How should I approach the investigation of a patient with suspected heart block, including the role of ECG and ambulatory monitoring?

When investigating a patient with suspected heart block, the initial and essential step is to perform a resting 12-lead electrocardiogram (ECG) to identify the presence and type of atrioventricular (AV) conduction abnormality, such as first-degree, second-degree (Mobitz type I or II), or third-degree (complete) heart block NICE CKS. The ECG provides immediate diagnostic information about the conduction delay or block and guides further management NICE CKS.

Given that heart block can be intermittent or rate-dependent, ambulatory ECG monitoring, such as 24- to 48-hour Holter monitoring, plays a crucial role in detecting transient or paroxysmal conduction abnormalities that may not be captured on a resting ECG NICE CKS. Holter monitoring is particularly valuable in patients with symptoms suggestive of intermittent heart block, such as syncope or presyncope, but with a normal or inconclusive resting ECG NICE CKS.

In some cases, extended ambulatory monitoring (e.g., event recorders or implantable loop recorders) may be warranted if symptoms are infrequent and initial Holter monitoring is non-diagnostic NICE CKS.

Exercise testing and pharmacological tests (e.g., atropine challenge) can be adjunctive tools to assess the conduction system's response to increased heart rates or autonomic modulation, especially in congenital or isolated AV block cases, as demonstrated in paediatric populations Chammas et al. 1991. These tests help differentiate between physiological and pathological conduction delays and may unmask latent conduction abnormalities [Chammas et al., 1991].

In summary, the investigation of suspected heart block should start with a resting ECG, followed by ambulatory ECG monitoring to detect intermittent blocks, with exercise and pharmacological testing reserved for specific clinical scenarios to further characterize conduction abnormalities NICE CKS; Chammas et al. 1991.