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(BQ) Part 2 book “Swanton’s cardiology” has contents: Disturbances of cardiac rhythm - tachycardias and ablation, infective endocarditis, pericardial disease, the heart in systemic disease, systemic hypertension, pulmonary hypertension and pulmonary embolism, cardiac investigations,… and other contents.
CHAPTER 7 Disturbances of Cardiac Rhythm: Bradycardias, Pacing, the ICD, Biventricular Pacing for Heart Failure 7.1 Indications for Temporary Pacing AV Block in Acute MI Complete AV block (Figure 7.1) In inferior infarction, complete AV block usually results from right coronary artery occlusion The AV nodal artery is a branch of the right coronary artery Second-degree AV block (Wenckebach type) does not always represent AV nodal artery occlusion because vagal hyperactivity may play a part A localized, small inferior infarct may thus cause complete AV block In anterior infarction, complete AV block usually represents massive septal necrosis with additional circumflex artery territory damage The prognosis in complete AV block is dependent on infarct size and site rather than the block itself Complete AV block in either type of infarction should be temporarily paced Second-degree AV Block (Figure 7.1) • Wenckebach (Mobitz type I): incremental increases in PR interval with intermittent complete blocking of the P wave This is decremental conduction at the AV node level In inferior infarction it does not necessarily require pacing unless the bradycardia is poorly tolerated by the patient It may respond to atropine In anterior infarction, Wenckebach AV block should be temporarily paced • Mobitz type II AV block: fixed PR interval with sudden failure of conduction of atrial impulse (blocking of the P wave) Often occurs in the presence of a wide QRS because this type of block is usually associated with distal fascicular disease It carries a high risk of developing complete AV block It Swanton’s Cardiology: A concise guide to clinical practice Sixth Edition By R H Swanton and S Banerjee © 2008 R H Swanton and S Banerjee ISBN: 978-1-405-17819-8 310 Bradycardias, Pacing, the ICD, Biventricular Pacing for Heart Failure 311 Figure 7.1 Second- and third-degree AV block usually occurs in association with anterior infarction, but should be prophylactically paced with either type of infarct First-degree AV Block This does not require temporary pacing, but approximately 40% will develop higher degrees of AV block, and observation is necessary 312 Chapter Bundle-branch Block (see Chapter 16, Figures 16.7 and 16.8) This is a more complex group with conflicting evidence from various series Patients with evidence of trifascicular disease or non-adjacent bifascicular disease complicating MIs should be prophylactically paced, i.e • Trifascicular disease • Alternating RBBB/LBBB • Long PR interval + new RBBB + LAHB or new RBBB + LPHB • Long PR + LBBB • Non-adjacent bifascicular disease: RBBB + new LPHB (see Chapter 16, Figure 16.8) There is no proof that LBBB with a long PR interval is genuine trifascicular disease without measurements from His bundle studies, but, if it develops in the presence of septal infarction, LBBB is assumed to be LAHB + LPHB One of the most common bundle-branch blocks complicating anterior infarction is RBBB and LAHB (usually manifest by RBBB + left axis deviation), as these two fascicles are in the anterior septum In anterior infarction this combination should be paced only if a long PR interval develops Measurement of the H–V interval is theoretically useful in acute infarction, but involves insertion of an electrode under fluoroscopy and is not generally practical Sinoatrial Disease Profound sinus bradycardia or sinus arrest may occur in acute infarction (typically inferior infarction and right coronary occlusion) The sinus node arterial supply is usually from the right coronary artery Vagal hyperactivity may contribute and be partially reversed by atropine However, sinus bradycardia or sinus arrest may need temporary pacing if not reversed by atropine and if poorly tolerated by the patient Temporary Pacing for General Anaesthesia The same principles apply as those in acute infarction: 24-hour monitoring for those thought to be at risk may provide useful information Notice should be taken of recent ECG deterioration (e.g lengthening of PR interval, additional LAHB) Asymptomatic patients with bifascicular block and a normal PR interval not need temporary pacing Patients with sinoatrial disease should have 24-hour ECG monitoring before surgery, because vagal influences may produce prolonged sinus arrest Temporary Pacing during Cardiac Surgery Temporary epicardial pacing may be necessary in surgery adjacent to the AV node and bundle of His, e.g • aortic valve replacement for calcific aortic stenosis (with calcium extending into the septum) • tricuspid valve surgery and Ebstein’s anomaly Bradycardias, Pacing, the ICD, Biventricular Pacing for Heart Failure 313 • AV canal defects and ostium primum ASD • corrected transposition and lesions with AV discordance A knowledge of the exact site of the AV node and His bundle can be obtained by endocardial mapping at the time of surgery Closure of a VSD in corrected transposition or of the ventricular component of a complete AV canal defect may damage the His bundle and permanent epicardial electrodes may be required Other Indications for Temporary Pacing Indications include termination of refractory tachyarrhythmias, during electrophysiological studies and drug overdose (e.g digoxin, β-blocking agents, verapamil) 7.2 Pacing Difficulties Failure to Pace or Sense Wire Displacement This is the most common reason for failure to pace and is a common problem with temporary wires that have no tines or screw-in mechanisms To some extent it can be avoided by stability manoeuvres during wire insertion Positions just across the tricuspid valve tend not to be very stable Positions in the RV apex are usually more stable but sometimes threshold measurements are not ideal here Wire displacement requires repositioning in either temporary or permanent systems Microdisplacement If not noticed on chest radiograph this may be overcome by increasing pacing output voltage or pulse width Otherwise repositioning is necessary Exit Block This may develop in the first weeks as a result of a rise in threshold As the electrode becomes fibrosed into the endocardium the threshold levels off With temporary units the threshold is checked daily and the voltage increased if necessary Occasionally a fibrotic reaction at the pacing tip results in exit block and the lead may have to be removed (Figure 7.2) With programmable permanent pacing units the programmer may be used to increase the output During temporary wire insertion a threshold of