Tachycardia

Articolo redatto e revisionato del Dott. Luca Moretti

Tachycardia is an increase in heart rate relative to the value expected for age and physiological condition. In resting adults, a threshold of 100 beats per minute is conventionally used, but exceeding it does not by itself identify a rhythm disorder. Exercise, fever, anemia and reduced circulating volume may appropriately accelerate the sinus node; a re-entry circuit or an ectopic focus may instead impose a rate that is inappropriate for the body's needs. This distinction changes both the clinical significance and the treatment.

The assessment must answer different questions: which structure generates the impulse, how it is conducted to the ventricles, what is causing the acceleration and whether the rhythm is compromising the circulation. A high heart rate may be the primary problem or a sign of another urgent condition. This monograph develops the cross-cutting clinical reasoning used for tachycardias; details of individual arrhythmias are addressed on their dedicated pages, beginning with sinus tachycardia, supraventricular tachycardias and ventricular tachycardia.

Functional definition and consequences of an increased heart rate

The rate detected at the pulse corresponds to effective peripheral pulsations, whereas the ECG records electrical activations. In the presence of premature beats or contractions with poor filling, a pulse deficit may occur: the palpable rate is lower than the electrical rate. Optical sensors and wearable devices may also be inaccurate during movement or irregular rhythms. Before assigning significance to an isolated value, its reliability, duration and relationship with posture, activity, temperature and symptoms must therefore be verified.

The atrial rate and ventricular rate may differ. Atrial flutter with 2:1 atrioventricular conduction may produce a regular pulse of approximately 150 beats per minute despite a much faster atrial rate. Atrial tachycardia with variable block may generate ventricular irregularity. The number reported by a monitor therefore does not describe the circuit and cannot distinguish sinus tachycardia from an organized arrhythmia; analysis of atrial waves remains essential.

An increased heart rate mainly shortens diastole. Ventricular filling may decrease, while myocardial oxygen consumption increases and the time available for coronary perfusion becomes shorter. The effect depends on diastolic function, contractility, blood pressure and coronary reserve. A healthy young person may tolerate a rate that causes ischemia or pulmonary edema in a patient with heart disease. Severity therefore cannot be inferred from a single numerical threshold that applies to everyone.

Atrioventricular synchrony further modifies tolerance. Atrial contraction contributes to filling and may become particularly important in stiff ventricles. When the atria and ventricles contract at unfavorable times, cardiac output may fall even without an extreme heart rate. Irregularity adds beat-to-beat variation in filling. Assessment of hemodynamic harm therefore requires consideration of rhythm, chamber relationship and substrate, in addition to rate alone.

The sinus response to hypovolemia or reduced vascular resistance may temporarily support cardiac output. Lowering the heart rate without correcting the problem may worsen perfusion. Conversely, a persistent tachyarrhythmia may itself cause ventricular dysfunction, creating a cycle of mutual deterioration. The first clinical task is to distinguish a compensatory response from an autonomously pathological rhythm, while recognizing that both components may coexist in the same patient.

Systemic causes and mechanisms of tachyarrhythmias

Sinus tachycardia accompanies exercise, pain, anxiety, fever and adrenergic stimulation, but it may also indicate anemia, hypoxemia, dehydration, hemorrhage, severe infection or thyrotoxicosis. Dyspnea and tachycardia may warrant evaluation for pulmonary embolism according to clinical probability. An emotional explanation should not be adopted merely because the patient is young or reports agitation: the perception of tachycardia may itself cause anxiety without clarifying its origin.

Drugs and substances may accelerate sinus rhythm or promote true arrhythmias. Beta-agonists, sympathomimetics, excess thyroid hormone and some recreational substances are relevant examples. Other medications alter conduction or repolarization, predisposing to ventricular tachycardias in the presence of interactions or electrolyte disturbances. The review should include dosages, recent changes, renal function and non-prescription products, without treating every exposure as equivalent in terms of risk.

Enhanced automaticity consists of faster spontaneous impulse generation by a pacemaker. It may involve the sinus node or ectopic atrial, junctional and ventricular structures. Gradual acceleration and deceleration may sometimes point to this mechanism, but they are not an absolute criterion. A persistent focus may impose a high rate for many hours and be clinically relevant even without abruptly paroxysmal episodes.

Triggered activity results from oscillations of the membrane potential following a previous activation. Early afterdepolarizations are important in some conditions with prolonged repolarization; delayed afterdepolarizations may depend on intracellular calcium overload and adrenergic stimulation. Re-entry, by contrast, requires a pathway in which the impulse recirculates and encounters tissue that has become excitable again. Nodal pathways, accessory connections, atrial tissue or ventricular scars may form different substrates for the circuit.

A sudden onset often suggests a re-entry mechanism, but the history and subjective perception are not sufficient to prove it. Likewise, the absence of known heart disease does not exclude idiopathic ventricular tachycardia or a channelopathy. Age, previous procedures, ischemia and family history modify the probability of the various mechanisms. Clinical classification should remain consistent with the level of documentation, reserving a precise diagnosis for cases in which the ECG or electrophysiological study supports it.

ECG interpretation and initial differential diagnosis

The ECG approach combines QRS width, regularity, atrial activity and the temporal relationship between atria and ventricles. A QRS duration below 120 ms suggests activation through the His-Purkinje system, although some specific ventricular tachycardias may be relatively narrow. A wide QRS may result from a ventricular origin, pre-existing bundle branch block, functional aberrancy or pre-excitation. Width structures the diagnostic reasoning but does not by itself identify the site of the arrhythmia.

In regular narrow-QRS tachycardia, sinus tachycardia, focal atrial tachycardia, flutter with fixed conduction, atrioventricular nodal re-entry and atrioventricular re-entry are considered. The search for P waves includes the terminal portions of the QRS and the ST segment, where retrograde atrial activity may be hidden. The relationship between RP and PR intervals helps narrow the possibilities, but overlap and atypical conduction prevent it from becoming an infallible rule. Tracings of onset and termination may be particularly informative.

An irregular narrow-complex rhythm points toward atrial fibrillation, atrial flutter or atrial tachycardia with variable conduction, and multifocal atrial tachycardia. Distinguishable P waves with different morphologies and variable intervals require careful interpretation rather than reliance on irregularity alone. Tremor artifacts may mimic fibrillatory activity; it is useful to look for normal complexes passing through the artifact and to compare multiple leads. Automated device interpretation must be verified on the original signal.

Regular wide-QRS tachycardia should raise primary consideration of a ventricular origin, especially in the presence of previous myocardial infarction or structural heart disease. Atrioventricular dissociation, capture beats and fusion beats are important findings when correctly documented. Morphology, axis and comparison with the baseline ECG complete the analysis, but no algorithm excludes every exception. Good hemodynamic tolerance does not prove a supraventricular origin; diagnostic error may lead to drugs that are inappropriate for the actual substrate.

Irregular wide-complex tachycardia requires differentiation among atrial fibrillation with aberrancy, pre-excited atrial fibrillation and polymorphic ventricular tachycardia. Marked variability of the complexes and very rapid rates may suggest abnormal conduction over an accessory pathway. In this setting, indiscriminate administration of drugs that block the atrioventricular node may be dangerous. The polymorphic form also requires assessment of the preceding QT interval and possible ischemia because treatment changes according to the mechanism.

Clinical assessment and selection of diagnostic tests

The immediate assessment looks for hypotension with hypoperfusion, altered mental status, syncope, ischemia and acute heart failure. It is necessary to establish whether these manifestations are caused by the tachyarrhythmia or by the condition that accelerated sinus rhythm. A patient with sepsis may be unstable with sinus tachycardia without benefiting from cardioversion; a re-entrant tachycardia may instead rapidly precipitate the same instability. The causal relationship guides intervention, together with the need to support breathing and circulation.

The history reconstructs onset, duration, perceived regularity, frequency of episodes and mode of termination. Posture, activity, meals, fever and recent exposures may provide clues. Heart disease, previous procedures, prior syncope and family history of sudden death are documented. A photograph of a monitor may help, but a twelve-lead ECG during the event is preferable when it can be obtained without delaying treatment. The baseline tracing after conversion may reveal pre-excitation or repolarization abnormalities.

Ambulatory monitoring is selected according to symptom frequency. A short Holter recording may be useful for daily events, whereas sporadic episodes require more prolonged recording or patient-activated devices; an implantable monitor is considered when appropriate. The result should establish a temporal correlation with the symptom rather than merely list maximum heart rates. A negative recording obtained in the absence of the usual symptoms does not exclude their possible arrhythmic origin.

Complete blood count, electrolytes, renal function, thyroid tests and other investigations are selected according to the presentation. Troponin and biomarkers require interpretation in context: prolonged tachycardia may be associated with myocardial injury without automatically demonstrating an acute coronary occlusion. Echocardiography defines cardiac function and structure; magnetic resonance imaging, coronary assessment and investigations for extracardiac causes are added when there is a pertinent clinical question. An indiscriminate battery of tests does not replace the assessment of clinical probability.

Orthostatic tachycardia requires assessment of the response to standing, symptom duration and secondary causes. Inappropriate sinus tachycardia is a specific diagnosis made after adequate explanations have been excluded, not a label to apply to every elevated resting heart rate. Electrophysiological study is useful when a circuit needs to be clarified or treated, whereas genetic testing concerns selected suspicion of inherited disease rather than all tachycardias.

Acute treatment and rhythm-control strategies

In a tachyarrhythmia with a pulse that is causing instability, synchronized cardioversion is indicated, with sedation when possible without dangerous delay. Synchronization to ventricular depolarization reduces the risk of delivering the shock during the vulnerable phase. If there is no pulse, management follows the cardiac arrest and shockable-rhythm pathway. In polymorphic forms, synchronization may be unreliable and urgent electrical treatment follows the specific indications. Initial stability and rhythm must be reassessed throughout every intervention.

For a stable, regular narrow-complex tachycardia, appropriate vagal maneuvers may terminate circuits that depend on the atrioventricular node. Adenosine may terminate some arrhythmias or reveal atrial activity through transient conduction block; failure to convert does not by itself identify the diagnosis. Indications, contraindications and availability of monitoring must be respected. In wide-complex tachycardias, its possible use requires a selected setting and appropriate expertise and does not extend to irregular pre-excited forms.

Appropriate sinus tachycardia is treated by addressing the underlying cause: oxygenation, circulating volume, pain, fever, anemia or another responsible condition. A negative chronotropic drug may be useful in selected situations but must not mask a necessary compensatory response. In supraventricular tachycardias, the choice between rate control, conversion and prevention of recurrence depends on the mechanism. Ventricular function, blood pressure, conduction and possible interactions limit the interchangeability of drugs.

In atrial fibrillation and flutter, the strategy also includes episode duration and thromboembolic risk. The need for anticoagulation does not arise from the high rate itself and does not automatically apply to every tachycardia. Planned cardioversion requires the assessment appropriate to the arrhythmia and its context; hemodynamic urgency changes the timing while maintaining appropriate management of thromboembolic protection. Simple restoration of sinus rhythm does not necessarily complete this part of treatment.

Catheter ablation may provide durable treatment for nodal circuits, accessory pathways and selected foci. In ventricular arrhythmias, its role depends on the substrate and therapeutic goals, often together with drugs and a defibrillator. The choice is based not only on the number of episodes but also on symptoms, risk, tolerability and likelihood of success. An antiarrhythmic drug that is effective in one form may be contraindicated in another: diagnosis of the mechanism precedes a stable pharmacological strategy.

Prognosis, arrhythmia-induced cardiomyopathy and follow-up

The prognosis of tachycardia ranges from a normal response to exercise to a manifestation of severe heart disease. Rate, duration and recurrences must be interpreted together with the substrate. A brief episode is not automatically benign when associated with syncope or inherited disease; a moderate sinus tachycardia may instead be appropriate but signal an important systemic condition. The prognostic objective is not to assign risk to the number of beats but to the diagnosis that explains it.

Arrhythmia-induced cardiomyopathy may develop with persistent or very frequent rhythms, even when the patient does not perceive marked palpitations. The relationship is often recognized by improvement in function after effective rate or rhythm control. Partial recovery may indicate coexistence of primary heart disease. Surveillance should therefore assess both the arrhythmia and ventricular function, without assuming that initial normalization of heart rate has eliminated every consequence.

Prevention of recurrence includes adherence, correction of facilitating factors and reassessment of exposures. There is no universal list of substances that must be prohibited in every patient: timing, amount and the specific disease determine recommendations. Physical activity and return to work should be adapted to the diagnosis and the degree of control achieved. Generic restrictions may be unnecessarily limiting, whereas a high-risk phenotype requires precise and verifiable recommendations.

Follow-up assesses drug efficacy and safety, any changes in conduction or the QT interval, and procedural results. Episodes recorded by devices should be confirmed when signal quality is uncertain. New symptoms, a different morphology or loss of the previous therapeutic response may indicate a change in substrate. A previous diagnosis does not justify automatically attributing every subsequent palpitation to the same arrhythmia, especially when the clinical presentation changes.

The patient should know which episodes to document and which manifestations require urgent assessment, such as syncope, persistent chest pain, significant dyspnea or marked weakness associated with a rapid rhythm. A shared plan may include recording methods and how to access the treatment center, in addition to any maneuvers already taught. Continuity of management helps avoid both repeated therapies without a diagnosis and underestimation of signs that modify risk.