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Biomedical subjects

A McCullen

Publications and source records attributed to A McCullen.

15 recordsLinked to original sources
PubMed1994

Reappraisal of effective refractory period testing.

To test the hypothesis that effective refractory period (ERP) testing is an accurate and reproducible measure of the cardiac refractory period, 4 pigs and 12 dogs were studied. We define the conventional ERP as the longest S1-S2 interval associated with noncapture and the triplicate ERP as the average of three ERP values determined by the up-down algorithm. We also determined the probability curve for noncapture after testing multiple S1-S2 intervals. The results showed that the difference between the coupling intervals associated with a 0 and a 100% probability of noncapture was 13 +/- 1 (range 12-16) ms for pigs and 9 +/- 2 (range 4-12) ms for dogs. The conventional and the triplicate ERP were associated with a 71 +/- 20% (range 26-100%) and a 60 +/- 21% (range 12-100%) probability of noncapture, respectively. We conclude that the ERP is a probability function. Therefore a single ERP determination cannot be used as an accurate and reproducible measure of cardiac refractory period unless it is used to detect potential differences in ERP that exceed 16 ms in pigs or 12 ms in dogs. A probability function curve determination will be needed if smaller differences in the ERP are to be detected.

Algorithms↗
PubMed1991

Biomechanical analysis of osseointegration of IMZ implants in goat mandibles and maxillae.

This study was designed to determine the effect of time on the biomechanical integration of cylindrical dental implants in the mandible and maxilla. A total of 136 IMZ dental implants was placed bilaterally in the endentulous maxillae and mandibles of 17 goats. Pullout tests were used to assess biomechanical integration at intervals from 2 to 24 weeks. This study indicated that there is a time-dependent, progressive increase in pullout force to 24 weeks. The forces were consistently higher for the mandible than the maxilla. Moreover, there was no correlation between the intraoperative stability of the implant and the postoperative pullout force.

Animals↗
PubMed1985

Spontaneous ventricular tachycardia associated with isolated right ventricular infarction, one day after right coronary artery occlusion in the dog: studies on the site of origin and mechanism.

The electrophysiologic and arrhythmic properties of isolated infarcted right ventricle (RV) were studied in 17 dogs during the first 24 hours after complete occlusion of the right coronary artery (RCA). During the 16-to-20-hour post occlusion period, spontaneously occurring sustained monomorphic ventricular tachycardia (VT) was present in all 17 dogs. Overdrive ventricular pacing (cycle lengths 200 to 250 msec) caused significant suppression of the VT when the rate of the VT was slower than 150 bpm (range 120 to 145 bpm) (n = 9), but had negligible effect when VT rate was higher than 150 bpm (range 160 to 245 bpm (n = 8). Overdrive pacing could not terminate either the slow or the fast type of VT. Bipolar intramural electrograms have showed electrical activity in the infarcted RV zone to precede Q wave of the VT by 15.4 +/- 5.8 msec regardless of VT rate. Microelectrode studies on isolated RV endocardial infarcted tissues 24 hours after RCA occlusions have shown the presence of spontaneous repetitive activity at a rate of 87 +/- 47 bpm, which was overdrive suppressed in dogs with slow VT, and spontaneous activity at a rate of 115.2 +/- 36 bpm (p less than 0.05) which was not overdrive suppressed in dogs with fast VT. Maximum diastolic potential, action potential amplitude, and Vmax of surviving subendocardial Purkinje fibers (SEPF) in the infarct zone were slightly but significantly depressed (p less than 0.05), and they manifested enhanced phase 4 depolarization, giving rise to automatic impulse initiation. Although action potential duration of these fibers was somewhat prolonged (p less than 0.05), no conduction delay occurred. Histopathologic examinations have shown necrosis of the basal two thirds of the RV, with no left ventricular involvement. Electron microscopy revealed lipid accumulation in the surviving SEPF as the sole abnormality. We conclude (1) that occlusion of the RCA in the dog is associated with high survival rate despite extensive necrosis involving exclusively the RV and (2) that VT seen during the 20 to 24 hours after occlusion arise in the infarcted zone of the RV, by an enhanced automatic mechanism in the surviving SEPF, possibly caused by cytoplasmic lipid accumulation. This model, by virtue of its high survival rate and frequency of late VTs, should be useful in providing clues to determine factors involved in the genesis of early VT/VF and for the evaluation of new pharmacologic agents during the 20- to 24-hour VT period.

Animals↗
PubMed1984

Electrophysiologic and hemodynamic effects of propafenone, a new antiarrhythmic agent, on the anesthetized, closed-chest dog: comparative study with lidocaine.

The relative hemodynamic and electrophysiologic effects of a new antiarrhythmic drug, propafenone, and lidocaine were evaluated in eight closed-chest, anesthetized dogs. Propafenone (4 mg/kg intravenously) significantly (p less than 0.05) lowered aortic and pulmonary systolic pressures and caused a rise in heart rate (p less than 0.05). Cardiac output decreased from 4.5 +/- 1 to 3.8 +/- 0.7 L/min (p less than 0.05) during atrial pacing at 400 msec cycle length. Propafenone had no effect on pulmonary and aortic diastolic pressures. Lidocaine (5 mg/kg intravenously) caused a significant (p less than 0.05) decrease in aortic systolic pressure and a rise in heart rate. Lidocaine had no significant effect on the other measured hemodynamic parameters. Propafenone, unlike lidocaine, significantly (p less than 0.05) increased atrioventricular nodal functional refractory period and right ventricular endocardial (apex) cathodal (0.5 +/- 0.1 mA to 1.9 +/- 0.3 mA) and bipolar (1.4 +/- 0.3 to 2.2 +/- 0.4 mA) diastolic excitability threshold. Propafenone, unlike lidocaine, also caused a significant (p = 0.05) intraatrial conduction delay; however, neither drug caused conduction slowing in the His-Purkinje system. Both drugs had no effect on sinus nodal recovery time and on the effective refractory period of the right ventricular endocardium (apex). Mean plasma propafenone levels during hemodynamic and electrophysiologic measurement ranged between 3.2 +/- 1.8 micrograms/ml and 1.7 +/- 1.1 micrograms/ml. All of the propafenone-induced effects were reversible within 90 minutes. We conclude that propafenone differs from lidocaine in its atrial, AV nodal, and ventricular electrophysiologic properties, and thus these may explain propafenone's greater efficacy over lidocaine against both certain atrial and ventricular arrhythmias.(ABSTRACT TRUNCATED AT 250 WORDS)

Anesthesia↗
PubMed1983

Electrophysiologic effects of bretylium on canine ventricular muscle during acute ischemia and reperfusion.

Electrophysiologic effects of bretylium were assessed on a recently developed animal model for analysis of conduction of premature impulses and excitation threshold. Bretylium was administered intravenously 10 mg/kg over 10 minutes followed by 2 mg/min of infusion immediately after coronary ligation. Conduction of the premature impulse was recorded in the epicardial and endocardial sites both in the base-to-apex and apex-to-base directions, in the normal, in the center, and across the border of ischemic myocardium. Compared to the control group of animals, bretylium did not cause any significant change in the conduction characteristics in the ischemic myocardium; however, it delayed the conduction of premature impulses in the normal myocardium. Thus the disparity in conduction times between the normal and the ischemic myocardium was lessened by bretylium. Further, conduction of impulses from normal tissue across the border of ischemia was also delayed. Bretylium also decreased the excitability threshold in the ischemic myocardium, although the normal myocardial excitation threshold was unaffected. These unique effects of bretylium on conduction and excitability in the normal, in the center, and across the border of ischemic myocardium, when a therapeutic dosage of the drug is used, further validate its antiarrhythmic potential and offer an insight into its mechanism of action in the setting of acute myocardial ischemia.

Animals↗
PubMed1977

Electrophysiologic effects of anterior septal arterial occlusion.

The effect of reduction in anterior septal arterial flow on the conduction system was studied in seven anesthetized dogs. After 2 hours of occlusion P-Q, A-H, and H-V intervals as well as atrioventricular nodal effective and functional refractory periods were significantly prolonged, sinoatrial conduction time was prolonged and the heart rate was decreased. The duration of the His bundle electrogram was significantly prolonged and the configuration altered. However, QRS duration did not prolong significantly. Fifteen minutes after reperfusion, A-H interval, duration of the His bundle electrogram, effective refractory period and functional refractory period returned toward control values. However, the H-V and QRS intervals as well as sinoatrial conduction time were unchanged after reperfusion. Thus, reduction of anterior septal arterial flow influences not only the distal but also the proximal portion of the conduction system; the most vulnerable part is probably the His bundle. The distal portion of the conduction system is directly influenced by ischemia itself, whereas the proximal portion is influenced through other mechanisms induced by reduction of anterior septal arterial flow.

Animals↗
PubMed1975

Cardiorenal effects of lidocaine and procaine amide in the conscious dog.

Simultaneous measurements of hemodynamics, arterioventricular (AV) conduction, and renal functioner were obtained in conscious dogs. Catheters were implanted for the long-term measurement of central aortic, right ventricular, and pulmonary artery pressure. AV conduction was assessed following surgical implantation of multipolar electrode plaques in the area of the bundle of His, as well as on the epicardium of the right and left atria and ventricles. Renal function was assessed utilizing standard techniques. Following control measurements, lidocaine, 1 mg/kg, or procaine amide, 10 mg/kg, was administered intravenously. Subsequently, serial measurements were obtained for a 90-min period. No significant changes in hemodynamics were observed following either drug. Procaine amide produced a significant increase in heart rate and a minimal increase in QRS duration associated with a decrease in low right atrial to His bundle conduction time. However, no significant changes in cardiac conduction were observed after lidocaine administration. Renal function was unaffected by lidocaine but significantly depressed by procaine amide, as demonstrated by a decrease in GFR and effective renal flow. In summary, acute administration of procaine amide significantly alters renal function in the conscious dog with minimal effects on AV conduction and hemodynamics.

Aminohippuric Acids↗