8 The ECG waveform

Abstract

Different portions of the ECG waveform are influenced differently. The QRS waveform is influenced principally by ventricular muscle mass but may be influenced by differences in ventricular filling which occur with cardiac failure or transiently with cord compression. The PR interval normally has a positive correlation with the R-R interval but this correlation becomes negative, probably as a result of catecholamines, when the fetus is stressed. This difference between PR/RR correlation may help distinguish vagally mediated decelerations from those associated with hypoxia. The ST waveform becomes elevated when myocardial oxygen delivery is insufficient to maintain aerobic metabolism to meet the energy demands of the myocardium, and anaerobic metabolism occurs with the production of lactate; this is a dynamic situation and may occur at different levels of oxygenation in different fetuses, depending on their ability to compensate and the workload on the myocardium. It is very likely that changes in PR/RR relationships and the ST waveform will be useful in intrapartum monitoring in the future, and this contention is supported by recent analysis of PR/RR and ST segment shifts (Figure 16). It seems clear that the detailed study of the ECG waveform provides more, and perhaps clinically more useful, information from the same signal and electrode as heart rate, and it is no longer reasonable to ignore it. However, as clinicians we have an extraordinary ability to move with disenchantment from one method of fetal monitoring to another of no proven value, with a firm but unreasoned conviction that it is better. ECG analysis is by no means ready for clinical use if we are to avoid the same pitfalls as continuous FHR monitoring. Further clinical assessment will not be easy. In modern obstetric practice asphyxiated infants are fortunately not common, so there are practical difficulties in monitoring sufficient numbers of patients to evaluate new indices of asphyxia, as well as the difficulties of defining an objective endpoint. Thorough clinical (probably multicentre) studies are needed with well defined endpoints and this is beginning. Further fetal lamb studies are also needed, particularly to examine the interrelationships between changes in heart rate, PR interval and ST waveform; it is hoped that this chapter has emphasized the importance of such animal studies in understanding the physiological and therefore clinical background to any change.(ABSTRACT TRUNCATED AT 400 WORDS)