Electrophysiological study (EPS) is a useful test in the evaluation of the cardiac conduction system. ^{1 , 2} Moreover, it has an additional role in investigating symptoms such as palpitations, lipothymia and syncope, especially in established structural heart disease or when electrocardiogram (ECG) shows abnormalities that suggest an arrhythmic cause for these symptoms. ³

A normal EPS is characterized by the following: no triggering of sustained arrhythmias (with clinical repercussion), no evidence of accessory pathway or ectopic focus, and sinus and nodal functions presenting expected normal responses as defined by the literature. In certain situations, patients with a normal EPS have a better prognosis regarding mortality. ⁴ Thus, knowledge of the normal, or reference intervals is fundamental to the interpretation of EPS results for each patient. Normal intervals should be established from a healthy population and address pathological implications of values out of these ranges. ⁵ The PA interval evaluates the conduction time of depolarization in the right atrium, from the sinus node to the atrioventricular (AV) node. The AH interval determines the conduction time within the compact AV node to the His bundle. The His-ventricular (HV) interval estimates the conduction time through the His-Purkinje system.

Several studies on cardiac electrophysiology have sought to establish a consensus of values of electrical stimulation conduction that would be associated with a higher risk of adverse events. ^{6 , 7} However, these values have not been established from systematic sampling yet, and the reference values commonly used were those obtained in studies published in the 60’s and 70’s. ⁸ No studies including diverse populations have been conducted and so far, no study has been performed in Brazil.

The aim of the present study was to establish the pattern of normal intracavitary values by EPS in the Brazilian population.

We evaluated the first 1,500 patients from 7,090 cases in our service. Of these, 124 had EPS considered normal, that is, no changes in the conduction system or evidence of sinus node dysfunction ( Figure 1 ). From the cases initially considered normal, three were excluded from analysis due to lack of clinical or demographic data. In the analysis of the pattern of normal intracavitary values - PA, AH, HV -, we identified those patients whose test results within the expected range. Outlying HV values were reviewed by one experienced electrophysiologist and then another four patients, with significant conduction disturbance and / or HV > 70 ms, were excluded.

Figure 1

Characteristics of the individuals included in the study are summarized in Table 1 .

Table 1

	n = 124	(%)
Male sex	63	50.8
Average age in years	52 ± 21
EPS indication
Pathway system rating	85	0.8
Syncope	15	12.1
Evaluate sinus function	10	8
Supraventricular tachycardia Hx	2	1.6
Hx PCR	2	1.6
Pre-excitation on ECG	2	1.6
Vertigo	1	0.8
Palpitations	1	0.8
Atrial flutter Hx	1	0.8
Hx of ventricular tachycardia	1	0.8
Other	4	4

EPS: electrophysiological study; Hx: previous history; CRP: cardiopulmonary arrest; ECG: electrocardiogram.

The normal patterns of intracavitary values used by the ICFUC electrophysiology service are based on international parameters, which were described and compiled between the 1960s and 1980s ( Table 3 ). ^{10 , 13 - 18} These studies have limitations, since intracavitary values were obtained not only from healthy patients but also from patients with some type of AV block or infra-Hisian block. Therefore, it is important to analyze the results with caution, not only to establish the patterns of our own population, but also to reevaluate the pattern of normality, considering that the studies were conducted decades ago.

Table 3

Laboratory	PA	AH	HV
Narula	25-60	50-120	35-45
Damato	24-45	60-140	30-55
Castellanos	20-50	50-120	25-55
Schulenburg	85-150	35-55	-
Peuch	30-55	45-100	35-55
Beckeit	10-50	50-125	35-45
Rosen	9-45	54-130	31-55
Josephson	9-45	54-130	31-55
IC -FUC	18-26	76-114	40-50

PA interval defined as the interval from the onset of the P wave to the first atrial deflection recorded on the HIS bundle electrocardiogram; the AH interval determined the conduction time within the compact AV node to the His bundle; the HV interval of the HIS bundle was measured from the onset of ventricular depolarization by ECG or local EC.

The prevalence of cardiovascular diseases increases with age. ¹⁰ The cardiac conduction system is affected by the increase in elastic and collagenous tissues associated with aging. There is more than 90% reduction in the number of cells with automatism by the age of 75 years. Age-associated calcification may affect the conduction system and increase the prevalence of atrioventricular block. ¹¹ The PR interval, a marker of atrioventricular conduction, increases from 159 ms (on average) at 20-35 years of age to 172 ms at 60 years of age. ¹² We believe that the differences in AH and in HV intervals between age groups observed in our study reflect these changes ( Figure 3 ).

Figure 3

Clinically, changes in conduction intervals should be interpreted in the context of the symptoms. ¹³ Shorter PA and AH ranges could reflect increased adrenergic tone or supranormal conduction. Shorter HV intervals generally represent the presence of accessory pathway. HV values are useful in the evaluation of patients with syncope and have a moderate accuracy to predict future occurrence of total atrioventricular block when the values exceed 70 ms. ¹⁴

The present study has some limitations. First, the study made a retrospective analysis of a prospectively collected data, and thus did not have a uniform methodology or adequate review; second, it presented data from a single center, with a single team, which increases the reliability of the measurements, but decreases the external validity of the results; third, the study did not evaluate the presence of comorbidities and the use or not of drugs; also, it did not present a multivariate analysis of possible independent factors that could influence interval values. Finally, all patients included had an indication for electrophysiological study with or without ablation regardless of participation in the research, that is, they did not necessarily represent a healthy population. This may have influenced the values considered “normal”.

This study showed that intracavitary conduction intervals in a sample of the Brazilian population were similar to previously published studies. Elderly patients tend to have higher values of intracavitary conduction intervals in EPS. However, it was not possible to infer from this study whether there is an association between the number of existing comorbidities and the increase in conduction intervals. Future studies including broader age ranges could enable the acquisition of more reliable and reproducible reference values.