OBJECTIVE To assess recording accuracy of right atrial and ventricular depolarization during 12-lead ECG when precordial lead V1 was positioned at each of 5 locations on the thorax of dogs with various thoracic conformations. ANIMALS 60 healthy client-owned dogs. PROCEDURES 20 dogs were allocated to each of 3 groups (brachymorphic, mesomorphic, or dolichomorphic) on the basis of thoracic conformation. Each dog remained unsedated and was positioned in right lateral recumbency for a series of five 12-lead surface ECGs, with V1 located adjacent to the sternum in the fifth intercostal space (ICS; control), at the costochondral junction (CCJ) of the right first ICS (1st-R), at the CCJ of the right third ICS, at the right third ICS where the thorax was the widest, and at the CCJ of the left first ICS. Electrocardiographic variables were compared among the 5 ECG tracings. RESULTS When V1 was at the control location, the P wave was positive for all dogs; however, consistent recording of right atrial and ventricular depolarization (ie, R wave-to-S wave ratio [R/S] < 1) occurred more frequently for brachymorphic dogs (16/20) than for dolichomorphic (7/20) and mesomorphic (6/20) dogs. When V1 was at the 1st-R location, the P wave was negative for most dogs, and R/S was < 1 for the majority of dogs in the brachymorphic (19/20), mesomorphic (17/20), and dolichomorphic (16/20) groups. The median R/S for V1 at the 1st-R location was significantly lower than that for the other 4 V1 locations. CONCLUSIONS AND CLINICAL RELEVANCE Results indicated that placement of V1 at the 1st-R location provided correct evaluation of right atrial and ventricular depolarization in most dogs regardless of thoracic conformation.

Objective-To characterize the electrocardiographic features of the atrial repolarization (Ta) wave in dogs with third-degree atrioventricular (AV) block. Sample-ECGs of 36 dogs with third-degree AV block and no identifiable structural heart diseases. Procedures-Standard 12-lead ECGs were acquired with a digital system, and measurements were manually edited. Results-A Ta wave was detectable in all dogs for at least 1 ECG lead. The Ta wave had negative polarity in leads I, II, III, and aVF and positive polarity in leads aVL and aVR, with a mean electrical axis of -114.26°. Mean duration and mean amplitude of the Ta wave in lead II were 140.2 milliseconds and -0.09 mV, respectively, with the ratio for the Ta-to-P wave duration of 2.3 and the ratio of Ta-to-P wave amplitude of -0.35. Significant correlations were found between the Ta wave duration and duration of the P-Ta interval, Ta wave amplitude and the ECG lead, Ta wave duration and body weight, and duration of the P-Ta interval and atrial rate. Measurements of the Ta wave were repeatable. Conclusions and Clinical Relevance-Measurements of the Ta wave in dogs with third-degree AV block were repeatable. The values for the Ta wave reported here can be used as reference values for dogs with AV conduction disturbances and an echocardiographically normal atrial size. Further studies are needed to validate these results in dogs with structural heart diseases.

Objective-To determine the effects of initial handling and training on autonomic nervous functions in young Thoroughbreds. Animals-63 healthy Thoroughbreds. Procedure-All horses were trained to be handled and initially ridden in September of the yearling year and then trained until the following April by conventional training regimens. To obtain the heart rate (HR), electrocardiograms were recorded in the stable before initial handling and training and following 7 months of training; variations in HR were then evaluated from the power spectrum in terms of the low frequency (LF; 0.01 to 0.07 Hz) power and high frequency (HF; 0.07 to 0.6 Hz) power as indices of autonomic nervous activity. To evaluate the fitness, the V200 (velocity at HR of 200 beat/min), which is reflective of the aerobic capacity of the horse, was measured. Results-Mean (+/- SE) resting HR decreased significantly from 41.5 +/- 0.8 to 38.7 +/- 0.4 beat/min following 7 months of training. The LF power of horses increased significantly from 1,037 +/- 128 milliseconds2 in September of the yearling year to 2,944 +/- 223 milliseconds2 in the following April. Similarly, the HF power increased significantly from 326 +/- 30 milliseconds2 to 576 +/- 39 milliseconds2 at the corresponding time points. The V200 increased significantly following training. Conclusions and Clinical Relevance-Increases in LF and HF powers indicate that parasympathetic nervous activity increases in horses by 7 months of training. The decrease in resting HR may be dependent on the training-induced increase of parasympathetic nervous activity in Thoroughbreds.

Although it is known that the QT interval is dependent on the preceding RR interval, QT interval does not vary during respiratory sinus arrhythmia, despite a wide variation in heart rate. To assess the rate of change of the QT interval following an abrupt increase or decrease in heart rate, QT intervals were measured from ECG of healthy, anesthetized, thoracotomized dogs in which a junctional rhythm had been induced by destroying the sinoatrial node. Atria were paced at 800- or 600-millisecond cycle durations until a steady state was reached, and then the cycle duration was changed suddenly to a new cycle duration (600 or 800 milliseconds, respectively). The time and number of heart beats required until the QT interval achieved a value of 63% (1 time constant) of the new steady state were calculated. Time constants for change in QT interval vs the number of beats following the change were 2.8 (SD = 1.3 s) seconds when heart rate was accelerated and 4.7 (SD = 2.1 s) seconds when heart rate was slowed. Differences were not statistically significant. The time constants for change in QT interval duration vs duration after the sudden change in heart rate were 1.7 (SD = 0.8 s) seconds when heart rate was accelerated and 3.7 (SD = 1.7 s) seconds when heart rate was slowed. These time constants differed significantly (P < 0.01). Response of QT interval, therefore, depended on the number of heart beats following sudden change in heart rate, but not time, except as time determined the number of heart beats. The QT interval did not change until 3 to 5 beats after the heart rate was suddenly changed. This number of beats would be more than that which would occur in 1 respiratory cycle in dogs; therefore, QT interval memory would prohibit changes in QT intervals that occur during respiratory sinus arrhythmia.

Medetomidine, an investigational drug indicated for clinical use as a short-term chemical restraint in dogs, was evaluated for its cardiopulmonary effects, in 10 naturally heartworm-infected (HW+) and 10 noninfected (HW-) Beagles. The drug was randomly administered IV (30 microgram/kg of body weight) and IM (40 microgram/kg) in single injections to all dogs. Heart rate, respiratory rate, ECG, blood gas tensions, blood pH, central venous and arterial pressures were measured at 0, 15, 30, 60, 90, 120, and 180 minutes. Medetomidine induced an immediate significant (P less than or equal to 0.001) increase in mean arterial blood pressure followed by decreased blood pressure that remained below normal throughout the study in both groups, irrespective of route of administration. Medetomidine increased central venous pressure, over time, for both groups and both routes of administration. Heart and respiratory rates were significantly (P less than or equal 0.001) decreased after medetomidine administration and remained reduced for the duration of the study in all dogs. The ECG variables were not significantly different between groups or between routes of administration. The HW+ dogs tended to have higher mean PaO2 than did HW- dogs at several postinjection determination times, particularly when the drug was administered IM. The PaO2 decreased during the first 30 minutes in both groups and tended to increase gradually thereafter. The pH decreased over time for both groups and both routes. A significant (P less than or equal to 0.05) decrease in pH was seen in the HW- dogs, compared with HW+ dogs at each measuring time for both routes. The PaCO2 did not significantly change for groups or routes. In general, bradycardia was the predominant cardiovascular effect seen after medetomidine administration in all dogs, irrespective of route. Lowering of blood pressure and heart rate (after a transient blood pressure increase) was synchronized with sedation in these dogs. The overall clinical response with regard to cardiopulmonary effects in HW+ dogs was similar to that in HW- dogs.

The influence of the QT, TQ, and ST intervals, and heart score on both cardiac cycle duration (RR) and diastole/systole (D/S) quotient were analyzed during the neonatal (1 day and 5 days) pigs belonging to 2 crossbreeds of different rusticity, Landrace X Belgian White (LBW) and Landrace X Duroc Jersey (LDJ). Our findings indicate that the shortening of the RR interval in 5-day-old pigs of both crossbreeds was determined by different variables in each breed. In LDJ pigs, this shortening was only associated with a shortening of ventricular activation, and in each age group, the systole and the diastole contributed equally to the RR value. The D/S quotient did not differ significantly in 1-day-old vs 5-day-old pigs, and at both ages, the quotient was only determined by the TQ value. In LBW pigs, the RR, QT, TQ, and ST were shortened, but only the shortening of QT was significant as a result of an acceleration of the ventricular recuperation process. Moreover, differences were found between 1-day-old vs 5-day-old pigs with regard to the contribution of the different intervals to the RR duration. In 1-day-old pigs, the RR depended closely on the TQ, whereas in 5-day-old pigs, all intervals contributed significantly to its duration. The D/S quotient was not significantly different in 1-day-old vs 5-day-old pigs, but a different contribution of the variables studied was observed at the 2 ages selected. In 1-day-old pigs, D/S quotient depended on the diastole duration, whereas in 5-day-old pigs, the diastole and systole contributed to its variation.

We investigated the influence of parasympathetic tone on the arrhythmogenic dose of dobutamine in horses premedicated with xylazine, anesthetized with guaifenesin and thiamylal, and maintained on halothane in oxygen. Six horses were used in 12 randomized trials. In each trial, after end-tidal halothane concentration was stabilized at 1.1% (1.25 times minimum alveolar concentration [MAC]) in oxygen, either saline solution (0.02 ml/kg of body weight) or atropine (0.04 mg/kg) was administered IV. Five minutes later, dobutamine infusion was started at dosage of 2.5 micrograms/kg/min, IV. The dobutamine infusion was continued for 10 minutes, or until 4 or more premature ventricular complexes occurred within 15 seconds, or sustained narrow-complex tachyarrhythmia clearly not sinus in nature occurred. If the criteria for termination were not met, dobutamine infusion was increased by 2.5 micrograms/kg/min, after the hemodynamic variables had returned to baseline. The horses were allowed to recover, and were rested for at least 1 week before the second trial. The arrhythmogenic dose of dobutamine was calculated by multiplying the infusion rate by the elapsed time into infusion when arrhythmia occurred. There was significant difference between the arrhythmogenic dose of dobutamine (ADD) in saline-treated horses (mean +/- SEM, ADD 105.6 +/- 16.3 micrograms/kg) and atropinized horses (ADD 36.2 +/- 8.7 micrograms/kg). There were no differences in the prearrhythmia or immediate postarrhythmia ventricular heart rate (HR) or systolic (SAP), diastolic (DAP), or mean (MAP) arterial pressures between treated and control groups. The change in hemodynamic variables from prearrhythmia to immediate postarrhythmia formation was not different between the 2 groups. Ventricular beats were clearly evident in 8 of the 12 arrhythmias meeting the criteria for establishing the ADD. These results indicate that atropine may lower the arrhythmogenic threshold for dobutamine in halothane-anesthetized horses.

Objective: To determine whether withholding of food affects autonomic nervous system balance by analysis of heart rate (HR), HR variability (HRV), and frequency of second-degree atrioventricular block in horses. Animals: 5 healthy Thoroughbreds. Procedures: For two 24-hour periods in a crossover study, food was withheld from horses or horses were maintained on their regular feeding schedule (control conditions) in their stalls and Holter monitor ECG recordings were obtained. The ECGs were analyzed by use of fast-Fourier transformation, and power spectrum densities were calculated for low-frequency (0.01 to 0.07 Hz) and high-frequency (0.07 to 0.6 Hz) variations in HR. Serum cortisol and plasma ACTH, norepinephrine, and glucose concentrations were measured at predetermined time points. Results: Withholding of food resulted in significantly lower HR and more frequent second-degree atrioventricular block (the frequency of which was inversely related to the HR), compared with findings for control conditions. Circadian rhythms were similar during food-withholding and control conditions; peak HR was detected from 7:00 pm to 8:00 pm, and the lowest HR was detected in the early morning. During food-withholding conditions, the low-frequency and high-frequency components of HRV were significantly higher, and the low-frequency-to-high-frequency ratio was lower than during control conditions. Serum cortisol concentration was higher and plasma glucose concentration was lower at 6:00 pm in horses when food was withheld, compared with findings during control conditions. Conclusions and Clinical Relevance: Indices of HRV seemed to be sensitive to changes in autonomic nervous activity and may be useful as clinical indices of the neuroendocrine response to stressors in horses.

Mosapride stimulated dietary motility was introduced because of the arrhythmogenic effect of cisapride. Cisapride, 5-HT receptor agonist, induces prolongation of QT interval. Additionally, this condition can raise the possibility of acute, "malignant" arrhythmias such as torsade de pointes. It is hard to find any reports about effects of mosapride on cardiac parameters in dogs. By confirming electrocardiogram (ECG) parameters, the surface extremity leads ECG that was obtained from the four-limb electrodes and which was recorded by an ECG recorder after administration of mosapride 3 mg/kg PO b.i.d, and mosapride 3 mg/kg with itraconazole 5 mg/kg PO b.i.d, respectively. QT interval was shortened on the days of 3, 5, and post-day 1 in both mosapride 3 mg/kg administrated group and mosapride with itraconazole group. Heart rate increased significantly. QTc was slightly prolonged in mosapride administration group and mosapride with itraconazole group. However, all dogs of QTc were in normal variation (150~250 msec). Besides, the dogs showed no side effects reported in human medicine during the administration with these drugs. Although mosapride can increase the heart rate, this study suggest that mosapride may be useful for the dogs with disorders of gastrointestinal motility because of no fatal arrhythmogenic effect inspite of administration with itraconazole in dogs.

Objective: To determine whether standard manual frequency filters in the ON and OFF settings affected P-QRS-T voltages, discover whether recorded P-QRS-T voltages vary between commercial electrocardiographs, assess effects of frequency filters on base-line artifact, and evaluate ECG frequency content by high-fidelity recordings subjected to digital filters with variable frequencies. Design: Sequential 10-lead ECG were recorded in 30 cats, using 3 commercial electrocardiographs to assess effects of manual frequency filters on the P-QRS-T wave forms. Three clinically normal cats were evaluated for ECG frequency content. Animals: Thirty cats (13 with hypertrophic cardiomyopathy; 4 with restrictive cardiomyopathy; 3 hyperthyroid; 1 with ventricular septal defect; 1 with aortic stenosis; and 8 with no detectible cardiovascular disease). Three additional clinically normal cats were studied for effects of frequency filters on the ECG frequency content. Procedure: Ten-lead ECG were recorded on each cat by use of 3 commercial electrocardiographs sequentially. For each machine, a recording was made with manual filters ON, immediately followed by a recording with manual filters OFF. High-fidelity lead-II ECG recordings were made with filters set with their rolloff frequency at 0.1 Hz and 3.0 kHz; output voltage (0.2 mV/V) was fed to an analog-to-digital converter, then to attendant software, which sampled the signal at 6 kHz with a 12-bit sampler, and were digitally filtered at various corner frequencies. Results: Voltages recorded by all 3 electrocardiographs were greatest when filters were OFF (most prominent on R- and S-wave voltages). In all recorded leads, it-wave voltage was significantly greater when filters were OFF than ON. Comparison of voltages indicated significant (P < 0.05) differences between R-wave voltages recorded in all leads with manual filters ON, but not with filters OFF. With filters ON, each electrocardiograph produced a smaller percentage of recordings with moderate to severe baseline artifact than with filters OFF. R-Wave amplitudes of high-fidelity lead-II ECG were significantly decreased with digital filters set at corner frequencies < 150 Hz. Conclusion: Significant (P < 0.05) voltage attenuation was recorded by each of the 3 commercial electrocardiographs when frequency filters were ON, compared with OFF. Comparison of waveform voltages among electrocardiographs with filters ON indicated significant variation in R-wave amplitudes in all leads. With manual filters ON, each electrocardiograph recorded a smaller percentage of recordings with baseline artifact than with filters OFF. Substantial frequency components greater than or equal to 150 Hz are present in the feline ECG waveform. Thus, filters with frequencies < 150 Hz markedly attenuate the feline R wave. Clinical Relevance: Attenuation of feline ECG signals occurs with use of commercial electrocardiographs and varies greatly between manufacturers. This is attributable largely to internal manual frequency filters. These consequences may be important when applying standard feline reference values or when equivocal voltage measurements are recorded.