Bundle branch block has been observed on surface electrocardiograms in 12–61 % of cases of CS, depending on study series [1–4]. Both the original and 2006 revised Japanese Ministry of Health and Welfare guidelines for the diagnosis of CS use RBBB as one of the ECG abnormalities constituting a minor diagnostic criterion [5]. While neither sensitive nor specific, both RBBB and LBBB are seen more commonly in patients with CS than those with sarcoidosis without myocardial involvement [4], and should prompt further investigations. Further studies are also needed to determine whether the presence of bundle branch block has prognostic implications for the future development of complete atrioventricular block or ventricular arrhythmias and sudden death in these patients.

With inflammation and subsequent scarring in CS, areas of delayed myocardial activation are expected, and this can manifest as fragmentation on the surface ECG. While not included in previously-published diagnostic guidelines, QRS complex fragmentation, defined by the presence of an additional R wave (R′), notching in the nadir of the S wave, or the presence of >1 R′ in two anatomically contiguous leads (Fig. 9.2), was shown in two study series to carry more diagnostic value than RBBB alone [4, 6].

The signal-averaged ECG (SAECG) has utility in the detection of abnormal activation characterized by late potentials, and may also have both diagnostic and prognostic value [7]. Unlike the standard 12-lead ECG recording, requiring only a few seconds, SAECG recording requires up to 10 min, as multiple QRS potentials are averaged to allow both for the removal of interference due to skeletal muscle and for the detection of low amplitude, high frequency late potentials. While originally developed as a means of risk stratification in patients with coronary disease and cardiomyopathies to detect substrate for reentry, further investigations are needed to assess the abnormal SAECG as a risk marker for ventricular arrhythmias in CS.

Conduction block can range from first-degree atrioventricular delay to complete atrioventricular block, and the severity of block can progress with progression of inflammation and scar. Complete atrioventricular block (CAVB) is the one of the most common findings in patients with clinically-evident cardiac sarcoidosis, with a prevalence of 25 % in one retrospective analysis [1]. While usually felt to be related to infiltration of the conduction system itself, granulomatous involvement of the AV nodal artery has also been described as a cause of atrioventricular block in sarcoidosis [3]. CAVB often occurs at a younger age in patients with sarcoidosis than in individuals with complete heart block due to other etiologies.

Treatment of patients with high grade atrioventricular block with permanent pacing should be performed in accordance with published guidelines [8]. However, the presence of AV block likely signifies extensive myocardial involvement from sarcoidosis granuloma and portends a higher risk of future ventricular arrhythmias [9]. For this reason, the recently-published expert consensus statement from the Heart Rhythm Society related to the management of CS gives a Class IIa recommendation for ICD implantation (regardless of left ventricular ejection fraction) for the primary prevention of sudden cardiac death in CS if the patient meets an indication for pacing [10]. And, while there are no specific data related to the use of cardiac resynchronization therapy in CS patients, relevant recommendations based on major trials investigating biventricular pacing from the general device guidelines should apply to CS patients [8].

CAVB can be reversible in CS (see case vignette on recovery of atrioventricular conduction in Chap.​ 14), and there may be a role for immunosuppression in attempt to reverse CAVB. Kato et al. described their experience with 20 patients with CAVB and preserved left ventricular function [11]. AVB resolved in four of seven patients treated with corticosteroids (57 %), but did not improve in any of the untreated patients. In another study, atrioventricular conduction improved to normal in four of eight CAVB patients with relatively preserved left ventricular function, but in none of the four patients with advanced left ventricular dysfunction when given steroid treatment [12]. Accordingly, current guidelines state (with a Class IIa recommendation) that immunosuppression can be useful in CS patients with Mobitz II or third degree heart block.

Lastly, it should be noted that CAVB in young patients (less than 60 years of age) should prompt evaluation for sarcoidosis, even in those who do not carry a previous diagnosis of extracardiac sarcoidosis. CAVB can be the initial manifestation of cardiac involvement in patients with a prior diagnosis of systemic sarcoidosis, as well as the first clinical manifestation of sarcoidosis from any organ. This was the case in a Japanese study of 89 consecutive patients with no known history of sarcoidosis with high-grade atrioventricular block requiring permanent dual chamber pacemaker implantation that were prospectively evaluated for cardiac sarcoidosis. Ten cases (11.2 %) of cardiac sarcoidosis were diagnosed, most frequently in young women aged 40–69 (32 %) [13]. Kandolin et al. investigated 72 patients (age <55 years) with unexplained AV block and found biopsy-verified CS in 14 (19 %) and ‘probable’ CS in 4 (6 %) of the 72 patients [14]. Patients with sarcoidosis had a significantly more adverse prognosis when compared to patients with idiopathic AV block in this series.

Extensive granulatomous lesions in the sinoatrial node subendocardium have previously been described in autopsy series [15, 16], and sinus node dysfunction may be an under-recognized manifestation of CS [17]. Figure 9.3 shows an electrocardiogram of a patient with sinus node dysfunction and CS, ultimately leading to sinus arrest, and ultimately a polymorphic VT arrest. Patients with sinus node dysfunction may have an indication for permanent pacing, and if this is the case, dual chamber ICD implantation may be reasonable.