How does autoregulation (automatic regulation) affect spinal cord blood flow?

Autoregulation of Spinal Cord Blood Flow

Spinal cord blood flow autoregulation functions similarly to cerebral autoregulation, maintaining stable perfusion across a range of blood pressures (60-120 mmHg) to protect against inappropriate fluctuations in blood flow despite changes in perfusion pressure. 1

Mechanism of Spinal Cord Autoregulation

Autoregulation in the spinal cord works primarily through arteriolar control, which is the most significant mechanism for regulating blood flow to specific tissues:

• Arteriole diameter changes can dramatically alter blood flow according to Poiseuille's law, where resistance is reduced by the fourth power of increases in vessel radius 2
• This allows the spinal cord to maintain constant blood flow despite fluctuations in systemic blood pressure
• The autoregulatory range for spinal cord blood flow mirrors that of the brain (60-120 mmHg) 1

Regional Differences and Similarities

• Autoregulation occurs regionally throughout the spinal cord (cervical, thoracic, and lumbar segments) 1
• Within the autoregulatory range, spinal cord blood flow (SCBF) averages 61.1 ± 3.6 ml/100g/min, which is remarkably similar to cerebral blood flow (CBF) at 59.2 ± 3.2 ml/100g/min 1
• Below a mean arterial pressure (MAP) of 50 mmHg, spinal cord blood flow falls passively with further decreases in pressure 3
• Above 135 mmHg, vasodilation occurs resulting in breakthrough of autoregulation and marked increases in spinal cord blood flow 3

Factors Affecting Spinal Cord Autoregulation

Anesthesia Effects

• Anesthetic agents can significantly alter spinal cord autoregulation
• Isoflurane anesthesia impairs autoregulation in a dose-dependent manner:
  • At 1.0 MAC isoflurane: spinal cord blood flow increases to 278% of awake control values
  • At 2.0 MAC isoflurane: spinal cord blood flow increases to 535% of awake control values 4

Carbon Dioxide Levels

• CO₂ levels significantly impact spinal cord blood flow
• Hypocapnia (low CO₂) can alter blood flow by widening the plateau on the autoregulatory curve 5
• Spontaneous hyperventilation (PaCO₂ < 35 mmHg) may affect spinal cord perfusion 5

Spinal Cord Injury Effects

• After spinal cord injury (SCI), especially high-level injuries above T6:
  • Static cerebral autoregulation appears to be maintained
  • Dynamic cerebral autoregulation, cerebrovascular reactivity, and neurovascular coupling are markedly altered 6
  • Patients with SCI who remain asymptomatic during orthostatic hypotension maintain better cerebral blood flow (69.3% of baseline) compared to symptomatic patients (55.5% of baseline) 7

Clinical Implications

• Monitoring and assessment of autoregulation may be useful in targeting perfusion management goals in acute brain injury 5
• Pressure autoregulation protects against inappropriate fluctuations in blood flow despite changes in cerebral perfusion pressure 5
• Failure of autoregulation is associated with worse outcomes in various acute neurological diseases 5
• In spinal cord injury patients, autoregulation of cerebral blood flow plays a more dominant role in adaptation to orthostatic hypotension than systemic blood pressure control 7

Monitoring Considerations

• Continuous bedside monitoring of autoregulation is feasible using various techniques:
  • Pressure reactivity index (PRx)
  • Transcranial Doppler ultrasound
  • Near-infrared spectroscopy (NIRS) 5
• These methods can help determine optimal perfusion pressure targets and aid in prognostication 5

Pitfalls and Caveats

• Intraspinal cord compliance might be adversely affected by anesthetics through their effects on autoregulation 1
• Autoregulation can be impaired by various factors including:
  • Systemic endothelial dysfunction
  • Sleep apnea
  • Dyslipidemia
  • Decentralization of sympathetic control
  • Dominant parasympathetic activity 6
• Dysfunctional cerebral and spinal blood flow regulation may increase risk of transient ischemic attacks, stroke, and cognitive dysfunction 6