Physiologic Effects on SSEPs

Question 1

Physiologic Factors

Answer 1

Blood pressure, Temperature, Oxygen/pCO2, Hematocrit

Question 2

Patients at greater risk for hypotension

Answer 2

Preop hypertension, Cardiovascular disease, Diabetes

Question 3

2 Factors that affect hypotensive tolerance

Answer 3

Spinal cord pathology (i.e. compression or trauma), Compromised blood flow during surgery

Question 4

What responses are most affected by a drop in blood pressure below lower range of autoregulation

Answer 4

Cortical responses - Upper/lower SSEPs, bilateral, drop in amp, increased latency

Question 5

Which neural structure is the most sensitive to hypo fusion and changes will be seen

Answer 5

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

Question 6

At what blood flow amount are SSEPs lost

Answer 6

15 - they start to change at 18

Question 7

Penlucida

Answer 7

Cells can keep themselves alive, but won’t be functioning normally

Question 8

Penumbra

Answer 8

In the timeline of brain cells under 18 - cells are beginning to lose time, right before they die.

Question 9

Infarction

Answer 9

Cell dies

Question 10

Cerebral autoregulation

Answer 10

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)

Question 11

Critical closing pressure (CCP)

Answer 11

Pressure at which a vessel’s walls collapse

Question 12

Critical opening pressure (COP)

Answer 12

Pressure at which a closed vessel will open - always higher than CCP

Question 13

How much time does it take for grey matter to be affected

Answer 13

Approx 2 min, due to its high metabolic rate

Question 14

How much time does it take for white matter to be affected

Answer 14

8-18 min

Question 15

In regards to spinal cord ischemia, which is the most sensitive modality

Answer 15

TcMEPs

Question 16

What nerve is the most resilient (in terms of ischemia)

Answer 16

Peripheral nerve

Question 17

What most frequently causes peripheral nerve ischemia

Answer 17

Positioning of the patient

Question 18

Neurogenic shock

Answer 18

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

Question 19

What does temperature affect the most

Answer 19

Latency

Question 20

Hypothermia effects

Answer 20

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

Question 21

Hyperthermia effects

Answer 21

May result in slightly elevated conduction velocities (decreased latency)

Question 22

Hypoxia

Answer 22

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

Question 23

Hypercapnia

Answer 23

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)

Question 24

Respiratory Acidosis

Answer 24

CO2 is increased, hyopmexia may occur also. Respiratory acidosis results from hyperventilation, ventilation: perfusion mismatch.

Question 25

Hematocrit

Answer 25

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.