The Cardiovascular System
An electrocardiogram (ECG) is a recording of the electrical changes that accompany the cardiac cycle; a typical ECG is shown in Figure 7.1.
Fig. 7.1. The structure and origin of the electrocardiogram. A lengthened PR interval may indicate impairment of conduction of impulses from the atrium to the ventricle; the QRS complex is associated with interventricular conduction.
When mice were exposed for 1000 hours to 100 kv/m, 50 Hz, the PR interval and the QRS duration were each lengthened by 19.5% (1). Guinea pigs exposed acutely (30 min.) to the same field exhibited sinusal arrhythmia that began 10-20 minutes after removal from the field, and lasted 10 minutes (1).
Fischer et al. exposed rats to 50 and 5300 v/m, 50 Hz, and observed bradycardia (decreased heart rate) at both field strengths as soon as 15 minutes after commencement of exposure (2). At the lower field strength the effect was about 8%, and this decrease remained statistically significant (p < 0.01) after 2, 10, 21, and 50 days of continuous exposure. At 5 300 v/m the decrease in heart-rate after 15 minutes' exposure was about 16%: it was not seen following 2, 10, or 21 days exposure, but it was present (about a 5% decrease) after 50 days.
Bradycardia was also reported in rabbits following exposure to 50 Hz electric fields (3); at 1000 v/m, the heart-rate decreased by about 9% after 30-60 days. The field also brought about a reduction in the amplitude of the ECG: the P, R, and T waves were each reduced by 40-50%. Another effect induced by the 1000-v/m field was a reduction in the physiological reserve capacity of the rabbits. When the control animals were forced to remain in an erect position the heart-rate increased by 22-32%, but among the animals exposed to the field the range was 34-46%. No effects on heart action were seen at 500 or 100 v/m.
Microwave EMFs have produced alterations in heart function that are remarkably similar to the changes observed at 50 Hz (4). Bradycardia was observed in rabbits after 2 weeks', but not after 2 months', exposure to 0.5 and 3 v/m. The amplitudes of the P, R, and T waves in the exposed animals were decreased by about 50% following 2 weeks' and 2 months' exposure. When pituitrin was injected intravenously into control and exposed (1.5 mo.) rabbits, the resulting coronary insufficiency was stronger, and disappeared more slowly, in the exposed animals.
In preliminary studies, dogs were exposed to 15 kv/m, 60 Hz, for 5 hours to determine whether such exposure altered the physiological response to a controlled hemorrhage (10 ml/kg, over a 3-minute period) (5). The cardiovascular changes (p < 0.05) at the end of the hemorrhage were mean arterial pressure fell an average of 5.9 mmHg in the control group and 16 mmHg in the exposed group; arterial pulse pressure fell 0.9 mmHg in the control group and 10.9 mmHg in the exposed group; average heart-rate decreased 9.3 beats per minute in the control group, but increased 57. 5 beats per minute in the exposed group.
Heart action is one of several factors that influence arterial blood pressure. In studies involving the exposure of rats to 153 µW/cm2, 3 GHz both a short-term hypertensive effect and a long-term hypotensive effect were reported (6). During the first month of the 1 hour/day exposure regimen an increased arterial pressure was seen: beginning with the second month's exposure, the arterial pressure of the exposed animals was consistently lower than that of the controls for the next 5 months. When the exposure was terminated the arterial-pressure difference disappeared within about 1 month.