Prolonged PQ Interval: Causes and Significance

The PR interval (PQ) is the time from the onset of the P wave to the onset of the QRS complex, reflecting intra-atrial conduction, AV nodal delay, and conduction via the His-Purkinje system. Normal values are 120–200 ms in adults; prolongation (PR > 200 ms) is traditionally interpreted as first-degree AV block. In most cases, this finding is asymptomatic, but it requires a thorough assessment of the cause and level of block. [1]

Clinical significance is determined not so much by the absolute number as by the context: the cause, QRS width (a hint at the level of delay), symptoms, PR response to exercise/atropine, and comorbidities. Marked prolongation (usually PR ≥300 ms) can cause symptoms of "pacemaker-free pacemaker syndrome" due to severe atrioventricular mismatch. [2]

Although first-degree AV block was long considered benign, data from the Framingham Heart Study showed that PR prolongation is associated with an increased risk of atrial fibrillation, pacemaker implantation, and overall mortality. The risk varies individually and depends on age, comorbidities, QRS width, and the extent of conduction system fibrosis. [3]

Current guidelines emphasize: it's not the number that matters, but the patient. In asymptomatic cases of PR prolongation and the absence of signs of infra-His disease, observation and correction of reversible factors are indicated. In cases of symptoms, extremely long PR, progression to high-grade blocks, or special conditions, cardiac pacing is considered—preferably with "physiological" stimulation of the conduction system. [4]

Epidemiology

The prevalence of first-degree AV block in the general population ranges from ~0.7 to 3-4% in adults and increases with age; in endurance athletes, it can reach ~8-9% as a manifestation of increased vagal tone and adaptation to exercise. The sex ratio is skewed toward men and the elderly. [5]

According to the Framingham Heart Study (observation of >7,000 participants), prolongation of PR is associated with a doubling of the risk of atrial fibrillation, a threefold increase in the likelihood of pacemaker implantation, and increased overall mortality (odds ratio ≈1.4). This association persists after adjustment for traditional factors. [6]

Geographical studies (e.g., rural northeast China, n≈10,926) show a prevalence of about 3.4% in people ≥40 years old, with a male predominance; independent factors included age, height, systolic blood pressure, triglyceride levels, and low HDL. This highlights the influence of both neurovegetative regulation and metabolic determinants. [7]

In young people, the incidence is less than 2%, but in athletes and with increased vagal tone, PR prolongation is more common and is generally safe. However, evaluation for occult infra-His disease is necessary in cases of wide QRS or symptoms. [8]

Reasons

Physiological factors: increased vagal tone (sleep, exercise), young age - cause predominantly nodal (AV node) slowing. In these cases, PR usually shortens with exercise/atropine, indicating a benign nature. [9]

Medications: beta-blockers, non-dihydropyridine calcium channel blockers (verapamil, diltiazem), digoxin, antiarrhythmics (flecainide, sotalol, amiodarone), sedatives, and some anti-ischemic agents. Discontinuation or dose adjustment often normalizes PR. [10]

Structural/inflammatory: fibrosis and sclerosis of the conduction system with age, ischemia or infarction, myocarditis, infiltrative diseases (sarcoidosis, amyloidosis), postoperative changes; with a wide QRS, involvement of the infra-His level is more likely. [11]

Infectious/metabolic/other: Lyme disease (including transient AV block), electrolyte disturbances, hypothyroidism, intoxication. In some patients, PR is prolonged due to an increase in the duration of the P wave in interatrial block (atrial conduction), rather than true "nodal" inhibition. [12]

Risk factors

Age is the main non-modifiable factor reflecting the accumulation of fibrosis in the AV junction and His-Purkinje system; men and tall individuals have a longer PR on average. Hypertension and dyslipidemia are also associated with prolonged PR. [13]

Endurance-type sports activities increase vagal tone and prolong PR at rest, remaining reversible with detraining; this does not exclude pathology if there are symptoms, a wide QRS, or atypical dynamics during exercise. [14]

Drug combinations (eg, beta-blocker + verapamil/diltiazem, digoxin in the elderly) increase the risk of excessive slowing of AV conduction, especially in CKD and electrolyte shifts. [15]

Atrial myopathy (Bayesian atrial block) prolongs the P wave and may contribute to PR prolongation; it is also associated with atrial fibrillation and stroke. This impacts prognosis and surveillance strategy. [16]

Pathogenesis

PR includes the sum of: intra-atrial conduction (P duration), delay in the AV node, and conduction along the His-Purkinje system. Therefore, prolongation can occur at any level - from the atria to the infra-His system; the QRS width and the PR response to load help localize the zone of delay. [17]

AV nodal (supraglottic) slowing is the most common and usually benign: with increased sympathetic tone (exercise, atropine), PR shortens; with vagal activation, it lengthens. This is the "normal physiology" of the node. [18]

Infra-His delay (below the AV node) is more dangerous and is often associated with a wide QRS (branch block, fascicular blocks) and a reduced ability of the PR to contract under stress. This phenotype tends to progress to high-grade blocks. Electrophysiological study (EPS) is used to determine the level. [19]

Interatrial block (IAB) prolongs the P wave itself; in this case, the "long PR" is partly explained by prolonged atrial conduction rather than by true AV delay. The presence of IAB is associated with the risk of AF and outcomes—a marker of "atrial disease." [20]

Symptoms

Most often, there are no problems: most people with a PR of 200-240 ms experience no problems, and the finding is incidental during an ECG or Holter monitoring. In these cases, a search for the cause, clarification of the delay level, and dynamic observation are required. [21]

With a marked prolongation of PR (≈≥300 ms), “atrioventricular dyssynchrony” appears: the atria contract too early relative to the ventricles, which can cause weakness, fatigue, exercise intolerance, shortness of breath, dizziness, and rare presyncopes - a phenomenon described as “pacemaker syndrome without a pacemaker”. [22]

In some patients, symptoms only appear during exercise: normal physiology is shortening of the PR; if the PR is not shortened or is lengthened, pathological delay and decreased cardiac output during exertion are possible. This is a reason for stress testing with an ECG and, if necessary, an electrophysiological study. [23]

Associated complaints (chest pain, fainting, severe bradycardia) require the exclusion of ischemia, myocarditis, high-grade blocks and other dangerous causes, especially against the background of a wide QRS. [24]

Forms and stages

According to ECG classification, PR prolongation usually corresponds to first-degree AV block (each P wave is conducted, PR >200 ms). High-degree blocks (Mobitz I/II, 2:1, complete) are different conditions; however, a “very long PR” can functionally mimic a synchrony disorder similar in symptoms to pacemaker syndrome. [25]

According to the level of delay, the following are distinguished: 1) suprafascicular (AV node, often narrow QRS, benign dynamics); 2) intrafascicular/infrafascicular (wide QRS, tendency to progression). Clarification of the level is important for prognosis and indications for stimulation. [26]

According to the load dynamics: 1) physiological response (PR shortens) - nodal benign delay; 2) paradox (PR does not change/lengthens) - suspicion of infra-HIS pathology. This is taken into account during stress tests and selection for EPS. [27]

Separately - the influence of interatrial block: long P and PR in IAB + characteristic morphology of P (± in II, III, aVF) - a marker of “atrial disease” and the risk of AF/stroke. [28]

Table 1. What does “long PR” mean: levels and tips

Sign	Probable level of delay	Clinical thought
Narrow QRS, PR shortens with exercise/atropine	AV node (supraglottic)	Often benign, observation
Wide QRS, PR does not shorten, possible branch block	Infra-GIS	Risk of progression, consider EPS/stimulation
Long P wave, signs of IAB (P ± in II, III, aVF)	Intra-atrial delay	Atrial disease, risk of AF/stroke
PR ≥300 ms with symptoms	Severe AV delay	“Pacemaker syndrome without a pacemaker” is possible [29]

Complications and consequences

Long-term risks include a higher likelihood of atrial fibrillation, pacemaker implantation, and a moderately increased overall mortality in the population, as demonstrated by analyses of the Framingham study. This does not automatically indicate treatment but requires risk stratification. [30]

At PR ≥300 ms, a decrease in stroke volume is possible due to dyssynchrony, especially in patients with existing left ventricular dysfunction - symptoms appear, exercise tolerance worsens, and atrial and ventricular arrhythmias may become more frequent. [31]

Infra-His delay in the setting of a wide QRS is more likely to progress to high-grade block - an important argument for observation with monitoring, testing, and early discussion of pacing when symptoms occur.[32]

The presence of IAB and prolonged P is associated with recurrent AF and stroke, shifting the strategy towards active screening for AF and aggressive risk factor correction. [33]

Diagnostics

A 12-lead ECG confirms PR prolongation; QRS width/morphology, P wave (duration/morphology), and associated abnormalities (branch block, repolarization) are analyzed. This is the starting point for localization of the level of delay and primary stratification. [34]

Holter monitoring and long-term wearable recorders help capture PR dynamics/transitions to higher degrees of block, their relationship with symptoms, and nocturnal vagal influences. An exercise test (or atropine test) evaluates "physiology": in a healthy person, PR shortens with increasing heart rate. [35]

Laboratory - according to the clinic: electrolytes (K, Mg), thyroid function, ischemia markers in pain syndrome, in case of epidemiology/symptoms - tests for borreliosis. EchoCG - to assess structural pathology (EF, valves, hypertrophy). [36]

Electrophysiological examination is indicated selectively—in cases of suspected infra-His delay, unexplained syncope, or discrepancy between clinical and ECG findings. It accurately localizes the level of the block and helps decide on stimulation. [37]

Table 2. Minimal diagnostic algorithm for prolonged PR

Step	What are we doing?	For what
1	ECG: PR, QRS, P/IAB wave	Delay level (initial assessment)
2	Holter/patch, symptom diary	Relationship between symptoms and conductivity
3	Exercise/atropine (as indicated)	Node physiology vs. infra-GIS
4	EchoCG, laboratory	Structure/reversible causes
5	EFI (selective)	Precise localization, solutions for ECS [38]

Differential diagnosis

"Long PR" due to long P wave (interatrial block) vs. true AV delay: in IAB the P wave is long (≥120 ms) and often "biphasic" (±) in II, III, aVF; risk of AF/stroke is higher, AV node may be normal. This is an important distinction. [39]

Benign nodal slowing (narrow QRS, normal response to exercise) versus infra-His disease (wide QRS, paradoxical response, 2:1/Mobitz II episodes). When in doubt, an electrophysiological study is the solution. [40]

Vagus-mediated slowing during sinus bradycardia/sleep tests is usually transient; structural damage (ischemia/myocarditis/infiltration) is persistent, may be associated with elevated troponins/echo abnormalities. [41]

Artifacts and technical reasons (errors in distinguishing the onset of P/QRS, baseline jitter) - revision of the ECG in different leads and correct calibration help to avoid false hyperdiagnosis. [42]

Table 3. PR extension: “red flags” requiring in-depth assessment

Sign	Why is it important?
PR ≥300 ms, especially with symptoms	Risk of hemodynamic dyssynchrony (pacemaker syndrome)
Wide QRS and/or bundle branch block	More likely infra-His delay → risk of progression
Syncopation, presyncopation	A transient high-grade block is possible.
No PR shortening under load	Suspected pathological conduction below the node
IAB signs (long P, P ± in II, III, aVF)	Atrial disease, risk of AF/stroke [43]

Treatment

1) Correction of reversible causes. Review and, if possible, discontinue/reduce the doses of negatively chronotropic drugs; normalize electrolytes, thyroid function, treat ischemia/myocarditis/infections (e.g., Lyme disease). This is often sufficient. [44]

2) Observation tactics. Asymptomatic patients with a narrow QRS and moderate PR (200-240 ms) usually do not require invasive treatment; ECG/Holter monitoring, assessment of the response to exercise in case of doubt, correction of risk factors and medication audit are recommended. [45]

3) When to consider electrical pacing (EP). According to the ACC/AHA/HRS and ESC, the indications are individual: symptomatic severe AV delay (usually PR ≥300 ms) with proven causal relationship; progressive or infra-His conduction with risk of high-grade block; severe dyssynchrony in patients with CHF/wide QRS. In controversial cases, EPS helps. [46]

4) "Physiological" stimulation of the conduction system. When necessary, pacemakers increasingly use pacing of the His bundle or left bundle branch (conduction system pacing) to preserve natural ventricular activation and reduce the risk of pacing-induced cardiomyopathy compared to classical apical RV pacing. Indications are determined on an individual basis. [47]

Table 4. Indications for pacemaker insertion in patients with prolonged PR (summary of guidelines)

Scenario	Approach
Symptomatic atrioventricular delay (PR ≥300 ms) with proven causality	Consider ECS (class IIa-IIb according to context)
Infra-His delay with wide QRS and signs of progression/syncope	ECS is reasonable/indicated
Combination of CHF + prolonged PR + wide QRS	Discuss CRT/physiological stimulation
Asymptomatic moderate PR prolongation with narrow QRS	Observation, elimination of causes [48]

Table 5. How to understand whether PR prolongation is “benign”

Criterion	Benign	Needs attention
Symptoms	No	Yes (weakness, shortness of breath during exertion, presyncope)
QRS	Narrow	Wide/leg block
Load/atropine	PR is shortened	Does not shorten/lengthen
PR duration	200-240 ms	≥300 ms
Additionally	No IAB	IAB signs (long P, P ±) [49]

Table 6. Drugs that prolong PR and what to do with them

Class	Examples	Tactics
Beta blockers	Metoprolol, bisoprolol	Reduce dose/replace if symptoms occur/PR↑
Non-dihydropyridine CCBs	Verapamil, diltiazem	Avoid combinations with BB; ECG monitoring
Cardiac glycosides	Digoxin	Monitor levels and doses, especially in the elderly
Antiarrhythmics	Flecainide, sotalol, amiodarone	Review readings/levels, ECG monitoring [50]

Table 7. Stress/pharmacological tests: what the PR response tells us

Impact	Expected effect with nodal delay	If PR does not shorten/lengthen
Physical activity	Shortening of PR (sympathetic effect)	Suspected infra-HIS pathology
Atropine/isoproterenol	Acceleration of AV conduction	Same
Carotid massage (vagus)	Additional PR extension	Non-specific, but confirms vagal sensitivity of the node [51]

Prevention

Primary prevention involves controlling factors affecting conductivity: rational administration/combination of bradycardiac medications, correction of electrolytes and thyroid function, and monitoring of blood pressure and metabolic disorders. For athletes, this involves periodizing exercise and adequate recovery to reduce vagus-mediated bradycardia at rest. [52]

Secondary prevention after detection of prolonged PR - dynamic monitoring with ECG/Holter, if necessary, stress tests; early correction of drugs and reversible causes; active screening for atrial fibrillation in the presence of interatrial block and other markers of "atrial disease". [53]

Forecast

Most asymptomatic patients with a narrow QRS and moderate PR have a favorable prognosis: progression to high-grade blocks is rare, and the risk of complications is low when factors are controlled. Observation and medication adjustments are sufficient in the vast majority of cases. [54]

However, PR prolongation is not an "innocent" finding in any context: population data indicate an association with AF, pacing requirements, and overall mortality. The detection of an infra-His component, marked prolongation (≥300 ms), or symptoms warrants further evaluation and consideration of "physiological" pacing. [55]

FAQ

• How dangerous is “long PR” in itself?

Moderate prolongation without symptoms and with a narrow QRS is usually benign. However, in the long term, PR prolongation is associated with a risk of AF and pacemaker implantation – it is important to monitor and control the causes. [56]

• When is a pacemaker needed?

In case of symptomatic, pronounced AV delay (often PR ≥300 ms) with proven causality; in case of infra-His pathology with risk of progression; in combination with CHF and wide depolarization - according to ESC/ACC guidelines. [57]

• Is it possible to "shorten" PR without a pacemaker?

If the cause is medications, vagal influences, or reversible disorders, yes: we adjust medications and electrolytes and evaluate the dynamics under load. Structural infra-His delay without symptoms does not require "digital treatment." [58]

• Is "long PR" associated with stroke?

Directly - no, but through "atrial disease" (interatrial block) and the risk of AF - yes. In case of IAB/LA enlargement, screening for AF and aggressive control of stroke risk factors are advisable. [59]

• What's new in stimulation for this problem?

The proportion of “physiological” stimulation of the conduction system (bundle of His/left leg region) increases, allowing for the preservation of natural activation and a reduction in the risk of stimulation-induced dyssynchrony compared to the apical pancreas. [60]