Preoperative electrocardiogram in an 89-year-old man.
An 89-year-old man with spinal stenosis and the inability to ambulate was referred for a preoperative laminectomy. He had no prior cardiac history and took no medications besides 100 mg oral tramadol every six hours as needed for pain. His only symptom was blurred vision in his left eye. A preoperative electrocardiogram (Figure 1) was performed for risk stratification.
The most probable explanation of the rhythm shown in the 12-lead ECG and 3-lead rhythm strip (at the bottom) is second-degree SA block. The pattern of two consecutive P-QRS complexes ("group beating") followed by a pause is likely produced by failure of every third sinus impulse to conduct to the atrium. There is also probable second-degree, type I AV block, as evidenced by progressive PR interval prolongation prior to the dropped beat. However, due to 3:2 SA block, the next sinus impulse is not conducted to the atrium so the Wenckebach sequence (non-conducted sinus P wave) cannot be completed. The origin of SA exit block commonly occurs in the tissue surrounding the sinus node (the "perinodal" area), which fails to conduct the sinus node (SN) impulse to the rest of the right atrium (RA) (Figure 2). While SA block is less well known than AV block, it shares a similar nomenclature, and block is designated first degree, second degree, or third degree.
First-degree SA nodal block is due to prolongation of conduction time between the SA node impulse and depolarization of the rest of the RA (designated by the P wave on ECG). First-degree SA nodal block is undetectable on a 12-lead ECG as the SA nodal impulse does not generate any detectable depolarization. Much like second-degree AV nodal block, second-degree SA nodal block is classified as two different types. Second-degree, type I SA nodal block (Wenckebach) is caused by an increasing delay between SA nodal depolarization and atrial depolarization until the SA node impulse completely fails to reach the atrium. This type of block is manifested on ECG by progressive shortening of P-P intervals until eventually a P-QRS-T sequence is dropped completely. In contrast, with second-degree, type II SA nodal block, the impulse travelling from the SA node to the atrium fails abruptly without prior prolongation of conduction. This type of block is manifested on ECG by constant P-P intervals until a P-QRS-T complex is dropped. The type of second-degree AV block cannot be assessed in the above ECG because there is only 1 P-P interval (two P waves) prior to each dropped complex. Finally, third-degree SA nodal block is caused by complete absence of SA nodal impulse propagation to the RA, which can give the ECG appearance of sinus arrest. SA block can be differentiated from true sinus arrest by the repetitive nature of the pattern where two or more P waves are regularly followed by a pause.
According to the 2008 ACC/AHA/HRS "Guidelines for Device-Based Therapy of Cardiac Rhythm Abnormalities: Executive Summary," permanent pacemaker implantation is indicated for symptomatic patients with documented bradycardia (which includes sinus pauses of any type). In addition to cardiac monitoring, our patient underwent extensive neurologic and ophthalmologic evaluation for his blurred vision, and it was decided that his visual symptoms were unrelated to his bradycardia. Consequently, it was determined that permanent pacemaker implantation was not warranted. Due to frequent episodes of bradycardia seen on the cardiac monitor, however, an intraoperative temporary transvenous pacemaker was recommended because of the possibility that a more rapid heart rate might be needed in the event of significant periprocedural blood loss. Ultimately, the patient and his family decided against having the laminectomy, and he was discharged from the hospital.
(1.) Epstein AE, Dimarco JP, Ellenbogen KA, et al. ACC/AHA/HRS 2008 guidelines for Device-Based Therapy of Cardiac Rhythm Abnormalities: executive summary. Heart Rhythm 2008;5: 934-55.
Matthew N. Peters, MD; Morgan J. Katz, MD; Anand M. Irimpen, MD; Patrice Delafontaine, MD
Drs. Peters and Katz are second-year Internal Medicine Residents at Tulane University Health Sciences Center in New Orleans. Drs. Delafontaine and Irimpen are Professors in the Section of Cardiology, Department of Medicine, Tulane University Health Sciences Center. Dr. Irimpen is also a Professor in the Section of Cardiology, Department of Medicine, New Orleans Veteran Affairs Medical Center.
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|Author:||Peters, Matthew N.; Katz, Morgan J.; Irimpen, Anand M.; Delafontaine, Patrice|
|Publication:||The Journal of the Louisiana State Medical Society|
|Article Type:||Clinical report|
|Date:||Jan 1, 2013|
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