The Importance of Autoregulation in the Spinal Cord
Arun V. Moorjani, MD and Michael Mahla, MD
Department of Anesthesiology, University of Florida, Gainesville, Florida
Autoregulation keeps the blood flow to the central nervous system
constant over a wide range of perfusion pressure. While in vivo evidence for
autoregulation in the human brain is compelling, direct evidence for
autoregulation of the human spinal cord is sparse.
An 80 year old man with lumbar spinal stenosis presented for a three level
lumbar decompression. His functional capacity was good and limited only by
the recent onset of lower back pain.. His only medication was aspirin 81mg
per day, which he had stopped one week prior to surgery.. His blood pressure
in the preoperative clinic was 126/80 mmHg, which was markedly different
from that obtained in the neurosurgical clinic, where it was 191/81 mmHg.
His preoperative hematocrit was 37.3. He had decreased neck extension due to
an old neck injury from World War II.
Anesthesia was induced with thiopental, vecuronium, and a remifentanil
infusion. Despite the limited neck extension, tracheal intubation was easily
accomplished with a Cormack and Lehane grade 2 view. Anesthesia was
maintained with isoflurane, remifentanil, and vecuronium. During surgery the
patient lost 2000 mL of blood, but remained hemodynamically stable. The
immediate postoperative neurological exam was normal.
In the recovery room hypertension despite adequate pain control was treated
with labetalol. Ongoing bleeding from wound and drain, thought to be due to
aspirin-induced platelet dysfunction, was treated with a platelet
transfusion. Because of the bleeding, there was concern for a lumbar
epidural hematoma. Therefore, frequent neurological exams were performed.
The labetalol decreased the patient’s blood pressure from 170-190/60 to
110/50mmHg. Soon thereafter, left sided weakness ensued followed by the
rapid onset of flaccid paralysis of all four extremities and respiratory
failure. The patient was still able to follow complex commands with his head
with normal strength. He was aware of his problem and very frightened. The
neurosurgical team and the attending anesthesiologist were called to the
bedside while the recovery room physician was bag-mask ventilating the
patient. Instead of intubating the patient, the anesthesiologist gave
phenylephrine’ which increased the blood pressure to 210/90 mmHg.
Spontaneous respirations resumed and the neurological exam normalized within
three minutes.
So what happened? The clinical picture strongly suggested a cervical spine
problem, remote from the operative site, which made a lumbar epidural
hematoma an unlikely etiology. The rationale for increasing the blood
pressure was based on the assumption that the old cervical spine injury had
resulted in subclinical cervical spine stenosis resulting in inadequate
perfusion at the decreased blood pressure. An MRI of the cervical spine
obtained immediately after the event confirmed that assumption.
This case provides clinical evidence that autoregulation does occur in the
spinal cord, likely over a similar range of pressures as in the brain. When
arterial hypertension shifts the autoregulatory range towards higher blood
pressures, spinal cord perfusion pressure may become inadequate when
subclinical extrinsic compression, which unlike intracranial pressure is
rarely quantified, combines with low-normal arterial blood pressures.
Intraoperatively, evidence for autoregulation may be seen with somatosensory
evoked potentials and transcranial motor evoked potentials, which are
commonly affected by blood pressure, both in brain surgery and spinal cord
surgery.
