Sudden intravenous injections of small amounts of angiotensin or phenylephrine were given to 30 subjects to produce modest, brief increases in directly measured systemic arterial pressure. A plot of each systolic pressure against the second succeeding cardiac cycle length produced a linear distribution, the slope of which was expressed as the millisecond increase in cycle length per mm Hg rise in systolic pressure. The slope is an index of baroreflex sensitivity and was found to have an average value of 12.8 in 18 subjects without hypertension and 2.8 in 12 others with hypertension. When all results were pooled, there was an inverse relationship between the resting mean arterial pressure and slope of the baroreflex regression lines. The findings demonstrate reduced sensitivity of the baroreflexes in hypertension, with respect to control of heart rate. A distinction is made between this change in sensitivity and simple resetting of the reflex.

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SUMMARY1. Leucocyte pyrogen has been injected bilaterally into various parts of the rabbit brain. It caused fever when injected into the pre-optic area and the anterior hypothalamus, but not when injected into the posterior hypothalamus, the mid-brain, the pons, the cerebellum or the cerebral cortex.2. The mean time which elapsed between a leucocyte pyrogen injection into the anterior hypothalamus and the onset of fever was 7-8 min. For similar injections of bacterial pyrogen the time lag was 24-8 min. The mean time lag between bilateral injections of noradrenaline into the anterior hypothalamus and the onset of fever was 7-4 min.3. The amount of leucocyte pyrogen required to cause fever when injected into the anterior hypothalamus was less than 1/100 of that required to cause a similar fever on intravenous injection. The quantity of bacterial pyrogen injected into the hypothalamus was of the same order as that which would cause a similar fever on intravenous injection.4. Control injections of saline, plasma, cerebrospinal fluid, heated leucocyte pyrogen and red cells into the anterior hypothalamus did not cause fever. 5. After attempts to deplete the hypothalamus of its monoamine stores by intraventricular injections of reserpine, the rabbit had fever as a result of an intravenous injection of bacterial pyrogen.6. We conclude that the anterior hypothalamus and the pre-optic area are sites at which leucocyte pyrogen acts to cause fever in the rabbit. The mechanism of this febrile response is not clear, but it appears that part, at least, of the response could be mediated by a mechanism other than release of noradrenaline or failure to release 5-HT.

(Johnson et al., 1969). This is a further demonstration of the biochemical consequences of imbalance between levels of emotional and physical activity. It provides some additional encouragement for those investigating the place of "exercise therapy" in the prevention and treatment of cardiovascular disease.In addition, this study indicates that sudden exposure to intense heat, as in sauna baths, Turkish baths, and presumably the "coffee baths" becoming fashionable in Japan, may contain an element of risk in those prone to coronary insufficiency. This group includes a number of middle-aged men with tiredness, lethargy, and "fibrositis" in the chest and shoulder who will seek out the most gruelling of these artificial environments and stay in longest to "sweat it out of their system." Apart from descriptions such as "my heart was thumping" and "I thought my heart was going to stop" appearing in recent articles on the subject (Observer, 1971), sudden syncopal attacks just on standing to leave the sauna are not uncommon.If any recommendations can be made on the basis of the limited number of subjects in this study perhaps a cautious approach for the middle-aged and elderly sauna novice might be indicated. The cooler, lower levels away from the heater should be tried in the high temperature saunas being promoted in Britain with a time limit of five minutes. However, at the lower temperature of 60-70'C more common in Finnish saunas, Eisalo's (1956) work has shown that a more leisurely approach can be used without apparently undue strain on the cardiovascular system. The recumbent posture customary in these Finnish saunas also helps adequate coronary artery perfusion, rather than sitting or standing in the smaller units found in Britain. For many healthy people the sense of relaxation and wellbeing induced by a sauna will continue to outweigh the potential dangers of the circulatory gymnastics involved. On the basis of these experiments, however, it appears to be neither healthgiving nor heroic to prolong a sauna bath past the point where a person feels he has had enough.