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Atrial Fibrillation

Author: Cliff Mann / Editor: Jason Kendall / Codes: CAP25, HAP23 / Published: 24/10/2013 / Review Date: 24/10/2016

 

Jump to:Top | Introduction | Clinical Assessment | Management | Safety Issues and Pitfalls | References

Introduction

Atrial Fibrillation affects a substantial number of patients. The prevalence roughly doubles with each decade beyond 50 years. It affects 0.5% of the population aged 50 to 60 but 9% of the population aged 80 to 89 years.

The significance of the dysrhythmia is highlighted by the associated odds ratios for death. In men with AF in comparison to non AF controls the Odds Ratio for death is 1.5 and in women 1.9. Most of the excess mortality occurs early after the diagnosis of AF.

The management of atrial fibrillation is seldom entirely straightforward; the rate versus rhythm debate and the efficacy and safety of anticoagulation are just two examples of key decisions the clinician and patient must face. This situation is often compounded in patients presenting with AF to the Emergency Department as there may be uncertainty regarding onset time and relevance to the presenting complaint.

Definition

Atrial Fibrillation (AF) is an atrial tachydysrhythmia characterised by predominantly uncoordinated atrial activation with consequent deterioration of atrial mechanical function.

The p waves which represent depolarisation of the atria, are absent during atrial fibrillation and the heart rhythm is irregularly irregular.

The main goals of treatment of atrial fibrillation are to minimize circulatory instability or insufficiency and to prevent stroke.

Circulatory instability or insufficiency are normally managed by either a rate or rhythm control strategy. In an emergency, when circulatory collapse is imminent due to an unacceptably high ventricular rate immediate cardioversion may be indicated.

The risk of stroke must be assessed in all AF patients and treated accordingly.

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Clinical Assessment

(i) Assessment of time of rhythm onset

The most important ED determination in AF is the probable duration of the dysrhythmia.

The currently recognised classification of AF relates to the duration and persistence of the AF:

  • Paroxysmal AF: discrete episodes of AF between which there are episodes of sinus rhythm. As such this definition applies to patients who have episodes only every few years as well as those who have dozens of episodes per day. The key point is that each episode terminates spontaneously
  • Persistent AF: atrial fibrillation that persists unremittingly unless and until it is successfully treated
  • Permanent: atrial fibrillation resistant to therapy or in which no therapy is attempted.

Many of the patients who present to the Emergency Department in AF have had the condition diagnosed previously and been commenced on appropriate treatment, ie rate or rhythm control. However some will not previously been known to be in AF and will therefore require a full assessment with a view to commencing therapy.

Some patients present with what has often been called Fast AF. This is a misnomer since all patients in AF have chaotic atrial electrical activity with no discernible pattern, so the description fast which implies a contradistinction to slow is incorrect. The correct description is AF with a fast /slow / controlled ventricular response.

(ii) Assessment of precipitating events

There are many illnesses which may precipitate new atrial fibrillation or worsen the cardiovascular consequences of pre-existing AF. These include fever, myocardial infarction, pneumonia, and thyrotoxicosis.

Alcohol is also a common precipitant the so called holiday heart.(1)

Learning Bite

Where there are identifiable precipitants of atrial fibrillation they should be treated. Failure to do so will diminish the chances of correcting or controlling the atrial fibrillation. (Evidence grade A)

(iii) Assessment of Haemodynamic Consequence

Most patients with AF are cardiovascularly stable. Two uncommon groups of patients may become unstable as a consequence of the onset of AF.

  1. Those patients in whom adequate LV function is dependent upon the 15% of ventricular filling provided by atrial contraction i.e. patients with poor LV function
  2. Patients in whom the ventricular response to AF results in very high heart rates (>150 bpm). In such cases there is inadequate time for LV filling with a consequent reduction in LV output and this is exacerbated by a reduction in time in diastole with a consequent reduction in coronary blood flow. This may exacerbate ischaemia and further compromise LV function.

In both groups restoration of sinus rhythm assumes increased importance and prompt cardioversion may be indicated. However for patients in the second group rate reduction may be sufficient.

(iv) Assessment of Stroke Risk

The greatest morbidity and mortality arising from AF is caused by thromboembolic sequelae. It follows that assessment and reduction of this risk is the single most important aspect of the treatment of AF. Patients with uncomplicated AF over the age of 70 have a sixfold increase in their risk of CVA. The risk of stroke is further increased by associated comorbidities e.g. hypertension and diabetes. All patients presenting with atrial fibrillation should be subject to a formal stroke risk assessment and there are a number of scoring tools. The algorithm below shows the recommended stratification system contained within the NICE guidelines (Figure 2)(2).

Figure 2: NICE Guidance on stroke risk stratification(2)

Atrial_Fibrillation_stroke_risk

Basic science and pathophysiology

The primary pathological change seen in atrial fibrillation is progressive atrial fibrosis. This fibrosis is usually associated with dilation of the atria and can occur as a result of a number of mechanisms including valvular and ischaemic heart disease. As a result of the dilation and fibrosis the atria undergo physical and electrical remodeling. The longer AF is present the greater and more irreversible these changes become.

Investigation

In AF the normal sinus rhythm produced by regular discharges from the sino-atrial node are overwhelmed by the rapid irregularly irregular electrical discharges generated and propagated through abnormal atrial muscle. The areas most commonly implicated as the source of the dysrhythmia are the areas around the pulmonary veins.

All patients with suspected AF must undergo a physical examination, 12 lead ECG and a blood sample sent for routine haematological and biochemical analysis.

The hall-mark feature of the ECG of a patient in AF is the presence of an irregularly irregular rhythm as shown on the ECG in Figure 1.

Figure 1: 12 lead ECG of AF note irregularly irregular rhythm and lack of p waves

Atrial Fibrillation_lead_ECG

Differential Diagnosis

The differential diagnoses are atrial flutter with variable block and multifocal atrial tachycardia.

Figure 3: Atrial flutter with variable block note the characteristic saw-tooth p waves often seen best in lead V1

Atrial Fibrillation_multifocal

Figure 4: Multifocal atrial tachycardia most commonly seen in association with severe pulmonary disease.

The key features are an atrial rate in excess of 100bpm and at least 3 morphologically different non-sinus p waves in the same ECG lead

Atrial Fibrillation_Multifocal_atrial_tachycardia

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Management

In the ED the approach to the patient in AF is determined primarily by the duration of the rhythm. In patients in whom the time of AF onset can be determined to be within 48hrs of presentation restoration of sinus rhythm is both safe and desirable. If sinus rhythm is restored patients will not require anticoagulation or further in-hospital intervention.

There is no robust evidence on which to base the choice of antidysrhythmic agent. Whilst DC cardioversion remains popular in the UK, pharmacological cardioversion with a variety of agents is established practice in many centres. Flecainide has the highest success rate with respect to restoration of sinus rhythm and is significantly quicker in onset than amiodarone. However it is recommended only in patients without evidence of ischaemic heart disease or left ventricular dysfunction. The following diagram highlights (in blue shading) the relevant sections of the algorithm published by the National Institute of Clinical Evidence(2) (see Figure 5).

Figure 5: Cardioversion Treatment Algorithm(2)

Atrial_Fibrillation_cardioversion_treatment

After 48hrs there is ample evidence that restoration of sinus rhythm risks dislodging thrombi from the left atrial appendage with the consequent risk of thromboembolic events including stroke.

In patients presenting after 48 hours of symptom onset or where there is doubt as to when the AF began the ED approach is limited to determining the stroke risk and controlling the ventricular rate (see Figure 6)(2). Subsequently it is necessary to decide whether the long term strategy should be one of rate or rhythm control.

Rate control, slowing the heart, increases the proportion of time and the amount of time spent in diastole thereby increasing the time available for coronary blood flow. Similarly, by increasing the time spent in diastolic filling of the ventricles, the cardiac output is increased.

Figure 6: Rate control algorithm for patients in permanent (and some persistent) AF:(2)

Atrial_Fibrillation_rate_control_treatment

Rhythm control produces the same benefits via the same mechanisms but also restores atrial transport and in doing so further increases diastolic filling of the ventricles. Furthermore the maintenance of sinus rhythm reduces the problems of atrial re-modelling.

Rhythm control should be considered in all patients with persistent AF in whom there are no contra-indications. In most circumstances relevant to the ED physician this will require commencement of anticoagulation and referral to cardiology for an elective interval (6 weeks) cardioversion. During this interval it may be necessary to control the ventricular rate with a beta blocker or calcium antagonist.

Although counterintuitive, for many patients restoring sinus rhythm has not been shown to provide morbidity or mortality benefits.(3,4) In particular patients over 65 have a significantly lower all cause mortality with a rate control strategy compared with a rhythm control strategy (Evidence Class A).

A rhythm control strategy is most appropriate in the following patient groups:(3-6)

  • Those who are symptomatic
  • Younger patients
  • Those presenting for the first time with lone atrial fibrillation
  • Those with atrial fibrillation secondary to a treated / corrected precipitant
  • Those with congestive heart failure.

Learning Bite

No study has found rate control to be inferior to rhythm control (Evidence grade A)

Haemodynamically unstable AF

Rarely but importantly patients may present with haemodynamic compromise secondary to AF. Such instability due to AF rarely occurs unless the ventricular rate remains above 150 bpm for prolonged periods. It may be associated with hypotension or ischaemic chest pain. In such cases unless the AF is known to be long standing DC cardioversion is the safest and most effective treatment option(2) (See figure 7). Where the AF is longstanding the choice of agent will be governed by the presenting features. Where the main feature is ischaemic (presumably rate related) chest pain a B blocker is the most rational choice, where the main feature is hypotension a B blocker is contraindicated and therefore amiodarone would be a more rational choice.

In patients with an accessory pathway (e.g. WPW syndrome), the onset of AF may bring about profound hypotension. This occurs because the accessory pathway can conduct the fibrillation waves much more rapidly than the AV node. Nodal conduction is limited to approximately 180bpm whereas in patients with accessory pathways the rate may rise to almost 300 bpm. At such rates there is virtually no time for ventricular filling with a consequent precipitous fall in cardiac output. It is for this reason that AV nodal blocking agents should never be given to patients in AF where an accessory pathway is suspected.

Figure 7: Treatment algorithm for patients with haemodynamic instability secondary to AF(2)

Atrial Fibrillation_haemodynamic_instability

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Safety Issues and Pitfalls

Mis-diagnosis of Atrial Fibrillation:

  • Where doubt exists a long rhythm strip is required.
  • Remember the faster the rate the more regular AF will look
  • AF is always irregularly irregular
  • AF may co-exist with bundle branch block
  • The possibility of Multi-focal atrial tachycardia and Atrial flutter should always be considered

Mis-treatment of Atrial Fibrillation:

  • Digoxin does not sufficiently limit AV nodal conduction when there is significant sympathetic discharge It should only be used as monotherapy in sedentary patients
  • The greatest risk arising from AF is thromboembolic complications including stroke. Failure to assess and treat the stroke risk is a significant error
  • Patients with accessory pathways are at risk of sudden death if AF occurs. This risk is increased if the AV node is blocked. Therefore any patient with a known or suspected accessory pathway and an irregular rhythm should not be treated with any AV blocking agent including adenosine and B blockers

References

  1. Lowenstein SR, Gabow PA, Cramer J, et al. The role of alcohol in new onset atrial fibrillation. Arch Intern Med 1983;143(10):1882-5.
  2. The management of atrial fibrillation. National Institute of Health and Clinical Excellence Clinical Guideline 36. London. June 2006.
  3. Wyse DG, Waldo AL, DiMarco JP, et al. A comparison of rate versus rhythm control in patients with atrial fibrillation. N Engl J Med 2002;347:1825-33.
  4. Van Gelder IC, Hagens VE, Bosker HA, et al. A comparison of rate control and rhythm control in patient with recurrent persistent atrial fibrillation. N Engl J Med 2002;347(23);1834-40.
  5. Carlson J, Miketic S, Windeler J et al. randomized trial of rate-control versus rhythm-control in persistent atrial fibrillation: the Strategies of Treatment of Atrial Fibrillation (STAF) Study. Journal of the American College of Cardiology. 2003;41(10):1690-6.
  6. Gronefeld GC, Lilienthal J, Kuck KH, et al. Impact of rate versus rhythm control on quality of life in patients with persistent atrial fibrillation. Results from a prospective randomised study. Eur Heart J 2003;24;1430-6.

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