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Biomedical subjects

M L Young

Publications and source records attributed to M L Young.

At least 73 records · Page 4Linked to original sources
PubMed1990

A reappraisal of atrioventricular nodal excitability during functional 2:1 block: what should be the gauge?

By using extrastimulation technique the effective refractory period (ERP) of the atrioventricular (AV) node is conventionally defined as the longest A1A2 interval at which conduction of A2 is blocked within the node. With the prolongation of the AV nodal transit time that is associated with this technique, the delayed AV nodal arrival of the A1 impulse will cause an overestimation of the true interval if the A1A2 interval is used as the gauge. Under this condition, the conventional approach will not accurately delineate the ERP of the AV node. However, this prolongation of the nodal transit time can be calculated by subtracting the basal conduction time from the conducted A-H interval. By subtracting out the prolongation from the longest A1A2 at which A2 impulse is blocked, not only an interval more indicative of the true AV nodal ERP can be obtained, but also the apparent discrepancies between the in vitro and human AV nodal recovery curves disappear.

Atrioventricular Node↗
PubMed1989

Effects of hypoxia on atrioventricular node of adult and neonatal rabbit hearts.

We used an isolated perfused heart model to assess the effects of graded hypoxia (95, 45, 20, 10, or 0% O2, exposure for 5 min) on the adult and neonatal (0-3 days) rabbit atrioventricular (AV) node. The AV nodal function was assessed by measuring the A-H interval at a constant atrial pacing cycle length, the longest pacing cycle length resulting in Wenckebach periodicity [Wenckebach cycle length (WCL)] and the AV nodal effective refractory period (AVNERP). The A-H intervals remained stable in neonatal hearts until O2 saturation was decreased to 10%. On the other hand, the A-H intervals began to increase in adult rabbit hearts at 20% O2. In 95% O2, the AV nodal WCL was longer in adult hearts than in the neonatal hearts (165 +/- 8 ms vs. 142 +/- 7 ms). The effect of hypoxia on the AV nodal WCL was significantly greater in adult hearts than in neonatal hearts when the O2 saturation was decreased to 20% (a 54% increase in adults vs. a 14% increase in neonates, P = 0.02). The difference was greater at lower levels of O2. In 95% O2 at comparable basis driving cycle length (240 ms), the A-H intervals were equal in neonatal and adult hearts (43 +/- 3 vs. 43 +/- 7 ms), but the AVNERP of the neonates was significantly longer than that of the adults (133 +/- 21 vs. 97 +/- 19 ms, P = 0.007).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1988

Correlation between directly measured and indirectly estimated sinoatrial conduction time in children.

The indirectly estimated sinoatrial conduction time has been widely used in children to assess sinus node (SN) function, but has never been validated in pediatric patients. Using a standard quadripolar catheter with high amplification and low-pass filters, SN electrograms were recorded in 11 of 26 (42%) pediatric cardiac patients (age range 10 months to 18 years) with normal SN function. The sinoatrial conduction time was measured directly from the SN electrogram and estimated indirectly by the premature stimulation and continuous pacing methods. The direct sinoatrial conduction time (15 to 91 ms) in these 11 patients (ages 1 to 18 years) correlated well with that estimated by the premature stimulation method (r = 0.81, n = 9, p less than 0.01). There was a poor correlation between the direct and continuous pacing methods (r = 0.53, n = 8, p = 0.17). It was concluded that SN electrograms can be recorded in children and that there is a good correlation between the directly measured sinoatrial conduction time and the indirectly estimated sinoatrial conduction time by the premature stimulation method.

Adolescent↗
PubMed1988

Concealed atrial parasystole following the Senning operation.

A patient with sick sinus syndrome following the Senning operation was also found to have concealed atrial parasystole during intracardiac electrophysiological study. This atrial parasystolic pattern could be converted to atrial bigeminy by changing the atrial drive rate.

Arrhythmias, Cardiac↗
PubMed1988

Electrophysiologic effects of intravenous propranolol in the intact immature mammalian heart.

Using intracardiac electrogram recording and stimulation techniques, 13 neonatal puppies (3-15 days) and 9 adult dogs were studied following cumulative doses of intravenous propranolol. Although propranolol caused a dose-dependent increase in the sinus cycle length in both groups, the increase was significantly less in the neonates than in the adults. In both groups the pacing interval resulting in atrioventricular (AV) nodal Wenckebach was increased significantly by propranolol; however, the increase was again less in the neonates than in the adults. There were comparable dose-dependent increases in atrial and ventricular refractoriness in both neonate and adult groups.

Aging↗
PubMed1988

Electrophysiologic and hemodynamic effects of bethanidine sulfate on the immature mammalian heart.

The cumulative electrophysiologic and hemodynamic effects of bethanidine sulfate (2.5, 5, 10 and 15 mg/kg i.v.) were studied in 9 canine neonates and 7 adult dogs. Increased heart rate, blood pressure and enhanced atrioventricular nodal function, observed in both groups, and decreased ventricular refractory periods and inducibility of ventricular arrhythmias seen in the adult, are probably related to a bethanidine-mediated release of catecholamines. However, in spite of the catecholamine release, bethanidine sulfate results in a significant prolongation of atrial refractoriness, this effect being quantitatively more important in the neonate than in the adult.

Aging↗
PubMed1987

Reverse alternating Wenckebach periodicity.

An atrial pacing-induced reverse conduction pattern of the alternating Wenckebach periodicity was observed in 5 of 42 children (12%) during electrophysiologic study. This conduction pattern is a reverse of the usual alternating Wenckebach periodicity: During an underlying 2:1 atrioventricular conduction block there is progressive shortening of the conduction time of the conducted impulses with termination in a lower degree of block. This reverse alternating Wenckebach periodicity may be caused by a mechanism similar to that in other Wenckebach phenomena.

Atrioventricular Node↗
PubMed1987

Adenosine and hypoxia effects on atrioventricular node of adult and neonatal rabbit hearts.

An isolated perfused heart model was used to assess the effects of hypoxia and adenosine on the adult and neonatal (1-5 days) rabbit atrioventricular (AV) node. The AV nodal function was assessed by the A-H interval at a constant atrial pacing cycle length and by the longest pacing cycle length resulting in Wenckebach periodicity. We defined the pacing cycle length at or below which the AV node demonstrated Wenckebach periodicity as the Wenckebach cycle length. Adenosine produced a smaller dose-dependent increase in A-H interval in neonates than in adults, but the increase in Wenckebach cycle length was similar in the two age groups. When the hearts were exposed to 5 min of hypoxia the increase of Wenckebach cycle length was greater for adults than for neonates. The change in Wenckebach cycle length in adults caused by hypoxia was significantly greater than that caused by 1 mM adenosine. In addition, in adults aminophylline could partially attenuate the increase in Wenckebach periodicity caused by adenosine, but aminophylline could not attenuate the increase in Wenckebach cycle length caused by hypoxia. We conclude that in the rabbit AV node 1) the adenosine effect in neonates is similar to that in adults; 2) neonates are relatively resistant to acute hypoxia compared with adults; and 3) the response to acute hypoxia in adults cannot be totally explained by the adenosine release theory.

Adenosine↗
PubMed1987

Alterations in endocardial activation of the canine papillary muscle early and late after myocardial infarction.

Permanent coronary occlusion produces time-dependent changes in surviving subendocardial cellular properties. We compared the functional alterations in Purkinje (P) and ventricular muscle (VM) activation early (24 hr) and late (4 weeks or greater) after permanent coronary occlusion in an in vitro preparation of canine papillary muscle. High-density extracellular (1 to 2 mm resolution) and selected intracellular recordings were made in five animals early and seven animals late during stimulation of a free-running P strand. Activation patterns of P and VM layers from ischemic and unaffected papillary muscles were compared in the same animal. Average P layer conduction velocity was determined in normal and ischemic regions with the use of a linear array of recording and stimulating electrodes. Purkinje activation was altered little in the early phase of infarction, while healing was associated with a generalized 25% reduction in P layer conduction velocity and localized block and fragmentation of P waveforms. Intracellular recordings at sites of nonsynchronous P activation revealed electrotonic interaction between cell groups. At 24 hr, small groups of VM were present but with abnormal activation patterns in regions of necrosis with fragmented and delayed extracellular waveforms produced by partially uncoupled groups of cells. Local delay and block could be modulated by rate and site of stimulation. After healing, VM activation abruptly stopped at the visual infarct border, marked by a characteristic "end potential." These studies demonstrate important differences in the functional attributes of the P and VM layers studied early and late after coronary occlusion. Alterations in cell-to-cell relationships are likely very important in determining abnormalities of activation in both settings.

Animals↗
PubMed1987

Effects of hypoxia, hyperkalemia, and metabolic acidosis on canine subendocardial action potential conduction.

We have studied the individual and combined effects of elevated external potassium concentration (8 mM [K+], metabolic acidosis (pH = 6.8), and hypoxia at different stimulation 400 milliseconds) on Purkinje (P) and ventricular (V) conduction velocities and on Purkinje-ventricular junctional conduction delay (PVJ delay) in in vitro preparations from canine ventricles. Elevated [K+] had opposite effects on P and V velocities, increasing V velocity by 8% while reducing P velocity by 7%. Acidosis reduced P velocity by 9% while reducing V velocity by only 4%. Hypoxia and rapid stimulation rates had no significant effect on either P or V velocities. All test solutions (except hypoxia alone) significantly increased the PVJ delay. The magnitude of the increase in PVJ delay was much greater than the effects on either P or V velocity. In addition, hypoxia and rapid stimulation augmented the increase in PVJ delay in the presence of elevated [K+] and/or acidosis. The special features of conduction at the PV junctional sites may produce altered pathways of excitation of the ventricles during myocardial ischemia.

Acidosis↗
PubMed1987

Management of behavioral symptoms in disturbed elderly patients: comparison of trifluoperazine and haloperidol.

This double-blind trial compared the efficacy of trifluoperazine 1 mg b.i.d. and haloperidol 0.5 mg b.i.d. as initial doses in the treatment of behavioral symptoms associated with chronic brain syndromes and senile psychoses in 54 institutionalized elderly patients. Forty-four of the patients were followed through the entire 6-week trial. Improvement in symptoms as measured by Clinical Global Impressions (CGI) scores was observed in 86% (N = 19) of those who received trifluoperazine and 90% (N = 20) of those given haloperidol. Twenty-seven percent (N = 6) of the trifluoperazine patients were rated at least moderately improved on the CGI, whereas only 9% (N = 2) of those treated with haloperidol exhibited such improvement. Marginally significant trends favoring trifluoperazine over haloperidol were observed in the ratings obtained on the Brief Psychiatric Rating Scale, the Nurses' Observation Scale for Inpatient Evaluation, and the Sandoz Clinical Assessment-Geriatric. The type and incidence of adverse reactions were similar for the two treatments, with sedation being the most common complaint. None of the patients developed any extrapyramidal motor symptoms.

Aged↗
PubMed1986

Electrophysiologic cardiac function before and after surgery in children with atrioventricular canal.

Thirty-two children with atrioventricular (AV) canal underwent electrophysiologic studies: 18 underwent preoperative studies at a median age of 3 years (range 6 months to 16 years); 14 underwent postoperative studies at a median age of 4 years (range 2 to 19); and 2 underwent both preoperative and postoperative matched studies. In the preoperative group the following abnormalities were observed: first-degree AV block in 5 patients (due to internodal conduction delay in 1, AV nodal conduction delay in 2 and normal intracardiac intervals in 2); internodal conduction delay but normal PR interval in 4; and disease of the sinus node in only 1. In the postoperative group the following abnormalities were observed: first-degree AV block in 9 (due to AV nodal conduction delay in 2, His-Purkinje system conduction delay in 1, upper normal intracardiac intervals in 3 and unidentified in 3); prolongation of the right ventricular apical activation time in 11 of 13 with right bundle branch block; abnormal sinus node function in 3; and abnormal AV nodal function in 4 (1 of whom had associated sinus node disease). Atrial and ventricular functions were normal in all preoperative and postoperative patients. Electrophysiologic dysfunction is rare in preoperative patients with AV canal; in postoperative patients electrophysiologic abnormalities occur in 38% and involve the sinus and AV nodes in 19 and 25%, respectively.

Adolescent↗
PubMed1986

Application of the Rosenblueth hypothesis to assess atrioventricular nodal behavior.

Based on the Rosenblueth hypothesis, atrioventricular nodal behavior is evaluated using a method involving the coupling interval at the level of the atrioventricular nodal step delay. If the sum of the coupling interval at the level of step delay and the resultant step delay is greater than the atrial coupling interval, Wenckebach periodicity results; if the sum is less than the atrial coupling interval, the reverse Wenckebach phenomenon occurs; and if both are equal, steady state conduction is reached.

Atrioventricular Node↗
PubMed1986

Atrial pacing-induced alternating Wenckebach periodicity and multilevel conduction block in children.

Multilevel block within the atrioventricular (AV) node has not been previously described in children. Six children with atrial pacing-induced repetitive block are presented. The conduction patterns satisfy the requisites for alternating Wenckebach periodicity or multilevel AV block. In 2 patients the block is documented in the AV node and infra-His region. In 4 patients multilevel block within the AV node is postulated by deductive reasoning. In this study, 2 patterns of alternating Wenckebach periodicity are reported for the first time: sequences of 3:1 block with progressive prolongation of the conducted impulses terminating in 4:1 block; and sequences of 2:1 block with progressive prolongation of the conducted impulses terminating in 2 series of 3:1 block, in which the first conducted impulse following the first 2 blocked beats is not the shortest one, whereas that following the second 2 blocked beats is the shortest.

Atrioventricular Node↗
PubMed1986

Application of the Rosenblueth hypothesis to assess cycle length effects on the refractoriness of the atrioventricular node.

In adults, the effective refractory period of the atrioventricular (AV) node is lengthened, whereas that of the atrium, His-Purkinje system or ventricular myocardium is shortened with a shorter atrial pacing cycle length. However, in children, the effective refractory period of the AV node at shorter cycle lengths is also shortened. Based on Rosenblueth's 1-step delay hypothesis, an index of refractoriness within the AV node is defined as the longest coupling interval at the level of step delay within the AV node of an impulse that cannot be conducted to the His bundle. The slopes relating cycle length and refractoriness of the AV node are determined by both the conventional and revised methods in 9 pediatric patients with heart disease. The slope is positive for all patients using the revised method. The difference in values between the 2 methods in older children is striking because the slope is converted from a negative to a positive value. It is concluded that the AV node has the same positive slope relating cycle length and refractoriness as other cardiac tissues.

Adolescent↗