A common level, SAN, AVN [ME] has been shown on Fig.4.6 at 0.33s [180 beats per minute]. Under the assumption of linearity this gives the gives the control lines on the right of Fig.4.7. intersecting at fs =1 p.u. ME denotes ‘maximal exercise’ and refers to our observation, in collaboration with Professor H. Thomason, that an e.c.g. manifestation which can be interpreted as alternation between SAN and AVN dominated rhythms sometimes occurs in very fit athletes approaching exhaustion under maximal exercise. The heart rate is then of the order of 180 beats per minute [and often higher]. More will be said on this observation in section 4.5.
The period control characteristic of Fig.4.7. implies that, of the two pacemakers, SAN is more sensitive to parasympathetic stimulation and AVN sympathetic stimulation. We take comfort in the fact that there may be physiological justification for this: Hirsch [1970] shows that parasympathetic terminations decrease as one goes down the heart form the SAN [and there are apparently none in the ventricles], whereas sympathetic terminations increase.