Physiologic Effects on SSEPs Flashcards
Physiologic Factors
Blood pressure, Temperature, Oxygen/pCO2, Hematocrit
Patients at greater risk for hypotension
Preop hypertension, Cardiovascular disease, Diabetes
2 Factors that affect hypotensive tolerance
Spinal cord pathology (i.e. compression or trauma), Compromised blood flow during surgery
What responses are most affected by a drop in blood pressure below lower range of autoregulation
Cortical responses - Upper/lower SSEPs, bilateral, drop in amp, increased latency
Which neural structure is the most sensitive to hypo fusion and changes will be seen
Cerebral cortex - has a high metabolic rate. Changes include a decrease in cortical SSEP, possible increased latency, decrease in EEG amps and slowing, possible loss of TcMEP
At what blood flow amount are SSEPs lost
15 - they start to change at 18
Penlucida
Cells can keep themselves alive, but won’t be functioning normally
Penumbra
In the timeline of brain cells under 18 - cells are beginning to lose time, right before they die.
Infarction
Cell dies
Cerebral autoregulation
The cerebral vasculature is designed to vasoconstrict or vasodilate to compensate for MAP changes in order to maintain a stable cerebral blood flow. If you fall outside of the range, blood flow becomes pressure dependent (resistance becomes fixed)
Critical closing pressure (CCP)
Pressure at which a vessel’s walls collapse
Critical opening pressure (COP)
Pressure at which a closed vessel will open - always higher than CCP
How much time does it take for grey matter to be affected
Approx 2 min, due to its high metabolic rate
How much time does it take for white matter to be affected
8-18 min
In regards to spinal cord ischemia, which is the most sensitive modality
TcMEPs
What nerve is the most resilient (in terms of ischemia)
Peripheral nerve
What most frequently causes peripheral nerve ischemia
Positioning of the patient
Neurogenic shock
Severe hypotension caused by autonomic control of peripheral vascular muscle tone due to a spinal injury, usually occurring at cervicothoracic levels. Loss of volume returns to the heart from wide scale vasodilation (to the rest of the body except heart) causes hypotension
What does temperature affect the most
Latency
Hypothermia effects
Slow nerve conduction. Approx 4% latency increase with every 1 C drop. Cortical amp decreases until below 32 degrees. Peripheral amp increase in amp until very low temps, then diminish. Cortical SSEPs disappear at about 22, followed by subcortical, then finally peripheral
Hyperthermia effects
May result in slightly elevated conduction velocities (decreased latency)
Hypoxia
May result from pulmonary or respiratory compromise. Decreased amp, increased latency. Cortical responses most affected. If continues, EEG will go flat (no O2 to brain), peripheral nerve excitability is lost and critical organ failure will occur
Hypercapnia
Excessive CO2. Excessive vasodilation of cerebral vessels, increased intracranial pressure (increased blood flow). When severe, vasoconstriction of cerebral vessels (to try to stop having CO2 being sent to brain). Decreases blood pH (more acidic)
Respiratory Acidosis
CO2 is increased, hyopmexia may occur also. Respiratory acidosis results from hyperventilation, ventilation: perfusion mismatch.
Hematocrit
Percentage of blood solids. Normal is 45%. Determines blood viscosity. Less hematocrit, less RBC, less O2 carry capacity. At 16-20%, SSEP may increase in amplitude. Below 10%, SSEP becomes decreased in amp and increased in latency.
