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Electrophysiological investigation
INVESTIGATIONS
Electrophysiological investigation
Arrhythmias often present with palpitations; however they can also present non-specifically with symptoms of breathlessness, tiredness, chest pain, dizziness and syncope. The investigation of an arrhythmia needs to define whether the rhythm disturbance is relevant to the patient’s symptoms and to identify the underlying cause and the electrophysiological mechanism; only then can one specify the most suitable treatment.
Martina Muggenthaler Razeen Gopal Richard Schilling
Ambulatory electrocardiography (AECG) AECG allows the recording of an ECG over a prolonged period of time and is the main tool used to decide whether an arrhythmia is the cause of the patient’s symptoms; it may also give details about the mechanism and therefore the best treatment. Less frequently, AECG can also be used for risk stratification, particularly in ischaemic heart disease, the detection of myocardial ischaemia, the monitoring of drug treatment and the evaluation of pacemaker function (Figure 1).
Nicholas Gall
Abstract Cardiac electrophysiology is the subspecialty of cardiology dealing with heart rhythm disorders, particularly the investigation and management of brady- and tachyarrhythmias, syncope and the prevention of sudden cardiac death. Recent years have seen significant developments in our understanding of arrhythmias. Recent technological developments include ever smaller and more complex implantable pacing devices, the ability to map an arrhythmia circuit in three dimensions, potentially in one beat and to destroy small areas of myocardium safely known to be critical to a particular arrhythmia. These have led to a dramatic expansion in our ability to treat heart rhythm problems. This article will focus on the diagnostic tools which can be used to investigate these conditions and its sister article will deal with some of the treatment strategies.
Selection of recording technique: the frequency and characteristics of the patient’s symptoms will dictate the choice of recording technique. Continuous ECG recording can be achieved throughout the day and night up to 7 days depending on the digital memory of the device used. Three leads are typically recorded but 12 lead versions are also available for 24e48 h and are used in selected cases where detailed ECG morphology may be important (e.g. defining ventricular ectopic morphology or investigating biventricular pacemaker function). These continuous recording techniques are particularly useful if symptoms occur on most days or if the patient is incapacitated by their symptoms, for example when an arrhythmia leads to syncope. Patients can mark events on the recording and should always keep a diary of their symptoms and activities to allow the correlation of symptoms and heart rhythm. With less frequent symptoms there are two alternatives. Event recorders record and store a brief period of ECG when activated by the patient. They are either attached via ECG leads or can be carried in a pocket and held to the chest wall at the time of symptoms. These devices can be used for weeks or months at a time and are useful for patients with infrequent symptoms provided the patient is able to activate the recorder at the time of symptoms.
Keywords ablation; arrhythmia; ECG; electrophysiological; Holter; palpitation
Non-invasive electrophysiology Electrophysiology is the subspecialty of cardiology dealing with heart rhythm disorders; it is a growing area because of technological developments and our developing understanding of arrhythmia mechanisms. The specialty involves the investigation and treatment of both brady- and tachyarrhythmias, including ablation and device therapy. The prediction and prevention of sudden cardiac death also constitute a major focus. This article will deal with invasive and non-invasive methods of investigation and its sister article will deal with the treatment of tachyarrhythmias. Other articles in this series will deal with device therapy.
Martina Muggenthaler MRCP is a Specialist Registrar in Cardiology and Electrophysiology, King’s College Hospital, London, UK. Competing interests: none declared. Razeen Gopal MBChB MMed (US) is a Cardiac Electrophysiology Fellow, London, UK. Competing interests: none declared. Richard Schilling MD FRCP is a Professor of Cardiology and Electrophysiology, St Bartholomew’s Hospital and Queen Mary University, London, UK. Competing interests: none declared.
Figure 1 Holter monitors provide a considerable amount of information, including the heart rate profile, the frequency of ectopics beats and more complex arrhythmias in addition to providing symptom-rhythm correlation. The heart rate trend is shown providing information about rate control in atrial flutter in this case. The bottom graph gives an idea of ventricular ectopic frequency.
Nicholas Gall MSc MD FRCP is Consultant Cardiologist and Cardiac Electrophysiologist, King’s College Hospital, and Honorary Senior Lecturer, King’s College London, UK. Competing interests: none declared.
MEDICINE 38:7
367
Ó 2010 Elsevier Ltd. All rights reserved.
INVESTIGATIONS
Loop recorders continuously record the ECG but only store a few minutes, which is constantly updated. When activated, the stored ECG is fixed in a separate memory for later downloading; several separate events can be stored in this manner. As a rule ECG before and after device activation is stored to ensure that the whole event is captured, particularly in cases where symptoms are brief or associated with incapacity. Loop recorders are either attached to the patient or can be implanted subcutaneously for prolonged monitoring. Patients activate the device in response to symptoms. Implanted devices have a battery life of up to three years and can also be programmed to store the ECG automatically (e.g. with heart rates above or below a defined cut-off). Implantable devices of the future will have better arrhythmia detection and may also be able to measure other physiological parameters, such as blood pressure and movement.
Indications for AECG to assess symptoms possibly related to rhythm disturbances Definite uses: 1. Patients with unexplained syncope, near syncope, or episodic dizziness in whom the cause is not obvious. 2. Patients with unexplained recurrent palpitation. Possible uses: 1. Patients with episodic shortness of breath, chest pain or fatigue that is not otherwise explained. 2. Patients with neurological events when transient atrial fibrillation or flutter is suspected. 3. Patients with symptoms such as syncope, dizziness, or palpitation in whom a probable cause other than an arrhythmia has been identified but in whom symptoms persist despite treatment of this other cause.
Normal findings during AECG monitoring: during a 24-h period the heart rate of a healthy person varies in response to autonomic influences, with a reduction in heart rate at night to below 50 beats per minute in some people but this is usually of little relevance when it causes no symptoms. Indeed, many asymptomatic abnormalities may be observed which have little significance. For instance, supraventricular and ventricular ectopics are frequently noted in healthy people; AV nodal Wenckebach overnight can also be normal in young people as a result of high vagal tone. However, very frequent ventricular ectopy or signs of higher grade AV block, particularly at times of low vagal tone should warrant further consideration.
Table 1
symptoms, such as the appearance of delta waves indicative of an accessory pathway, the likely cause of a supraventricular tachycardia. Assessment of risk in patients without symptoms of arrhythmia (Table 2): patients who have survived a myocardial infarction are at increased risk of sudden cardiac death, usually because of ventricular arrhythmia from a re-entrant circuit formed inside the infarction scar. Even though the positive predictive value of ventricular ectopy on AECG (10 premature ventricular ectopics/h, non-sustained VT) in most studies for arrhythmogenic events is low (between 5 and 15%), it increases if associated with a low ejection fraction to 15e34%. Current UK guidelines (NICE) for implantable cardioverter defibrillators (ICDs) reflect this finding: one indication in post-infarct patients is an ejection fraction of 35e40%, non-sustained VT on Holter monitoring and a positive VT stimulation study (see later). Heart rate varies from beat to beat through autonomic influences. Reduced heart rate variability, which can be assessed using a number of mathematical formulae, is associated with increased mortality in post-infarct patients although its predictive value if used alone is very low. Heart rate variability is not routinely used in clinical practice but can be assessed from Holter monitoring. Implanted devices can also record this information and the information may be used in the
Assessment of symptoms with AECG (Table 1): the most common indication for the use of AECG is to correlate symptoms with a cardiac rhythm disturbance. Four different scenarios are possible: Characteristic symptoms correlate with the recording of a cardiac arrhythmia. This outcome is most useful and can direct therapy. Typical symptoms occur at the time of normal rhythm. This may represent a patient’s abnormal perception of a normal heart rhythm or the fact that the symptoms are not related to a cardiac arrhythmia. The AECG detects an arrhythmia whilst the patient remains asymptomatic. This finding may be relevant, for example if the recording shows a bradycardia, which might indicate that the patient experiences more severe episodes of bradycardia at other times. The patient remains asymptomatic throughout and no arrhythmias are recorded. Further monitoring is necessary. The diagnostic yield will depend on whether an event occurs during the recording period. For syncope this is often low. Causative arrhythmia detection in this situation is reported to be up to 25% with loop recorders and considerably less with 24-h Holter recording. This low diagnostic yield relates to nonarrhythmic causes for syncope and the infrequency of symptoms. Implantable loop recorders can increase the diagnostic yield to about 50% and may be the investigation of choice in unexplained syncope. For palpitations, with Holter monitoring the pick-up rate can be up to 35% but with the use of an event recorder for two weeks a diagnosis can be reached in a larger proportion. Apart from the recording of the arrhythmia itself, recording may reveal more subtle clues as to the aetiology of the patient’s
MEDICINE 38:7
Indications for AECG arrhythmia detection to assess risk for future cardiac events in patients without symptoms from arrhythmia Definite 1. NSVT in post-MI patients with LV dysfunction (ejection fraction <40%). 2. NSVT in patients with HCM. Possible 1. Heart rate variability assessment in heart failure patients. Table 2
368
Ó 2010 Elsevier Ltd. All rights reserved.
INVESTIGATIONS
future to detect heart failure deterioration. In patients with hypertrophic cardiomyopathy (HCM) the presence of NSVT on Holter monitoring may also provide evidence for a risk of sudden death.
invisible on a standard ECG. An SAECG can detect these ‘late potentials’ by greatly amplifying the ECG. Averaging is used to cancel out random electrical noise leaving the true signal behind. In ischaemic cardiomyopathy late potentials have been shown to be a sensitive but non-specific marker of arrhythmic risk. An SAECG may also be helpful in the diagnosis of arrhythmogenic right ventricular cardiomyopathy, a rare inherited cardiomyopathy. This is a specialized test used in only a few centres and has not found general clinical use.
Other indications for AECG monitoring: AECG monitoring can also be used to assess the efficacy of therapy. The most common indication in this context is the evaluation of rate control in atrial fibrillation. Following ablations for atrial fibrillation or ventricular ectopics/tachycardia, AECG is used to detect asymptomatic episodes of arrhythmia in order to assess the success of the procedure. On occasion Holter monitoring can be helpful in identifying pacemaker or defibrillator malfunction if the intrinsic storage and interrogation facilities of the device are insufficient. The analysis of transient ST-segment deviation as a measure for cardiac ischaemia remains controversial but has been used to assess the efficacy of anti-ischaemic therapy in coronary disease or in patients who cannot perform an exercise test.
Tilt testing and carotid sinus massage Tilt testing is used to evaluate syncope, particularly if reflex syncope or orthostatic hypotension is suspected. Reflex syncope describes conditions in which the cardiovascular reflexes that usually control blood pressure become intermittently inappropriate following a certain trigger (orthostatic stress, emotion, micturition, etc.). This leads to vasodilatation and/or bradycardia. Orthostatic hypotension is caused by a more pervasive blood pressure control failure that results in an inadequate increase in vascular resistance in response to standing. Tilt testing reproduces orthostatic stress in a laboratory setting. It involves strapping the patient to a bed that can be tilted, head-up at about 60 with constant blood pressure and heart rate monitoring. A passive head-up tilt can be performed
Signal averaged ECG (SAECG) Myocardial scars create areas of slow conduction that can be the substrate for re-entrant ventricular arrhythmias. These areas show delayed and fragmented activation of very low amplitude occurring after the QRS complex and are usually so small as to be
Adenosine blocks the AV node and thus can highlight the presence or the absence of an accessory pathway. a The upper trace shows an apparently normal QRS at the start which becomes broader with the pattern of Wolff-Parkinson-White syndrome. The AV node is blocked and activation spreads preferentially down the pathway. The pathways is far on the left-hand side of the heart and thus at rest most of the ventricles are activated by the AV node and therefore the QRS appears narrow. b The lower panel shows AV block after adenosine thus excluding a potentially dangerous antegradely conducting accessory pathway. Figure 2
MEDICINE 38:7
369
Ó 2010 Elsevier Ltd. All rights reserved.
INVESTIGATIONS
for 20e45 min. Pharmacological provocation (active test) with GTN or isoprenaline is often added to increase the likelihood of collapse. Hypotension, bradycardia and symptom reproduction are end-points, a positive response may be seen in up to 49% of passive tests and 66% of active tests. Depending on whether the predominant feature is hypotension or bradycardia the response to tilt testing is labelled respectively as vasodepressor, cardioinhibitory or mixed. However a negative tilt test does not exclude the diagnosis of reflex syncope; only the reproduction of usual symptoms with classical haemodynamic changes can be viewed as diagnostically important. Carotid sinus massage (CSM) is indicated in patients over the age of 40 with unexplained syncope, to diagnose carotid sinus hypersensitivity. Pressure at the common carotid artery bifurcation produces a fall in HR and BP and this reflex can be exaggerated in some people. CSM is performed on each side for 10 s in both the supine and erect position with continuous HR and BP monitoring. Falls in BP or HR associated with typical symptoms define carotid sinus hypersensitivity (CSH). Carotid sinus syndrome (CSS) is diagnosed in patients with spontaneous syncope and CSH and a pacemaker is indicated for those with a bradycardic response. Neurological complications are rare but CSM should be avoided in patients with recent TIA or stroke or with carotid bruits, unless carotid Doppler studies have shown no significant stenosis.
more obvious. Knowing that a pathway is present can be critical because AF conducted down a pathway can be dangerous; whereas in the absence of a pathway a patient with an SVT is at no risk from dangerous arrhythmias (Figure 2). Electrophysiological studies (EPS) In some circumstances it is possible to define the mechanism of a tachycardia from the ECG. It may be difficult to differentiate ventricular tachycardia from a supraventricular tachycardia conducted with aberrancy. Some patients are keen to explore a definitive cure through ablation (see later). For others it is important to define the possible risk of VT. In these situations, an invasive electrophysiological study (EPS) is indicated. The procedure is performed in the catheter lab with similar preparation to a coronary angiogram (nil by mouth, sedation, local anaesthetic). Flexible wires are passed either through a vein or an artery up to the heart. These have electrodes through which intrinsic cardiac activity can be sensed and the heart can be paced. Wires are generally positioned near the sinus node and AV node, in the ventricle and in the coronary sinus. This latter structure runs around the mitral valve annulus and an in-dwelling electrode will therefore detect electrical information from both the left atrium and left ventricle. By increasing the heart rate with pacing, introducing ectopics or burst pacing it is possible to define if the conducting system is functioning normally and if there are additional connections (e.g. an accessory pathway). It is also possible to induce many different arrhythmias and using various techniques to define the arrhythmia mechanism. This may allow risk stratification (distinguishing VT from SVT) but more critically it is usually possible to define a small area in the heart that is vital for the tachycardia mechanism. Destruction of this area (ablation) can therefore cure the patient (see later). Electrophysiological studies are performed as a day-case procedure and have perhaps a 1/500 risk of damage to vascular structures from the access sites. Other complications are much less common so it is regarded as a safe procedure. Indications include: Differentiating SVT mechanisms to allow risk stratification and the possibility of cure through ablation. Differentiating SVT with aberrancy from VT. Risk stratification of post-infarct patients by assessing the inducibility of VT. Investigating the cause of a collapse although EPS is much less sensitive for bradycardia than tachycardia. A
Ajmaline test Brugada syndrome is a genetically determined channelopathy with a risk of sudden cardiac death secondary to ventricular fibrillation in young adults. The diagnostic ECG in Brugada syndrome shows coved type ST-segment elevation in the right pre-cordial leads. These changes can sometimes remain concealed but can be unmasked by sodium channel blockers such as ajmaline (the preferred drug owing to its short half-life). The test is indicated in patients with a suspicious ECG or if there is a family history of premature sudden death. Adenosine test Adenosine is widely used for the termination of supraventricular tachycardias (SVTs). It blocks conduction in the atrioventricular (AV) node and can therefore interrupt the re-entry circuit in AV nodal re-entry tachycardia (AVNRT) and AV re-entry tachycardia (AVRT). Despite having a very short half-life of less than 10 s adenosine commonly causes flushing, nausea and chest discomfort and is contraindicated in patients with severe asthma. Adenosine can also be helpful in the assessment of patients with SVT. Typical AVNRT is caused by a re-entry circuit formed by a slow and a fast pathway in the area of the AV node. In patients with these two distinct pathways (dual AV node physiology) adenosine can block the fast pathway allowing conduction via the slow pathway to become visible with a sudden increase in the PR interval of >50 ms on the ECG. AVRT is caused by a re-entry circuit involving the AV node and an accessory pathway. The presence of a delta wave on an ECG in sinus rhythm indicates that the accessory pathway can conduct from atrium to ventricle. Some pathways between the left atrium and ventricle may be invisible on the ECG because of rapid conduction through the AV node. As adenosine blocks the AV node, pre-excitation in the form of a delta wave will become
MEDICINE 38:7
FURTHER READING Antzelevitch C, Brugada P, Borggrefe M. Brugada syndrome: report of the second consensus conference. Circulation 2005; 111: 659e70. Crawford MH, Bernstein SJ, Prakash CD, et al. ACC/AHA guidelines for ambulatory electrocardiography. A report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines. J Am Coll Cardiol 1999; 34: 912e48. Enseleit F, Duru F. Long-term continuous external electrocardiographic recording: a review. Europace 2006; 8: 255e66. Kapoor WN, Smith MA, Miller NL. Upright tilt testing in evaluating syncope: a comprehensive literature review. Am J Med 1994; 97: 78e88. Moya A, Sutton R, Ammirati F, et al. Guidelines for the diagnosis and management of syncope (version 2009). Eur Heart J 2009; 30: 2631e71.
370
Ó 2010 Elsevier Ltd. All rights reserved.
Electrophysiological investigation
Arrhythmias often present with palpitations; however they can also present non-specifically with symptoms of breathlessness, tiredness, chest pain, dizziness and syncope. The investigation of an arrhythmia needs to define whether the rhythm disturbance is relevant to the patient’s symptoms and to identify the underlying cause and the electrophysiological mechanism; only then can one specify the most suitable treatment.
Martina Muggenthaler Razeen Gopal Richard Schilling
Ambulatory electrocardiography (AECG) AECG allows the recording of an ECG over a prolonged period of time and is the main tool used to decide whether an arrhythmia is the cause of the patient’s symptoms; it may also give details about the mechanism and therefore the best treatment. Less frequently, AECG can also be used for risk stratification, particularly in ischaemic heart disease, the detection of myocardial ischaemia, the monitoring of drug treatment and the evaluation of pacemaker function (Figure 1).
Nicholas Gall
Abstract Cardiac electrophysiology is the subspecialty of cardiology dealing with heart rhythm disorders, particularly the investigation and management of brady- and tachyarrhythmias, syncope and the prevention of sudden cardiac death. Recent years have seen significant developments in our understanding of arrhythmias. Recent technological developments include ever smaller and more complex implantable pacing devices, the ability to map an arrhythmia circuit in three dimensions, potentially in one beat and to destroy small areas of myocardium safely known to be critical to a particular arrhythmia. These have led to a dramatic expansion in our ability to treat heart rhythm problems. This article will focus on the diagnostic tools which can be used to investigate these conditions and its sister article will deal with some of the treatment strategies.
Selection of recording technique: the frequency and characteristics of the patient’s symptoms will dictate the choice of recording technique. Continuous ECG recording can be achieved throughout the day and night up to 7 days depending on the digital memory of the device used. Three leads are typically recorded but 12 lead versions are also available for 24e48 h and are used in selected cases where detailed ECG morphology may be important (e.g. defining ventricular ectopic morphology or investigating biventricular pacemaker function). These continuous recording techniques are particularly useful if symptoms occur on most days or if the patient is incapacitated by their symptoms, for example when an arrhythmia leads to syncope. Patients can mark events on the recording and should always keep a diary of their symptoms and activities to allow the correlation of symptoms and heart rhythm. With less frequent symptoms there are two alternatives. Event recorders record and store a brief period of ECG when activated by the patient. They are either attached via ECG leads or can be carried in a pocket and held to the chest wall at the time of symptoms. These devices can be used for weeks or months at a time and are useful for patients with infrequent symptoms provided the patient is able to activate the recorder at the time of symptoms.
Keywords ablation; arrhythmia; ECG; electrophysiological; Holter; palpitation
Non-invasive electrophysiology Electrophysiology is the subspecialty of cardiology dealing with heart rhythm disorders; it is a growing area because of technological developments and our developing understanding of arrhythmia mechanisms. The specialty involves the investigation and treatment of both brady- and tachyarrhythmias, including ablation and device therapy. The prediction and prevention of sudden cardiac death also constitute a major focus. This article will deal with invasive and non-invasive methods of investigation and its sister article will deal with the treatment of tachyarrhythmias. Other articles in this series will deal with device therapy.
Martina Muggenthaler MRCP is a Specialist Registrar in Cardiology and Electrophysiology, King’s College Hospital, London, UK. Competing interests: none declared. Razeen Gopal MBChB MMed (US) is a Cardiac Electrophysiology Fellow, London, UK. Competing interests: none declared. Richard Schilling MD FRCP is a Professor of Cardiology and Electrophysiology, St Bartholomew’s Hospital and Queen Mary University, London, UK. Competing interests: none declared.
Figure 1 Holter monitors provide a considerable amount of information, including the heart rate profile, the frequency of ectopics beats and more complex arrhythmias in addition to providing symptom-rhythm correlation. The heart rate trend is shown providing information about rate control in atrial flutter in this case. The bottom graph gives an idea of ventricular ectopic frequency.
Nicholas Gall MSc MD FRCP is Consultant Cardiologist and Cardiac Electrophysiologist, King’s College Hospital, and Honorary Senior Lecturer, King’s College London, UK. Competing interests: none declared.
MEDICINE 38:7
367
Ó 2010 Elsevier Ltd. All rights reserved.
INVESTIGATIONS
Loop recorders continuously record the ECG but only store a few minutes, which is constantly updated. When activated, the stored ECG is fixed in a separate memory for later downloading; several separate events can be stored in this manner. As a rule ECG before and after device activation is stored to ensure that the whole event is captured, particularly in cases where symptoms are brief or associated with incapacity. Loop recorders are either attached to the patient or can be implanted subcutaneously for prolonged monitoring. Patients activate the device in response to symptoms. Implanted devices have a battery life of up to three years and can also be programmed to store the ECG automatically (e.g. with heart rates above or below a defined cut-off). Implantable devices of the future will have better arrhythmia detection and may also be able to measure other physiological parameters, such as blood pressure and movement.
Indications for AECG to assess symptoms possibly related to rhythm disturbances Definite uses: 1. Patients with unexplained syncope, near syncope, or episodic dizziness in whom the cause is not obvious. 2. Patients with unexplained recurrent palpitation. Possible uses: 1. Patients with episodic shortness of breath, chest pain or fatigue that is not otherwise explained. 2. Patients with neurological events when transient atrial fibrillation or flutter is suspected. 3. Patients with symptoms such as syncope, dizziness, or palpitation in whom a probable cause other than an arrhythmia has been identified but in whom symptoms persist despite treatment of this other cause.
Normal findings during AECG monitoring: during a 24-h period the heart rate of a healthy person varies in response to autonomic influences, with a reduction in heart rate at night to below 50 beats per minute in some people but this is usually of little relevance when it causes no symptoms. Indeed, many asymptomatic abnormalities may be observed which have little significance. For instance, supraventricular and ventricular ectopics are frequently noted in healthy people; AV nodal Wenckebach overnight can also be normal in young people as a result of high vagal tone. However, very frequent ventricular ectopy or signs of higher grade AV block, particularly at times of low vagal tone should warrant further consideration.
Table 1
symptoms, such as the appearance of delta waves indicative of an accessory pathway, the likely cause of a supraventricular tachycardia. Assessment of risk in patients without symptoms of arrhythmia (Table 2): patients who have survived a myocardial infarction are at increased risk of sudden cardiac death, usually because of ventricular arrhythmia from a re-entrant circuit formed inside the infarction scar. Even though the positive predictive value of ventricular ectopy on AECG (10 premature ventricular ectopics/h, non-sustained VT) in most studies for arrhythmogenic events is low (between 5 and 15%), it increases if associated with a low ejection fraction to 15e34%. Current UK guidelines (NICE) for implantable cardioverter defibrillators (ICDs) reflect this finding: one indication in post-infarct patients is an ejection fraction of 35e40%, non-sustained VT on Holter monitoring and a positive VT stimulation study (see later). Heart rate varies from beat to beat through autonomic influences. Reduced heart rate variability, which can be assessed using a number of mathematical formulae, is associated with increased mortality in post-infarct patients although its predictive value if used alone is very low. Heart rate variability is not routinely used in clinical practice but can be assessed from Holter monitoring. Implanted devices can also record this information and the information may be used in the
Assessment of symptoms with AECG (Table 1): the most common indication for the use of AECG is to correlate symptoms with a cardiac rhythm disturbance. Four different scenarios are possible: Characteristic symptoms correlate with the recording of a cardiac arrhythmia. This outcome is most useful and can direct therapy. Typical symptoms occur at the time of normal rhythm. This may represent a patient’s abnormal perception of a normal heart rhythm or the fact that the symptoms are not related to a cardiac arrhythmia. The AECG detects an arrhythmia whilst the patient remains asymptomatic. This finding may be relevant, for example if the recording shows a bradycardia, which might indicate that the patient experiences more severe episodes of bradycardia at other times. The patient remains asymptomatic throughout and no arrhythmias are recorded. Further monitoring is necessary. The diagnostic yield will depend on whether an event occurs during the recording period. For syncope this is often low. Causative arrhythmia detection in this situation is reported to be up to 25% with loop recorders and considerably less with 24-h Holter recording. This low diagnostic yield relates to nonarrhythmic causes for syncope and the infrequency of symptoms. Implantable loop recorders can increase the diagnostic yield to about 50% and may be the investigation of choice in unexplained syncope. For palpitations, with Holter monitoring the pick-up rate can be up to 35% but with the use of an event recorder for two weeks a diagnosis can be reached in a larger proportion. Apart from the recording of the arrhythmia itself, recording may reveal more subtle clues as to the aetiology of the patient’s
MEDICINE 38:7
Indications for AECG arrhythmia detection to assess risk for future cardiac events in patients without symptoms from arrhythmia Definite 1. NSVT in post-MI patients with LV dysfunction (ejection fraction <40%). 2. NSVT in patients with HCM. Possible 1. Heart rate variability assessment in heart failure patients. Table 2
368
Ó 2010 Elsevier Ltd. All rights reserved.
INVESTIGATIONS
future to detect heart failure deterioration. In patients with hypertrophic cardiomyopathy (HCM) the presence of NSVT on Holter monitoring may also provide evidence for a risk of sudden death.
invisible on a standard ECG. An SAECG can detect these ‘late potentials’ by greatly amplifying the ECG. Averaging is used to cancel out random electrical noise leaving the true signal behind. In ischaemic cardiomyopathy late potentials have been shown to be a sensitive but non-specific marker of arrhythmic risk. An SAECG may also be helpful in the diagnosis of arrhythmogenic right ventricular cardiomyopathy, a rare inherited cardiomyopathy. This is a specialized test used in only a few centres and has not found general clinical use.
Other indications for AECG monitoring: AECG monitoring can also be used to assess the efficacy of therapy. The most common indication in this context is the evaluation of rate control in atrial fibrillation. Following ablations for atrial fibrillation or ventricular ectopics/tachycardia, AECG is used to detect asymptomatic episodes of arrhythmia in order to assess the success of the procedure. On occasion Holter monitoring can be helpful in identifying pacemaker or defibrillator malfunction if the intrinsic storage and interrogation facilities of the device are insufficient. The analysis of transient ST-segment deviation as a measure for cardiac ischaemia remains controversial but has been used to assess the efficacy of anti-ischaemic therapy in coronary disease or in patients who cannot perform an exercise test.
Tilt testing and carotid sinus massage Tilt testing is used to evaluate syncope, particularly if reflex syncope or orthostatic hypotension is suspected. Reflex syncope describes conditions in which the cardiovascular reflexes that usually control blood pressure become intermittently inappropriate following a certain trigger (orthostatic stress, emotion, micturition, etc.). This leads to vasodilatation and/or bradycardia. Orthostatic hypotension is caused by a more pervasive blood pressure control failure that results in an inadequate increase in vascular resistance in response to standing. Tilt testing reproduces orthostatic stress in a laboratory setting. It involves strapping the patient to a bed that can be tilted, head-up at about 60 with constant blood pressure and heart rate monitoring. A passive head-up tilt can be performed
Signal averaged ECG (SAECG) Myocardial scars create areas of slow conduction that can be the substrate for re-entrant ventricular arrhythmias. These areas show delayed and fragmented activation of very low amplitude occurring after the QRS complex and are usually so small as to be
Adenosine blocks the AV node and thus can highlight the presence or the absence of an accessory pathway. a The upper trace shows an apparently normal QRS at the start which becomes broader with the pattern of Wolff-Parkinson-White syndrome. The AV node is blocked and activation spreads preferentially down the pathway. The pathways is far on the left-hand side of the heart and thus at rest most of the ventricles are activated by the AV node and therefore the QRS appears narrow. b The lower panel shows AV block after adenosine thus excluding a potentially dangerous antegradely conducting accessory pathway. Figure 2
MEDICINE 38:7
369
Ó 2010 Elsevier Ltd. All rights reserved.
INVESTIGATIONS
for 20e45 min. Pharmacological provocation (active test) with GTN or isoprenaline is often added to increase the likelihood of collapse. Hypotension, bradycardia and symptom reproduction are end-points, a positive response may be seen in up to 49% of passive tests and 66% of active tests. Depending on whether the predominant feature is hypotension or bradycardia the response to tilt testing is labelled respectively as vasodepressor, cardioinhibitory or mixed. However a negative tilt test does not exclude the diagnosis of reflex syncope; only the reproduction of usual symptoms with classical haemodynamic changes can be viewed as diagnostically important. Carotid sinus massage (CSM) is indicated in patients over the age of 40 with unexplained syncope, to diagnose carotid sinus hypersensitivity. Pressure at the common carotid artery bifurcation produces a fall in HR and BP and this reflex can be exaggerated in some people. CSM is performed on each side for 10 s in both the supine and erect position with continuous HR and BP monitoring. Falls in BP or HR associated with typical symptoms define carotid sinus hypersensitivity (CSH). Carotid sinus syndrome (CSS) is diagnosed in patients with spontaneous syncope and CSH and a pacemaker is indicated for those with a bradycardic response. Neurological complications are rare but CSM should be avoided in patients with recent TIA or stroke or with carotid bruits, unless carotid Doppler studies have shown no significant stenosis.
more obvious. Knowing that a pathway is present can be critical because AF conducted down a pathway can be dangerous; whereas in the absence of a pathway a patient with an SVT is at no risk from dangerous arrhythmias (Figure 2). Electrophysiological studies (EPS) In some circumstances it is possible to define the mechanism of a tachycardia from the ECG. It may be difficult to differentiate ventricular tachycardia from a supraventricular tachycardia conducted with aberrancy. Some patients are keen to explore a definitive cure through ablation (see later). For others it is important to define the possible risk of VT. In these situations, an invasive electrophysiological study (EPS) is indicated. The procedure is performed in the catheter lab with similar preparation to a coronary angiogram (nil by mouth, sedation, local anaesthetic). Flexible wires are passed either through a vein or an artery up to the heart. These have electrodes through which intrinsic cardiac activity can be sensed and the heart can be paced. Wires are generally positioned near the sinus node and AV node, in the ventricle and in the coronary sinus. This latter structure runs around the mitral valve annulus and an in-dwelling electrode will therefore detect electrical information from both the left atrium and left ventricle. By increasing the heart rate with pacing, introducing ectopics or burst pacing it is possible to define if the conducting system is functioning normally and if there are additional connections (e.g. an accessory pathway). It is also possible to induce many different arrhythmias and using various techniques to define the arrhythmia mechanism. This may allow risk stratification (distinguishing VT from SVT) but more critically it is usually possible to define a small area in the heart that is vital for the tachycardia mechanism. Destruction of this area (ablation) can therefore cure the patient (see later). Electrophysiological studies are performed as a day-case procedure and have perhaps a 1/500 risk of damage to vascular structures from the access sites. Other complications are much less common so it is regarded as a safe procedure. Indications include: Differentiating SVT mechanisms to allow risk stratification and the possibility of cure through ablation. Differentiating SVT with aberrancy from VT. Risk stratification of post-infarct patients by assessing the inducibility of VT. Investigating the cause of a collapse although EPS is much less sensitive for bradycardia than tachycardia. A
Ajmaline test Brugada syndrome is a genetically determined channelopathy with a risk of sudden cardiac death secondary to ventricular fibrillation in young adults. The diagnostic ECG in Brugada syndrome shows coved type ST-segment elevation in the right pre-cordial leads. These changes can sometimes remain concealed but can be unmasked by sodium channel blockers such as ajmaline (the preferred drug owing to its short half-life). The test is indicated in patients with a suspicious ECG or if there is a family history of premature sudden death. Adenosine test Adenosine is widely used for the termination of supraventricular tachycardias (SVTs). It blocks conduction in the atrioventricular (AV) node and can therefore interrupt the re-entry circuit in AV nodal re-entry tachycardia (AVNRT) and AV re-entry tachycardia (AVRT). Despite having a very short half-life of less than 10 s adenosine commonly causes flushing, nausea and chest discomfort and is contraindicated in patients with severe asthma. Adenosine can also be helpful in the assessment of patients with SVT. Typical AVNRT is caused by a re-entry circuit formed by a slow and a fast pathway in the area of the AV node. In patients with these two distinct pathways (dual AV node physiology) adenosine can block the fast pathway allowing conduction via the slow pathway to become visible with a sudden increase in the PR interval of >50 ms on the ECG. AVRT is caused by a re-entry circuit involving the AV node and an accessory pathway. The presence of a delta wave on an ECG in sinus rhythm indicates that the accessory pathway can conduct from atrium to ventricle. Some pathways between the left atrium and ventricle may be invisible on the ECG because of rapid conduction through the AV node. As adenosine blocks the AV node, pre-excitation in the form of a delta wave will become
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FURTHER READING Antzelevitch C, Brugada P, Borggrefe M. Brugada syndrome: report of the second consensus conference. Circulation 2005; 111: 659e70. Crawford MH, Bernstein SJ, Prakash CD, et al. ACC/AHA guidelines for ambulatory electrocardiography. A report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines. J Am Coll Cardiol 1999; 34: 912e48. Enseleit F, Duru F. Long-term continuous external electrocardiographic recording: a review. Europace 2006; 8: 255e66. Kapoor WN, Smith MA, Miller NL. Upright tilt testing in evaluating syncope: a comprehensive literature review. Am J Med 1994; 97: 78e88. Moya A, Sutton R, Ammirati F, et al. Guidelines for the diagnosis and management of syncope (version 2009). Eur Heart J 2009; 30: 2631e71.
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