, brain oxygenation. Other widely used tools are helpful, but do not address this critical resuscitation factor. Capnography is a proxy measure for gas exchange and perfusion, but does not correlate reliably with cerebral perfusion specifically. Transthoracic and transesophgeal ultrasound can evaluate compression effectiveness and potentially distinguish true electromechanical dissociation from pseudo-PEA. However, neither is widely available prehospital nor can they assess cerebral perfusion. Recently, cerebral oximetry, a device well-known in many hospital settings, has begun to be used in the prehospital setting for exactly this purpose.

Cerebral Oximetry

Cerebral oximetry has been used for years in cardiac surgery and neurointensive care settings. Initially, this was accomplished with invasive monitors either placed direcly into brain tissue or in the internal jugular vein. Non-invasive technology is now available from several manufacturers using near infrared spectrosocopy (NIRS) via sensors applied to the forehead. NIRS takes advantage of infrared light that can penetrate the skull and measure hemoglobin oxygenation/deoxygenation at a predefined depth. Given the location of the sensors, NIRS measures regional oxygenation even without pulsatile flow, which is helpful in cardiac arrest and ECMO. The blood in that compartment are both venous and arterial, in a roughly 70% venous/30% arterial ratio. A normal value of this regional oxygen saturation (rSO2) in a healthy awake subject is 60 to 80% but lower values have been seen in patients with intact mentation.1

The wide range in measured values is due to differences in manufacturers technology as well as to the patient’s anatomy and current condition. As a result trends are considered more meaningful than isolated readings. 

The Literature

One of the most comprehensive papers addressing the utility of cerebral oximetry in predicting neurologic outcomes after cardiac arrest is that of Parnia et al.2 This study involved in-hospital cardiac arrest cases at five medical centers in the United States and United Kingdom from 2011 to 2014. The authors concluded that higher peak pre-ROSC rSO2 values correlated with the likelihood of ROSC, and the length of time with an rSO2 over 50% is a helpful marker for positive neurologic outcome. The data are promising for indicating meaningful measurements but the authors admitted the exrapolation to OHCA should not be made. Additionally, they acknowledged its design as a convenience sample and its small number of survivors with “good” neurologic outcomes (Cerebral Performance Category 1 or 2) show the need for 24/7 enrollment and a larger sample size. 

A study in Japan bridged the gap between prehospital and hospital data by measuring the rSO2 for OHCA patients immediately upon arrival to the hospital. (Ito et al)3 The primary endpoint was neurological outcome at 90 days. The authors concluded that future research is needed but that all indications are that rSO2 values have sufficient sensitivity and specificity to be useful for neuro prognostication.

The only true prehospital study to date was led by Prosen and colleagues in Slovenia.4 The goal of this small prospective observational study of 58 patients was to characterize rSO2 values all the way from early in the prehospital CPR process to post ROSC/termination of efforts. They found a higher rSO2 value (average 47%) for patients with ROSC than those whose resusication was terminated (31%) but did not have enough patients to correlate a specific rSO2 threshold with neurologic outcomes.   

Observed Cerebral Oximetry in OHCA in Albuquerque, New Mexico

As mentioned previously, the data supporting prehospital use of cerebral oximetry is not yet sufficient to confidently determine the values or trends that may be meaningful in OHCA. Conceptually, use of such a device could assist the quality of the resuscitation by giving feedback that could lead to corrections. Additionally, it seems likely that it could indicate futility or viability more accurately than current tools. With this in mind, the University of New Mexico EMS Consortium and Albuquerque Fire Rescue (AFR) in 2019 sought a loaner device from Masimo Corporation as a proof of concept. This loaner was obtained coincident with the development of our prehospital ECMO program. We were curious to see if the rSO2 values would show any patterns in both prehospital ECMO as well as in standard OHCA resuscitation. As the device is not yet FDA-approved for prehospital use, the unit was  deployed purely for observational purposes, and had no impact on patient care.   

The monitors were placed with the AFR EMS captains who respond to most cardiac arrests within Albuquerque — although they do not normally arrive on scene until resuscitation has been on-going for 5 to 20 minutes. We have used this technology in over 100 cases and found it easy to deploy. Because the cerebral oximetry devise was deployed for observational purposes, placement was prioritized after the supervisor ensured that all measures were in place for a smooth event; this included applying the mechanical compression device, securing the airway, gaining IV/IO access and beginning to administer ACLS medications. The long Masimo cables allowed for deployment from a position that did not interfere with the resuscitation and placing the probe on the patient’s forehead was simple with readings consistently obtained. 

Values observed ranged from 20% to 90%. Interestingly, the highest reading was in a patient that walked out of the hospital neurologically intact. There were numerous incidents where cerebral oximetry increased markedly along with quantitative capnography in patients that soon thereafter experienced a return of spontaneous circulation (ROSC), although this was not always the case. Anecdotally, it does appear that high values correlate with ROSC and low values with poor outcomes. We have also seen that poor rSO2 values may be an indicator of poor CPR quality that warrants rapid review of mechanical compression puck placement, oxygen supply, airway position, etc.

The Future

More research, particularly robust prospective studies in the prehospital setting, is needed to garner FDA approval for these devices and determine the exact role they can play in OCHA resuscitation. Our hope is that cerebral oximetry will provide valuable information beyond capnography, ultrasound and other monitoring to improve the quality of resuscitation, allow more sophisticated decisions about termination of resuscitation and better apply advanced interventions such as ECMO.

References

  1. W Tosh, FRCA, M Patteril, MD FRCA DipClinEdu (RCS), Cerebral oximetry, BJA Education, Volume 16, Issue 12, December 2016, Pages 417—421, https://doi.org/10.1093/bjaed/mkw024.
  2. Parnia S, Yang J, Nguyen R, et al. Cerebral Oximetry During Cardiac Arrest: A Multicenter Study of Neurologic Outcomes and Survival. Crit Care Med. 2016;44(9):1663″1674. doi:10.1097/CCM.0000000000001723.
  3. Ito N, Nishiyama K, Callaway CW, et al. Noninvasive regional cerebral oxygen saturation for neurological prognostication of patients with out-of-hospital cardiac arrest: a prospective multicenter observational study. Resuscitation. 2014;85(6):778″784. doi:10.1016/j.resuscitation.2014.02.012.
  4. Prosen G, Strnad M, Doniger SJ, et al. Cerebral tissue oximetry levels during prehospital management of cardiac arrest – A prospective observational study. Resuscitation. 2018;129:141″145. doi:10.1016/j.resuscitation.2018.05.014.

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