EFFECTS OF NERVE STIMULATION AND HORMONES ON THE HEART 539 



CUMULATIVE WORK 



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FIG. 3. In healthy alert dogs, the principal variables fluctuate continuously in response to stimuli 

 from the external environment or from spontaneous changes in level of activity. In the dog, the 

 changes in ventricular performance during eating are similar to those occurring during exercise 

 and may be as pronounced. 



event detected by an observer are frequently recorded. 

 Most of the spontaneous fluctuations disappear under 

 the influence of standard anesthetic agents, as though 

 the cardiovascular system were completely severed 

 from tlie higher levels of the central nervous system. 



The cardiac responses are altered by repetition of 

 events and by "conditioning." For example, a very 

 prominent cardiac response can be elicited by making 

 a loud noise — as by dropping a metal vvastebasket. 

 If this noise is repeated, the cardio\"ascular response 

 is greatly attenuated, and by the third or fourth 

 trial no vestige of a response may be observed. Even 

 on subsequent days it may not be possible to produce 

 a response as intense as that from the original stimulus. 

 Similarly, the first few cardiovascular adaptations to 

 exercise on a motor-driven treadmill show a progres- 

 sive reduction in the initial overshoot (39). The 

 responses become quite reproducible after the four 

 or five trials in those animals that are easily trained. 

 Some animals do not adapt well to the laboratory 

 situation and resist all efl'orts at training. In these 

 animals, the cardiac responses stabilize very slowly. 



Variation in responses from animal to animal is 

 consistently noted. Although the response patterns in 



a group of animals may be generally similar, they 

 differ in important and distinctive details. In fact, 

 with experience, the records froin a specific dog can 

 be identified in a group of records with quite remark- 

 able accuracy. Obviously, the individual characteris- 

 tics are more consistently recognized in records 

 obtained after the animals are well trained. 



In normal human subjects, certain cardiovascular 

 variables can be recorded continuously (such as, 

 heart rate, systemic arterial pressure, or finger 

 volume). Such records exhibit the same type of 

 spontaneous fluctuations under normal conditions. 

 Very subtle changes in the environment may induce 

 obvious changes indicating the interplay of multiple 

 factors in cardiovascular regulation. The difficulties 

 encountered in attempts to obtain reliable and repro- 

 ducible values for systemic arterial pressure represent 

 a case in point. Also, when finger volume is measured 

 both intrinsic and extrinsic factors produce continuous 

 fluctuations indicating adaptations in the calibre of 

 the blood vessels in the finger. Individual differences in 

 cardiovascular responses are a most challenging 

 aspect of clinical management. Thus, there is little 

 reason to doubt that spontaneous fluctuation of 



