384 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION I 



13. RELATIONS BETWEEN THE ECG AND THE 

 MECHANICAL EVENTS IN THE HEART 



Time Relations in the M'ho/e Heart 



The problem of the electromechanical relations, 

 as far as the heart is concerned, is divided into two 

 different parts : the time relations between mechanical 

 and electrical events, the determination of which is 

 more or less a matter of recording technique; and the 

 causal relations between the membrane potential 

 and the accompanying mechanical changes. In the 

 time relations, the mechanical latency has often 

 been studied. It is obvious that the results depend 

 upon the sensitivity of the mechanical recorder. In 

 skeletal muscles, the mechanical latency is extremely 

 short [less than o. i msec (227)], but such latencies 

 have never been observed in hearts with a reliable 

 mechanical recorder. The first phase of the latency 

 starts with Q (which precedes every mechanical 

 event in the ventricle) and ends with the very first 

 movements visible in the beginning of the heart 

 sounds or the first movement detectable at the ven- 

 tricular surface. Its duration is about 15 msec. The 

 time between the onset of Q and the start of intra- 

 ventricular pressure rise (Umformungszeit, electro- 

 pressor latency) is much longer; about 40 to 50 

 msec (123, 264, 355). It is followed by the rising 

 pressure time of 32 msec, so that the isometric tension 

 time, as the sum of electropressor latency and rising 

 pressure time, lasts about 82 msec (355)- The result 

 of the subdivision, however, is controversial, because 

 different methods lead to very different results. As a 

 whole, the end of QRS is not precisely reflected in 

 the mechanical events, but may coincide more or 

 less with the beginning of a steep rise in intraven- 

 tricular pressure. The mechanical latencies reported 

 for the atria differ considerably, ranging from 50 to 

 90 msec in man (123). 



The latencies so far discussed have been found for 

 the heart as a whole. Local observations of both 

 electrical and mechanical events have been made 

 with the aid of differential mechanographs and a 

 simultaneous record of the electrical events with 

 close bipolar electrodes. Such records in dogs con- 

 firmed previous findings in the turtle, indicating 

 that the mechanical events start in the septal region 

 (apparently near the so-called source region, section 

 8). Local mechanical events start at the peak of the 

 bipolar action potential recorded at that region or 

 not more than 2 msec later, i.e., in the moment when 

 the electrical potentials are at their maximal rate of 

 change (189). 



The question of how long mechanical systole lasts 

 as compared with the ECG has already been dis- 

 cussed (section 11). The end of the systole is a 

 secondary event and cannot yield any correlation to 

 the electrical potentials. In small heart muscle strips, 

 a certain correlation exists and will be discussed 

 later. For the whole heart, the time relations are 

 apparently indirect. Doubtlessly, a pronounced 

 lengthening of QT, combined either with a normal 

 or a shortened mechanical s\-stole, is abnormal and, 

 in the case of shortened mechanical systoles, the 

 sign of a so-called "energetico-mechanical insuffi- 

 ciency"' (33). The problem is, however, extremely 

 complicated and mostly a matter of ionic balance 

 (265), especially in the case of hypopotassemia. De- 

 crease in serum potassium seems to lead to severe myo- 

 cardial damage, an intimate interdependency between 

 QT and metabolic conditions having been proved (73). 

 Nevertheless, we should agree with Burch (136) that 

 the ECG "more reliably indicates the existence, 

 rather than the type and degree, of electrolytic 

 disturbance." Therefore, we here omit all detailed 

 discussion which may be read in the original papers. 



Coupling of Electrical and Mechanical Events 



Doui)tlessl)' the action potential is the first step 

 toward contraction. But we are unable to explain 

 fully this apparently simple relationship. There is 

 no action potential without a mechanical contraction, 

 though it may be extremely feeble. The spread of 

 the mechanical events even seems to imitate the 

 path taken by the electrical excitation: in dying 

 hearts, the contraction wave runs clearly toward the 

 base of the heart (255). There is a real peristalsis 

 running over the heart of the tortoise (229). In 

 very slowly conducting hearts, the contraction, 

 damaged by digitalis, is preceded by a wave of 

 relaxation or distention (120), which seems to cor- 

 relate with the relaxation phase preceding every 

 contraction of skeletal inuscles. 



The linkage of mechanical and electrical events is 

 fairly complicated and can be discussed here only in 

 regard to its most important points. It may be put 

 thus : a close correlation usually does exist, yet, at 

 times, this correlation may appear to be completely 

 absent. Correlations are found in the following 

 observations. Simultaneous recordings of the action 

 potential and contraction of small bundles show that 

 the peak of contraction coincides more or less with 

 the onset of the steep slope of the repolarization 

 wave. The duration of the total action potential 



