By: Shakira Tassone
Cardiac neuromonitoring is an underutilized tool in the surgical world. It is critical for stroke identification intraoperatively and optimal patient management postoperatively in various procedures. Cardiothoracic surgery carries a high risk of cerebral hypoperfusion, which is why it is pertinent to have cerebral blood flow continuously monitored to ensure anoxic events do not go undetected.
The University of Pittsburgh Medical Center has recent publications that display how neuromonitoring can optimize patient outcomes by utilizing multimodality monitoring (1). Raw and processed EEG (electroencephalography) combined with somatosensory evoked potentials (SSEPs) are the standard monitoring protocol for these procedures.
Multimodality neuromonitoring provides rapid identification of evolving neuronal injury. A twelve-channel EEG setup is utilized to detect ischemic events to the cortex. This allows the neurophysiologist to optimally monitor each lobe of the brain for early detection of events. If there is a decline in waveform amplitude or beta waves, that is an early indication of hypoperfusion to the cortex. SSEP’s of the median and posterior tibial nerves are monitored to assess the blood flow of the middle cerebral arteries (MCA) and the anterior cerebral arteries (ACA). If hypoperfusion occurs, the latency will increase, and the amplitude will decline.
In addition, the transcranial motor evoked potentials (tceMEP’s) can be an ancillary modality for assessment of the deeper structures of the brain. This modality can detect ischemic events such as lacunar strokes that can cause pure motor hemiparesis. The use of these modalities in tandem not only identifies a stroke sooner but allows the surgeon to put the right actions in motion so the patient can recover faster.
It is known that most of the population has an incomplete Circle of Willis, which is associated with an increased possibility of acute ischemic stroke (2). Cardiothoracic surgeons do not routinely utilize transcranial doppler, brain MRIs, or angiograms after surgery. Therefore, without neuromonitoring present, it is unlikely that the surgeon would know if the patient had a stroke intraoperatively.
Even so, cardiac surgeons rely on cerebral oximetry for the measurement of brain perfusion and oxygenation. This modality has limitations due to regionality and sensitivity. A prime example of uncaptured strokes would be during the period of hypothermia generally utilized for aortic dissections. If neuromonitoring is not utilized, it is unknown whether the brain is in its neuroprotective state or not, and cerebral oximetry does not always change with changes in temperature.
The general guideline is to have anesthesia apply ice to the head, perfusion will cool the patient to a certain temperature, and the surgeon assumes neuroprotection is in place. This differs from the utilization of neuromonitoring during a state of hypothermia, which should change in a systematic and reliable way. This means the SSEP’s, tceMEP’s, and EEG will slowly suppress until near or total flatness occurs in the waveforms. The EEG declining in this fashion demonstrates a rhythm called burst suppression, which means the brain is inactive and in the desired neuroprotective state. When rewarming begins, the previously suppressed signals of each modality should come back slowly and uniformly. This is a clear data driven demonstration of a metabolically preserved brain.
It should be noted that, in a scenario where the waveforms were not to return symmetrically and only the left or the right side of the SSEP’s, tceMEP’s, or the EEG returns, then it is probable that a stroke has transpired. The odds of vegetation or plaque traveling during critical periods such as clamping, cooling, and flow reversals are about 3-9% (3). If continuous neuromonitoring is not in place, brain ischemia may go undetected.
Above all, continuous neuromonitoring provides prompt identification of hypoxia to mitigate adverse neurologic events. Whether the mechanism is embolic or hypoperfusion, constant assessment of oxygenation and cerebral blood flow should occur in a reliable and measurable way. This will not only provide the surgeon with an extra level of confidence, but will also optimize patient outcomes, and preserve brain function (4).
- Amorim E, Rittenberger JC, Zheng JJ, Westover MB, Baldwin ME, Callaway CW, Popescu A; Post Cardiac Arrest Service. Continuous EEG monitoring enhances multimodal outcome prediction in hypoxic-ischemic brain injury. Resuscitation. 2016 Dec.
- Rosner J, Reddy V, Lui F. Neuroanatomy, Circle of Willis. [Updated 2023 Jul 24]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 Jan-.
- https://newsroom.heart.org/news/steps-outlined-to-reduce-the-risk-of-stroke-during-after-heart-surgery. American Heart Association
- Roach GW, Kanchuger M, Mangano CM, Newman M, Nussmeier N, Wolman R, et al. Adverse cerebral outcomes after coronary bypass surgery. N Engl J Med. 1996;335:1857–63.
About the Author:
Shakira Tassone is a seasoned Senior Neurophysiologist with SpecialtyCare, bringing over nine years of experience in neurodiagnostics. Her journey began as an EEG tech in an EP lab, which laid the foundation for her current role. She’s a R.EEGT., CNIM certified, and a notable contributor to the spine surgery field, co-authoring a significant publication in The Spine Journal. In addition to writing for ASNM, Shakira serves on the ASET research task force committee, and contributes as an independent rater for ABRET.