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PubMed1994

Characterization of the excitable gap in a functionally determined reentrant circuit. Studies in the sterile pericarditis model of atrial flutter.

BACKGROUND: Single premature beats were introduced in the reentrant circuit during stable atrial flutter in the canine sterile pericarditis model to test the hypotheses that (1) despite the fact that the reentrant circuit is functionally determined, there is a fully excitable gap; (2) the excitable gap in the reentrant circuit is not uniform; and (3) inhomogeneities of conduction in the reentrant circuit explain the effects of premature beats. METHODS AND RESULTS: A multiplexing system was used to record 190 unipolar electrograms from the right atrial free wall during 18 atrial flutter episodes in 9 dogs. In all 18 episodes, premature stimuli captured the atrial flutter reentrant circuit. At the longest coupling intervals, the return cycle at the site closest to the pacing site did not prolong. As the coupling interval of the premature stimulus decreased, the return cycle then progressively increased, associated with changes in conduction in the reentrant circuit that were not uniform. The result was that coupling intervals associated with introduction of the premature beat also were not constant. The mean duration of the total (ie, fully plus partially) excitable gap was 12 +/- 4 ms in areas of slow conduction, and it was always shorter than the total excitable gap in other areas (22 +/- 6 ms, P < .001). The mean duration of the fully excitable gap based on analysis of the return cycle was 4 +/- 1 ms in the reentrant circuit. In 13 of 18 atrial flutter episodes, a premature stimulus terminated atrial flutter by causing block of the orthodromic wave front of the premature beat in an area of slow conduction. The mean coupling interval that caused orthodromic block was 113 +/- 5 ms (recorded at the site just proximal to the area of block), and it was always longer than the delivered stimulus coupling interval at the pacing site (96 +/- 8 ms, P < .001). CONCLUSIONS: We conclude that in this functionally determined atrial flutter reentrant circuit in the canine sterile pericarditis model, (1) a fully excitable gap is present in at least part of the reentrant circuit; (2) the duration of the excitable gap in the reentrant circuit is shortest in areas of slow conduction; and (3) when a premature beat encounters the partially excitable gap of the reentrant circuit, it results in changes in conduction such that the coupling intervals are not uniform throughout in the reentrant circuit.

Animals↗
PubMed2004

Endothelin levels in experimental diabetes combined with cardiac hypertrophy.

Diabetes mellitus is associated with endothelial and cardiac dysfunction, and endothelin has been suggested to alter cardiac function by being a positive inotropic agent, modulating the Frank-Starling response, contracting the coronary arteries and inducing tissue proliferation. We investigated endothelin levels in diabetic and in healthy dogs, 1 and 3 days after placing arteriovenous shunts (8 weeks after diabetes induction) in the femoral regions. Right and left ventricular weight/body weight ratios and Nterminal- atrial natriuretic peptide were increased in shunted animals (P < 0.05). Plasma endothelin levels were comparable in healthy and diabetic dogs. Shunted circulation did not change systemic endothelin levels in healthy dogs but reduced endothelin levels in diabetic dogs. The functional significance of altered endothelin responses to acute hemodynamic burden in experimental diabetes needs further investigation.

Animals↗
PubMed1995

Nonlinear analysis of epicardial atrial electrograms of electrically induced atrial fibrillation in man.

INTRODUCTION: We applied methods from the theory of nonlinear dynamics to characterize unipolar epicardial right atrial electrograms of electrically induced atrial fibrillation (AF) in man. METHODS AND RESULTS: Electrograms were selected from a high-density mapping study, which confirmed the existence of at least 3 different types of induced AF (types I, II, and III) in patients undergoing open chest surgery. We analyzed sets of 5 electrograms (4 sec, sampling frequency 1 kHz, resolution 8 bits) in 9 patients (AF type I, n = 3; type II, n = 3; type III, n = 3). The Grassberger-Procaccia method was applied to estimate the correlation dimension and correlation entropy from the electrograms. In 2 patients (AF type I) some electrograms (2 of 5 and 3 of 5, respectively) showed scaling at normalized distances ranging from 0.2 to 0.5 in phase space. Correlation dimension D ranged from 1.8 to 3.2 and correlation entropy K from 2.2 to 3.8 nats/sec. The patients were ranked according to increasing coarse-grained correlation dimension Dcg (range 3.7 to 7.9) and coarse-grained correlation entropy Kcg (range 5.6 to 18.6 nats/sec). The method of surrogate data was applied to detect nonlinearity in the electrograms. Using the correlation integral as test statistic, it could be excluded that electrograms of type I AF have been generated by linear stochastic dynamics. Episodes of sinus rhythm (D ranging from 1.0 to 5.1 and K from 2.0 to 8.6 nats/sec) and induced atrial flutter (D ranging from 2.7 to 4.2 and K from 2.2 to 4.2 nats/sec) in 2 different patients showed features of low-dimensional chaos. CONCLUSION: Nonlinear analysis discriminated between electrograms during electrically induced AF in humans. The results are consistent with a classification of AF into 3 types based on the spatiotemporal complexity of right atrial activation patterns.

Adult↗
PubMed1998

[Atrial defibrillators or implantable atrioverters. Initial results].

The atrial defibrillator is a new non-pharmacological treatment of atrial fibrillation (AF) for restoration of sinus rhythm. This device has two programmable modes: automatic or activated by the physician or patient. In the automatic mode, the device delivers a shock synchronous with the R wave to restore sinus rhythm when AF is detected. Two patients with paroxysmal AF resistant to pharmacological therapy were included in a study to assess the efficacy and safety of the atrial defibrillator in the mode activated by the physician. The device implanted in the pectoral region is connected to 3 electrodes, two for atrial defibrillation and sensing positioned in the coronary sinus and right atrium respectively and a sensing and pacing electrode in the right ventricle. The right ventricle is paced if a post-shock pause is detected. It is possible to interrogate the device with a programmer using its Holter function and so determine the number of episodes of AF sensed and treated. The number, intensity and energy of the shocks and the parameters of ventricular stimulation are programmable. In these two patients, the atrial defibrillator effectively reduced prolonged episodes of AF with a follow-up of 12 and 7 months. No pro-arrhythmic effects were observed. Further clinical evaluation is under way to assess this new mode of treatment, including the mode activated by the patient, safety and tolerance of the shocks. In our two patients, the treatment of prolonged episodes of AF was followed by reduction of many short or asymptomatic episodes.

Aged↗
PubMed1986

Long-term results after Fontan procedure and its modifications.

The Fontan procedure is an effective method of treatment for patients with tricuspid atresia, univentricular heart, and other complex lesions. Modifications of the Fontan procedure have been developed to treat various anatomic malformations. From 1975 to 1984, 31 patients (17 male and 14 female) have undergone 35 Fontan operations. The youngest patient was 4 and the oldest 26 years of age (average 12.2 years). Sixteen patients had tricuspid atresia, and 15 univentricular heart. Twenty-three patients had undergone 37 palliative operations before the Fontan procedure. The original Fontan procedure was performed in 10 patients until 1977, and there were three early deaths (30%). After that time, the operative technique was modified and the early mortality dropped to 12% (3/25 patients). A valved prosthetic conduit was used in 11 patients (right atrium to pulmonary artery in five; right atrium to right ventricle in six), a valveless conduit in 14 patients (right atrium to pulmonary artery in nine; right atrium to right ventricle in five), and direct anastomosis (right atrium to pulmonary artery) in 10 patients. Postoperative cardiac catheterization was performed in 20 patients and revealed excellent results in 15. The remaining five had reduced oxygen saturation; three of them had had a Glenn anastomosis 8 and 10 years before the Fontan procedure, and one had an anomaly of the coronary sinus with a shunt to the left atrium. This patient required reoperation. Three other patients underwent reoperation because of calcified stenotic and/or outgrown conduits. Postoperative mean right atrial pressure varied from 10 to 20 mm Hg. The hemodynamic ventricular parameters were within the normal range. We conclude that in patients with tricuspid atresia, univentricular heart, and other complex cardiac lesions, inactivating the right ventricular pump function by means of the Fontan procedure does not adversely affect the systemic ventricle. Although the response to exercise was abnormal, the clinical condition of these patients was good to excellent.

Adolescent↗
PubMed1984

Considerations on the nature of irregularity of the sequence of RR intervals and the function of the atrioventricular node in atrial fibrillation in man based on time series analysis.

Using sinus arrhythmia as a control, we elucidated the random nature of RR intervals during atrial fibrillation in man and determined the function of the atrioventricular (AV) node from the variability of RR intervals. The major difference between the characteristics of sinus arrhythmia and those of atrial fibrillation is the presence of a significant correlation between successive intervals in the former. Since the pattern of distribution of RR intervals in atrial fibrillation is unimodal and skewed to the right and so can be fitted to an Erlang distribution, atrial inputs can be considered to be summated to a certain threshold for ventricular activation in the N region of the AV node, the number of cumulative atrial inputs corresponding to this threshold being the phase of this Erlang distribution. The function of the AV node during atrial fibrillation is to transform an exponentially distributed input into an Erlang-distributed output. Loss of inputs occurs between the atria and the N region and the greater the loss of inputs the slower the ventricular response. However, the greatest loss occurs in the N region for summation of atrial inputs required to elicit ventricular activation.

Adolescent↗
PubMedDate not supplied

Evaluation of right ventricular performance long after the atrial switch operation for transposition of the great arteries using the Doppler Tei index.

Right ventricular (RV) dysfunction may occur in patients after the atrial switch operation for d-transposition of the great arteries (d-TGA) and can be an important complication. Noninvasive assessment of RV function using echocardiography is necessary for following up these patients. We evaluated RV function using the Doppler Tei index (RV Tei index). The RV Tei index measures the ratio of total time intervals in isovolumetric contraction and relaxation to the ejection time. The subjects consisted of 33 patients who had undergone the atrial switch operation for d-TGA (the atrial switch group) (mean age, 21 +/- 4 years), 10 patients with pulmonary artery stenosis after the arterial switch operation for d-TGA (the arterial switch group) (mean age, 17 +/- 1 years), and 10 patients with a normally structured heart as a control group (mean age, 26 +/- 7 years). The RV Tei index (0.65 +/- 0.16) was significantly higher in the atrial switch group than in the arterial switch group (0.29 +/- 0.07) and the control group (0.29 +/- 0.01) (p < 0.05). The fractional area change of the RV obtained from the four-chamber view in the atrial switch group showed a significant negative correlation with the RV Tei index (r = -0.58, p < 0.01). The RV Tei index increased with increasing severity of the NYHA functional class. In the moderate/severe tricuspid regurgitation group, it was significantly higher than in patients with none/trivial tricuspid regurgitation. We conclude that the RV Tei index is useful for evaluating RV junction long after the atrial switch operation for d-TGA.

Adolescent↗
PubMed1994

Lung management with perfluorocarbon liquid ventilation improves pulmonary function and gas exchange during extracorporeal membrane oxygenation (ECMO).

We investigated whether pulmonary function and gas exchange would improve with liquid perfluorocarbon ventilation (LV) during ECMO for severe respiratory failure. Lung injury was induced in 11 young sheep 15.1 +/- 3.7 kg in weight utilizing right atrial injection of 0.07 cc/kg oleic acid followed by saline pulmonary lavage. When (A-a)DO2 > or = 600 mmHg and PaO2 < or = 50 mmHg with FiO2 = 1.0, ECMO was instituted. Animals were then ventilated with either standard ECMO "lung rest" gas ventilator settings (ECMO, n = 5) or with "total" liquid ventilation at standard ventilator device settings (LIQ-ECMO, n = 6) utilizing perflubron (perfluooctyl bromide, Liquivent; Alliance Pharmaceutical Corp.). After 3 hours on ECMO, pulmonary physiologic shunt decreased (ECMO = 88 +/- 11% vs LIQ-ECMO = 31 +/- 1%; p < .001) and pulmonary compliance increased (ECMO = 0.50 +/- 0.06 cc/cmH2O/kg vs. LIQ-ECMO = 1.04 +/- 0.19 cc/cmH2O/kg; p < .001). The ECMO flow rate required to maintain the PaO2 in the 50-80 mmHg range was decreased significantly (ECMO = 116 +/- 14 ml/kg/min vs. LIQ-ECMO = 14 +/- 5 ml/kg/min; p < .001). In this model requiring extracorporeal support for severe respiratory failure, lung management with liquid ventilation improves pulmonary function and gas exchange.

Animals↗
PubMed1979

The vulnerability of the right atrium. III. Electrophysiologic correlates of atrial vulnerability.

70 patients were investigated by means of the atrial extrastimulus method at three different driving rates: 80, 100 and 120/min. At each rate the effective, the relative, the total and the functional refractory periods were measured. 30 patients who showed signs of atrial vulnerability at least one of the tested rates were included in the so called vulnerability group. The remaining 40 patients, who did not fulfill the criteria for atrial vulnerability, were included in the nonvulnerability group. When the two groups were compared to each other there were significant larger P waves (p less than 0.005), shorter effective refractory periods (p less than 0.001) and longer relative refractory periods (p less than 0.001) in the vulnerability group. With increasing driving rate there was an increased tendency to repletitive firing in the vulnerability group. The phenomenon of vulnerability correlated well with the rate-induced shortening of the effective and the lengthening of the relative refractory period. The above described phenomena are compatible with the concept of re-entry as the electrophysiologic mechanism of atrial vulnerability in man.

Atrial Fibrillation↗
PubMed1981

[Tricuspid valve replacement by Hancock's prosthesis].

The medium term results of tricuspid valve replacement with the Hancock bioprosthesis are reported. Twenty eight patients underwent tricuspid valve replacement with this prosthesis between December 1974 and January 1978: mitral valve replacement with a Starr-Edwards or Cooley-Cutter prosthesis was associated in all cases and aortic valve replacement with a Björk-Shiley prosthesis in 11 cases. Follow-up at an average of 36,2 months after operation examined functional status, cardiac size and haemodynamics (in 12 patients). Three patients died in the immediate postoperative period and four others died later: the number of survivors was greater in the triple valve replacement (9/11) than in the double valve replacement group (12/17) but the difference was not statistically significant. Of the 23 surviving patients (average follow-up of 36,2 +/- 9,6 months), 17 were classified in functional Classes I or II of the NYHA classification. All patients had been Class III or IV before operation. The cardiothoracic ratio did not decrease significantly in patients undergoing triple valve replacement. Control cardiac catheterisation showed a significant increase in cardiac index (2,53 +/- 0,11 1/mn/m2, compared to 1,87 +/- 0,35 1/mn/m2 before operation; p less than 0,001) without significant reduction in pulmonary artery or right atrial pressures. The resting gradient across the Hancock bioprosthesis was not related to the size of the prosthesis (No 29-30: 2,17 +/- 2,57 mm Hg; No 31-33: 2,78 +/- 3,53 mm Hg) or with the quality of the functional result. However, on exercise, the gradient across the prosthesis was high, reaching an average of 10,3 +/- 5,2 mm Hg). The operative mortality of tricuspid valve replacement is relatively bioprosthesis associated with mitral and/or aortic valve replacement is relatively bioprosthesis associated with mitral and/or aortic valve replacement is relatively low (about 10%) and could be an argument in favour of broadening the indications for tricuspid valve replacement as resting tricuspid function with a bioprosthesis is satisfactory. However, the stenotic effects on exercise and the uncertainty over the long-term outcome of bioprostheses suggest that surgery should be limited to severe tricuspid stenoses and/or major tricuspid regurgitation organic or functional uncontrolled by digitalis and diuretic therapy.

Adult↗
PubMed1984

[Determination of the size of the right atrium in patients with pulmonary hypertension using 2-dimensional echocardiography].

Normal values for right atrial area and tricuspid annular diameter and their percentage shortening measured from the two-dimensional echocardiographic four-chamber view are now available. In this study right atrial size of patients with pulmonary hypertension is evaluated and the results compared with the M-mode findings of the right ventricle in order to detect pulmonary hypertension. Hemodynamics and echocardiograms of 60 patients (mean age 57 +/- 10 years) with mitral stenosis III-IV NYHA and concomitant pulmonary hypertension were examined. 20 patients in group I with atrial fibrillation had additional functional tricuspid incompetence. Unlike the 20 patients in group II, who had atrial fibrillation, the remaining 20 in group III were still in sinus rhythm. The mean pulmonary artery pressure was 44.1 +/- 9.3 mmHg in group I, in group II 33.9 +/- 6.3 mmHg (grp. I vs. grp. II p less than 0.001) and in group III 33.2 +/- 7.1 mmHg (grp. II vs. grp. III ns). The end-systolic index of the right atrial area in group I was 19.6 +/- 5.7 cm2/m2 and thus significantly greater than in group II with 10.6 +/- 2.3 cm2/m2 (p less than 0.001) and in group III with 9.1 +/- 2.5 cm2/m2 (p less than 0.001). The maximal diameter of the tricuspid annulus measured 24.6 +/- 5.5 mm/m2 in group I, 18.9 +/- 3.5 mm/m2 in group II (grp. I vs. grp. II p less than 0.001) and 20.2 +/- 2.2 mm/m2 in group II (grp. I vs. grp. II p less than 0.001) and 20.2 +/- 2.2 mm/m2 in group III.(ABSTRACT TRUNCATED AT 250 WORDS)

Blood Pressure↗
PubMed1984

The auricular myocardiocytes of the heart constitute an endocrine organ. Characterization of a porcine cardiac peptide hormone, cardiodilatin-126.

A peptide hormone was extracted from the porcine right atrium following a bioassay for differential vasorelaxant effects on smooth muscle strips from aorta and renal and inferior mesenteric arteries. The isolation procedure included several steps of gel-permeation and ion-exchange chromatography, and high performance liquid chromatography. During the isolation procedure, other peptides of smaller molecular weight were also found, which, in relation to cardiodilatin-126 (CDD-126), are shorter at their N-terminal. Among these, CDD-88 has also been isolated and characterized, and has been established as a prominent member of the cardiac hormone family. The N-terminal and C-terminal segments of the 126 amino acid-containing molecule were synthesized and used to raise region-specific antibodies. The natural peptide was then localized within myoendocrine cells of the right atrium where specific atrial granules are located. Renal effects of cardiodilation were studied in conscious dogs and showed strong diuretic and natriuretic activities. According to our functional studies, cardiodilatin-126 and cardiodilatin-88 possess qualities of a significant hormone family regarding the regulation of extracellular fluid volume and blood pressure.

Amino Acid Sequence↗
PubMed1998

Hemodynamic profile in severe ARDS: results of the European Collaborative ARDS Study.

OBJECTIVE: Although the acute respiratory distress syndrome (ARDS) was identified as long as 30 years ago, potential therapeutic objectives have been defined from small series rather than large trials. Moreover, relationships between ARDS and hemodynamics are unclear. The European Collaborative ARDS Study was designed to identify factors influencing the pathogenesis, severity, and prognosis of ARDS. Analysis of the hemodynamic profiles collected during this study and of their contribution to the above-mentioned facets of ARDS is the focus of the present report. DESIGN: Prospective clinical study. SETTING: 38 European intensive care units (ICUs). PATIENTS AND METHODS: We collected 2758 sets of data from 586 patients, including baseline data, data on proven or suspected causes of ARDS differentiating direct and nondirect lung injury, and data on baseline status including multiple organ dysfunction (MOD) with differentiation of primary ARDS from ARDS secondary to severe systemic disorders. Events during follow-up were also recorded, including whether the acute respiratory failure did or did not improve after 24 h (groups A and B, respectively). When available, hemodynamic data were recorded at enrollment (day 0), on days 1-3, 7, 14, and 21, and at discharge or at the time of death in the ICU. RESULTS: Although the rate of pre-existing disease and the nature and rate of complications varied widely among etiologic categories, differences in the hemodynamic profile occurred only between primary and secondary ARDS. Both at inclusion and during the course of the illness, variables that were used to investigate Va/Q mismatch [arterial oxygen tension (PaO2, arterial oxygen saturation, right-to-left shunt, and the PaO2/fractional inspired oxygen (FIO2) ratio] predicted survival. High pulmonary artery pressure (PAP) and low systemic artery pressure (SAP) were also related to the prognosis. In the logistic regression model including hemodynamic and oxygen-related variables, however, the only independent predictors of survival were the ratio of right over left ventricular stroke work (RVSW/LVSW) and the PaO2/FIO2 ratio at admission. On day 2, the best prognostic model included: age [odds ratio (OR) = 1.04, p = 0.0004], opportunistic pneumonia as the cause of ARDS (OR = 3.2, p = 0.03), existence of MOD (OR = 1.9, p = 0.03), PaO2/FIO2 (OR = 0.96, p = 0.005), and RVSW/LVSW (OR = 25, p = 0.02). A high RVSW/LVSW ratio, high systolic PAP, low diastolic SAP, and low PaO2/FIO2, and increased right atrial pressure were negative prognostic indicators during follow-up. CONCLUSION: In addition to the cause of ARDS and the early time-course of lung function, a high systolic PAP and a low diastolic SAP were strong independent indicators of survival.

Adult↗
PubMed2005

Coronary sinus flow reversal in congestive heart failure detected during biventricular pacing.

This report describes a phenomenon observed during implantation of a left ventricular lead through the coronary sinus. During coronary sinus venography there was reflux of dye from the coronary sinus ostium towards the distal part of the sinus against the anticipated direction of flow. This was correlated to elevation of the right atrial pressure above the pressure within the coronary sinus. A review of data is provided and suggests several deleterious effects of this phenomenon on left ventricular function.

Aged↗
PubMed1992

A substantial proportion of cardiac Gs is not associated with the plasma membrane.

The precise interactions between the subunits of Gs (alpha s, beta, gamma) and the plasma membrane remain to be established. If alpha s is associated loosely with the inner membrane, is labile during activation, or is always present to some extent in the cytoplasm, then it should fractionate to the supernatant of a high-speed centrifugation. We identified abundant alpha s (52-66% of total cellular) in the supernatant fraction of right atrial and left ventricular membrane preparations of porcine heart as shown by two distinct measures of alpha s (immunoblotting and ADP ribosylation by cholera toxin). However, functional assays utilizing reconstitution of cardiac alpha s with cyc- S49 membranes revealed that the supernatant fraction contained approximately 16% of total cellular alpha s activity. The alpha s present in the supernatant fraction did not result from contamination by sarcolemmal membrane fragments. We conclude that traditional methods for quantifying alpha s which utilize only detergent extracts from high-speed pellets do not account for a sizable proportion of total cellular alpha s, but that the majority of this population of cardiac alpha s may not be functional, at least with respect to adenylyl cyclase activation.

Adenosine Diphosphate Ribose↗
PubMed1999

Impact of arrhythmia circuit cryoablation during Fontan conversion for refractory atrial tachycardia.

Refractory atrial arrhythmias in late postoperative Fontan patients are usually associated with residual hemodynamic abnormalities and result in significant morbidity and mortality. Surgical revision of the Fontan anastomosis may improve hemodynamics without eliminating tachycardia. This study sought to assess the impact of surgical cryoablation of the arrhythmia circuit at the time of Fontan conversion on the clinical recurrence of tachycardia. Sixteen consecutive atriopulmonary Fontan patients with refractory atrial arrhythmias underwent surgical conversion to lateral tunnel total cavopulmonary anastomosis (15) or Fontan revision (1 patient). The initial 4 patients underwent Fontan conversions alone, without specific arrhythmia surgery. The subsequent 12 patients underwent electrophysiologically guided cryoablation of the tachycardia circuits at the time of surgical conversion. The mean age at Fontan revision was 15.6 +/- 3.8 years. Cryoablation was directed to 3 identified major tachycardia circuits: the inferomedial right atrium, the superior rim of the prior atrial septal defect patch, and along the lateral right atrial wall. Transmural antitachycardia pacemakers were implanted in 11 of the 16 patients. There was no surgical mortality in either group, and all patients improved in functional classification. All patients not undergoing cryoablation experienced recurrent symptomatic tachycardia requiring antiarrhythmic therapy (median follow-up, 54 months) versus 2 of 12 patients receiving cryoablation (median follow-up, 25 months; p <0.02). Thus, surgical cryoablation of the arrhythmia circuit at the time of Fontan conversion is highly effective in the management of refractory atrial arrhythmias, and is superior to Fontan conversion alone.

Adolescent↗
PubMed1984

Maturational changes in ventriculoatrial conduction in the intact canine heart.

This study reports on the changes in ventriculoatrial (VA) conduction that occur with maturation. Programmed atrial and ventricular premature extra-stimulation (coupled to a fixed paced cycle length) and rapid atrial pacing were performed in three groups of dogs: Group I = 8 neonates aged 5 to 14 days, Group II = 9 young dogs aged 6 to 9 weeks and Group III = 10 adult dogs. High right atrial, His bundle and right ventricular electrograms were recorded. There were no differences in the AH intervals at rest. In all but five animals, atrioventricular conduction was limited by the atrial functional refractory period (Group I, 109 +/- 12 ms; Group II, 152 +/- 22 ms; Group III, 167 +/- 19 ms). As expected, with rapid atrial pacing, Wenckebach conduction developed at a shorter cycle length in the younger animals (Group I, 145 +/- 20 ms; Group II, 153 +/- 15 ms; Group III, 200 +/- 25 ms, p less than 0.01). Ventriculoatrial conduction was documented in 87% of Group I puppies and 100% of Group II, but only 40% of Group III dogs. The effective and functional refractory periods of the VA conduction system were significantly shorter in the more immature groups of dogs (effective/functional: Group I, 124 +/- 27/168 +/- 22 ms; Group II, 139 +/- 23/202 +/- 13 ms; Group III, 270 +/- 28/326 +/- 25 ms; p less than 0.01). Relative to the adult dog, the immature heart showed a greater incidence of VA conduction and shorter VA refractory periods. This enhanced VA conduction may be of physiologic importance in the initiation and perpetuation of certain supraventricular arrhythmias.

Age Factors↗