CARDIAC EXCTTA IITT. IT V— BROOKS 



299 



200 300 



TIME — msec 



Fig. 8. — Electrogram and ventricular pressure pulse showing temporal relationships. Ampli- 

 tude of pressure change in closed ventricle during an extra electrical systole at times (X) and 

 the pressure developed by subsequent systole (Extra + H (o). (Brooks rf al., 1955.) 



free energy changes in muscle and their dependence on temperature. There is a 

 straight-Hne relationship between temperature and the free energy in muscle slices 

 when contracting; at higher temperatures there Js a higher energy change. Ac- 

 tomyosin threads behave in a similar manner. Studies of pressure pulses during 

 hypothermia have shown that the observed decline of blood pressure is not a mani- 

 festation of myocardial failure ( fjerne, 1954). In arousal of hibernating mam- 

 mals pulse pressure increases transiently, then declines as the heart is warmed 

 (Lyman and Chatfield, 1955). In the cooled heart (30° C.) concentrations of the 

 drug dinitrophenol. which decreases work capacity of the normothermic heart 

 (39° C), has no negative inotropic effect (Rothlin ct a!., 1955). Other studies 

 (Hegnauer and D'Amato, 1954) have shown that at 17° C. the work output of 

 the heart per minute is only 7.3 per cent of the control value obtained at normal 

 heart temperatures. The work per stroke, however, is reduced by only 50 per cent 

 at this low temperature. Trautwein and Dudel in a series of papers (1954) showed 

 that progressive cooling of isolated cat papillary muscle and Purkinje tissue of 

 the dog from 38° C. to 19° C. prodticed the following changes. At constant rates 

 of beat there was still a gradual prolongation of isometric contraction and relaxa- 

 tion. Amplitude of the isometric contraction was increased up to a maximum and 

 then decreased progressively. Amplitude of the action potentials increased with 

 the fall in temperatiu'e to a critical point and then decreased. Apparently no single 



