Electrogram-guided radiofrequency catheter ablation of atrial tissue comparison with thermometry-guide ablation: comparison with thermometry-guide ablation

OBJECTIVES

To characterize a new method for radiofrequency energy titration during ablation of atrial tissue based on reduction in electrogram amplitude. To compare this method with energy titration using electrode thermometry.

BACKGROUND

Complications associated with "anatomy-based" atrial endocardial radiofrequency ablation for suppression of atrial fibrillation may be due to flawed methods of energy titration.

METHODS

The effect of radiofrequency ablation on electrogram amplitude was characterized in a porcine model. A method for energy titration guided by electrogram amplitude reduction ("electrogram-guided") was developed and validated prospectively. Focal (smooth and trabeculated endocardial areas) and linear (smooth endocardial areas) ablation was performed comparing energy titration guided by amplitude reduction with electrode thermometry.

RESULTS

Amplitude reduction during radiofrequency application was not necessarily equal among unipolar and bipolar electrograms in the ablation region; specific patterns of reduction could be discerned, based on factors such as catheter-endocardial orientation. A criterion of >90 % reduction of unipolar and/or bipolar amplitude best predicted pathologic lesion success. Electrogram-guided focal and linear lesions in smooth areas were free of lesion complications such as endocardial charring, barotrauma, or damage to contiguous extraatrial structures. However, there was a significant incidence of insufficient lesion size, principally non-transmurality, probably due to undertitration of energy. Thermometry-guided focal and linear lesions in smooth areas were uniformly transmural but frequently evidenced complications, due to overtitration of energy. Electrogram-guided focal lesions in trabeculated areas could usually not be achieved, probably due to insufficient contact of the ablation electrode with adjacent pectinate muscles. Thermometry-guided focal lesions in trabeculated areas were smaller than electrogram-guided lesions and did not evidence complications.

CONCLUSIONS

Electrogram-guided lesions in smooth endocardial areas were uncomplicated but had a significant incidence of non-transmurality. Thermometry-guided lesions were uniformly transmural but were frequently complicated.

Changes in local wall thickness correlate with pathologic lesion size following radiofrequency catheter ablation: an intracardiac echocardiographic imaging study

INTRODUCTION

High-resolution intracardiac echocardiographic (ICE) imaging can accurately assess wall thickness during radiofrequency (RF) catheter ablation procedures. This study investigated the correlation of changes in wall thickness at the ablation site with pathologic lesion size.

METHODS AND RESULTS

ICE image-guided 31 RF applications (30-50 W, up to 120 sec) were performed in five anesthetized closed chest swine (n = 5, body weight 35-60 kg). Twenty-four lesions were delivered in the right and left atria with standard RF; seven lesions were delivered in the left ventricle (LV) with irrigated (30-40 ml/min) RF. Wall thickness and tissue echo density measured by ICE imaging (pre- and 1-minute post-RF delivery) with increased focal echo density following RF deployment in the atria (4.5 +/- 1.5 vs 2.3 +/- 1.0 mm pre-RF) and the LV (9.8 +/- 2.3 vs 6.8 +/- 2.2 mm pre-RF; P < 0.01). The observed changes in wall thickness (DeltaWT) following ablation in the LV were greater than in the atria (3.0 +/- 1.4 vs 2.2 +/- 1.2 mm; P < 0.05). A significant correlation between DeltaWT and lesion depth (ventricular: r = 0.85, P < 0.05; atrial: r = 0.82, P < 0.01) was demonstrated at all ablation sites. Local wall thickness measured post-RF also significantly correlated with lesion depth (r = 0.89, P < 0.01), especially with that of transmural lesions (r = 0.95, n = 23, P < 0.001) at atrial and LV sites.

CONCLUSION

Therapeutic RF ablation results in mural swelling and increased echo density. These changes can be detected by ICE imaging and correlate with pathologic lesion size. ICE imaging may be useful in online quantification of lesion size, especially for transmural lesions during clinical catheter ablation procedures.

Echocardiographic transponder-guided catheter ablation feasibility and accuracy

BACKGROUND

The utility of echocardiography for catheter guidance during percutaneous endocardial ablation is increasingly apparent. However, the technique is currently imperfect due to limitations in discerning the ablation electrode from other parts of the catheter shaft.

PURPOSE

To examine the feasibility and accuracy of echocardiography-guided ablation using commercial ablation catheters fitted with a transponder to improve localization of the ablation electrode.

METHODS

Fifteen healthy pigs and five pigs with chronic anterior myocardial infarction were studied. In healthy animals, echocardiographically distinct endocardial sites in right and left cardiac chambers were targeted for ablation. In infarcted animals, the left ventricular infarction border zone was targeted. Both intracardiac (ICE; 12.5 megahertz and 5 megahertz) and transesophageal echocardiographic (TEE) techniques were utilized.

RESULTS

In healthy animals, transponder-guided ablation was feasible with each of the echocardiographic techniques. Accuracy was 82 % (45 of 55 lesions) with ICE-12.5 MHz, 87 % (27 of 31 lesions) with ICE-5 MHz, and 81 % (22 of 27 lesions) with TEE. In infarcted animals, the accuracy was 38 % (3 of 8 lesions) for ICE-5 MHz and 38 % (3 of 8 lesions) for TEE. Errant lesions in healthy animals were observed in earlier experiments, due to operator misinterpretation of the plane of imaging. Errant lesions in infarcted animals were observed throughout the experimental series, and may have been due to a variable relationship between echocardiographic and histologic infarction border zones.

CONCLUSIONS

Echocardiographic transponder-guided catheter ablation is feasible. Accuracy for normal endocardial targets was excellent, less so for chronic infarction border.

Intracardiac echocardiographic evaluation of ventricular mural swelling from radiofrequency ablation in chronic myocardial infarction: irrigated-tip versus standard catheter

INTRODUCTION

The production of larger, particularly deeper lesions may improve the success rate for radiofrequency (RF) ablation of post infarction ventricular tachycardia (VT). Therapeutic RF ablation causes left ventricular (LV) mural swelling. This swelling can be detected as increased wall thickness at the ablation site by intracardiac echocardiography (ICE) and correlates with pathologic lesion size. This study compared the extent of mural swelling caused by linear ablation lesions created with irrigated tip and standard RF ablation in a porcine model of healed anterior infarction.

METHODS AND RESULTS

In anesthetized closed-chest swine ICE guided multiple RF applications to construct linear lesions at the border zone of the infarct region using an irrigated RF (n=6 swine) and a standard RF (n=6 swine) ablation catheter. 47 individual lesions were created with irrigated RF ablation; 57 lesions created with standard RF ablation. At all sites, wall thickness (measured at end-diastole Pre- and 1 min Post-RF delivery) increased following either irrigated (p<0.0001) or standard (p<0.004) RF deployment. Irrigated RF ablation produced more mural swelling at border zone sites than standard RF ablation (wall thickness increase of 21.2 versus 15.1 %, p<0.003). This difference was more pronounced at RF sites within the infarct (40.7 versus 12.0 %, p<0.0007). Thrombus formation or intramural explosion were not observed; surface crater formation was not more frequent with irrigated compared to standard RF ablation (14/47 versus 12/57 lesions, p=NS).

CONCLUSION

Irrigated RF ablation may produce larger lesions than standard RF ablation, particularly for ablation targets within infarcted tissue. ICE imaging provides on line data about the characteristics of the developing lesion which may prove useful in dosing irrigated-tip RF energy application.

Electrical impedance properties of normal and chronically infarcted left ventricular myocardium

BACKGROUND

Previous reports have disclosed that a significant difference exists between the electrical impedance properties of healthy and chronically infarcted ventricular myocardium.

PURPOSE

To assess the potential utility of electrical impedance as the basis for mapping in chronically infarcted left ventricular myocardium. Specifically: (1) to delineate electrical impedance properties of healthy and chronically infarcted ventricular myocardium, with special emphasis on the infarction border zone; (2) to correlate impedance properties with tissue histology; (3) to correlate impedance properties with electrogram amplitude and duration; (4) To demonstrate that endocardial impedance can be measured effectively in vivo using an electrode mounted on a catheter inserted percutaneously.

METHODS

An ovine model of chronic left ventricular infarction was utilized. Sites of healthy myocardium, densely infarcted myocardium and the infarction border zone were investigated. Bulk impedance was measured in vitro using capacitor cell, four-electrode and unipolar techniques. Epicardial and endocardial impedances were measured in vivo using four-electrode and unipolar techniques. Impedance was measured at multiple frequencies. Electrographic amplitude, duration and amplitude/duration ratio were measured using bipolar electrograms during sinus rhythm. Quantitation of tissue content of myocytes, collagen, elastin and neurovascular elements was performed.

RESULTS

Densely infarcted myocardial impedance was significantly lower than healthy myocardium. Impedance gradually decreased in the border zone transitioning between healthy myocardium and dense infarction. Decreasing impedance correlated with a decrease in tissue myocyte content. The magnitude of the difference in impedance between densely infarcted and healthy myocardium increased as the measurement frequency decreased. Healthy myocardium exhibited a marked frequency dependence in its impedance properties; this phenomenon was not observed in densely infarcted myocardium. There was a direct association between impedance and both electrogram amplitude and amplitude/duration ratio. There was an inverse association between impedance and electrogram duration. Endocardial impedance, measured in vivo using a electrode catheter inserted percutaneously, was demonstrated to distinguish between healthy and infarcted myocardium.

CONCLUSIONS

The electrical impedance properties of healthy and infarcted left ventricular myocardium differ markedly. The properties of the infarction border zone are intermediate between healthy and infarcted myocardium. Impedance may be a useful assay of cardiac tissue content and adaptable for cardiac mapping in vivo. Condensed Abstract. To delineate the electrical impedance properties of healthy and chronically infarcted left ventricular myocardium emphasizing the infarction border zone, impedance was measured in chronically infarcted ovine hearts. Densely infarcted myocardial impedance was significantly lower than healthy myocardium. Impedance gradually decreased in the infarction border zone in transition between healthy myocardium and dense infarction. This correlated with a decreasing myocyte content. The magnitude of the difference in impedance between densely infarcted and healthy myocardium increased as measurement frequency decreased. There was a direct association between impedance and electrogram characteristics. Endocardial impedance, measured in vivo using an electrode catheter inserted percutaneously, distinguished between healthy and infarcted myocardium

Selective stimulation of latissimus dorsi muscle for cardiac assist

The contractile power of the latissimus dorsi muscle (LDM) is used in skeletal muscle cardiac assist (SMCA) to augment the blood pumping ability of a failing heart. The LDM has three anatomically distinct, independently innervate segments-the transverse, oblique, and lateral. There are potential advantages to selectively stimulating these LDM regions. We hypothesized that (1) the three nerve branches could be stimulated selectively to activate individual muscle regions with little or no functional overlap, (2) the three muscle regions would generate similar force, and (3) nerves stimulated in combinations would generate forces corresponding to the sum of forces generated by the individual regions. In acute studies of canine LDM (n=5), regional electromyogram (EMG) and isometric force were recorded while branches of the thoracodorsal nerve were stimulated (via nerve-cuff electrodes) individually and in combinations. Analysis of regional EMG and force confirmed selective activation. Stimulation of lateral, oblique, and transverse branches of thoracodorsal nerve activated 53+/-5%, 20+/-9%, and 36+/-9% of the muscle, respectively; with corresponding developed forces of 48+/-6%, 21+/-8%, and 31+/-8% of total muscle force (R=0.98, p<0.05). Selective activation of LDM is possible with little or no functional overlap; however, the muscle regions were nonuniform. Selective stimulation may ultimately facilitate the use of performance enhancing stimulus protocols for SMCA.

Recording of pacing stimulus artifacts by endovascular defibrillation lead systems: comparison of true and integrated bipolar circuits

BACKGROUND

The occurrence ICD undersensing of ventricular fibrillation due to the presence of a pacing stimulus artifact (PSA) is in part related to the amplitude of the artifact recorded on the ICD rate sensing circuit. There is little comparative data regarding PSA amplitude recorded by commercial ICD rate-sensing circuits.

PURPOSE

To compare PSA amplitude recorded by commercial endovascular defibrillation leads utilizing integrated or true bipolar sensing circuits.

METHODS

Nineteen large (60-120 kg) pigs were utilized. Two different commercial endovascular defibrillation leads were evaluated, each with its distal tip located at the right ventricular apex: (1) Medtronic Transvene; and (2) CPI Endotak. Three different rate-sensing circuits were evaluated: (1) Transvene true bipolar (tip-ring); (2) Transvene integrated bipolar (tip-coil); and (3) Endotak integrated bipolar (tip-coil). Using a separate pacing lead located at the left ventricular apex (n = 19 animals) or right ventricular outflow tract (n = 10 animals), pacing was performed at a pulse width of 0.5 milliseconds at outputs of 1.5, 5 and 10 volts. PSA amplitude was recorded at each output by each circuit.

RESULTS

During pacing from the left ventricular apex, at each pacing output voltage the PSA amplitude recorded by the true bipolar circuit (0.6 +/- 0.1 mV at 1.5 volts, 2.0 +/- 0.5 mV at 5 volts, 3.7 +/- 0.8 mV at 10 volts) was significantly smaller than recorded by the Transvene integrated circuit (1.4 +/- 0.3 mV at 1.5 volts, 3.8 +/- 0.7 mV at 5 volts, 4.1 +/- 0.8 mV at 10 volts) or the Endotak integrated circuit (1.8 +/- 0.4 mV at 1.5 volts, 4.2 +/- 1.0 mV at 5 volts, 6.3 +/- 1.8 mV at 10 volts). During pacing from the right ventricular outflow tract, at each pacing output voltage the PSA amplitude recorded by the true bipolar circuit (0.7 +/- 0.1 mV at 1.5 volts, 1.7 +/- 0.4 mV at 5 volts, 4.0 +/- 0.7 mV at 10 volts) was significantly smaller than recorded by the Transvene integrated circuit (1.1 +/- 0.4 mV at 1.5 volts, 3.9 +/- 1.2 mV at 5 volts, 7.5 +/- 1.8 mV at 10 volts) or the Endotak integrated circuit (1.6 +/- 0.7 mV at 1.5 volts, 4.3 +/- 1.7 mV at 5 volts, 7.5 +/- 2.6 mV at 10 volts). For both pacing sites, the PSA amplitude recorded by the two integrated circuits was not significantly different.

CONCLUSIONS

For a given pacing output voltage, PSA amplitude recorded by commercial endovascular rate sensing/defibrillation leads is greater when the sensing circuit is integrated than when it is true bipolar. These data may be helpful in planning ICD implantation in patients with previously implanted permanent pacemakers.

Cardiomyoplasty

Cardiomyoplasty is a new surgical treatment for heart failure in which skeletal muscle assists the heart. However, for the first 2 weeks postoperatively, the latissimus dorsi muscle (LDM) remains unstimulated, and during the next 2 weeks, the LDM is stimulated with only one pulse every other heart beat. Thus, for the initial 4 postoperative weeks, minimal systolic assistance is provided. The present study determined if the LDM is capable of providing early assistance. Cardiomyoplasty surgery involves severing the perforating intercostal arteries to the LDM, detaching the LDM from its distal insertion, and wrapping it around the heart. At each of these steps, we measured LDM force development, shortening, and blood flow in six dogs. At control, LDM shortening, work, and power decreased during a 2 min fatigue test: fatigue indices (final/ initial value) for shortening, work, and power were 47.6 +/- 6.9%, 47.5 +/- 7.1%, and 46.9 +/- 6.6%, respectively. Blood flow increased in the proximal (P), mid (M), and distal (D) LDM during the fatigue test. After partial vascular isolation, initial shortening, work, and power decreased by 29.4%, 32.5%, and 31.7% from their respective control values. During the fatigue test, fatigue indices for shortening, work, and power were 24.7 +/- 3.3%, 19.5 +/- 4.6%, and 22.2 +/- 4.7%, respectively, all significantly (p < 0.05) less than control values. Resting blood flows were unaltered. During exercise, flow to the P increased, whereas flow did not increase in M (p < 0.05). Loss of LDM function was most apparent after mobilizing and reattaching the muscle. Initial shortening, work, and power significantly decreased (p < 0.05) by 74.1%, 76.8%, and 74.4%, from their respective control values. During a fatigue test, final values for shortening, work, and power were all near zero. Resting blood flow decreased in the M and D (p < 0.05) and, during exercise, blood flow increased only in P. Thus, LDM function was severely depressed during the isolation procedure. This functional loss is associated with inadequate blood flow responses. Therefore, preconditioning and/or revascularization is needed if the LDM is to provide cardiac assistance shortly after cardiomyoplasty surgery.

Lower frequency (5 MHZ) intracardiac echocardiography in a large swine model: imaging views and research applications

Our previous investigation indicated that, in the 50-114-kg weight range, the swine model provides transeosophageal echocardiographic normal values for cardiac structures comparable to those found in human adults. Intracardiac echocardiographic imaging using a 12.5-MHz ultrasound catheter is limited, due to ultrasonic attenuation. Transesophageal echocardiographic imaging of the right heart is also limited with its anterior anatomic location. To further study the utility of intracardiac imaging, we placed a 5-MHz (30 Fr) multiplane transducer at the junction of the superior vena cava and right atrium, in the right atrium and right ventricle in 8 closed-chest swine (weight 129 +/- 61 kg). In each animal, complete whole heart imaging was obtained, with tomographic views including the cardiac 4-chamber, right atrium, right ventricle and outflow, left atrium and ventricle, and basal great vessels. Major intracardiac anatomic landmarks (i.e., crista terminalis, right atrial appendage, coronary sinus orifice, interatrial septum, tricuspid valve, right ventricular outflow, pulmonary veins, mitral valve and left ventricular papillary muscles) were visualized in every swine. Thus, this 5-MHz multiplane transducer, as a prototype for a steerable low-frequency intracardiac ultrasound catheter, improved both whole heart and individual cardiac structure imaging from a single intracardiac location. Further technological development and refinement is needed for routine use in research and clinical imaging practice.

Seasonal and latitudinal occurrence of cerebral vasospasm and subarachnoid hemorrhage in the northern hemisphere

Using patient data obtained from the International Cooperative Aneurysm Study that evaluated clinical records from 68 neurosurgical centers in 14 countries, we evaluated the data as to the monthly occurrence of cerebral vasospasm and subarachnoid hemorrhage relative to the latitude of the medical center where the data were gathered. Using the Edwards analysis for cyclic patterns, we examined peak to trough ratios and months of peak to assess the strength and nature of the cyclic variations between December 1980 and July 1983. Of the 3,521 subarachnoid hemorrhage patient records examined, 685 developed some grade of vasospasm as defined symptomatically, angiographically, and by the Fisher Grading Scale. Cyclic analysis demonstrated a strong seasonal occurrence for the incidence of subarachnoid hemorrhage, with a peak in February, in the northern hemisphere. Cerebral vasospasm incidence, after controlling for subarachnoid hemorrhage occurrence, exhibited only a small peak to trough ratio (1.15) relative to the larger ratio (1.74) seen with unadjusted vasospasm data. Age was related to both incidence and location. In populations at high risk for cerebrovascular diseases, climatic conditions may act as synchronizers of pathologic vascular events.

Effects of intracisternally injected serotonin on cerebral blood flow in cats during winter and summer, and after dark exposure

This investigation reports on two experiments: (a) The examination of the effects of intracisternally injected serotonin on cerebral blood flow and systemic pressure in cats during winter (1/10-3/7) and summer (7/6-9/3) at a latitude of 39' 40", and (b) the effects of similarly injected serotonin on the same parameters in 11 cats after exposure to 3 days of continuous darkness during the summer months. Serotonin (5 mg) injected intracisternally produced significant decreases in cerebral blood flow and systemic pressure that lasted from 60 to 180 min after injection. Blood flow decreases, expressed as percentage change from baseline, seen during winter (n = 24) periods were significantly greater [analysis of variance (ANOVA), p < 0.02] from decreases seen during summer (n = 25) periods. Following serotonin injection, systemic pressure decreases between the winter and summer cats were not significantly different. Eleven summer cats were subjected to 3 days of continuous darkness before being injected with serotonin. Blood flow decreases in the dark-exposed cats were significantly (p < 0.05) greater than those seen in normal light-exposed cats, whereas systemic pressure changes were not different. These studies suggest that the seasonal photoperiod may affect the sensitivity of cerebral vessels to intracisternally injected serotonin in cats.

Effects of cardiomyoplasty on biventricular function in canine chronic heart failure

Dynamic cardiomyoplasty, the use of skeletal muscle to assist the heart, is a new therapy for the treatment of heart failure. However, the effects of cardiomyoplasty on biventricular function and the synchrony of ventricular contraction are not fully known. We assessed the efficacy of latissimus dorsi muscle (LDM) dynamic cardiomyoplasty in a chronic model of biventricular failure. Five dogs received doxorubicin (1 mg.kg-1.wk-1) for up to 12 weeks to induce heart failure and then underwent a biventricular cardiomyoplasty. After operation, the muscle was progressively stimulated according to an established protocol. When training was complete (10 weeks), radionuclide ventriculographic and catheterization data were obtained. Peak left ventricular (LV) systolic pressure and its first derivative were unchanged, whereas LV end-diastolic pressure decreased slightly with LDM assistance (11.0 +/- 1.6 to 9.6 +/- 1.5 mm Hg; p < 0.05). Right ventricular (RV) systolic pressure increased significantly with LDM assistance from 21 +/- 2 to 26 +/- 3 mm Hg (p < 0.05), whereas its first derivative and RV end-diastolic pressure were unchanged. Dynamic cardiomyoplasty significantly improved LV ejection fraction from 0.18 +/- 0.07 without LDM assistance to 0.31 +/- 0.05 with LDM assistance (p < 0.05); similarly RV ejection fraction increased from 0.32 +/- 0.07 to 0.45 +/- 0.06 with LDM assistance (p < 0.05). The temporal sequence of LV wall motion was assessed by phase analysis of the radionuclide ventriculograms. With skeletal muscle assistance, standard deviation ("spread") decreased from 31.6 +/- 17.4 to 20.0 +/- 15.4 degrees (p < 0.06), whereas skewness ("symmetry") was unchanged. Dynamic cardiomyoplasty improved both LV and RV ejection fractions without increasing diastolic pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of intraluminally versus extraluminally administered nimodipine on serotonin-induced cerebral vascular responses in vitro and in situ

The purpose of our study was to compare the ability of intraluminally and extraluminally administered nimodipine to inhibit serotonin-induced cerebral vascular responses in vitro and in situ. No difference was noted in the ability of nimodipine, whether administered intraluminally or extraluminally, to reduce the contractile response of extraluminally administered serotonin in a closed, pressurized, in vitro bovine middle cerebral artery preparation; histologic studies indicated that the tight endothelial junctions normally found in cerebral arteries remained intact in this preparation. In cats, pretreatment with nimodipine did not significantly reduce the ability of intracisternally injected serotonin to decrease cerebral blood flow; however, nimodipine did reduce the changes in cerebral artery diameter normally noted angiographically after serotonin injection. Although minor differences were noted between the intraluminal and extraluminal routes of administration of nimodipine in situ, in general the effects were comparable.

Extended latency of the cortical component of the somatosensory-evoked potential accompanying moderate increases in cerebral blood flow during systemic hypoxia in cats

In 24 adult cats, the somatosensory-evoked potential (SEP) and cerebral blood flow (CBF) were measured under paralyzed, anesthetized conditions during exposure to two different ventilatory regimens. Group I cats (ventilated from 20 to 2% oxygen) responded with a significant increase in white matter blood flow from 25.0 +/- 7.8 to 43.8 +/- 10.5 ml/100 g/min recorded at 7% O2. Gray matter blood flows in these animals increased but not to significant levels above the control blood flow measured at 20%. No significant changes in blood flow were observed in group II animals ventilated over the range of 25-3% oxygen as gray matter rose slightly (but not significantly) with hypoxia and white matter flows remained at levels of 25-30 ml/100 g/min. The latency of the cortical component of the SEP was related to the degree of hypoxia. For both groups, significant extensions in the latency to the occurrence of the cortical component of the SEP (normalized to the % of control SEP) occurred in each case (P less than 0.05). An inverse, linear relationship existed between the latency to the appearance of cortical component (ms) and the percentage oxygen concentration of the ventilatory mixture. No significant changes in thalamocortical conduction times were found, which indicates that hypoxia may have generalized effects on the synaptic pathways supporting the conduction of the SEP. The variation in blood flow and the latency of the cortical component observed between groups I and II may reflect the oxygen concentration used at the beginning of the experiment (25 vs 20%) and the gradations between them vs 3 and 2%.(ABSTRACT TRUNCATED AT 250 WORDS)

Latency of the cortical component of the somatosensory evoked potential in relation to cerebral blood flow measured in the white matter of the cat brain during focal ischemia

In a focal model of cerebral ischemia (middle cerebral artery (MCA) occlusion in the cat), the existence of a relationship between the latency from the onset of stimulation to the appearance of the cortical component of the somatosensory evoked response and blood flow was explored. White matter blood flow was determined in the left and right hemispheres under control conditions. During left MCA occlusion, the blood flow and the latency of the cortical component recorded from the right hemisphere deviated from their preexisting linear relationship obtained from plotting latency as a function of blood flow. In an effort to increase flow during focal ischemia, hemodilution and volume expansion with dextran 40 was used. Application of this technique only mildly increased blood flows and did not restore the correlation of latencies of the cortical component of the evoked response and blood flows observed before MCA occlusion. This finding suggests that latencies and blood flows are not correlated in the contralateral hemisphere during focal ischemia and only show a mild degree of relationship in the unperturbed brain. During ischemia, flow and electrical activities may not respond to the same stimuli and, if they do respond, it may not be unidirectionally. These findings are discussed in light of other reports on the Significance of the evoked response in monitoring cerebrovascular insults and the observation of diaschisis.

Cerebral blood flows and tissue oxygen levels associated with maintenance of the somatosensory evoked potential and cortical neuronal activity in focal ischemia

The middle cerebral artery was occluded in 18 cats to evaluate the physiological consequences of cerebral blood flow reductions on the somatosensory evoked potential, spontaneous neuronal activity, and oxygen availability in the ipsilateral and contralateral hemispheres. In the ipsilateral ectosylvian gyrus high-grade ischemia was produced as blood flow in the gray matter was reduced from 52.1 +/- 8.6 (mean +/- SE) to 13.3 +/- 9.0 ml/100 g/min and in the white matter from 33.8 +/- 5.6 to 6.1 +/- 6.4 ml/100 g/min. This significant reduction (p less than 0.05) was associated with abolition of the cortical component of the somatosensory evoked potentials. In all animals occlusion resulted in a predictable extended latency change and a variable amplitude response of the cortical component of the contralaterally recorded somatosensory evoked potentials. In 5 animals, oxygen availability was measured and spontaneous neuronal activity in the contralateral hemisphere was recorded. Volume expansion and hemodilution with either dextran or saline infusions elevated cerebral blood flow in the contralateral gray matter significantly (p less than 0.05) compared with the control and clip values. Ipsilateral spontaneous activity stopped within 4-12 minutes of occlusion, while contralateral spike activity persisted at rates at least equal to those recorded immediately following occlusion.

Comparison of the somatosensory evoked potential and the direct cortical response following severe incomplete global ischemia: selective vulnerability of the white matter conduction pathways

Eight cats were subjected to graded hemorrhagic hypotension following bilateral carotid ligation to produce incomplete global cerebral ischemia. Three additional cats served as controls. The somatosensory evoked potential (SEP) and direct cortical response (DCR) were monitored in all animals and in each case, the cortical component of the SEP was abolished during progressive ischemia while the morphology of the DCR was well-preserved but with reduced amplitude. Determinations of adenosine triphosphate (ATP), phosphocreatine (PCr), and lactate levels in cerebral cortex and white matter were made in five experimental cats and the three controls. At the time of failure of the cortical SEP, PCr was dramatically reduced and lactate moderately elevated in the white matter while ATP remained unchanged. Cortical lactate was only mildly elevated and PCr and ATP were unchanged accounting for preservation of the DCR. In this model of global ischemia, abolition of the cortical SEP is due to a block of stimulus conduction in white matter projection pathways. A hypothesis to explain the observed metabolic changes is presented and correlation is made to clinical situations.

Failure of the somatosensory evoked potential following middle cerebral artery occlusion and high-grade ischemia in the cat--effects of hemodilution

Acute focal ischemia was created in 10 cats by unilateral retro-orbital middle cerebral artery (MCA) occlusion. Regional cerebral blood flow (CBF) was determined utilizing the hydrogen clearance technique from electrode recordings within the gray matter and white matter of the ectosylvian gyrus of both hemispheres. The somatosensory evoked potential (SSEP) was obtained during contralateral median nerve stimulation. When the MCA was clipped the white and gray matter blood flows in the ipsilateral ectosylvian gyrus were reduced to 14.8 +/- 19.6% and 19.3 +/- 23.7% of control, and the cortical component of the SSEP was abolished. In the contralateral hemisphere an average increase of 3.5% above the control latency and a 10% mean depression in the amplitude of the cortical component of the SSEP were observed following occlusion. CBF in the contralateral hemisphere was unaffected by the MCA clip. Infusion of saline or dextran to lower the hematocrit by approximately 45% did not significantly improve blood flow or restore the SSEP in the hemisphere ipsilateral to the MCA clip. However, significant increases in the contralateral hemisphere gray matter CBF occurred following hemodilution while the latency of the cortical component of the SSEP in this same hemisphere was significantly extended. Elevations in gray and white matter blood flows were achieved in the experimental hemisphere of 3 of 10 cats suggesting a wide range of variation in the collateral circulation.

Effects of chronic beta-adrenergic blockade on hemodynamic and metabolic responses to endurance training

To study the influence of chronic propranolol therapy on hemodynamic and metabolic adaptations associated with endurance training, five dogs given 250 mg X d-1 propranolol therapy (P) and five control dogs (NP) were endurance trained for 7 wk using a continuously increasing treadmill workload. The P group was also evaluated pre- and post-training off the drug (P-OD) to separate drug and training effects. Training lowered mean heart rate score (HR) to a standardized multistage dog treadmill test in the NP and P-OD (P less than 0.05) and had no effect on the HR of the P group while on propranolol. At a fixed submaximal workload there were also slight reductions in cardiac output in the P group which were more pronounced (P less than 0.10) following training, and a corresponding increase in a-vDO2. Pre- and post-training metabolic studies were performed at rest, during a fixed submaximal workload, and following 30 min of recovery. During exercise, blood glucose levels were significantly lower in the P group both before and after training. While NP showed no significant change in high-density lipoprotein bound cholesterol after training, the P-OD group demonstrated a significant fall in high-density lipoprotein bound cholesterol after training (P less than 0.05). These data indicate that endurance exercise training done in the presence of chronic beta-adrenergic blockade produces training-induced hemodynamic adaptations to exercise, but beta-blockade inhibits the changes in serum lipids and lipoprotein fractions normally seen in response to an exercise conditioning program.

Alteration of somatosensory evoked potentials in response to global ischemia

The somatosensory evoked potential (SEP) measured in response to median nerve stimulation was correlated with cortical and white matter cerebral blood flow (CBF), adenosine triphosphate (ATP), and lactate levels in 14 cats subjected to graded hemorrhagic hypotension following bilateral carotid artery ligation. Three additional cats served as controls. Regional CBF was determined by the hydrogen clearance method, and the time for conduction of the sensory stimulus from the thalamus to the cortex (the thalamocortical conduction time), was used to assess SEP latency changes. A reproducible sequence of changes occurred in the SEP as ischemia developed. There was an early conduction delay that correlated well with mild white matter ischemia. Amplitude reductions in the SEP began as significant cortical ischemia occurred. The cortical SEP was abolished when white matter CBF and ATP fell to critical levels.