AND RHYTHM Dr Abdollahi 4112014 1 The ECG is a valuable tool for diagnosing disturbances of cardiac conduction and rhythm Ambulatory ECG monitoring Holter monitoring is useful in documenting ID: 733759
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DISTURBANCES OF CARDIAC CONDUCTIONAND RHYTHM
Dr Abdollahi
4/11/2014
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The ECG is a valuable tool for diagnosing
disturbances of cardiac conduction and rhythm. Ambulatory ECG monitoring (Holter monitoring) is useful in documenting the occurrence of life-threatening cardiac dysrhythmias and assessing the efficacy of
antidysrhythmic drug therapy.
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The
incidence of intraoperative cardiac dysrhythmiasdepends on the definition (benign versus life-threatening),patient characteristics, and the type of surgery (frequentincidence during cardiothoracic surgery).
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The
following questions should be asked when interpreting the ECG:1. What is the heart rate?2. Are P waves present, and what is their relationship tothe QRS complexes?3. What is the duration of the PR interval (normal 120 to200
msec)?4. What is the duration of the QRS complex (normal 50
to 120
msec
)?5.
Is the ventricular rhythm regular?
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6. Are there early cardiac beats or abnormal pauses after
a preceding QRS complex?7. Is there evidence of prior myocardial infarction orventricular hypertrophy?8. Is there evidence of myocardial ischemia?9. Is there a conduction disturbance such as left bundle
branch block, right bundle branch block, or intraventricular
conduction delay?
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Heart Block
Disturbances of conduction of cardiac impulses can beclassified according to the site of the conduction blockrelative to the atrioventricular node (AV node).Heart block occurring above the
atrioventricular node isusually benign and transient. Heart block occurringbelow
the
atrioventricular
node tends to be progressiveand permanent.
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Classification of Heart Block
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A theoretical concern in patients with
bifascicular heart block is that perioperative events, such as alterations in systemic blood pressure, arterial oxygenation, or electrolyte concentrations, might compromise conduction in the one remaining intact fascicle, leading to the acute
onset intraoperatively of third-degree atrioventricular heart block.
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However
, surgery performed during a general or regional anesthetic has not been shown to predispose to the development of third-degree atrioventricular heart block in patients with coexisting bifascicular block. Therefore, placement of a prophylactic artificial cardiac pacemaker is
not required before anesthesia and surgery, but it should be available.
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Third-degree atrioventricular
heart blockThird-degree atrioventricular heart block is treated by placement of an artificial cardiac pacemaker
. An artificial cardiac pacemaker can be inserted intravenously (
endocardial
lead) or by the subcostal approach (epicardial
or myocardial lead). An alternative to emergency transvenous
artificial
cardiac
pacemaker
placement is
noninvasive
transcutaneous
or temporary esophageal
cardiac pacing
.
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Third-degree atrioventricular
heart block4/11/2014
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A continuous intravenous infusion of
isoproterenol acting as a pharmacologic cardiac pacemaker may be necessary to maintain an adequate heart rate until artificial electrical cardiac pacing can be established.4/11/201416Slide17
Sick Sinus Syndrome
Sick sinus syndrome is characterized by inappropriate sinus bradycardia associated with degenerative changes in the sinoatrial node. Frequently, bradycardia due to this syndrome is complicated by episodes of
supraventricular tachycardia.
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Artificial
cardiac pacemakers may be indicated when therapeutic plasma concentrations of drugs necessary to control tachycardia result in bradycardia. The increased incidence of pulmonary embolism in these patients may be a reason to initiate anticoagulation.
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Ventricular Premature Beats
Ventricular premature beats (VPCs) are recognized on the ECG by :Premature occurrence, T
he absence of a P wave preceding the QRS complex,
A
wide and
often bizarre QRS complex,
Inverted
T wave,
Compensatory
pause that follows the premature beat
.
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The primary goal with VPCs should be
to identify anyunderlying cause (myocardial ischemia, arterial hypoxemia, hypercarbia, hypertension, hypokalemia, mechanical irritation of the ventricles) if possible and correct it.
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Treatment
Ventricular premature beats can be treated with lidocaine (1 to 2 mg/kg iv) when they : (1) are frequent (more than 6 premature beats/min), (2) are multifocal, (3) occur in salvos of 3 or more,
(4) take place during the ascending limb of the T wave (R on T phenomenon) that corresponds
to the
relative refractory
period of the ventricle.
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Ventricular Tachycardia
Ventricular tachycardia is defined as the appearance of atleast three consecutive wide QRS complexes (longer than120 msec) on the ECG occurring at an effective heartrate more rapid than 120 beats/min. Ventricular tachycardia not associated with hypotension is initially
treated with the intravenous administration of amiodarone
lidocaine
, or procainamide.
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Torsade-de-point
responds to magnesium. Symptomatic ventricular tachycardia is best treated with external electrical cardioversion. The presence of ventricular tachycardia should elicit an immediate search for a
cause such as myocardial ischemia, hypoxia, electrolyte abnormalities, or
myocardial stimulation
by the surgeons.
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Torsade-de-point
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Pre-Excitation Syndromes
Pre-excitation syndromes are characterized by activationof a portion of the ventricles by cardiac impulses thattravel from the atria via accessory (anomalous) conduction pathways. These pathways bypass the atrioventricular node such that activation of the ventricles
occurs earlier than it would if impulses reached the ventricles by normal pathways.
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WOLFF-PARKINSON-WHITE SYNDROME (WPW)
The Wolff-Parkinson-White syndrome is the most ommonpre-excitation syndrome, with an incidence of approximately 0.3% of the general population. The lackof physiologic delay in transmission of cardiac impulsesalong the Kent fibers
results in the characteristic short PR interval (less than 120
msec
) on the ECG.
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WPW
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The
wide QRS complex and delta wave on the ECG reflect the composite of cardiac impulses conducted by normal and accessory pathways. Paroxysmal atrial tachycardia is the most frequent cardiac dysrhythmia. More patients with Wolff Parkinson- White syndrome are frequently treated by
catheter ablation of accessory pathways as identified by
electrophysiologic
mapping.
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Supraventricular
tachycardias such as atrial fibrillation or atrial flutter with one-to-one conduction may lead to hemodynamic collapse in patients with Wolff-Parkinson-White syndrome.4/11/2014
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MANAGEMENT OF ANESTHESIA
The goal during management of anesthesia in the presence of a pre-excitation syndrome is to avoid events (anxiety) or drugs (anticholinergics, ketamine, pancuronium) that might
increase sympathetic nervous system activity and predispose to tachydysrhythmias
.
All cardiac antidysrhythmic drugs
should be continued throughout the perioperative period. Anesthesia can be induced with intravenous anesthetic, with the possible
exception
of ketamine
.
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Tracheal intubation should be performed
only after a sufficient concentration or dose of anesthetic has been given to reliably blunt sympathetic nervous system stimulation evoked by instrumentation of the upperairway. Intravenous B-adrenergic blockers (atenolol, metoprolol
, propranolol, or esmolol) can be used to avoidtachycardia during induction of anesthesia.
Neuromuscularblocking
drugs with minimal effects on heart rate should be used.
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The onset of paroxysmal atrial tachycardia or fibrillation
in the perioperative period can be treated with theintravenous administration of drugs that abruptly prolongthe refractory period of the atrioventricular node(adenosine) or lengthen the refractory period of accessorypathways (procainamide).B-Adrenergic blockers may be
used to control heart rate.
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Digitalis and verapamil
may decrease the refractory period of accessory pathways responsible for atrial fibrillation, resulting in an increase in ventricular response rate during this dysrhythmia and should be avoided. Electrical
cardioversion is indicated when tachydysrhythmias
are life-threatening.
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Prolonged QT Interval Syndrome
A prolonged QT interval (longer than 440 msec on theECG) syndrome is associated with ventricular dysrhythmias, syncope, and sudden death. Treatment probably should include B-adrenergic antagonists or left stellate ganglion block
. The effectiveness of a left stellate ganglion block supports the hypothesis that this syndromeresults from a congenital imbalance of autonomic
innervation to
the heart produced by decreases in right
cardiac sympathetic nerve activity
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Management of anesthesia
Includes avoidance of events or drugs that are likely toactivate the sympathetic nervous system and availabilityof B-antagonists (metoprolol, atenolol, propranolol, oresmolol) or electrical cardioversion to treat life-threateningventricular dysrhythmias. Inhaled
and intravenousanesthetics can prolong the QT interval on the ECG in
normal patients.
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Fortunately, these anesthetics do
not produce additional prolonged QT interval in those patients with this syndrome in a predictable manner. Many medications have the potential to prolong the QT interval (droperidrol) and
should be avoided if possible, in patients with prolonged QT syndrome.
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ARTIFICIAL CARDIAC PACEMAKERS
Preoperative evaluation of the patient with an artificialcardiac pacemaker in place includes determination ofthe reason for placing the pacemaker, assessment of itspresent function, as well as the brand, model, magnetmode, and availability of a programmer for this specificdevice and a person who knows how to operate theprogrammer.
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Many implanted electrical devices can be used. A device under the skin may not be a pacemaker.Implanted devices include deep brain stimulators, automatic implantable cardiac defibrillators, intravenouspumps, spinal stimulators for chronic pain, bladder stimulators for neurogenic bladder, gastric stimulatorsfor the treatment of obesity, intravenous ports, and vagal
stimulators for sleep.
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Special considerations are necessary for devices where
the patient's life depends on the device. If a device is acardiac pacemaker placed for third-degree heart block, appropriate experts regarding the continuous operation of that device and monitoring of its operation need to be immediately available.4/11/2014
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If a pacemaker implanted for third-degree heart block is to be disconnected to change the stimulator, transvenous pacing may be needed. If the device is an automatic defibrillator, it will need to be inactivated during electrical-surgical cautery to avoid the device erroneously detecting ventricular dysrhythmias and defibrillating, which would waste battery life and possibly cause R on T phenomena and ventricular fibrillation.
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The device should be
reactivated after the surgical procedure and interrogated for proper function. The magnet mode of many implanted devices is now programmable. The magnet mode cannot automatically be assumed to be "safe." The specific magnet mode for a patient's device should be identified as some magnet modes
change with device state or are programmable.Magnet mode for many pacemakers is asynchronous at99 beats/min. If the patient has a spontaneous heart rateof 60 to 80 beats/min, the asynchronous mode
at 99
beats/min would be safe.
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However, in some devices
, the magnet mode shifts to asynchronous at 50 beats/min at the end of battery life. Asynchronous pacing at 50 beats/min may lead to R on T phenomena if the patient has a spontaneous heart rate above 50 beats/min. The specific magnet mode should be identified and used only when needed given the circumstances of the case.
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Intraoperative monitoring of patients with artificial
cardiac pacemakers includes the ECG and possible intraarterial pressure monitoring so as to detect the appearance of asystole promptly. Atropine, isoproterenol, and an external pacemaker should be available if the artificial cardiac
pacemaker ceases to function. If electrocautery interferes with the ECG, monitoring intra-arterial pressure
, or
arterial oxygenation, auscultation through
an esophageal stethoscope or a palpable pulse
confirms continued cardiac activity.
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Inhibition of pulse generator activity by electromagnetic
interference most commonly from electrosurgical cautery,which is interpreted as spontaneous cardiac activityby the artificial cardiac pacemaker, is most likely whenthe ground plate for electrocautery is placed too nearthe pulse generator or unipolar cautery is used. For thisreason, the ground plate should be placed as far as possible from
the pulse generator.
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Bipolar
electrocautery may also reduce interference between electrosurgical cautery and the pacemaker. If surface pads are placed for external pacing or defibrillation, they should be placed away from the implanted device to reduce current passing down the
pacing lead and hyperpolarizing a small segment of myocardium, which could interfere with pacemaker capture after defibrillation. Automatic implantable
cardioversion
devices sense ventricular fibrillation or
ventricular tachycardia.
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They provide a
cardioversion shock through implanted cardiac leads. Electrocautery signals can be misinterpreted as ventricular dysrhythmias, thus triggering unnecessary shocks and decreasing battery life. These devices should be reprogrammed to the standby mode prior to elective surgery and returned postoperatively
to full function with interrogation of proper operation
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Selection of drugs or techniques for anesthesia
is not influenced by the presence of artificial cardiac pacemakers as there is no evidence that the threshold and subsequent response of these devices is altered by drugs administered in the perioperative period. However, patients with artificial cardiac pacemakers or implanted cardiaversion devices have a frequent incidence of
coexisting cardiac disease and should be monitored carefully
and anesthetized
with care.
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Patients with defibrillators
frequently have poor ventricular function. Insertion of a pulmonary artery catheter will not disturb epicardial electrodes but might dislodge recently placed (within 2 weeks) transvenous
endocardial electrodes. Pulmonary artery catheters are not necessary for cases with minimal fluid shifts.
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