Scott M. Eleff

Correlation of the Average Water Diffusion Constant with Cerebral Blood Flow and Ischemic Damage After Transient Middle Cerebral Artery Occlusion in Cats

Magnetic resonance water diffusion imaging can detect early ischemic changes in stroke. Using a middle cerebral artery occlusion model, we examined which range of values of the orientation-independent diffusion quantity is an early noninvasive indicator of reduced cerebral perfusion and focal brain injury. Cats underwent either a 30-min occlusion followed by 3.5 h reperfusion (n = 7) or a 60-min occlusion followed by 4-h reperfusion (n = 6). Repeated measurements of CBF were made with radiolabeled microspheres, and acute focal injury was measured with triphenyltetrazolium chloride (TTC) staining. During occlusion, the decrease in Dav correlated with CBF for caudate [30-min occlusion (n = 13): p < 0.0001; 60-min occlusion (n = 6): p < 0.02] and for cortex [30-min occlusion (n = 12): p < 0.0001; 60-min occlusion (n = 5): p < 0.04]. Variable caudate and hemispheric injury levels were found among cats in both groups. The area of tissue injury demarcated by TTC began to correlate with the area of reduced Dav by 30 min of occlusion (p < 0.02), and this correlation improved (p < 0.0001) at 1, 1.5, and 2.0 h after the onset of occlusion. The time necessary to reach a one-to-one correspondence between the percent of hemisphere injured and the percent of hemispheric area with Dav < 0.65 × 10−9 m2/s was 2 h after occlusion. Thus, the absolute value of Dav is a good indicator of the risk of tissue injury, whereas the combination of Dav and the length of time of Dav reduction is an excellent predictor of acute focal tissue injury demarcated by TTC staining.

Sodium, ATP, and intracellular pH transients during reversible complete ischemia of dog cerebrum

We tested the hypotheses that with the onset of cerebral ischemia, massive cellular sodium influx does not occur until adenosine triphosphate is fully depleted and that on reperfusion, neuronal sodium efflux does not occur until adenosine triphosphate is fully restored. We examined the temporal relationships among transcellular sodium, energy metabolism, and intracellular pH with sodium and phosphorus magnetic resonance spectroscopy in a new, hemodynamically stable, brain stem-sparing model of reversible, complete cerebral ischemia in eight anesthetized dogs. Inflation of a neck tourniquet after placement of glue at the tip of the basilar artery resulted in decreased blood flow to the cerebrum from 29 +/- 5 to 0.3 +/- 0.5 ml/min/100 g. Medullary blood flow was not significantly affected, and arterial blood pressure was unchanged. Sodium signal intensity decreased and did not lag behind the fall in adenosine triphosphate. After 12 minutes of ischemia, reperfusion resulted in a more rapid recovery of sodium intensity (12.4 +/- 4.8 minutes) than either adenosine triphosphate (16.5 +/- 3.7 minutes) or intracellular pH (38.9 +/- 1.8 minutes). Because intracellular sodium has a weaker signal than extracellular sodium, the decreased sodium intensity is interpreted as sodium influx and indicates that sodium influx does not require full depletion of adenosine triphosphate. Rapid recovery of sodium intensity during early reperfusion may represent sodium efflux, although increased plasma volume and sodium uptake from plasma may also contribute. If our interpretation of the sodium signal is correct, delayed recovery of adenosine triphosphate may be due to the utilization of adenosine triphosphate for the restoration of transcellular sodium gradient.

Effect of arrest time and cerebral perfusion pressure during cardiopulmonary resuscitation on cerebral blood flow, metabolism, adenosine triphosphate recovery, and pH in dogs

OBJECTIVES To test the hypothesis that greater cerebral perfusion pressure (CPP) is required to restore cerebral blood flow (CBF), oxygen metabolism, adenosine triphosphate (ATP), and intracellular pH (pHi) levels after variable periods of no-flow than to maintain them when cardiopulmonary resuscitation (CPR) is started immediately. DESIGN Prospective, randomized, comparison of three arrest times and two perfusion pressures during CPR in 24 anesthetized dogs. SETTING University cerebral resuscitation laboratory. INTERVENTIONS We used radiolabeled microspheres to determine CBF and magnetic resonance spectroscopy to derive ATP and pHi levels before and during CPR. Ventricular fibrillation was induced, epinephrine administered, and thoracic vest CPR adjusted to provide a CPP of 25 or 35 mm Hg after arrest times of O, 6, or 12 mins. MEASUREMENTS AND MAIN RESULTS When CPR was started immediately after arrest with a CPP of 25 mm Hg, CBF and ATP were 57 +/- 10% and 64 +/- 14% of prearrest (at 10 mins of CPR). In contrast, CBF and ATP were minimally restored with a CPP at 25 mm Hg after a 6-min arrest time (23 +/- 5%, 16 +/- 5%, respectively). With a CPP of 35 mm Hg, extending the no-flow arrest time from 6 to 12 mins reduced reflow from 71 +/- 11% to 37 +/- 7% of pre-arrest and reduced ATP recovery from 60 +/- 11% to 2 +/- 1% of pre-arrest. After 6- or 12-min arrest times, brainstem blood flow was restored more than supratentorial blood flow, but cerebral pHi was never restored. CONCLUSIONS A CPP of 25 mm Hg maintains supratentorial blood flow and ATP at 60% to 70% when CPR starts immediately on arrest, but not after a 6-min delay. A higher CPP of 35 mm Hg is required to restore CBF and ATP when CPR is delayed for 6 mins. After a 12-min delay, even the CPP of 35 mm Hg is unable to restore CBF and ATP. Therefore, increasing the arrest time at these perfusion pressures increases the resistance to reflow sufficient to impair restoration of cerebral ATP.

Intracellular pH during reperfusion influences evoked potential recovery after complete cerebral ischemia.

Background and Purpose Increasing ischemic duration delays recovery of intracellular pH and depresses recovery of somatosensory evoked potentials. We tested whether manipulation of the rate of pH recovery influences evoked potential recovery after complete ischemia. Methods Four groups of eight anesthetized dogs underwent 12 minutes of complete ischemia followed by 4 hours of reperfusion with either 1) normocapnia, 2) normocapnia and acetazolamide (25 mg/kg at reperfusion plus 12.5 mg/kg per hour, 3) hypocapnia, or 4) hypercapnia. Intracellular pH was measured by phosphorus magnetic resonance spectroscopy, and intracellular bicarbonate was calculated using sagittal sinus partial pressure of C02 during reperfusion. Results In the normocapnic control group, intracellular pH decreased from 7.10 ±0.04 (±SEM) to 6.13 ±0.08 during ischemia and recovered to 6.90 ±0.08 by 30 minutes of reperfusion. Bicarbonate also largely recovered (9.9±1.6 mM). With acetazolamide pH (6.51±0.10) and estimated bicarbonate (4.8±13 mM) remained depressed at 30 minutes and did not fully recover until 60–75 minutes. However, percent recovery of somatosensory evoked potential amplitude at 4 hours of reperfusion was less with acetazolamide (23 ±4%) than in the control group (52 ±5%). With hypercapnic reperfusion, which delayed pH recovery but not bicarbonate recovery, evoked potential recovery was also depressed (27 ±5%). With hypocapnic reperfusion, which delayed bicarbonate recovery but not pH recovery, evoked potential recovery (52 ±6%) was not depressed compared with controls. Recovery of adenosine triphosphate and oxygen consumption was similar among groups. Conclusions Delayed recovery of intracellular pH with or without delayed recovery of bicarbonate during reperfusion further impairs somatosensory evoked potential recovery independent of recovery of high-energy phosphates. Persistence of acidosis during reperfusion can contribute to postischemic electrophysiological deficit.

Brain bioenergetics during cardiopulmonary resuscitation in dogs.

Cardiac arrest causes a rapid loss of cerebral adenosine triphosphate [corrected] (ATP) and a decrease in cerebral intracellular pH (pHi). Depending on the efficacy of cardiopulmonary resuscitation (CPR), cerebral blood flow levels (CBF) ranging from near zero to near normal have been reported experimentally. Using 31P magnetic resonance spectroscopy, the authors tested whether experimental CPR with normal levels of cerebral blood flow can rapidly restore cerebral ATP and pHi despite the progressive systemic acidemia associated with CPR. After 6 min of ventricular fibrillation in six dogs anesthetized with fentanyl and pentobarbital, ATP was reduced to undetectable concentrations and pHi decreased from 7.11 +/- 0.02 to 6.28 +/- 0.09 (+/- SE) as measured by 31P magnetic resonance spectroscopy. Application of cyclic chest compression by an inflatable vest placed around the thorax and infusion of epinephrine (40 micrograms/kg bolus plus 8 micrograms/kg/min, intravenously) maintained cerebral perfusion pressure greater than 70 mmHg for 50 min with the dog remaining in the magnet. Prearrest cerebral blood flows were generated. Cerebral pHi recovered to 7.03 +/- 0.03 by 35 min of CPR, whereas arterial pH decreased from 7.41 +/- 0.4 to 7.08 +/- 0.04 and cerebral venous pH decreased from 7.29 +/- 0.03 to 7.01 +/- 0.04. Cerebral ATP levels recovered to 86 +/- 7% (+/- SE) of prearrest concentration by 6 min of CPR. There was no further recovery of ATP, which remained significantly less than control. Therefore, in contrast to hyperemic reperfusion with spontaneous circulation and full ATP recovery, experimental CPR may not be able to restore ATP completely after 6 min of global ischemia despite restoration of CBF and brain pHi to prearrest levels.

Determination of cerebral glucose transport and metabolic kinetics by dynamic MR spectroscopy

A new in vivo nuclear magnetic resonance (NMR) spectroscopy method is introduced that dynamically measures cerebral utilization of magnetically labeled [1-13C]glucose from the change in total brain glucose signals on infusion. Kinetic equations are derived using a four-compartment model incorporating glucose transport and phosphorylation. Brain extract data show that the glucose 6-phosphate concentration is negligible relative to glucose, simplifying the kinetics to three compartments and allowing direct determination of the glucose-utilization half-life time [ t ½ = ln2/( k 2 + k 3)] from the time dependence of the NMR signal. Results on isofluorane ( n = 5)- and halothane ( n = 7)- anesthetized cats give a hyperglycemic t ½ = 5.10 ± 0.11 min-1 (SE). Using Michaelis-Menten kinetics and an assumed half-saturation constant Kt = 5 ± 1 mM, we determined a maximal transport rate T max = 0.83 ± 0.19 μmol ⋅ g-1 ⋅ min-1, a cerebral metabolic rate of glucose CMRGlc = 0.22 ± 0.03 μmol ⋅ g-1 ⋅ min-1, and a normoglycemic cerebral influx rate CIRGlc = 0.37 ± 0.05 μmol ⋅ g-1 ⋅ min-1. Possible extension of this approach to positron emission tomography and proton NMR is discussed.

Poor hemodynamic and metabolic recovery after global incomplete cerebral ischemia associated with short-term diabetes in dogs.

We determined the effect of 4–5 weeks of diabetes on ATP recovery following global incomplete cerebral ischemia. 31P magnetic resonance spectra of ATP, intracellular pH (pHi), and CBF (radiolabeled microspheres) were measured in three groups of anesthetized dogs (n = 8/group): chronic hyperglycemic diabetes (pancreatectomy followed by blood glucose of >10 mM for 4–5 weeks); acute hyperglycemia (blood glucose of >10 mM) during ischemia and reperfusion in nondiabetic dogs; and normoglycemic controls. Twenty minutes of incomplete ischemia was produced by ventricular fluid infusion to keep cerebral perfusion pressure (CPP) at 10 mm Hg during spontaneous variations in MABP. Intracranial pressure was increased initially to similar levels, resulting in a similar Cushing response among the groups. However, during the final 8 min of ischemia, MABP decreased to a greater extent in diabetic (86 ± 42 mm Hg) than in hyperglycemic (162 ± 30 mm Hg) and normoglycemic (135 ± 54 mm Hg) groups and remained lower throughout 3 h of reperfusion. CPP was kept constant during ischemia, but was lower throughout reperfusion in diabetic dogs. During ischemia CBF was reduced similarly among groups: 5 ± 3 ml · min−1 · 100 g−1 in hyperglycemic and normoglycemic and 4 ± 3 ml · min−1 · 100 g−1 in diabetic dogs. During reperfusion early hyperemia was attenuated and delayed hypoperfusion was augmented (7 ± 17 ml · min−1 · 100 g−1 by 180 min) as a result of low perfusion pressure in diabetics. However, medullary blood flow was similar among groups. Endischemic ATP was similar among groups (25 ± 34, 30 ± 25, and 4 ± 11% of baseline in normoglycemic, hyperglycemic and diabetic dogs, respectively). Following 180 min of reperfusion ATP was 87 ± 15, 45 ± 38, and 7 ± 19% of control in normoglycemic, hyperglycemic and diabetic dogs, respectively. End-ischemic pHi was lower in hyperglycemic (5.94 ± 0.16) and diabetic (5.96 ± 0.08) than normoglycemic (6.32 ± 0.04) groups. By 45 min of reperfusion, pHi recovered in normoglycemic (7.02 ± 0.09) but not in diabetic (6.13 ± 0.38) or hyperglycemic dogs (6.44 ± 0.28). This study shows that MABP, CBF, ATP, and pHi following global incomplete ischemia did not recover in diabetic dogs. In diabetics poor metabolic recovery was related to the MABP decreases during ischemia and reperfusion. These results contrast with previous studies from our laboratory showing good metabolic recovery with longer-term diabetes (3–4 months). Our findings indicate that in this diabetic model, the cerebral ischemic response is dependent on duration of diabetes.

Diabetic chronic hyperglycemia and cerebral pH recovery following global ischemia in dogs.

Background and Purpose We determined the effect of chronic hyperglycemia associated with diabetes on recovery of cerebral pH after global incomplete cerebral ischemia. Methods 31P magnetic resonance spectra and cerebral blood flow (radiolabeled microspheres) were measured in three groups of dogs: (1) chronic hyperglycemic diabetes (pancreatectomy followed by blood glucose >10 mmol/L for 3 months; n=8); (2) acute hyperglycemia during ischemia and reperfusion in nondiabetic dogs (n=8); and (3) normoglycemic controls (n=8). Incomplete ischemia was produced for 20 minutes by ventricular fluid infusion followed by 3 hours of reperfusion. Results Cerebral blood flow was reduced to approximately 5 mL/min per 100 g in all groups during ischemia with individual values ranging from 1 to 11 mL/min per 100 g. Blood flow returned to preischemic values by 30 minutes of reperfusion in the normoglycemia group but remained elevated during reperfusion in the acute hyperglycemia and diabetes groups. Cerebral pH at the end of ischemia was lower in acute hyperglycemia (5.94±0.05; ±SE) and diabetes (5.97±0.08) groups than in the normoglycemia group (6.27±0.02). However, recovery of pH through 90 minutes of reperfusion in the normoglycemia (7.08±0.05) and diabetes (7.00±0.04) groups was significantly greater than in the acute hyperglycemia group (6.74±0.11). Persistent acidosis in the acute hyperglycemia group was associated with a delayed reduction of cerebral oxygen consumption and high‐energy phosphates and with greater cortical water content and impairment of somatosensory evoked potentials compared with the diabetes group. Conclusions This study shows that cerebral pH recovery after global incomplete ischemia is improved in chronic hyperglycemia compared with acute hyperglycemia, despite similar decreases in blood flow and pH during ischemia and similar levels of blood flow and glucose levels during ischemia and reperfusion. In addition, cerebral pH recovery in chronic hyperglycemic dogs was not different from that in normoglycemic controls. These results suggest that an adaptation occurs with chronic hyperglycemia that improves recovery of cerebral pH during reperfusion and that is associated with better maintenance of energy metabolism and evoked potentials and with less edema over 3 hours of reperfusion compared with acute hyperglycemia. (Stroke. 1994;25:1449‐1455.)

Visual Activation in α‐Chloralose‐anaesthetized Cats Does Not Cause Lactate Accumulation in the Visual Cortex as Detected by [1HINMR Difference Spectroscopy

The hypothesis that neuronal activation results in lactate accumulation due to mismatch between glucose and oxygen consumption was tested in the cat model of visual activation by monitoring cerebral metabolism with localized 1H nuclear magnetic resonance spectroscopy (MRS). Adult cats were anaesthetized with α‐chloralose, paralysed and mechanically ventilated. Visual evoked potentials measured over the occipital cortex showed maximal amplitude at 2 Hz stimulation, but the latencies of the early cortical potentials, N1 and P1, were independent of stimulation frequency. High signal‐to‐noise ratio, short echo time volume‐selected [1H]MRS was used to monitor cerebral lactate with a temporal resolution of 70 s. Difference proton spectroscopy unambiguously showed no lactate peak in the visual cortex during visual activation at stimulation frequencies ranging from 1 to 16 Hz. Absence of change in lactate concentration during visual stimulation was confirmed by averaging all the spectra acquired during activation and subtracting them from reference spectra collected in darkness, a procedure that had a calculated lactate detection limit of 0.17 mM. We also reduced the O2 in the inspired air to 13%, which decreased pO2 from 94.5 ± 8.9 to 47.0 ± 6.8 mmHg, during visual stimulation at 2 or 4 Hz. At this low pO2 level, visual stimulation did not cause lactate accumulation in the visual cortex, however. The present data show that neuronal activation to this degree in the cat brain is not associated with aerobic lactate production to an extent that can be detected with 1H MRS.

Effect of cerebral blood flow generated during cardiopulmonary resuscitation in dogs on maintenance versus recovery of ATP and pH

Background and Purpose Cardiopulmonary resuscitation with external chest compression generates low perfusion pressures that may be inadequate for restoring cerebral metabolism and may worsen intracellular pH. We tested the hypothesis that cerebral reperfusion with a low perfusion pressure after arrest restores brain adenosine triphosphate (ATP) and pH to levels attained at the same perfusion pressure without preceding complete ischemia. Methods Brain ATP and intracellular pH were measured by magnetic resonance spectroscopy, and cerebral blood flow was measured with microspheres in anesthetized dogs. External chest compressions were begun in group A (n=6) immediately after the onset of arrest (ie, arrest time zero) and in group B (n=10) after 6 minutes of arrest (ie, arrest time 6 minutes). In both groups, mean cerebral perfusion pressure was regulated at 30 mm Hg for 70 minutes by adjustment of inflation pressure of a pneumatic thoracic vest. Results At 12 minutes of resuscitation, cerebral blood flow was 27 ±4 mL/min per 100 g in group A and 21 ±4 mL/min per 100 g in group B, but ATP in group B (58 ±10% of prearrest) was less than in group A (105 ±6%). With prolonged resuscitation, ATP deteriorated to near zero levels in dogs in group B, with blood flow less than 15 mL/min per 100 g. Dogs with greater blood flow never achieved complete metabolic recovery. In group B, intracellular pH was unchanged from the 6.3 value at the start of resuscitation, even in those dogs with extremely low blood flows. Conclusions Levels of cerebral perfusion pressure sufficient to maintain cerebral oxidative metabolism without complete ischemia during cardiopulmonary resuscitation are not sufficient to restore metabolism after complete ischemia during cardiopulmonary resuscitation. However, low “trickle” blood flow did not worsen intracellular acidosis.