Local blood flow, oxygen tension, and oxygen consumption in the rat spinal cord. Part 2: Relation to segmental level

Delayed detection of motor pathway dysfunction after selective reduction of thoracic spinal cord blood flow in pigs

OBJECTIVE

Clinical monitoring of myogenic motor evoked potentials to transcranial stimulation provides rapid evaluation of motor-pathway function during surgical procedures in which spinal cord ischemia can occur. However, a severe reduction of spinal cord blood flow that remains confined to the thoracic spinal cord might render ischemic only the descending axons of the corticospinal pathway. In this situation lower-limb motor evoked potentials could respond relatively late compared with a similar spinal cord blood flow reduction of the lumbar spinal cord that renders predominantly motoneurons ischemic.

METHODS

Selective thoracic and lumbar spinal cord ischemia was induced by sequential clamping of segmental arteries during continuous assessment of laser-Doppler spinal cord blood flow at the thoracic and lumbar spinal cord. Myogenic motor evoked potentials were recorded from the upper and lower limbs. The time to loss of motor evoked potentials was compared (n = 11) during reduction of laser-Doppler spinal cord blood flow below 25% of baseline (ischemic segment), and flow was maintained at greater than 75% of baseline in the nonischemic segment, both during thoracic and lumbar spinal cord ischemia.

RESULTS

Average laser-Doppler spinal cord blood flow in the ischemic segment was similar during thoracic (26% +/- 15% [+/- SD]) and lumbar (26% +/- 16%) ischemia, whereas normal flow was maintained in the nonischemic segment. The time to motor evoked potentials loss was considerably longer after thoracic spinal cord ischemia (15 +/- 11 minutes) than after lumbar spinal cord ischemia (3 +/- 2 minutes, P <.005).

CONCLUSION

In this experimental model of selective spinal cord ischemia, a severe reduction of lumbar spinal cord blood flow results in rapid loss of myogenic motor evoked potentials, whereas a similar blood flow reduction in the thoracic spinal cord results in relatively slow loss of motor evoked potentials. The effectiveness of motor evoked potentials to rapidly assess spinal cord integrity might be limited when spinal cord ischemia is confined to the thoracic segments.

A model of extravascular bubble evolution: effect of changes in breathing gas composition

Observations of bubble evolution in rats after decompression from air dives (O. Hyldegaard and J. Madsen. Undersea Biomed. Res. 16: 185-193, 1989; O. Hyldegaard and J. Madsen. Undersea Hyperbaric Med. 21: 413-424, 1994; O. Hyldegaard, M. Moller, and J. Madsen. Undersea Biomed. Res. 18: 361-371, 1991) suggest that bubbles may resolve more safely when the breathing gas is a heliox mixture than when it is pure O(2). This is due to a transient period of bubble growth seen during switches to O(2) breathing. In an attempt to understand these experimental results, we have developed a multigas-multipressure mathematical model of bubble evolution, which consists of a bubble in a well-stirred liquid. The liquid exchanges gas with the bubble via diffusion, and the exchange between liquid and blood is described by a single-exponential time constant for each inert gas. The model indicates that bubbles resolve most rapidly in spinal tissue, in adipose tissue, and in aqueous tissues when the breathing gas is switched to O(2) after surfacing. In addition, the model suggests that switching to heliox breathing may prolong the existence of the bubble relative to breathing air for bubbles in spinal and adipose tissues. Some possible explanations for the discrepancy between model and experiment are discussed.

A preliminary report on the use of laser-Doppler flowmetry during tethered spinal cord release

Neurological deterioration in the tethered cord syndrome has been postulated to result from a compromise of blood flow in the distal spinal cord. In order to evaluate vascular perfusion in human subjects, a new technique of laser-Doppler flowmetry was used to monitor continuously the microcirculation of the distal spinal cord during surgery for tethered cord release in 10 children. For further comparison, five children undergoing selective dorsal rhizotomy were also monitored. In the tethered cord syndrome group, spinal cord blood flow before untethering was a mean of 12.6 ml/min per 100 g of tissue and increased in all cases after release to a mean of 29.4 ml/min per 100 g of tissue. All patients improved neurologically. The selective dorsal rhizotomy group had a preoperative mean spinal cord blood flow of 30.8 ml/min per 100 g of tissue, which was not altered by the operative procedure. Significant improvement occurs in distal spinal cord blood flow after tethered cord release, which may be representative of an important mechanism in the pathophysiology of the tethered cord syndrome.

Regulation of spinal cord blood flow in the rat as measured by quantitative autoradiography

Spinal cord blood flow (SCBF) and its response to alterations in blood gases and to systemic hypotension, haemodilution and barbiturate anaesthesia were measured in 47 rats at the level of Th 7-8 by quantitative autoradiography with [14C]iodoantipyrine ([14C]IAP) as tracer. During normocapnia and normoxia the mean SCBF values in the grey and white matter were 94 +/- 21 and 17 +/- 3 ml min-1 100 g-1, respectively. SCBF was highly dependent on PaCO2. Thus in hypercapnic animals (PaCO2 greater than 9 kPa) SCBF was increased in grey and white matter to 228 +/- 22 and 54 +/- 7 ml min-1 100 g-1, while during hypocapnia (PaCO2 less than 3.9 kPa) it was decreased to 64 +/- 14 and 11 +/- 2 ml min-1 100 g-1, respectively. Mean arterial blood pressure (MABP) was reduced by withdrawal of blood to 80 +/- 8 mmHg in a light hypotension group and to 53 +/- 11 mmHg in a severe hypotension group, compared with 126 +/- 13 mmHg in a control group. There was no significant difference in SCBF between the control group and the hypotension groups, suggesting that autoregulation is maintained down to an MABP of at least 50 mmHg. Normovolaemic haemodilution, with a reduction of the haematocrit from 50 +/- 2 to 33 +/- 3, increased SCBF to 113 +/- 9 ml min-1 100 g-1 in grey matter and to 21 +/- 12 ml min-1 100 g-1 in white matter.(ABSTRACT TRUNCATED AT 250 WORDS)

Regional peripheral and CNS hemodynamic effects of intrathecal administration of a substance P receptor agonist

Regional CNS and peripheral hemodynamic effects of the intrathecal (i.t.) administration of a substance P receptor agonist, [pGlu5,MePhe8,MeGly9]-substance P5-11 ([DiMe]-SP), were studied in anesthetized rats with the radioactive microsphere technique. It was previously shown that [DiMe]-SP caused a sympathetically mediated increase in mean arterial pressure (MAP) by an action within the spinal cord. In this study, [DiMe]-SP (5 and 33 nmol, i.t.) increased MAP. The 5 nmol dose increased resistance in cutaneous, renal, splanchnic, and adrenal vascular beds but decreased resistance, and increased blood flow in some skeletal muscle beds. Total peripheral resistance was unchanged. The 33 nmol dose increased resistance in each peripheral vascular bed analyzed and increased total peripheral resistance. Whereas each dose increased heart rate, stroke volume and cardiac output were unchanged with the 5 nmol dose and were reduced with the 33 nmol dose. Neither dose of [DiMe]-SP significantly altered regional brain or spinal cord blood flows. These data show that the i.t. administration of the SP agonist, [DiMe]-SP, increased vascular tone to most peripheral vascular beds whereas the low dose caused a vasodilation of skeletal muscle. These effects are consistent with the notion of a dose-related activation of SP receptors in the spinal cord affecting sympathetic outflow to the adrenals and to the vasculature.

Microangioarchitecture of the feline spinal cord. Three-dimensional observation of blood vessel corrosion casts by scanning electron microscopy

The microangioarchitecture of corrosion casts of the cat spinal cord was studied by scanning electron microscopy. On the ventral surface of the spinal cord, the anterior spinal artery and the anterior spinal vein ran parallel along the anterior median fissure. Many central arteries branching from the anterior spinal artery coursed in a wavelike manner in the anterior median fissure. The number of central arteries was lowest in the thoracic spinal cord. Central arteries at some spinal cord levels revealed well-developed anastomoses with other central arteries in the anterior median fissure. These well-developed anastomotic central arteries were frequently observed in the thoracic spinal cord, in which the number of central arteries was lowest. On the dorsal surface of the spinal cord, the posterior spinal vein ran longitudinally at the midline and was drained by circumferential veins and posterior central veins. This vein formed a characteristic anastomotic plexus. Small arterioles (20 microns in diameter) in the spinal parenchyma revealed a ring-like compression at the branching site.

Increased concentrations of uric acid in the spinal cord of rats with chronic relapsing experimental allergic encephalomyelitis

We investigated the effects of chronic relapsing experimental allergic encephalomyelitis (first attack and first recovery) on uric acid (UR) concentrations in Lewis rat spinal cord. Perchloric extracts of 9 regions between spinal cord segments C3 and Co1 were injected into a reversed-phase high-performance liquid chromatographic system, and UR was quantified electrochemically. In all regions UR, which was lower than 0.5 micrograms/g wet tissue in rats injected with adjuvant (controls), increased to a variable extent during the attack (5-12 micrograms/g, maximum in T11-L2) and returned during recovery to a residual level of 1-2 micrograms/g. These findings can be partly attributed to blood-brain barrier damage.

The influence of hemorrhagic hypotension on spinal cord tissue oxygen tension

In order to investigate the spinal cord surface PO2 (sPO2) reaction to hypovolemic hypotension, nine female pigs (25 kg bw) were premedicated, anesthetized, intubated and artificially ventilated with N2O:O2 = 3:1. Following laminectomy from L3 to L5, sPO2 was measured on the dorsal side of the exposed spinal cord using six gold cathodes (luminal diameter 15 microns) while MAP (mean +/- SD) was lowered in steps of about 10 Torr by bleeding into a reservoir from 69.3 +/- 10.1 Torr to extreme low values of 13.3 +/- 3.1 Torr. Only a slight decrease of lumbar sPO2 (mean +/- SD) from 33.5 +/- 7.2 Torr to 27.6 +/- 4.8 Torr was evaluated from the data in response to MAP reduction to 50.2 +/- 3.5 Torr. Below that value, a marked proportional decrease of sPO2 and MAP was observed (0.65 Torr/Torr) corresponding to pressure passive flow behavior of the Hagen Poisseuille Law ("loss of autoregulation"). Four to five minutes after start of reinfusion both sPO2 and MAP showed an overshoot with maximum values of 54.0 +/- 11.1 Torr resp. 102.6 +/- 18 Torr. Initial values were approximated about 15 min. later. Histograms plotted from the individual sPO2 values of all pigs and of all the different experimental stages showed signs of severe hypoxia only if MAP was reduced below about 30 Torr. In general, this situation was reversible within reinfusion, only one of the nine pigs did not tolerate hemorrhagic hypoxia induced by MAP reduction to less than 30 Torr for at least 5-10 min. Nevertheless, the experiments showed a considerable circulatory stability in the investigated pigs.

Response of spinal cord oxygen tension to aortic occlusion

In order to study the factors influencing spinal cord injury resulting from occlusion of the descending aorta, the local surface PO2 (sPO2) was measured in the lumbar region (L4-5) of the exposed spinal cord after laminectomy in the pig. This is the area supplied by the arteria radicularis magna anterior (ARMA). The following aspects were considered: duration of occlusion, blood supply by the ARMA, distribution and width of spinal cord collaterals. The animals were divided into two groups. In group I (n = 7) the descending aorta was occluded immediately below the left subclavian artery; in group II (n = 7) the abdominal aorta was occluded immediately superior to the ARMA. Occlusion induced a rapid sPO2 decrease in all animals, from 29 to 3 mm Hg in group I (-14 mm Hg/min) and from 28 to 8 mm Hg in group II (-12 mm Hg/min). After 45 minutes of occlusion the blood flow through the aorta was released giving a pronounced sPO2 overshoot (36 mm Hg) about 10 minutes later. After 20 minutes there had been a return to initial sPO2 values. The results of this study confirm the hypothesis that spinal cord injury during occlusion of the descending aorta is primarily due to ischemic hypoxia/anoxia.

Local blood flow, oxygen tension, and oxygen consumption in the rat spinal cord. Part 1: Oxygen metabolism and neuronal function

A reliable and reproducible microelectrode technique provided consistent simultaneous measurements of local spinal cord blood flow (local SCBF), tissue oxygen tension (TO2), and tissue oxygen consumption (TO2C) in the rat. Local SCBF was measured by the hydrogen clearance technique, local TO2 was measured polarographically, and local TO2C was calculated from the declining slope of local TO2 following temporary arrest of local SCBF. Mean local TO2 values varied only slightly between gray and white matter, while local TO2C and SCBF maintained a 3 to 1 ratio between gray and white matter areas. Measurements were also made of these parameters in specific white matter tracts and laminae of Rexed. Local white matter SCBF was relatively homogeneous throughout the ventral, lateral, and dorsal funiculi. Gray matter blood flow was more variable with topography. The highest local SCBF was recorded in areas rich in internuncial neurons. The somatic motor regions had values slightly higher than recorded in sensory regions.

Simultaneous measurement of local blood flow and tissue oxygen in rat spinal cord

Measurement of local blood flow by hydrogen clearance is a useful technique and is compatible with simultaneous measurement of oxygen tension over long periods. However, existing methods present serious limitations of spatial resolution due to high diffusion rates and other factors. Improved methods permit local measurements in both gray and white matter of the rat spinal cord that correlate well with data from autoradiographic techniques, and indeed distinguish between individual gray matter laminae. Applications of similar methods should be useful in other systems where high spatial resolution is required.