Although the changes were small, and not likely to be risky for normal healthy individuals, the team from University College London, King's College London and Guy's and St Thomas' Hospital found that watching an emotionally charged film clip caused a disturbance to the normal heartbeat and a significant increase in blood pressure.
"Our findings help us to better understand the impact mental and emotional stress can have on the human heart," said Dr Ben Hanson from UCL Mechanical Engineering.
"This is the first time that the effects have been directly measured and although the results varied from person to person we consistently saw changes in the cardiac muscle.
"If someone already has a weakened heart, or if they experience a much more extreme stress, the effect could be much more destabilising and dangerous," Hanson said.
The team devised a procedure where 19 patients, who were undergoing routine cardiac catheterisation treatments using local anaesthetic, were shown a clip from the film Vertical Limit.
Explaining why the researchers chose to use film clips as an emotional trigger, study author Professor Peter Taggart from the neurocardiology unit at University College London Hospitals (UCLH) said: "Film clips are considered to be among the most powerful stimuli to elicit affective responses in the laboratory setting, and have several advantages including their dynamic nature, a sustained effect and the combination of visual and auditory inputs."
The novel method used in the study means that for the first time the biological effects of mental and emotional stress have been recorded in healthy conscious patients.
Electrodes were placed in the ventricles of the heart to measure the changes in cardiac muscle, whilst the team simultaneously recorded changes to blood pressure and breathing speed.
A second part of this study involved the participants recreating the same breathing patterns they exhibited whilst viewing the clip. High levels of mental and emotional stress are well known to increase breathing rates, but until now the effects of this on cardiac muscle have been uncertain.
The results showed that neither blood pressure nor heartbeat were altered by replicating the breathing patterns, suggesting that changes in breathing brought on by a shock do not trigger the observed changes in heartbeat.
"This is the first study where the direct effect of mental and emotional stress on the heart has been observed. It helps us understand the mechanisms involved. Our results are really very exciting," Hanson said.
The study was published in the journal Circulation, Arrhythmia and Electrophysiology.