NB: See chapter 5 on preoperative EKG for sample EKG strips.
- Conduction blocks can occur at any point along the pathway (Figure 15-1)
- The lower the level of block along the electrical system, the worse the prognosis
- That is, block in the His–Purkinje system carries a much higher risk of sudden cardiac death, as compared with block at the level of the AV node
For most patients the level of block can be determined using the surface EKG findings (see preoperative EKG chapter 5 for examples of EKG tracings).
Figure 15-1. Pathway of Electrical Impulses in the Heart
In the normal heart, electrical impulses originate in the SA node, travel through the AV node, and then to the His–Purkinje system as depicted in the figure. Reproduced from Fuster V, Walsh RA, Harrington RA. Hurst’s The Heart. 13th ed. Figure 43-1. Available at: www.accessmedicine.com. Copyright © The McGraw-Hill Companies, Inc. All rights reserved.
Table 15-1 Determining Level of Block Based on Surface EKG |Favorite Table|Download (.pdf)
Table 15-1 Determining Level of Block Based on Surface EKG
|Location of disease||Name of disorder||EKG findings|
|1. SA nodal disease||Sinus pause||No P waves|
|2. AV nodal disease||First-degree AV block||Prolonged PR interval|
|Second-degree, type I (Wenckebach)||PR interval gets longer until a nonconducted P wave is seen|
|Third-degree (complete) w/narrow escape rhythm||P’s and QRS’s are dissociated. However, QRS complex is narrow|
|3. Infranodal disease||Second-degree, type II||PR interval constant with intermittent nonconducted P waves|
|Third-degree (complete) w/wide escape rhythm||P’s and QRS’s are dissociated. However, QRS complex is wide|
- The conduction system is autonomically innervated until the level of the His-Purkinje system. Increased vagal or parasympathetic tone will lead to slowing of sinus rhythm and of the conduction through the SA node and AV node, while sympathomimetics or vagolytics will improve or hasten sinus node and AV nodal conduction
- However, if the block is infranodal, then drugs that target the autonomic nervous system will not allow for better conduction. They may lead to an increased supranodal heart rate but will not improve the infranodal rate. The degree of block may worsen if sympathomimetics are given to patients with infranodal disease. The only therapy for patients with infranodal disease is implantation of a pacemaker
- Obtain thorough history and baseline EKG. Ensure that plasma electrolytes are normal
- If symptomatic (unexplained syncope, fatigue, lightheadedness), consider evaluation by an electrophysiologist
- Longer recordings should be considered
- Patients with evidence of infranodal disease will likely need implantation of either a temporary or permanent pacemaker prior to surgery
- Routine precautions with special attention to succinylcholine. Succinylcholine can trigger bradycardia, specifically if a second dose is given:
- Likely due to stimulation of muscarinic receptors in the sinus node...