Thirty day electrocardiogram (ECG) monitoring in patients with hypertrophic cardiomyopathy (HCM) detects more arrhythmias than the standard 24 to 48 hours, according to late breaking science presented at EHRA 2023, a scientific congress of the European Society of Cardiology (ESC).

Up to 20% of patients with HCM develop atrial fibrillation during the course of the disease and are at particularly high risk of stroke. Therefore, guidelines do not recommend the CHA₂DS₂-VASc score to calculate stroke risk but advise starting anticoagulant treatment in all patients with HCM diagnosed with atrial fibrillation. Approximately 20-30% of patients with HCM have non-sustained ventricular tachycardia (NSVT). NSVT increases the risk of sudden cardiac death and has been incorporated into decision algorithms to indicate an implantable cardioverter defibrillator (ICD) – for example HCM Risk-SCD, the prediction model recommended by ESC Guidelines.

The use of 24-48 hour Holter monitoring is recommended to detect atrial fibrillation and NSVT in patients with HCM. Extended ECG monitoring has previously been shown to improve detection of atrial fibrillation in patients with cryptogenic stroke or after pulmonary vein isolation.

TEMPO-HCM examined whether extended ECG monitoring of patients with HCM using a continuous recording system would identify a significantly greater number of clinically relevant arrhythmias compared with shorter measurement. This prospective observational study in five hospitals included consecutive patients with a diagnosis of HCM and a clinical indication to undergo conventional ECG Holter monitoring to screen for atrial fibrillation or for risk stratification of sudden cardiac death. Patients with HCM phenocopies or an ICD were excluded. Participants underwent extended ECG monitoring for 30 days using a dedicated device. The primary outcome was the detection of clinically relevant arrhythmias (atrial fibrillation/atrial flutter and NSVT) during the first 24 hours of monitoring versus the whole 30 day period.

A total of 100 patients were included. The average age was 57 years and 22% were women. Extended ECG monitoring detected a higher incidence of clinically relevant arrhythmias than 24 hour monitoring: 65% vs. 11% (p<0.001). The majority of patients had NSVT, which was detected in 62% of patients across 30 days compared with 8% in the first 24 hours. In patients who developed NSVT during the first 24 hours, tachycardias during the whole monitoring period were faster (174 vs. 152 beats per minute; p=0.001), longer (14 vs. 8 beats; p=0.029) and more frequent (11 vs. 2 episodes; p<0.001). Principal investigator Dr. Juan Caro Codon of La Paz University Hospital, Madrid, Spain said: "The increased detection of arrhythmias was mainly driven by the shockingly elevated prevalence of NSVT among a non-selected and non-high-risk HCM population. Most studies, largely using conventional Holter monitoring, have shown a prevalence of 20-30%."

The median estimated five-year risk for sudden cardiac death according to the HCM Risk-SCD calculator was 1.74% using data from the first 24 hours versus 2.92% using extended monitoring data (p<0.001). Extended monitoring led to reclassification of more than one in five (22.2%) patients to a higher risk category, resulting in 13 (14.4%) additional patients in which an ICD may be considered and 7 (7.8%) additional patients in which an ICD should be considered. Dr. Caro noted: "This was an exploratory analysis because the risk algorithm was developed and validated using 24-48 hour conventional Holter monitoring."

Caution is needed regarding the significance of NSVT episodes during prolonged ECG monitoring. If a certain risk factor is so prevalent among a certain population, it is possible that it does not adequately discriminate the true risk of sudden cardiac death. This is especially relevant in the era of wearables, when a large number of patients will consult due to abnormal findings detected by their own devices. In fact, patients with NSVT during the first 24 hours, which would be detected by conventional Holter, had more aggressive NSVT. In the future, more complete phenotyping of the arrhythmic profile of a specific patient may aid in risk stratification. Extended ECG monitoring may also help, but further research is needed before it is ready for clinical practice."

Dr. Juan Caro Codon of La Paz University Hospital, Madrid, Spain

Regarding atrial fibrillation, extended monitoring detected four more cases than 24 hour monitoring, including three patients without a prior diagnosis of this arrhythmia. Dr. Caro said: "Although the difference between the two study periods did not achieve statistical significance, that may have been true if the sample size had been larger. This may be a signal of true benefit for atrial fibrillation screening and our findings justify further research on extended ECG monitoring for this indication."