Scaling of Atrioventricular Transmission in Mammalian Species: An Evolutionary Riddle!

"Scaling deals with the structural and functional consequences of changes in size or scale among otherwise similar organisms." It plays a key role in all studies on comparative mammalian physiology and morphology. Heart weight is proportionally related to body weight and can be described by a straightforward, so-called allometric equation. We studied sealing of AV transmission times (PR intervals on the ECG) in 375 mammals of different dimensions and species. Scaling of AV transmission times versus heart length (third root of weight) is statistieally best described by a S-shaped curve. This implies that AV transmission time in mammals is not linearly related to heart length and does not depend solely on the length of the AV transmission system. The AV node fine-tunes AV transmission times at rest and during exercise in individuals; it protects the ventricles against high-rate atrial arrhythmias such as atrial fibrillation; and it regulates basal AV transmission times in mammalian species of varying sizes. We call the "how" and "why" of the sealing of AV transmission time in mammals an evolutionary riddle that deserves further study

ECG of the "Newborn" Mouse (Mus domesticus) with Specific Reference to Comparative AV Transmission

The objective of this study was to record the ECG of the smallest living mammal to extend the domain of data for comparative AV-nodal electrophysiologic purposes. These data are needed to establish the relationship between the PR interval and heart size in mammalian species of all sizes. Methods and Results: In recently born mice (age 1.5 to 8 weeks) weighing between 2.5 and 10 g and with estimated heart weights between 15 and 60 mg, ECGs, using bipolar limb leads, were recorded during general anesthesia. The PR interval, representing AV transmission time was about 40 msec, which is quite long for hearts of this size. On the basis of detailed analysis of the data, we postulate the presence of a fixed delay or discontinuous propagation in the AV node not only in newborn mice, but in mammals of all sizes. Conclusion: AV transmission times obtained in mammals (including humans) cannot be explained on the basis of generally accepted, classic AV conduction theories. The acceptance of the presence of a fixed delay in the AV node may ultimately be of value to better understand AV node function during sinus rhythm and supraventricular arrhythmias

Evaluation of pulsatile plasma concentrations of growth hormone in healthy dogs and dogs with dilated cardiomyopathy

Abstract OBJECTIVE: To evaluate plasma concentrations of growth hormone (GH) and insulin-like growth factor I (IGF-I) in healthy dogs and large-breed dogs with dilated cardiomyopathy (DCM). ANIMALS: 8 dogs with DCM and 8 healthy control dogs of comparable age and body weight. PROCEDURES: Blood samples for determination of the pulsatile plasma GH profile were collected from all dogs at 10-minute intervals between 8:00 am and 8:00 pm. Plasma IGF-I concentration was determined in the blood sample collected at 8:00 am. RESULTS: No significant differences in plasma IGF-I concentrations, basal plasma GH concentration, GH pulse frequency, area under the curve above the zero line and above the baseline for GH, and GH pulse amplitude were found between dogs with DCM and control dogs. CONCLUSIONS AND CLINICAL RELEVANCE: Results did not provide evidence for an association between DCM in dogs and a reduction in plasma concentrations of GH or IGF-I. Therefore, reported positive effects of GH administration are most likely attributable to local effects in the heart. PMID:21194336[PubMed - indexed for MEDLINE

Atrioventricular conduction in mammalian species: Hemodynamic and electrical scaling

OBJECTIVES The purpose of this study was to investigate scaling of the duration of late diastolic left ventricular (LV) filling in relation to AV conduction time (delay) (PR interval on the ECG) in mammals. BACKGROUND From mouse to whale, AV delay increases 10-fold, whereas body mass increases one million-fold. The apparent "mismatch" results from scaling of AV delay versus body and heart mass. METHODS We measured (1) mitral orifice diameter in 138 postmortem hearts of 48 mammalian species weighing between 17 g and 250 kg and (2) transmitral diastolic flow using magnetic resonance imaging (MRI) recordings of 10 healthy human individuals. (3) We visually inspected early and late diastolic LV filling. (4) We developed two physical models to explain scaling of late diastolic LV filling time. RESULTS (1) Diameter of the mitral orifice proportionally relates to heart length (third root of heart mass). (2) Atrial contraction starts at a fixed instant (± 80%) of the (normalized) cardiac cycle and contributes 31% ± 5% to LV filling. (3) MRI shows that during diastole, the left atrium (LA) and LV form a single space. (4) The physical models relate the duration of late diastolic LV filling directly to heart length, the third root of heart mass. CONCLUSIONS (1) Late diastolic (LV) filling time scales with heart length (third root of heart mass). (2) No "mismateh" exists between AV delay and heart size. (3) Knowledge of the actual starting time of atrial contraction may contribute to better treatment of patients with heart failure. (4) The findings suggest that in evolution of mammalian species, hemodynamics commands electrical behavior of the heart

Evaluation of pulsatile plasma concentrations of growth hormone in healthy dogs and dogs with dilated cardiomyopathy

Abstract OBJECTIVE: To evaluate plasma concentrations of growth hormone (GH) and insulin-like growth factor I (IGF-I) in healthy dogs and large-breed dogs with dilated cardiomyopathy (DCM). ANIMALS: 8 dogs with DCM and 8 healthy control dogs of comparable age and body weight. PROCEDURES: Blood samples for determination of the pulsatile plasma GH profile were collected from all dogs at 10-minute intervals between 8:00 am and 8:00 pm. Plasma IGF-I concentration was determined in the blood sample collected at 8:00 am. RESULTS: No significant differences in plasma IGF-I concentrations, basal plasma GH concentration, GH pulse frequency, area under the curve above the zero line and above the baseline for GH, and GH pulse amplitude were found between dogs with DCM and control dogs. CONCLUSIONS AND CLINICAL RELEVANCE: Results did not provide evidence for an association between DCM in dogs and a reduction in plasma concentrations of GH or IGF-I. Therefore, reported positive effects of GH administration are most likely attributable to local effects in the heart. PMID:21194336[PubMed - indexed for MEDLINE