CHAPTER 2 ANATOMY AND PHYSIOLOGY OF UTERUS AND FOETAL HEART

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1 10 CHAPTER 2 ANATOMY AND PHYSIOLOGY OF UTERUS AND FOETAL HEART the foetal heart. This chapter describes the anatomy and physiology of the uterus and 2.1 ANATOMY OF THE UTERUS The uterus is a pear shaped organ lying in the midline in the true pelvis. It is composed principally of involuntary muscle. The regions of the uterus are the body (corpus) which is the main part containing the growing conceptus and which has the highest proportion of muscle fibres providing the main expulsive force during labour, the cervix and the isthmus. The uterine musculature consists of a thin outer layer of longitudinal muscle which passes anteriorly from the front of the isthmus over the fundus of the uterus and down to the cervix, a thick layer of spiral myometrial fibres which encircles the cavity and an inner layer of circular muscle as in Figure 2.1.

2 11 Figure 2.1 Uterine Musculature (Dewhurst, 1984) 2.2 NATURE OF UTERINE ACTION The inherent physiological properties of uterine muscle are contraction, retraction which is the permanent shortening of the muscle Fibres, contributing to the dilatation of the cervix and mainly the rhythmicity. The rhythmicity results from the propagation of impulses from the pacemakers which are situated close to the point of entry of the uterine tubes analogous to the pacemakers of the cardiac system (Figure 2.2). It is likely that either one is dominant and the impulses from this spread through the uterine muscle like a ripple passing downwards as in atria of the heart rather than travelling along preformed pathways as in the Bundle of His, at a rate of 2 cm/sec. so that the whole of the uterine muscle is activated within about 15 seconds. At the end of contraction, relaxation of the muscle fibres starts synchronously throughout the uterus so that overall the cervix is contracting for a shorter period than the fundus (Hibbard. 1988).

3 12 Figure 2.2 Propagation of impulse (Hibbard, 1988) 4 The contractions initially occur once in 30 minutes which may last for half a minute. The frequency then increases and by the time active labour is reached, they occur once in every 2 to 3 minutes lasting for 45 to 60 seconds (Chamberlain, 1989). During labour, uterine muscle fibres contract and retract due to which the muscle will not regain its original length. 2.3 AMNIOTIC SAC AND FLUID The chorion and amnion are foetal tissues which form a sac to contain the foetus and the amniotic fluid. The amniotic fluid is derived from the maternal plasma in very early pregnancy and later receives its major contribution from the foetus as extracellular fluid equilibrates across the foetal skin. The main function of this fluid is to provide protection to the foetus. It provides a cushioning effect against external trauma and allows freedom of movement for foetal activity. It also provides protection from infection. In

4 labour, (he pressure on the amniotic fluid allows for the force of uterine contraction to be applied evenly to the cervix ELEC TROHYSTEROGRAM EHG is the recording of the electrical signals due to uterine activity. The EHG recorded simultaneously by internal and abdominal electrodes has good temporal relationships. EHG manifests as burst. These bursts can appear anywhere in the uterus but remain restricted to a small area. As labour progresses, the amplitude and the frequency of the burst increase where each burst associated with an increase in IUP. No uterine electrical activity is recorded between contractions as in Figure 2.3 (Marque, 1986). These suggest that the EHG gives information about the excitation and propagation of uterine activity. (a) RResSURe: TKAcrNG,. (b) H Crt Figure 2.3 Intrauterine pressure Vs EHG tracing

5 14 Marque (1986) indicated in her paper that the EHG recorded by abdominal electrodes is described as a slow electrical wave (0.03 Hz to 0.1 Hz, amplitude lmv to 5mV) on which a fast electrical activity (0.3Hz to 2 Hz, amplitude 50 pv to 1 mv) is superimposed. 2.5 FOETAL HEART This section briefly explains about the foetal heart Development and function Early rudiments of the foetal heart develop in the mesoderm of the cardiogenic area in the form of two parallel endothelial tubes. Soon these tubes fuse across the midline to form a primitive tubular heart. As the tubular heart lengthens, it forms a peculiar S shaped fold, thus bringing the various developing chambers more or less to the definite position. In the mean time the septum is also formed. In the atrioventricular node, the Bundle of His and the sinoatrial node are also developed. The foetal heart is fully developed by the eighth week of gestation Electrical characteristics of FECG The FECG obtained by the means of invasive technique has a QRS magnitude of 100 pv to 200 pv. When the same FECG is obtained from the maternal abdominal electrodes, it has a very low magnitude since the intervening media of tissue between the foetus and the maternal abdominal wall greatly attenuates the signal. The QRS complex of this FECG has a peak to peak magnitude ranging from less than 50 pv to 60 pv depending on electrode location, foetal position and gestational age (Oldenburg, 1977). Since the R

6 15 wave is the only prominent complex in FECG, it is deciding the bandwidth of the signal to be recorded. The highest power density of the foetal R wave lies between 15 Hz and 40 Hz (Bemmel, 1966). This FECG can be recorded from the maternal abdomen during the last 5 months of pregnancy (Favret,1966). From FECG, foetal heart rate (FHR) can be calculated. The uterine activity is found to have effect on FHR value. Hence, EHG and FECG are the two important parameters required in monitoring the progress of labour and the response of the foetus to the uterine contraction.