Performance on heartbeat counting tasks is usually interpreted in terms of cardiac sensitivity. We tested the hypothesis that heartbeat counting is influenced by beliefs about heart rates by dissociating beliefs about heart rates and actual heart rates. In a within-subjects design, heart rates of 50 patients with cardiac pacemakers were set to a low (50 bpm), medium (75 bpm), or high (110 bpm) pacing rate unknown to the patients via remote control while they performed a heartbeat tracking task. Results showed that patients' heartbeat counting did not follow the shifts in their actual heart rates adequately, although their overall performance was comparable to that of young and healthy control participants. As a result, tracking scores decreased significantly in the high pacing rate condition where beliefs about heart rates and actual heart rates were most extremely dissociated. The findings suggest that tracking scores reflect beliefs about heart rates rather than cardiac sensitivity.

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The circadian variation of acute cardiovascular events may be discussed as an essential factor influencing the circadian variation of habitual daily activities in patients with cardiac disease.

Results confirm the presence of specific psychophysiological stress reaction patterns in ICD-patients. The stress reaction pattern "Play Dead Reflex" could be discussed as a potential risk factor in developing life-threatening tachyarrhythmias leading to an increase in defibrillator discharges in patients with ICD. This hypothesis should be considered in prospective studies and psychosomatic treatment of concerned patients.

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